Chapter 19 Systemic Bacterial Infectious Diseases
A large number of bacteria can infect dogs and cats. This chapter focuses on leptospirosis, brucellosis, and bartonellosis. Chapter 18 is devoted to Lyme borreliosis. Other infectious diseases caused by bacteria are summarized in Table 19-1, and most are described in the respective organ-system chapters. Bordetella bronchiseptica is associated with the canine infectious tracheobronchitis (see Chapter 12).Mycoplasma haemofelis and Mycoplasma haemominutum are hemotropic infections (see Chapter 22). Mycobacterial infections are associated with chronic skin disease (see Chapter 39). Actinomycosis and nocardiosis are causes of pyothorax (see Chapter 164). Tetanus and botulism cause severe neuromuscular dysfunction (see Chapters 128 and 130, respectively). Salmonellosis, campylobacteriosis, yersiniosis, and Clostridium perfringens and Cl. difficile primarily involve the intestinal tract (see Chapter 69).
Leptospira are filamentous, motile, spirochete bacteria that infect many wild and domestic animals and humans. Over 200 leptospiral serovars have been described. A universal feature of pathogenic serovars is the ability to colonize the proximal renal tubules resulting in a prolonged renal carrier state and urine shedding.
In North America, canine leptospirosis is most frequently caused by serovars ofLeptospira interrogans (serovarsL. icterohaemorrhagiae, L. canicola, L. pomona, andL. bratislava) andLeptospira kirschneri (serovarL. grippotyphosa). SerovarL. autumnalis may also be emerging in some regions.
Canine leptospirosis appears to be a reemerging disease with increasing prevalence, especially involving serovars L. grippotyphosa and L. pomona. Middle-aged, male, large-breed outdoor dogs that live in peri-urban areas are most commonly affected; however, all dogs are at risk. The incidence is highest in late summer and early fall. Infection rates increase after wet periods of rainfall and flooding.
Leptospiremia occurs 4 to 12 days post-infection. The primary targets in leptospirosis are the kidneys and the liver. Vasculitis and disseminated intravascular coagulopathy (DIC) may result from widespread acute endothelial injury.
Leptospira organisms are fastidious, slow-growing, and difficult to culture, and they are difficult to identify in fluids or tissues; thus, the combination of serology and clinical signs is the most practical means of diagnosis.
The MA test is the standard method for presumptive clinical diagnosis of leptospirosis. MA titers become positive after 1 week, peak at 3 to 4 weeks, and remain positive for several months after either natural infection or vaccination.
Combined immunoglobulin M (IgM)-immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA) titers differentiate the early IgM antibody response of acute infection from the later IgG response that is seen with recovery, prior infection, or vaccination. Although the combined IgM-IgG ELISA helps to identify natural infection in vaccinated dogs, it does not distinguish between different serovars. The ELISA test is not as readily available as the MA test.
Leptospirosis can be confirmed by identification of Leptospira organisms by microscopy and culture. These tests are most useful for confirming positive serologic tests, and only positive results are meaningful. Because of low detection rates, negative results never exclude the possibility of leptospirosis. In addition, antibiotics rapidly eliminate the organisms from blood and urine so they will be undetectable by culture. Polymerase chain reaction (PCR) identification of leptospiral DNA may be a more reliable means of documenting infection.
Culture for Leptospira is technically difficult and has a low detection rate; thus, it is rarely used for clinical diagnosis. The organisms are fastidious and may take several weeks to grow. The preferred specimen is fresh urine taken by cystocentesis prior to administration of antibiotics. In hydrated dogs, a low dose of furosemide (0.5 mg/kg, IV) prior to urine collection may optimize the recovery rate. Collected urine must be alkalinized to >pH 8 and transported in special media.
PCR detection of leptospiral DNA in blood, urine, or tissues is highly sensitive and specific. It detects infection earlier than serology, and may be useful for identifying subclinical urine shedding. Unfortunately, validated PCR assays are not yet readily available to the clinician.
Effective treatment requires general supportive therapy, treatment of the leptospiremic phase of infection, followed by treatment of the leptospiruric carrier phase of infection. With intensive management, survival rates of 75 to 90% can be expected. Most dogs recover completely, but residual chronic renal failure is a potential outcome.
Because wild animal reservoirs harbor and shed leptospiral organisms that contaminate the environment, prevention of exposure is not a realistic expectation. Routine vaccination for leptospirosis helps to decrease the incidence and severity of canine leptospirosis and to reduce the zoonotic risks to pet owners.
Leptospirosis is a widespread and potentially fatal zoonotic infection. Most humans are infected through outdoor recreational activities in and around water, but animal caretakers and veterinarians are considered at risk because direct contact of mucous membranes or damaged skin with the urine of infected animals is a potential source of infection. Therefore, caretakers and pet owners must take proper precautions and maintain strict sanitation when treating dogs with leptospirosis. Consider the following preventive measures.
Canine brucellosis is caused by Brucella canis, a gram-negative aerobic coccobacillus. In addition, rare infections caused by B. abortus, B. suis, and B. melitensis have been reported in dogs in contact with livestock. Cats can be infected with B. canis experimentally, but are resistant and do not develop clinical signs. Rare zoonotic B. canis infections are seen in humans in contact with infected dogs.
B. canis is transmitted by infected semen (venereal), vaginal discharges (at estrus, breeding, and post-abortion), aborted fetal tissues, and urine. Infection occurs by penetration of oronasal, conjunctival, and genital mucous membranes.
Brucella canis is an intracellular pathogen that can persist for years within the mononuclear phagocytes of the host. Brucellosis is characterized by a prolonged leukocyte-associated bacteremia that begins 1 to 4 weeks after infection, and can last from 6 months to over 5 years. Spontaneous recovery after 1 to 5 years depends on cell-mediated immunity. Brucella organisms most often localize in the following areas.
Most infected animals have no overt clinical signs. Generalized lymphadenopathy, splenomegaly, and reproductive failure are the principal manifestations. Diskospondylitis and uveitis are less common. Fever and systemic illness are rare.
Brucellosis causes infertility and physical findings of scrotal swelling, scrotal dermatitis, enlarged epididymides (epididymitis), and testicular atrophy. Various semen abnormalities result in sterility (see “Diagnosis” section).
Brucellosis causes abortion of dead, partially autolyzed fetuses at 40 to 60 days of gestation without any other signs of illness; persistent discharge for 1 to 6 weeks after abortion; and failure to conceive (because of early fetal resorption).
The diagnosis of brucellosis is a 2-step process. First, use a screening serologic test for presumptive diagnosis, then confirm positives with a high-specificity serologic test, a blood culture, or, ideally, both.
Because other bacteria elicit antibodies that crossreact with B. canis, false-positive results are common with agglutination tests. Hemolysis (hemoglobin) also causes false-positive results. False-negative titers are rare, but can result from sequestration of infection or recent antibiotics. In recent infections, it can take up to 4 weeks to seroconvert; thus, when screening dogs for entry into a breeding kennel, a negative test result on day 1 and again after 4 weeks is required.
The rapid slide agglutination test (RSAT) (D-Tec CB; Synbiotics) is an inexpensive, in-office screening test that is used to identify suspects that need further testing. The RSAT detects antibodies to B canis cell-wall antigen. This highly sensitive screening test has an accurate negative predictive value, but false positives are common. Nearly 99% of negatives are true negatives. Only one half to two thirds of positives are confirmed to be truly infected; thus, all RSAT positives must be confirmed by a more specific serologic test, a blood culture, or, ideally, both.