Chapter 15 Miscellaneous Infectious Diseases
Clostridial myositis is a highly fatal disease of cattle caused by the anaerobic spore-forming bacteria Clostridium septicum, Clostridium chauvoei, Clostridium novyi, Clostridium sordelli, Clostridium perfringens, and occasionally other opportunistic Clostridial spp. For clostridial myositis to develop, both the organism and a suitable anaerobic environment for its vegetative growth must be present. Therefore muscle that has been damaged by trauma, penetrating or puncture wounds, lacerations, surgical incisions, or intramuscular (IM) injections of irritating drugs or chemicals is susceptible.
C. septicum and C. perfringens are normal inhabitants of the gastrointestinal tract of most warm-blooded animals. Therefore contamination or inoculation of muscle with these organisms is by exogenous routes (Figure 15-1). Soil and feces may contain C. septicum or C. perfringens. C. septicum has been identified specifically as the cause of malignant edema, whereas C. chauvoei infections are referred to as “blackleg.” It probably is easier to refer to all clostridial infections as clostridial myositis because clinical differentiation of the species involved is sometimes difficult, and laboratory assistance is usually required. Malignant edema—implying any clostridial myositis rather than specific C. septicum infections—also has been used as a general term for clostridial myositis.
C. chauvoei has the most confusing pathogenesis. The organism survives in soil, but it is not known whether it survives in both the vegetative and spore forms or only the spore form. Ingestion of C. chauvoei by cattle apparently allows the vegetative form to proliferate in the gut and then gain entrance to the lymphatics, bloodstream, and finally seed muscle and liver. Having reached the muscle and liver, the organism remains innocuously in the spore form unless the surrounding tissue is injured in some way that creates an anaerobic environment suitable for vegetative growth of C. chauvoei. Exogenous infections of muscle also are possible with C. chauvoei if soil contamination or inoculation of damaged tissue occurs. Farms and soils that harbor C. chauvoei create endemic risk of clostridial myositis for cattle grazing this ground or ingesting crops harvested from such soil. Young cattle appear to be at greatest risk for C. chauvoei muscular infections, and most cases occur in well cared for animals 6 to 24 months of age. However, we investigated a herd epidemic of C. chauvoei myositis that involved several first-lactation cows that ranged between 2 and 3 years of age. The cows in this outbreak had grazed pastures the previous summer, but the epidemic occurred during the winter months and was triggered by muscle bruising and trauma as a result of crowding through a narrow passage created by a frozen doorway (Figure 15-2). C. sordelli may have a similar pathogenesis because it has been associated with muscle bruising in rapidly growing beef cattle.
Regardless of the species of Clostridium causing infection, toxemia and severe myositis ensue. Clostridial exotoxins promote spread of the infection, are detrimental to host defense mechanisms, and propagate the anaerobic environment essential for vegetative growth. C. chauvoei, C. septicum, and C. perfringens produce alpha-toxin, which is a hemolytic and necrotizing lecithinase that is leukocidal and increases capillary permeability. In addition, C. chauvoei produces other toxins such as hyaluronidase; C. septicum produces beta-toxin (deoxyribonuclease and leukocidal), gamma-toxin (hyaluronidase), and delta-toxin (hemolysin); C. perfringens also may produce toxins other than the alpha-toxin, depending on the serotype involved. As a general statement, C. chauvoei and C. sordelli are linked with highest mortality, C. perfringens the least, and C. septicum in between. Occasional cases or specific geographic areas may encounter clostridial myositis as a result of C. novyi, C. sordelli, Clostridium fallax, or other species. C. novyi may cause exogenous infection or endogenous infections (“black disease”) as a result of fluke-induced hepatic activation of spores to vegetative forms.
Regardless of the causative species, most clostridial myositis in dairy cattle occurs by exogenous routes. Any procedure that allows feces or dirt to gain entrance to subcutaneous locations constitutes a risk. Minor surgical procedures, tail docking, IM injections, and neglected wounds predispose to clostridial infections. Tail docking is a relatively new management practice in the northeastern United States, and farmers heretofore unfamiliar with the procedure have been performing it. Clostridial myositis, tetanus, and ascending wound infections caused by other organisms have resulted from dirty tail docking techniques. An epidemic of C. perfringens myositis occurred following tail docking of an entire adult herd (Figure 15-3).
Currently the most common cause of clostridial myositis in dairy cattle is IM injections. Frequent use of prostaglandin makes this drug the most common offender. However, it may not be so much the drug itself but the fact that some people injecting a drug do not clean, nor discriminate among, injection sites. Nor do some always use sterile needles and syringes. Therefore while injecting a drug, they also inoculate the IM site with clostridial organisms that are present on the hair coat or skin of the cow, present in their multiuse syringe and needle, or present in a contaminated multidose vial.
The signs of clostridial myositis include fever, depression, inappetence, toxemia, and a progressively enlarging region of swollen muscle. Lameness is severe if the myopathy involves limb musculature. Initially the skin over the affected muscle is warm, soft, and may have pitting edema. However, the primary muscle site eventually becomes firm, and the overlying skin is dark, taut, cool, and necrotic. Crepitus caused by gas formation may be palpable in the infected muscle. Soft tissue swelling progresses along fascial planes and ascends or descends, depending on anatomic location. Systemic signs are referable to toxemia induced by the potent clostridial exotoxins. Fever usually is present initially, but some patients may become so ill as a result of toxemia that normal or subnormal temperatures are recorded. Heart and respiratory rates are elevated progressively as the pathology worsens. Signs progress rapidly over a 24- to 48-hour course, and few cows survive after 3 days unless therapy is instituted. The clinical course may be so rapid as to be thought a sudden death. Usually, however, progressive swelling, toxemia, and lameness are observed before death.
Dehydration, severe lameness or recumbency, apparent neurologic signs, and shock eventually appear in advanced cases. Terminally, some cattle develop disseminated intravascular coagulation (DIC) or multiple organ failure.
Recent tail docking, recent dystocia with vulvar or vaginal lacerations, or other wounds may provide diagnostic clues. Recent injections may be suspected by “needle tracks” or be reported in the history.
Clostridial myositis must be differentiated from soft tissue cellulitis, phlegmon, abscessation, seroma, or hematoma. In general, the progression of signs is too rapid for consideration of abscessation, and seroma is ruled out by fever and toxemia. Hematoma is ruled out by fever, toxemia, and absence of anemia.
Direct means to ascertain the cause of the infection and differentiate the condition from other causes of cellulitis are indicated immediately. The skin overlying the point of maximal muscular swelling should be clipped and prepared for aseptic aspiration. An aspirate usually reveals serosanguineous or brownish fluid and some gas. Gram staining and culturing the aspirate are indicated. Gram staining can allow a rapid diagnosis because large gram-positive rods are easily found. Muscle biopsies also provide excellent diagnostic samples for cytology, fluorescent antibody identification of clostridial species, and culture. Aspirate or biopsy sites do not bleed as healthy tissue would. In fact, incisions into obviously involved muscle ooze serum and serosanguineous fluid, but the blood supply to the most severely affected muscle is greatly reduced or absent. Gram stains and fluorescent antibody preparations provide the most rapid means of definitive diagnosis. Culture is helpful to identify the causative species but usually is completed too late to help an individual patient.
Although generally C. chauvoei produces more gas and C. septicum more edema (malignant edema organism), much overlap exists in the pathology. It is not possible to speciate clostridial organisms accurately based on the clinical signs they produce in affected cattle.
Blood work is not helpful to the diagnosis because neither a complete blood count (CBC) nor serum chemistry panel offers significant data. The leukogram result is extremely variable in clostridial myositis patients and most often is normal, despite the patient’s overwhelming infection and toxemia. Perhaps even more surprising is the fact that serum creatine kinase and aspartate aminotransferase values are sometimes only mildly elevated. In fact, muscle enzymes released from the region of profound myositis cannot gain access to the peripheral blood because of the self-serving vascular thrombosis and destruction created by clostridial exotoxins. Therefore absorption of enzymes and potassium from affected muscle is prevented by diminished blood supply to the lesion. Necropsy confirms the presence of black, deep red, or greenish red necrotic muscle with gas and fluid in C. chauvoei infections (Figure 15-4). Gas also may be present in other types of clostridial myositis, but edema and discolored muscles are the major lesions in C. septicum and C. perfringens. Serosal hemorrhages also can be observed in many tissues. Dr. John King, veterinary pathologist at Cornell University, likens the odor of affected tissue to the sickeningly sweet odor of rancid butter.
Treatment seldom is successful unless the disease is diagnosed early in its course. Penicillin is the antibiotic of choice to kill vegetative Clostridium spp., and the drug should be used at very high levels (44,000 U/kg IM or subcutaneously [SQ], twice daily). Intravenous (IV) sodium or potassium penicillin given four to six times daily at the same dose is an excellent choice but may be too expensive for use in cattle. Some clinicians believe it is important to inject some of the penicillin into the region of the infection or proximal to the lesion in an affected limb. Sulfa drugs or tetracyclines also have been used successfully against clostridial myositis infections and were responsible for some of the earliest successful treatment in grade cattle. Systemic antibiotic therapy kills only those organisms that can be reached by viable circulation. Therefore it tends to counteract spread into new tissue but may not be able to attain inhibitory concentrations in the most severely affected muscle because of loss of blood supply in this tissue.
Fenestration of affected muscle with skin and fascial incisions allows direct oxygenation of the tissue and subsequent interference with the anaerobic environment required for continued replication of the organism. Acute cases should be fenestrated surgically and the wounds lavaged with saline or hydrogen peroxide. Extensive debridement is not necessary or indicated in acute cases but may become necessary if the patient survives the acute infection and progresses to a sloughing wound.
Analgesics such as nonsteroidal antiinflammatory drugs (NSAIDs) may be used to aid patient comfort, and judicious dosages are necessary if such drugs are used as maintenance therapy, lest toxic gastrointestinal or renal side effects occur.
Improvement is signaled by stabilization of the progressive swelling, resolution of fever, reduced depression, and increased appetite. Antibiotic therapy is required for 1 to 4 weeks and can be reduced according to clinical response. Infected muscle frequently sloughs and may necessitate long-term wound care. Lameness may persist in animals that have prolonged wound healing or that suffer fibrosis and contraction of major muscle groups.
Client education may help prevent the disease when faulty injection techniques, multiple-dose vials, or dirty syringes and needles are suspected as causes. Similarly instruction in tail docking may be helpful. When C. chauvoei is identified as the cause, all young animals should be vaccinated against this organism. Vaccination is highly effective against blackleg and the other causes of clostridial myositis when C. chauvoei has been identified or whenever ongoing management conditions may predispose to clostridial myositis caused by any species. Many effective toxoids incorporating new adjuvants are available commercially and should be used according to manufacturer’s recommendations because some vaccines now claim effective immunization with just one dose. In the past, with the exception of C. chauvoei, most bacterin-toxoids required two initial doses at a 2- to 4-week interval to protect against most species of Clostridium capable of causing myositis. Bacterin-toxoids are best administered after passive maternal antibodies have dwindled. If administered at less than 4 months of age, these vaccines should be repeated at 4 months or older. Herd vaccination programs should include an initial primary course and boosters such that animals have adequate protection before tail docking if performed. The preventative value of annual boosters in protecting adult dairy cattle against clostridial myositis is uncertain but makes empiric sense for both endemic farms and those with no recent history of the disease.
Clostridium hemolytica, an anaerobic organism now renamed C. novyi type D, is the cause of bacillary hemoglobinuria in cattle. This fulminant disease results from peracute proliferation of C. novyi in the liver, resulting in a large necrotic infarct. The infarct and systemic signs of hemolysis and toxemia are caused by a potent beta-toxin, phospholipase C, produced by the organism.
C. novyi is endemic in some geographic areas, especially moist or swampy areas that maintain a high soil pH (approximately 8.0). The spores of C. novyi are extremely hardy and remain in contaminated soil for long periods. Ingested spores apparently are transported to liver and other tissue by lymphatics and blood as happens with C. chauvoei. Cattle harboring the organism can shed it in feces and urine but may remain healthy because C. novyi is a normal part of the gastrointestinal flora and can be found in the liver of healthy cattle.
Livers harboring C. novyi are at risk for converting spores to vegetative organisms if hepatocellular damage occurs. Unvaccinated cattle harboring C. novyi can have the organism activated to the virulent form by liver lesions associated with flukes, liver abscesses secondary to rumenitis, septicemia, and metabolic anoxia of the liver, hepatotoxins, and biopsy. Similar to other clostridia, the organism simply seeks a damaged area of liver with reduced oxygen tension such that anaerobic vegetative growth can occur. Vegetative growth is associated with production of potent exotoxins, including phospholipase C, which then induce hepatic necrosis, hemolysis, and profound toxemia. Liver flukes are the major biologic contributor to disease; therefore bacillary hemoglobinuria is more common in some geographic areas than others and is more common during pasturing of cattle.
The disease usually occurs in adult cattle. Peracute illness with high fever (104.0 to 106.0° F/40.0 to 41.11° C), elevated heart rate, gastrointestinal stasis, cessation of milk production, loss of appetite, an arched stance, and evidence of abdominal pain ensue. Intravascular hemolysis causes progressive anemia, eventual dyspnea, and hemoglobinuria that appears after a significant loss of red blood cells has already occurred. Icterus also may be apparent. The course of the disease is rapid, with most patients dying in 12 to 48 hours.
The disease must be differentiated from acute leptospirosis (usually in calves), postparturient hemoglobinuria (fresh cows in phosphorus-deficient areas), acute pyelonephritis (hematuria, pyuria), hemorrhagic cystitis associated with malignant catarrhal fever, and bracken fern toxicity (enzootic hematuria). In certain geographic areas hemoglobinuria and/or anemia associated with parasitemic diseases such as babesiosis and anaplasmosis should be considered, but these diseases do not tend to be so peracutely fatal and anaplasmosis does not cause hemoglobinuria.
Gross necropsy findings are somewhat pathognomonic in that a large anemic liver infarct and “blackened” kidney are present (Figure 15-5, A and B and C). novyi can be cultured from the lesion. Despite the fact that C. hemolytica can be cultured from normal livers, the presence of the characteristic infarct associated with the organism usually is sufficient for diagnosis. Clostridial fluorescent antibody (FA) tests also can be helpful but may cross-react with C. novyi types B and C. Toxin identification is conclusive but may not be available.
When flukes are involved in the pathogenesis of bacillary hemoglobinuria, infected pastures should be kept off limits to cattle, and the animals should be treated with appropriate and approved drugs to kill flukes. Feeding practices that predispose to rumenitis should be corrected.
Vaccination of calves with commercial bacterin-toxoids including C. novyi type D are protective if administered twice 3 to 4 weeks apart after maternal antibodies have worn off. Subsequent vaccination boosters should be administered twice yearly to cattle at risk.
Leptospirosis in cattle may be caused by non–host-specific serotypes such as Leptospira pomona, Leptospira icterohaemorrhagiae, and Leptospira canicola or host-specific types such as Leptospira hardjo. Pathogenic leptospira are now divided into 13 named species and four genomospecies based on DNA-DNA reassociation studies, with Leptospira interrogans being the most common. The species are further divided into serovars/serogroups and then into strains. The two genospecies L. interrogans and Leptospira borgpetersenii are most important in cattle. Six serovars have been identified in cattle in the United States: pomona, canicola, icterohaemorrhagiae, hardjo, grippotyphosa, and szwajizak. L. borgpetersenii hardjo-bovis appears to be the most common serovar of cattle in the United States. Leptospira australis and Leptospira hebdomadis also have been identified in cattle in Japan.
Leptospira spp. are spirochetes that are considered saprophytic aquatic organisms, and those pathogenic for humans and animals do not appear to multiply outside the host. Infection occurs by penetration of the organism through the mucous membranes of conjunctiva, digestive tract, reproductive tract, skin wounds, or moisture-damaged skin. Hematogenous spread of the organism can result in seeding of multiple organs, including the uterus, and establishment of renal infection. Most Leptospira spp. colonize the renal tubules and are shed in urine for variable periods of time following infection.
Many natural domestic and wild reservoirs of L. interrogans exist that can shed the organism into the environment of cattle. It is difficult to blame any single species in all instances because most of the serotypes are not host adapted. Dogs, swine, rats, mice, horses, deer, and other wild animals may contaminate the environment of susceptible cattle. Cattle are the maintenance host of L. hardjo and appear to be the only reservoir.
Following infection and bacteremia, immunoglobulin (Ig) M antibodies that are agglutinins appear within a few days, whereas IgG antibodies with neutralizing activity appear later. Although agglutinating antibodies help clear the bacteremia, they do not result in resolution of residual renal infection. Non–host-adapted Leptospira spp. may persist in cattle for 10 days to 4 months.
The clinical consequences of leptospira infection in cattle include both septicemic and reproductive disorders, but many leptospiral infections are subclinical and detected by serologic evidence or by presence of lesions of interstitial nephritis at slaughter. The exact prevalence of leptospirosis is not known, but serovar hardjo infection seems to be increasing, whereas serovar pomona infection rates seem to be decreasing. Some estimates suggest herd infection prevalence in U.S. dairies between 35% and 50%, mostly attributable to serovar hardjo.
Both experimental and natural infections with L. pomona have an incubation period of 3 to 9 days. Acute leptospirosis with L. pomona is most common in calves but can be seen in adult dairy cattle. Calves have an acute onset of fever (104.0 to 107.0° F/41.11 to 41.67° C), septicemia, hemolytic anemia, hemoglobinuria, inappetence, increased heart and respiratory rates, and depression. Petechial hemorrhages and jaundice also are possible. Mortality is high in calves less than 2 months of age. Adult cattle with acute L. pomona infections are septicemic, have high fever and a complete cessation of milk flow, accompanied by a slack udder with a characteristic thick mastitis secretion that is red, orange, or dark yellow in all quarters. Adult cattle may show hemoglobinuria and may abort during the septicemic phase.
Subacute or chronic infections are most common in adult dairy cattle and, unless fever, hemoglobinuria, jaundice, or mastitis appears, may go undiagnosed unless epidemic abortions occur. Abortion usually happens several weeks—on average 3 weeks—following septicemic infection of the fetus, and a cluster of animals may abort within a few days or few weeks. Aborted fetuses characteristically are in the last trimester of pregnancy but can be anywhere from 4 months gestation to term. Calves infected in utero during the terminal stages of gestation may be born weak or dead. Because abortion follows infection by such a long time, aborted fetuses are dead and may be somewhat autolyzed. It follows that serum collected from the aborting cow usually will show seroconversion and, in effect, be a convalescent titer because the cow was infected several weeks earlier. Certain geographic areas that support L. interrogans serovar pomona or other serovars pathogenic to cattle have a high incidence of leptospira abortion unless intensive vaccination is practiced. Heifers allowed access to pasture typically abort in late summer or early fall in the northeastern United States. Failure to establish adequate primary immunity in bred heifers that are pastured is the leading management problem predisposing to abortion in this area. A different situation occurs in free stalls, where infection can occur at any time of the year in susceptible cattle exposed to the organism.
Recently L. borgpetersenii serovar hardjo has been reported to cause epidemic or endemic reproductive problems in cattle in the United States. Definitive proof of a causative relationship between L. hardjo infection and abortion in cattle is lacking! This host-associated serovar (hardjo) may have a pathogenesis slightly different from other serovars in cattle in that L. hardjo primarily infects the uterus and mammary gland following septicemia. The subacute to chronic form of infection is most commonly associated with reproductive problems. Studies have demonstrated that cattle naturally infected with L. borgpetersenii serovar hardjo can shed the organism in their urine for indefinite periods, with the maximal shed occurring early in infection. Acute systemic signs are possible when the disease is introduced into a herd and include fever, depression, inappetence, and a flaccid udder that secretes thick yellow to orange milk from all quarters. Abortion is believed to occur most commonly 4 to 12 weeks following initial infection of pregnant cows.
Subclinical infection and possibly abortion are most likely in herds having endemic infection caused by L. borgpetersenii serovar hardjo. Such endemic herds may have resistant adult cows but persistent reproductive problems in first-calf heifers joining the herd. Infertility and early embryonic death are seen with increased services per conception, prolonged calving intervals, and delayed return to heat. The organism is shed from the reproductive tract for several days following abortion and persists in the oviducts and uterus of infected cows for prolonged periods of weeks to months. In addition, the organism can be cultured from the oviducts up to 3 weeks following abortion or calving. Venereal spread also is possible in bull-bred herds.
For acute infections in young calves showing hemoglobinuria, water intoxication is the major differential. Adult cattle showing acute septicemic disease and hemoglobinuria require differentiation from many diseases, including postparturient hemoglobinuria, bacillary hemoglobinuria, babesiosis, hemorrhagic cystitis associated with malignant catarrhal fever (MCF), enzootic hematuria, pyelonephritis, and other diseases causing “red urine.” Seroconversion assessed by comparative acute and convalescent titers is the best diagnostic proof of infection. Although several antibody tests are available, the microscopic agglutination test and enzyme-linked immunosorbent assay (ELISA) are used most commonly. FA techniques or dark field examination also can be used to detect leptospira in urine during acute infections with L. interrogans serovar pomona. A fourfold increase in convalescent titer over acute titer is considered significant and is even expected with most serovars. Vaccination of cattle generally causes a relatively low agglutination titer (400 or usually less).
Leptospira borgpetersenii serovar hardjo does not play by the same rules, however, and titers are more difficult to interpret and quite variable. Titers of antibody against serovar hardjo may be low or negative at the time of abortion.
Because aborted fetuses are long dead and autolyzed, they generally are not helpful to the diagnosis. Therefore serology is indicated for abortion epidemics suspected to be L. interrogans serovar pomona or other non-hardjo serovars and serology coupled with detection of the organism in uterine tissue, fluids, or urine in L. borgpetersenii serovar hardjo abortions. Leptospires or their DNA can be detected by culture, immunofluorescence, special stains of tissue, or polymerase chain reaction (PCR) assay.
Acute cases caused by L. interrogans serovar pomona can be treated with tetracycline or tilmicosin. Because streptomycin has been withdrawn from the market and causes prolonged meat residues, this highly successful treatment in cattle no longer can be recommended. Whole blood transfusions and IV fluids may be necessary supportive measures in the treatment of acute septicemic calves or cattle.
L. hardjo has been treated successfully with a single dose of long-acting oxytetracycline at 20 mg/kg IM, tilmicosin at 10 mg/kg SQ, or multiple injections of ceftiofur sodium (2.2 or 5 mg/kg IM, once daily for 5 days, or 20 mg/kg IM, once daily for 3 days). All have some efficacy in eliminating urinary shedding of L. borgpetersenii hardjo. Amoxicillin administered IM at 15 mg/kg, in two doses 48 hours apart, has likewise been shown to eliminate shedding of L. borgpetersenii hardjo in urine. Following treatment of shedding heifers with a single dose of amoxicillin at 15 mg/kg, no leptospires were isolated from the kidneys at slaughter.
Because treatment of leptospirosis often is unsuccessful, prevention using vaccination is imperative. Whole cell bacterins must be serovar specific for protection to occur. Five-way leptospirosis bacterins (pomona, canicola, icterohaemorrhagiae, grippotyphosa, and hardjo) are most commonly used. Effective prevention against these serovars—with the exception of hardjo—is possible when primary vaccination of calves is followed by twice-yearly boosters. Calves should be vaccinated after maternal antibodies have diminished at 4 to 6 months of age, and two doses of vaccine are essential to establish primary immunity. Boosters are administered at 4- to 6-month intervals thereafter. The most common mistake that prevents effective vaccination is administering a single dose of bacterin to heifers and then not giving them booster shots until 6 to 12 months later, thereby never effecting primary immunization. Effective immunization against L. borgpetersenii serovar hardjo is more difficult, and only a few vaccines have demonstrated efficacy against L. hardjo infections. Monovalent serovar hardjo vaccines have been shown to protect cattle from infection, whereas pentavalent vaccines have not. Currently available monovalent vaccines formulated with L. borgpetersenii serovar hardjo (Spirovac,. Pfizer Animal Health, New York, NY, and Leptavoid, Schering Plough, Coopers Animal Health, Wellington, New Zealand) have been demonstrated to induce both humoral IgG responses and cellular immune responses that confer protection against L. hardjo infection. Although proven disease due to L. hardjo is controversial, vaccination is recommended because other control measures are not available. Isolation of aborting or acutely ill cattle and prompt removal of aborted fetuses may decrease spread of the organism but is seldom a practical means of control. Antibiotic treatment to eliminate the organism in infected cattle should be part of the control strategy because vaccination will not eliminate infection.
Few diseases of cattle (other than perhaps brucellosis) generate the emotional, economic, and public health concerns that tuberculosis does. The consequences of a positive tuberculosis reactor cow or cows may entail depopulation of the herd and economic ruin—despite salvage and indemnity or compensation available through regulatory efforts. Few veterinarians in this generation have experience with the disease in dairy cattle and therefore have assumed the disease to be nearly eradicated and of little concern. However, eradication efforts directed toward tuberculosis have been hampered by confirmation of the disease in captive Cervidae, exotic imports and zoo animals, and cattle from Mexico. Additionally, since 1994 Michigan has recognized bovine tuberculosis caused by Mycobacterium bovis in wild white-tailed deer, with the discovery of tuberculosis in cattle populations since 1998. It is highly unusual to have self-sustaining bovine tuberculosis in a wild, free-ranging cervid population in North America, and it appears that high deer densities and the focal concentration caused by baiting (the practice of hunting deer over feed) and feeding may be responsible for this problem. A resurgence of surveillance efforts currently is underway to safeguard dairy cattle in the United States under the cooperative auspices of state and federal regulatory veterinary services. Surveillance programs have been diminished overall because of fiscal cutbacks at both the federal and state levels, but high-risk herds in areas where the disease has been confirmed or where cattle have had contact with infected Cervidae or Mexican cattle are still supported. Some states and some milkshed regions still mandate periodic tuberculin testing of all herds producing milk or supplying milk to the milkshed. Coupled with this concern of increased risk for certain cattle populations, the resurgence of tuberculosis in people has raised great concern.
M. bovis is the usual cause of tuberculosis in cattle, and the organism is capable of infecting many other species, including humans. Mycobacterium tuberculosis is the causative organism in people and may infect pigs, monkeys, and more rarely cattle, dogs, and parrots. M. bovis is very similar to M. tuberculosis and can infect cattle, pigs, horses, people, and rarelyw cats and sheep. Mycobacterium avium is a distinct species that rarely infects cattle, pigs, sheep, or humans. All three organisms are acid-fast, alcohol-fast, and gram-positive rods. Growth requirements are stringent, and specific media and laboratory techniques are necessary for culturing. Virulence factors include surface lipids such as 6,6′-dimycolyltrehalose or “cord factor” and other factors. The organism can survive in macrophages, in part as a result of interfering with cellular fusion of lysozymes to phagosomes and therefore are intracellular bacteria. M. bovis also produces proteins (stress or heat-shock proteins) that protect the organisms within phagosomes. Metabolic products of M. bovis are toxic for neutrophils, and immune responses to the organism eventually recruit cytotoxic T lymphocytes that kill macrophages harboring M. bovis.
Infection may occur following inhalation or ingestion by susceptible cattle. Inhalation is thought to be the major route of infection for adult cattle, whereas younger animals can be infected by ingestion—especially of infected milk. Following infection, primary lesions form in the infected organ or lymph nodes draining this area. Therefore inhalation of the organism usually results in small primary lesions in the lung. Because of the small size of early primary lesions, however, these lesions may be overlooked grossly, whereas larger lymph node lesions draining the organ may be more apparent. Lymph nodes may confine or “arrest” the infection for a variable length of time before spread to other lymph nodes and viscera, or generalized spread, which can occur in the most severe cases, immunosuppressed patients, or with extremely virulent types. In resistant host species or in highly resistant individuals, the tuberculosis organisms may be confined for extended periods to lymph nodes. Genetic resistance, mediated through macrophage killing of intracellular bacteria, may play a role in relative resistance to M. bovis in many species. Tubercles are the classic pathologic lesions that evolve in primary lesions and subsequent lymph nodes that drain the region. Tubercles result from a frustrated cellular response by the host and microscopically consist of necrotic centers with a halo of macrophages and other mononuclear cells. Calcification is common, and older lesions are calcareous and caseated. In adult cattle, the lesions are most common in the thorax because inhalation is the major source of infection. Advanced or generalized cases can have diffuse lesions. In calves, for which ingestion of the organism appears to be the major route of infection, mesenteric and other visceral lymph nodes usually are affected, and the pharyngeal lymph nodes may also develop lesions. Lesions in the gut itself are uncommon in calves.
Infected cattle shed the organism in sputum, aerosol tracheal exudates, feces (ingestion or swallowing of respiratory discharges), and other secretions, depending on the site and extent of their lesions. M. bovis may remain infective for weeks in feces and also persists for days in moist environments or stagnant water. Reproductive spread, although rare, is possible.
Infected cattle that have clinically detectable lesions represent the minority of infected cattle. When present, clinical signs are extremely variable and often nonspecific. Loss of body condition and failure to thrive with progressive emaciation may occur in patients with more advanced disease. Classic respiratory signs of a chronic moist cough and thoracic abnormalities on auscultation may be the most suspicious signs but do not occur with great frequency. Lymph node enlargement coupled with chronic respiratory disease may result in a higher index of suspicion. Retropharyngeal lymph node involvement may cause either respiratory signs or difficulty in swallowing or eructation. Apparent forestomach or intestinal obstruction may accompany visceral lymph node enlargement. This is usually painless and may be associated with drainage in advanced cases. Udder infections occur in the minority of cases but, when present, have drastic public health ramifications if infected unpasteurized milk is consumed by humans or animals. Fortunately pasteurization destroys M. bovis in milk. Reproductive tract lesions also are rare. Both reproductive and mammary tissue infections usually are accompanied by associated lymph node enlargement.
The majority of positive tuberculin reactors have minimal, if any, detectable lung lesions but are more likely to have detectable lymph node lesions. More frustrating is the fact that some severely infected cattle with generalized lesions may occasionally fail to react at all to tuberculin.
Routine surveillance through intradermal tuberculin tests of herds for milk market regulations and individual cattle for sale (interstate or foreign) and slaughterhouse inspection of carcasses comprise the major means of detection of infected cattle in the United States. Accredited veterinarians perform intradermal skin testing utilizing 0.1 ml of purified protein derivative (PPD) tuberculin into either of the caudal tail folds. The test is read at 72 hours and interpreted as negative, suspicious, or positive. Any suspicious or positive reactor cattle are retested by regulatory veterinary personnel by means of a comparative (avian and bovine PPD) cervical skin test. Historically, many other tests have been used, but few other than the intradermal tests are used currently. Slaughterhouse surveillance and subsequent traceback has been the primary large-scale diagnostic test. Slaughterhouse inspection, however, suffers from a lack of sensitivity because of the small size of lesions in many cattle. Currently a gamma-interferon test coupled with tail fold intradermal testing is being used in the El Paso milkshed area, where endemic tuberculosis exists in several large dairy operations. The gamma-interferon test detects specific lymphokines produced by lymphocytes in response to tuberculosis organisms.
Because eradication of tuberculosis in cattle remains the goal in the United States, positive tuberculin reactors usually are quarantined, identified, and sent to approved slaughter plants. Owners may collect indemnity for these animals and salvage value. When infection is confirmed in positive reactors, depopulation of the herd is recommended, and traceback measures are instituted to test herds that have sold cows to or purchased cows from the infected herd. Large herds, as in the El Paso study, may undergo a quarantine procedure with removal of positive reactors, at least two negative herd tests at 60-day intervals, and finally another test 6 months later. Unfortunately this procedure does not always rid the herd of infection. The U.S. Department of Agriculture, Animal and Plant Health Inspection Service, Veterinary Services has found that, in large infected herds, up to 40% of infected herds remain infected despite testing and quarantine procedures. Therefore depopulation of infected herds is the most helpful procedure when eradication is desired. Inherent errors in skin testing constitute the major reasons for failure of compromise programs. False-positive reaction (no gross lesions) may occur in cattle sensitized to other mycobacteria, including human or avian tuberculosis, Johne’s disease, and “skin tuberculosis.” False-negative reactions may occur in advanced cases, recently infected cattle, desensitized cattle, or old cattle. The current PPD tuberculin test is considered to have approximately 85% sensitivity and 98% specificity. The gamma-interferon test has similar sensitivity and specificity. However, on a herd basis—because of the current low level of tuberculosis—skin tests may have a low predictive value. In infected herds, the predictive value increases. In addition, attention to detail and technique by the testing veterinarian also can influence results.
Many states, including New York, no longer support regular tuberculin testing of all cattle but do require testing of cattle in a “high risk” category. Animals considered at high risk may include herds associated with captive Cervidae or those near exotic animal farms or zoos. High-risk herds obviously also include those found by traceback epidemiology from infected herds.
Accredited free states have had no known tuberculosis herds for 5 years. Such states will have this classification suspended or revoked when one or more cattle or bison herds are identified within a 48-month period. Vaccination using Bacillus Calmette GuÃ(c)rin (BCG) as practiced in some areas is not recommended in the United States. Similarly treatment of infected cattle is not allowed.
Lymphangitis of the lower limbs occurs sporadically in cattle. The lesion has occasionally caused a false-positive tuberculin test. Although usually present in tuberculosis-free cattle, the lesion also has been found in infected cattle. The major concern raised by the lesion is the frequency with which affected cattle react as suspicious or positive to tuberculin testing. In the past, such cattle have been labeled as “skin reactors.”
Organisms that probably are saprophytic and acid-fast have been observed within the lesions, but classification of these organisms and isolation on selected media have not been accomplished. Intradermal transmission of infection through ground tissue samples has been successful in only one report. The lesions are theorized to develop secondary to front or lower limb injuries that allow seeding of the lymphatics. Affected cattle usually are healthy otherwise.
Similar lesions have been identified in cattle associated with infection by Corynebacterium pseudotuberculosis. In these cases, the lesions are restricted to the lower limbs with or without lymph node enlargement. The ulcerative lesions may discharge a clear, gelatinous exudate. Infection of cattle with C. pseudotuberculosis can also cause granulomatous cutaneous abscesses, typically located on the exposed lateral face, neck, thorax and abdomen, or less commonly mastitis and visceral infections. Because the clinical signs are markedly different than ulcerative lymphangitis, these will be discussed below.
Multiple subcutaneous nodules in the metacarpal or metatarsal region are the primary lesions. One or more limbs may be affected. The nodules ulcerate periodically and discharge pus that varies from serous to caseated. Mild lameness may be apparent before ulceration and discharge as the nodules swell and become inflamed. Lameness resolves as drainage occurs. Over time, the nodules may coalesce or form knotted cords of tissue that mainly is subcutaneous but may have a dermal component as well (Figure 15-6). Other than periodic mild lameness and ulceration, systemic signs are absent.
Smears of pus or biopsy may allow identification of acid-fast organisms. Culture may be unrewarding, and saprophytic acid-fast bacilli are suspected to be the cause, but in other cases C. pseudotuberculosis may be identified. Suspicious tuberculin reactions in such cattle in noninfected herds usually are considered “skin reactors,” but a positive reaction may require notification of regulatory veterinarians who may elect a comparative cervical test. Parenteral administration of penicillin or tetracycline may be useful in treatment.
C. pseudotuberculosis is commonly known as the cause of caseous lymphadenitis in sheep and goats and pigeon fever in horses but seldom is mentioned as a cause of disease in cattle. However, in California and occasionally other areas of the United States, it is identified as the cause of ulcerative necrotic skin lesions in cattle. As in small ruminants, the organism tends to become endemic in certain herds, and clinical manifestations occur as sporadic instances.
The organism survives in soil, the environment, and within infected tissues for long periods. It is generally believed to require an entry site such as mucosal or skin injury, abrasion, or laceration to infect a host. Once through the skin or mucosal barrier, the organism travels through lymphatics to lymph nodes or other tissues. In the horse, C. pseudotuberculosis becomes a facultative intracellular organism that survives in phagocytes and also possesses many potential weapons to maintain itself in the host such as an exotoxin (phospholipase D) that attacks sphingomyelin in erythrocytes and capillary endothelial cells. The organism also possesses a pyogenic factor and surface lipids, which may be toxic to phagocytic cells. All of these factors contribute to chronicity and maintenance of host infection by the organism and are well recognized in small ruminants.
In cattle, the cutaneous ulcerative lesions exude pus and are typically located on the exposed lateral face, neck, thorax, and abdomen. Affected cattle do not usually show other signs of disease, and the lesions may heal spontaneously in 2 to 4 weeks, although healing may be enhanced by drainage or surgical debridement. The infection often occurs as a herd problem, and up to 10% of cattle in a herd may be affected. The disease occurs more frequently in adult cattle than primiparous or nulliparous heifers. Spread of infection is apparently enhanced where housing and handling facilities cause abrasion to the lateral body surfaces. It has been assumed that skin trauma and contamination of minor skin abrasions by the organism are causative features of the disease. Affected animals are often culled. The disease has most commonly been seen in dairy cows in the arid western United States and Israel and occurs more frequently in the summer months.
Signs in cattle consist of large ulcerative lesions on the sides of the face, neck, or trunk. Necrotic material accumulates in the lesions, and granulation tissue is present deep to the necrotic material. Affected cattle do not otherwise appear ill, although decreased milk production by 4% to 6% may be noted. Sporadic cases may be recognized in endemic herds.
The lesions must be differentiated from actinobacillosis granulomas, other granulomatous masses, and tumors. Culture confirms the diagnosis, and biopsies differentiate the lesion from tumors and granulomas.
Babesiosis is a protozoan disease of cattle that has been eradicated from the United States thanks to control of the causative ixodid ticks. The disease is also called Texas fever, redwater, piroplasmosis, or tick fever in cattle. Babesiosis may be caused by six or more species of Babesia that are divided morphologically into large or small types. The major large species is Babesia bigemina, and the major small species is Babesia bovis. The disease is seen primarily in tropical and subtropical climates but remains a threat to the United States from Central America and Mexico.
B. bigemina appears as paired pear-shaped bodies within erythrocytes and is transmitted by Boophilus spp., usually Boophilus annulatus. Ticks are infected by feeding on infected animals and subsequently infect their larvae through transovarian passage. B. bigemina continues to develop in the larvae, nymph, and eventually adult ticks, which then transmit the disease to susceptible cattle through bites. Other insects and blood-contaminated instruments also may transmit infection, but ticks are the major vector. Infection is most likely to cause clinical disease in cattle older than 6 months of age because calves are thought to have colostral passive protection or unique erythrocyte protective factors (or both) that protect against infection before 6 to 9 months of age.
B. bovis appears as a single, multiple, or paired complex within erythrocytes. The single and multiple organisms are rounded, whereas pairs may be pear shaped but joined at a more obtuse angle than B. bigemina. Erythrocytes infected with B. bovis are less numerous and more difficult to identify than with B. bigemina infection, and the propensity of B. bovis infection to localize in the capillaries of the brain has made microscopic examination of the brain a successful diagnostic test. Multiple Boophilus spp., including B. annulatus and Boophilus microplus, can transmit B. bovis, and the larval stage is the major source of infection.
Fever, anemia, hemoglobinuria, icterus, weakness, anorexia, depression, and gastrointestinal stasis are frequent signs of B. bigemina infections. Tachycardia, dyspnea, and pallor progress as erythrocyte destruction increases. Abortion sometimes is observed. A hemolytic anemia is responsible for intravascular hemolysis and the subsequent hemoglobinuria and jaundice. Mortality may exceed 50%. Ticks are present on affected cattle or herdmates.
B. bovis infections in cattle may be indistinguishable from those caused by B. bigemina, but the degree of anemia and hemoglobinuria frequently are less severe than that observed in B. bigemina. Relative host resistance and the organism can have an impact on the severity of disease. Neurologic signs, including opisthotonos, seizures, excitability, depression, or coma, are common in B. bovis infections and may explain mortality in cattle that are judged to not have life-threatening anemia. Neurologic signs are related to the propensity of infected erythrocytes to accumulate within capillaries in the brain.
Cattle that survive after the acute signs of babesiosis may have chronic disease, remain carriers, suffer recurrent infections, or die from secondary infections. Recovering cattle experience prolonged production compromise.
Babesiosis must be differentiated from other causes of hemoglobinuria, hemolysis, fever, and jaundice. Therefore bacillary hemoglobinuria, leptospirosis, postparturient hemoglobinuria, toxic hepatopathies, and chronic copper poisoning may be considered in the differential diagnosis. When neurologic signs appear in B. bovis infections, differentiation from other diseases of the central nervous system is required. Few other neurologic diseases cause hemolysis and hemoglobinuria, however. Anaplasmosis can lead to similar signs, but hemoglobinuria is absent in anaplasmosis.
Confirmation of babesiosis requires ancillary tests in addition to the suggestive clinical signs. B. bigemina is more likely to be observed on Giemsa-stained blood smears than B. bovis, but both organisms are more likely to be found in acute infections than in chronic cases. Antibodies against Babesia sp. may appear in the blood of infected cattle within 1 to 3 weeks and are sought by complement fixation (CF) or indirect FA tests. The FA test may be more sensitive and can detect antibodies for a longer period following infection than the CF. Brain biopsies also have been used in the diagnosis of B. bovis infections but are obviously of no value antemortem. Other serologic tests are being evaluated, but all suffer from a lack of availability.
Successful treatment is possible with a number of chemotherapeutic agents. A dilemma exists in that early effective therapeutic intervention that kills all parasites may deter effective immune responses and leave the patient subject to rapid reinfection.
Trypan blue, an early effective treatment for B. bigemina, is not effective against B. bovis and other small Babesia spp. Imidocarb (1 to 3 mg/kg) successfully treats both infections, as do several other diamidine derivatives such as diminazene diaceturate (3 to 5 mg/kg) and aminocarbalide disethionate (5 to 10 mg/kg). Other successful treatments include quinoline and acridine derivatives.
Tick control is essential and certainly, based on the U.S. experience, necessary for eradication of babesiosis. Many effective acaricides currently are available (see Chapter 7). In some countries, tick control rather than complete eradication is practiced in the hopes of maintaining a low level of vectors to effectively immunize cattle but not enough to result in severe or widespread disease.
Vaccines for both B. bigemina and B. bovis have been used in some areas (e.g., Australia) but are not available commercially and may require judicious use of chemotherapy when live organisms are used. With recent progress toward completion of the B. bovis genome project, more effective vaccination strategies combining genomic and proteonomic approaches may be forthcoming.
A rickettsial organism, Anaplasma marginale, is the cause of anaplasmosis in cattle. The organism parasitizes red blood cells following infection of susceptible cattle and is transmitted by ticks, biting insects, and introduced mechanically by blood-contaminated instruments that penetrate skin.
Dermacentor andersoni, other Dermacentor species, and B. annulatus are biologic vectors that pass A. marginale through their eggs into the next generation of ticks. Other ticks, tabanids, and mosquitoes may be mechanical vectors of the disease as they inject blood from infected cattle to susceptible cattle while feeding. Needles and veterinary instruments that become contaminated with blood during herd-wide procedures can transmit the infection. Similarly, blood-contaminated instruments used for reproductive work such as infusion cannulas, embryo transfer instruments, and insemination equipment occasionally can spread the infection.
Chronically infected cattle that usually are asymptomatic act as reservoirs of anaplasmosis, and spread of the disease tends to occur during peak vector seasons or following common surgical procedures that result in iatrogenic spread.
Cattle less than 1 year of age tend to either be resistant to infection or have very mild signs of illness. The opposite is true for adult cattle because susceptible animals more than 2 years of age often have severe illness and possible high mortality. The resistance of young animals to infection may be explained partially by passive antibodies obtained from colostrum. However, other factors appear to be important because infection in susceptible cattle up to 1 to 2 years typically results in mild signs, if any, whereas infection of susceptible cattle older than 2 years frequently causes acute, severe disease. In addition, sources of stress such as shipment, starvation, weather extremes, and experimental splenectomy apparently can overcome the natural resistance of young cattle to anaplasmosis, thereby resulting in acute disease.
Natural infection of young cattle results in a carrier state that may persist for the life of the animal. A biologic balance appears necessary to maintain immunity because clearing of infection eventually may allow susceptibility to reinfection. Seroconversion may occur despite the chronic carrier state, although it does not occur in all infected carrier cattle. Protective immunity requires both humoral and cellular immune components, including antibody against the outer cell membrane plus macrophage activation. The immune response can clear the acute rickettsemia but fail to completely clear the infection because of development of antigenic variants of the agent. Seropositive cattle are assumed to be carriers. Seronegative cattle in endemic areas are more difficult to categorize and the subject of much research. Further confusion is added by studies that demonstrate acquired immunity to clinical disease persisting following clearance of infection by chemotherapy. This immunity following chemotherapy with imidocarb or tetracycline persisted regardless of seropositive or seronegative status of the treated cattle. However, in endemic regions harboring anaplasmosis, seronegative cattle within A. marginale-infected herds appear susceptible to infection and illness. Therefore seronegative cows in positive herds have not necessarily developed effective immunity even if they had been seropositive previously and naturally cleared the infection later. Relative exposure rate, concurrent stresses, vector loads, and length of time between clearance of infection and subsequent reinfection all may influence the susceptibility of seronegative cattle that once had been seropositive.
As previously stated, the likelihood of clinical illness associated with A. marginale infection is typically proportional to the age of the susceptible animal. Exceptions do occur, especially when extraordinary stress, heavy infective doses, heavy vector parasitism, or concurrent diseases overwhelm the apparent resistance in younger cattle. Many, if not most, animals less than 1 year of age have inapparent infection or very mild signs. Incubation in natural infection ranges from 20 to 40 days and is followed by acute disease characterized by dramatic signs of fever (104.0 to 107.0° F/40.0 to 41.7° C), depression, anorexia, gastrointestinal stasis, anemia, dehydration, and cessation of milk flow. The severity of clinical signs is proportional to the degree of anemia. Icterus is present in many acute cases but may not appear unless the affected animal survives 2 or more days. Hemoglobinuria does not occur. Hemolysis results from erythrocyte destruction by the reticuloendothelial system and therefore is primarily extravascular. Mortality varies but may reach 50% in acute cases. Infected cattle that survive acute signs may remain weak, anemic, jaundiced, and lose significant condition. Susceptible adult cattle introduced into endemic herds may suffer peracute signs and die within 1 to 2 days after onset of signs. Infected animals are assumed to remain carriers of the organism regardless of the degree of subsequent seropositive status. Recovery from acute disease may require weeks. Abortion may occur during the acute or convalescent period.
The CF and rapid card agglutination tests are the most common means of confirmation of infection but may not become positive until 1 week following acute infection. These same serologic tests are very useful to detect chronic carrier cattle that may be free of clinical signs. Diagnosis in acute cases is aided by ancillary tests that verify the severe anemia (low packed cell volume and regenerative) and also rule out liver disease as a cause of jaundice. Microscopic examination of whole blood smears stained by Wright’s, new methylene blue, or Giemsa stains may allow identification of A. marginale in erythrocytes (Figure 15-7). The organisms appear as one or more spherical bodies in the periphery of erythrocytes and must be differentiated from basophilic stippling and Howell-Jolly bodies. PCR and competitive ELISA are newer and more sensitive tests and should be used to help determine infection and clearance of infection following treatment.
Treatment with several chemotherapeutic agents is possible but inconsistently effective in clearing the organism. The most current recommendations in North America indicate oxytetracycline to be the treatment of choice. In Europe the fluoroquinolones could be used. A variety of tetracyclines can be used, and intensity of treatment may dictate whether the organism is eliminated or simply reduced in number within the host. Imidocarb dipropionate (5.0 mg/kg IM in two doses at 14-day intervals) will sterilize infected cattle, but this drug is not used in cattle in North America. Long-acting tetracycline (20 mg/kg IM—four times at 3-day intervals; Liquamycin LA 200, Pfizer, Inc., Animal Health Division) also eliminates the infection in some calves. Lesser numbers of injections of this same long-acting tetracycline may control acute infections but not eliminate the organism completely. Cattle cleared of infection may eventually be susceptible to infection again. Whole blood transfusions also may be necessary when anemia is judged to be life-threatening in acutely infected cattle.