CHAPTER 49 Bacterial Causes of Bovine Infertility and Abortion
Bacteria generally are cited as the most common agents of infection causing abortion in cattle. In large retrospective studies, bacterial infections of the fetoplacental unit were identified in 15% to 16.2% of aborted bovines and accounted for 48% to 58% of abortions caused by infectious agents. Abortion in cattle has been associated with more than 25 different species of bacteria, which vary considerably in their pathogenicity for livestock (Table 49-1). It appears that if a bacterium can survive transit in the maternal bloodstream to reach the fetoplacental unit, it has the potential to cause abortion. The fetus may be particularly susceptible to infection by a wide variety of organisms as a result of its immature immune system and suppression of the immune response at the junction of the maternal and fetal placenta.
|Agent||Frequency (% of total bacteria isolated)|
|Campylobacter fetus spp. veneralis||1.5|
|Campylobacter fetus spp. fetus||1.3|
* Bacterial species encountered in 8962 bovine abortions or stillbirths.
Data from Kirkbride CA: Bacterial agents detected in a 10-year study of bovine abortions and stillbirths. J Vet Diagn Invest 1993;5:64.
Diagnostic laboratory data have provided valuable insight into the relative prevalence of bacterial species that contribute to the development of bovine abortion. Unfortunately, similar information is not available for bacterial causes of infertility. Campylobacter fetus spp. veneralis, Leptospira, Mycoplasma, Ureaplasma, Chlamydia, and Haemophilus somnus all have been associated with bovine infertility, but at present the relative prevalence and importance of these agents are unknown.
A number of the bacterial causes of bovine abortion and infertility are potential zoonoses. The agents of these diseases include Leptospira, Brucella, Listeria, Chlamydia, Salmonella, and Campylobacter spp. Care should be taken in handling and transporting the uterine discharge or products of abortion, to minimize or eliminate opportunities for human exposure.
BOVINE ABORTION CAUSED BY OPPORTUNISTIC BACTERIA
By far the larger portion of this chapter discusses the specific agents recognized as potential causes of bovine abortion. The vast majority of bacterial abortions in cattle (82%), however, are caused by organisms generally considered to be opportunists. These opportunistic organisms fall broadly into two overlapping categories: (1) bacteria that are part of the normal microflora of the mucosal surfaces (Arcanobacterium pyogenes, Pasteurella multocida, Mannheimia hemolytica, Haemophilus somnus, Escherichia coli, Campylobacter spp., Staphylococcus spp. Streptococcus spp.), and (2) common environmental bacteria (Bacillus spp., Pseudomonas sp, E. coli). These ubiquitous bacteria occasionally are able to enter the maternal bloodstream, survive transit to the fetoplacental unit, and cause abortion. Such agents are not considered to be contagious or transmissible causes of abortion.
The significance of isolating one of these agents from an aborted bovine conceptus depends on the incidence of abortion and the clinical signs in the herd. Recovery of any of the aforementioned organisms in isolated abortions generally is of minimal significance from a herd health standpoint. Isolation of one or more species of opportunistic bacteria from fetuses during an outbreak or ongoing abortion problem suggests that (1) access of these ubiquitous organisms to the maternal bloodstream has been enhanced or (2) abortions are a consequence of another disease process.
Although difficult to determine with certainty in retrospect, the occurrence of abortions may be a function of factors that enhance access of ubiquitous bacteria to the maternal bloodstream. Such factors could include subclinical or clinical acidosis (rumenitis), external lesions, or poor feed quality leading to microtrauma in the upper digestive tract. Anecdotal information from diagnostic laboratory submissions would support a link between subclinical or clinical acidosis and bacterial abortion in both dairy and beef cattle.
Pathogens of adult animals, such as Salmonella, Mannheimia hemolytica, H. somnus, and P. multocida, can cause abortion in one of two ways. With the exception of Salmonella, these organisms are ubiquitous members of the normal microflora and can behave as opportunists. They also may cause abortion as a result of disease processes in the adult, in which case abortion is a secondary consequence of the organism’s ability to cause enteric or respiratory disease, with bacteremia leading to fetal infection. Although these agents can be considered contagious pathogens, they typically are not classified as contagious causes of abortion.
Diagnosis is based on the isolation of a moderate to heavy and relatively pure growth of a bacterium from fetal tissues and/or placenta in conjunction with confirmatory gross and microscopic lesions. The four tissues from which bacteria are most commonly isolated are placenta, abomasal contents, lung, and liver. Culture of placenta is somewhat problematic because it often is contaminated by bacteria from the vaginal or vulvar region or the environment. Nonetheless, it is imperative to submit placenta for culture because the infectious process may be con fined to the placenta, so that the organism will not be recovered from fetal tissues or fluids. Gross lesions that may be observed in association with abortions due to opportunistic bacteria include placentitis, fine strands of fibrin covering the viscera, and occasionally epicarditis (in abortion due to Bacillus spp.). Microscopic lesions typically include changes characteristic of a placentitis, suppurative fetal bronchopneumonia, and occasionally inflammation of the liver.
Treatment and Control
Control of multiple abortions due to opportunistic bacteria is based on diminishing opportunities for ubiquitous organisms to gain access to the maternal bloodstream and survive transit to the fetus. This is accomplished by optimizing the health status of the cow (nutrition, stress, environment) and addressing factors associated with enhanced access of these bacteria to the maternal bloodstream, such as acidosis, poor feed quality, and presence of external lesions. Control of abortions that occur as a consequence of an infectious process in the dam is based on controlling the underlying disease process in the cow.
BOVINE ABORTION AND INFERTILITY RELATED TO INFECTION BY SPECIFIC PATHOGENS
Brucellosis once was considered to be the most important reproductive disease of cattle. Today, however, the State-Federal Brucellosis Cooperative Eradication Program, launched in 1934, as well as subsequent programs, has resulted in the elimination of this disease from commercial herds in the Unites States. As of December 2000, no known commercial cattle or bison herds were infected with Brucella in the United States. Currently the only remaining focus of brucellosis in the United States is in the Greater Yellowstone Area, which includes Yellowstone National Park, Grand Teton National Park, the National Elk Refuge, and portions of Idaho, Montana, and Wyoming, where free-ranging bison and elk are infected. These animals pose a disease threat to cattle in surrounding brucellosis-free areas and serve as a potential source for disease reintroduction into domestic cattle and bison herds. Transmission from bison to cattle has been demonstrated when Brucella-infected bison were penned with unvaccinated, seronegative cattle. Documented cases of natural transmission of Brucella from bison or elk to cattle have not been reported, however.
Currently, 46 states, Puerto Rico, and the U.S. Virgin Islands are classified as brucellosis-free, meaning that they have had no infected cattle or domestic bison herds for at least 1 year. Four states—Florida, Missouri, Oklahoma, and Texas–are close to complete eradication of the disease and, with no known infection at this time, are now in the final 1-year countdown phase.
Continued surveillance is necessary to prevent this insidious disease from once again gaining a foothold in U.S. domestic populations. It has been estimated that if brucellosis were allowed to spread, beef and dairy production costs would increase by $80 million within 10 years.
Agent, Source, and Epidemiology
Brucella abortus is a small, nonmotile, nonsporulating, nonencapsulated, gram-negative coccobacillus. Brucellae are facultative intracellular pathogens that are able to survive and multiply within phagocytic cells and lymphoid tissues as a result of their ability to prevent fusion of lysosomes with the phagosome.
The infected cow is the principal source of Brucella. The aborted fetus, placenta, and uterine discharge contain large numbers of organisms. Transmission typically is the result of ingestion of the products of a Brucella-induced abortion (fetus, placenta, uterine discharge) or ingestion of material contaminated by these products. Brucella may be present in uterine discharges beginning 2 weeks before calving or abortion, and lasting 2 to 3 weeks thereafter. The incubation period is quite variable, ranging from 2 weeks to 1 year or longer. The minimum incubation period from infection to abortion is approximately 30 days.
In the bull, B. abortus infection may produce orchitis, epididymitis, and inflammation of the accessory reproductive organs. Orchitis may lead to reduced libido and spermatogenesis and impaired fertility. Although the organism can be transmitted in semen, the bull appears to play a minor role in the transmission of Brucella.
Brucella penetrates the mucosa of the nasal or oral cavities after ingestion. B. abortus initially localizes in lymph nodes, infects the gravid uterus during bacteremia, and multiplies to massive numbers in chorioallantoic tro phoblasts, leading to trophoblast necrosis and chorioallantoic ulceration. Fetal bacteremia follows replication of Brucella organisms in trophoblasts. Once infection is established in sexually mature animals, it tends to persist indefinitely. Not all infections result in abortion, however, and less than 20% of infected cows abort more than once.
Intracellular replication in chorioallantoic trophoblasts is responsible for the massive accumulation of B. abortus in the placenta. The growth of Brucella organisms in trophoblasts may be enhanced by their erythritol and/or hormone content, because both erythritol and progesterone augment the growth of B. abortus in vitro.
The primary sign of brucellosis is abortion. Abortions generally occur after the fifth month of gestation. Retained placenta and metritis are frequent sequelae to abortion. Brucella-infected cows also may give birth to weak calves that die shortly thereafter.
The most consistent lesion observed in the conceptus is placentitis. In severe cases, the intercotyledonary placenta may be dry, thickened, and cracked and has been described as having the appearance of Moroccan leather. The intercotyledonary placenta may be covered by a thick, yellowish exudate. Cotyledons may demonstrate foci of necrosis and be covered by an exudate. Fetal lungs occasionally are enlarged, firm on palpation, and covered by fine strands of fibrin.
A definitive diagnosis of Brucella abortion requires isolation of the organism from fetal tissues or uterine discharge. The organism can be readily isolated from fetal abomasal contents, lung, placenta, and uterine fluids. It also may be recovered from colostrum, milk, and meconium. In the face of suspicious lesions and negative culture results, immunohistochemistry (IHC) studies can be used to confirm Brucella infection. After a diagnosis of Brucella abortion, additional infected animals generally are identified by serologic testing. Such testing may not identify all infected animals, because up to 15% of infected cows do not seroconvert until after abortion or calving.
A new systemic vaccine for brucellosis, Strain RB51, has been approved for use in cattle. The advantage of this vaccine is that it induces a protective response without stimulating the development of antibodies that react with standard assays based on B. abortus LPS oligosaccharide side chains.
Treatment and Control
Control of brucellosis traditionally has been based on vaccination of calves to reduce the population at risk, control of cattle movement to diminish the spread of disease, and testing to identify reactors. Currently, treatment has no role in the control of bovine brucellosis. Brucellosis is a reportable disease, and regulations require herd quarantine and elimination of all reactors. The current focus of brucellosis control is on surveillance, with rapid elimination of new Brucella infections.
Campylobacter fetus spp. veneralis Infection
Bovine genital campylobacteriosis is a sexually transmitted disease caused by the gram-negative, microaerophilic rod Campylobacter fetus subsp. veneralis. This organism is an obligate parasite of the bovine reproductive tract. Bulls are asymptomatic carriers. The clinical effects of infection are manifested in the cow. Historically, campylobacteriosis has been one of the most important sexually transmitted diseases of cattle. Economic losses are the result of poor conception rates, increased culling due to infertility, decreased average weaning weights, and increased management costs related to prolongation of the calving season.
Agent, Source, and Epidemiology
Campylobacter fetus spp. veneralis is an extracellular, motile, gram-negative, microaerophilic rod. Cattle constitute the primary host and main reservoir for this organism. Reproductive tract infection in cows and young bulls typically is transient. Bulls younger than 3 years of age tend to be resistant to infection and clear the organism within a few weeks. Mature bulls (4 years of age and older) become chronic carriers.
The disease is transmitted by coitus. The transmission rate from infected bulls to susceptible cows may approach 100%. Infected bulls may carry the organism in the preputial cavity indefinitely. Bulls do not become permanent carriers until they are at least 4 years of age, however, and most not until 5 to 6 years of age. The development of epithelial crypts in the penile mucosa with advancing age provides a favorable habitat for the bacteria. Because infection in young bulls is transient, transmission by these animals relies on sexual contact with an infected cow. Bull-to-bull transmission can occur from contaminated semen collection equipment or through mounting activity when bulls are held in common areas.
Cows become infected after natural service by an infected bull or after insemination with contaminated semen. Infection may be spread from cow to cow through the use of poorly sanitized instruments used for reproductive procedures. Infected cows develop immunity and generally clear the organism within 3 to 6 months of infection, and in a majority of cows, Campylobacter will not survive a normal gestation. Some persistently infected cows may harbor the organism for over a year, however, and it has been recovered from cows as long as 196 days beyond the end of pregnancy initiated by infected semen. Failure to eliminate the infection may be due in part to the organism’s ability to undergo substantial antigenic change during the course of natural infection. After clearance of infection, cows are resistant to reinfection for a short period of time.
The disease is transmitted by coitus. After exposure, the anterior vagina and cervix are colonized, and the infection spreads to the uterus and oviducts within 12 to 14 days. Infection typically does not interfere with fertilization and early embryonic development. Uterine infection leads to early embryonic death as a result of the inflammatory response in the uterus and oviducts. The clinical manifestations of infection in the female seem to depend on the number of organisms contained in the original infective dose and on the rate of multiplication within the uterus. Rapid replication occurs most commonly, causing death of the developing embryo or fetus on days 15 to 80. Because embryonic death typically takes place after maternal recognition of pregnancy (days 15–17), the dam’s return to estrus will be delayed. Less rapid replication leads to midterm abortion. Campylobacter fetus spp. veneralis is progressively eliminated from the oviducts and uterus, and fertility returns. Frequently, however, the organism persists in the cervix and vagina for several more months, during which time the cow remains a source of infection.
Detection of clinical signs is dependent on how closely the herd is monitored. Although rarely observed, infection is associated with vaginitis, cervicitis, and endometritis. In closely monitored herds, an increased number of repeat breeders will be identified. The hallmark of Campylobacter infection is irregular and delayed returns to estrus. “Infertility” is actually the result of early embryonic death. Herd pregnancy rates will be low (40%–70%), and a wide range of gestational ages may be found at the time of pregnancy examination. Less than 10% of infected cows will abort a detectable fetus. Cows typically abort between the fourth and seventh months of gestation. In Campylobacter-endemic herds, clinical signs are most commonly seen in young cows or newly introduced animals.
We emphasize that this organism does not cause specific gross lesions in the aborted fetus or placenta. A history of infertility accompanied by a low number of midterm abortions is more indicative of the potential for Campylobacter abortion than of gross lesions. Aborted fetuses typically have a placentitis, which is consistently identified microscopically, but gross evidence for a placentitis is variable and often inconspicuous. Changes characteristic of a suppurative fetal pneumonia and hepatitis often are observed on microscopic examination of fetal tissues.
Diagnosis is based on (1) isolation of the organism, (2) demonstration of the agent in fetal tissues, preputial scrapings, or vaginal mucus with direct fluorescent antibody (FA) tests, or (3) detection of antibodies in vaginal mucus by an enzyme-linked immunosorbent assay (ELISA) or agglutination tests.
A definitive diagnosis typically is based on isolation of the organism under microaerophilic conditions on selective media. Culture of preputial scrapings tends to be more productive than that of preputial washes because the organism resides deep in the preputial crypts. Culture of vaginal mucus is most useful when samples are obtained early in the course of infection and when the cows are in estrus. The recovery rate drops substantially in chronically infected cows.
Proper handling of samples is necessary to optimize organism recovery. A transport medium should be used, such as Clark’s transport media or Weybridge TEM. Practitioners should contact their regional diagnostic laboratory for media and specific transport recommendations.
Serologic diagnosis of campylobacteriosis is not practical because infection does not stimulate the production of sufficient serum antibody levels to be accurately measured by commercial methods. Genital tract infection in the cow, however, stimulates local immunity, which can be detected by the vaginal mucus agglutination test or an ELISA measuring immunoglobulin A (IgA) antibodies. Both of these tests are used most appropriately on a herd rather than individual animal basis. An indirect FA assay may be used to detect the organism in preputial scrapings from bulls.