Leptospirosis is caused by spirochete bacteria in the genus Leptospira. In horses, serovars Pomona and Grippotyphosa are the most common in North America. Clinical findings include abortion, renal failure, and recurrent uveitis. Diagnosis may be made by means of serology, immunohistochemistry, or PCR assay. Antimicrobials can be given in acute infections, but are not helpful for recurrent uveitis. A bacterin against Leptospira Pomona may help prevent bacteremia.
Leptospirosis is the disease caused by the gram-negative spirochete bacteria Leptospira spp. Leptospira infection occurs in horses when the organism colonizes the mucous membranes of a susceptible horse which then becomes bacteremic. The bacteremia provides an opportunity for the organism to invade organs of trophism such as kidneys, placenta, fetus, and eye. The bacteremia can last for several days and can cause persistent fever, but rarely are clinical signs noted unless renal failure or abortion occurs.
Epidemiology of Leptospirosis in Horses
In the US and Canada, Leptospira interrogans serovar Pomona type kennewicki and serovar Grippotyphosa are the most common causes of equine leptospirosis.
The prevalence of leptospirosis in horses is unknown, but serologic evidence indicates a much higher incidence of exposure than of clinical disease. Antibodies against serovar Bratislava are reported frequently in horses in the US and in Europe, but clinical disease has not been confirmed in horses in North America after Bratislava infections. LeptospiraBratislava is thought to be a host-adapted serovar in horses, which would explain the absence of clinical signs after infection; however, unlike most host-adapted serovars in other species, Bratislava causes a marked serologic response in horses after exposure.
Clinical Findings of Leptospirosis in Horses
Leptospira interrogans serovar Pomona abortions may account for a high percentage of infectious (bacterial) abortions in mares in Leptospira-endemic regions, although incidence varies considerably between years. The reason for the yearly variation in incidence of abortions is not clear but may be climate related, because higher numbers of abortions have been reported when late fall and early winter conditions are milder and wetter than typical.
Serovar Pomona type kennewicki is responsible for most of the leptospiral abortions in North America, but serovars Grippotyphosa and Hardjo have also been reported. Skunks, raccoons, and red foxes are known to harbor L Pomona type kennewicki. Most abortions occur after 9 months of gestation; rarely a live foal may be born with clinical signs due to leptospirosis infection. Some foals infected in utero are normal at birth and have presuckle serum antibodies against L Pomona, indicating that the fetal immune response effectively eliminated the infection without severe disease occurring.
Macroscopic lesions are edema, areas of necrosis in the chorion, and placentitis that does not involve the cervical star. Microscopic lesions include necrosis and calcification of the placenta. Placental disease may result in the mare developing hydroallantois. Macroscopically, the fetal liver may have yellow discoloration. Liver disease in the fetus is a multifocal necrosis and giant cell hepatopathy. Tubulonephrosis and interstitial nephritis may be detected in the kidneys of the aborted fetus. Inflammation of the umbilical cord (funisitis) may be recognized by diffuse yellowish discoloration. Mares experiencing abortion typically have very high leptospiral antibody titers at the time of abortion; and, although quite variable, the time period of urine shedding of leptospires after an abortion is often 2–3 months.
Occasionally, serovar Pomona causes fever and acute renal failure in horses. The kidneys become swollen due to tubulointerstitial nephritis, and urinalysis may reveal hematuria and pyuria without visible bacteria. On rare occasions, multiple weanlings or yearlings may be affected with fever and acute renal failure after infection. Antibody titers are generally high at the time of the diagnosis of renal failure.
The most important clinical disease associated with L interrogans serovar Pomona infection in adult horses in North America and L kirschneri serovar Grippotyphosa in Europe and North America is equine recurrent uveitis (ERU). ERU is believed to be an immune-mediated disease sometimes involving antibody against certain Leptospira antigens, specifically the LruC outer membrane protein, which cross-reacts with tissues of the lens, cornea, and possibly the retina. Live Leptospira organisms can be found in the aqueous or vitreous fluid of many horses with ERU. A high concentration of antibodies against serovar Pomona in the aqueous humor, compared with serum titers, -suggests persistent local antigenic stimulation. Survival of the organism in the setting of high ocular antibody titers indicates an absence of cells or molecules (eg, complement) involved in bacterial clearance, suggesting an ocular immune privilege similar to that of the CNS. Recurrent episodes of the disease may be related to a Th17 response of autoreactivity following mimicry and inter- or intramolecular epitope spreading, or both.
Genetic factors are likely involved in the disease process, helping to explain why only some horses infected with Leptospira develop uveitis. Appaloosas and warmblood breeds are thought to be genetically predisposed, and specific MHC markers on ECA1, ELA class 1, and an ELA class II microsatellite are strongly associated with the disease. Warmblood breeds with Leptospira-associated uveitis commonly develop the initial signs of ERU as young adults.
The prevalence of ERU in the general horse population is unknown, but reports suggest that ≥1% of horses will develop the disease during their lifetime. It is probable that some cases of ERU are not associated with Leptospira infection, and this may vary by geographic region. In some regions, more than 50% of ERU cases are associated with persistent ocular infections with Leptospira. Leptospira-associated uveitis may cause corneal, anterior chamber, and posterior chamber disease. Therefore, clinical findings may vary from corneal edema, clinically silent retinal lesions observed on funduscopic examination; and, most dramatically, recurrent and progressive painful uveitis. Chronic disease of the globe may cause cataracts, retinal degeneration, or even glaucoma.
Diagnosis of Leptospirosis in Horses
Diagnosis of Leptospira abortion is best accomplished by means of fluorescent antibody testing (FAT) or immunohistochemical evaluation of tissue samples of the placenta, umbilical cord, fetal liver, or fetal kidney. The sensitivity and specificity of the FAT of these tissues (vs urine) are nearly 100%. Examination of silver-stained kidney samples in horses with renal disease does not yield high accuracy because there may be false-negative and false-positive findings, likely a result of nonpathogenic serovars. PCR testing is therefore preferred for evaluation of fluids, such as urine, ocular fluids, and blood samples. PCR tests for Leptospira are based upon detection of a surface-exposed lipoprotein found only in pathogenic Leptospira sp.
Marked increases in serum antibody titers often accompany Leptospira abortions or acute renal failure, but serum titers may be low or negative in horses with recurrent uveitis because of the chronic and localized nature of infection. Acute L interrogans serovar Pomona infections often cause marked increases in antibody titers against several serovars (especially Icterohemorrhagiae and Bratislava, but the noninfecting serovar titers usually decline much more quickly over several weeks than the titers to the actual infecting serovar.
Collection of a voided urine sample after furosemide administration may improve sensitivity of PCR testing, darkfield staining, or culture testing. A combination of serologic testing, culture, and PCR testing of aqueous or vitreous fluid may be the only way to confirm Leptospira-associated uveitis. In ERU, the organism is most commonly found in the vitreous rather than aqueous fluid, which limits the practical application of ocular fluid PCR testing.
Treatment and Prevention of Leptospirosis in Horses
In acute leptospirosis, systemic antimicrobials such as enrofloxacin, penicillin, tetracyclines, or aminoglycosides are useful, but this is not the case with recurrent uveitis. Intravitreal injections of low-dose, preservative-free gentamicin has been reported to be effective in preventing further episodes of uveitis. Local and topical immunosuppressive agents have previously been the standard treatments for ERU.
There is a Leptospira Pomona bacterin approved for use in horses to prevent L Pomona bacteremia. By preventing bacteremia, it might be inferred that Leptospira-associated abortions, renal failure, and even uveitis may be prevented. It would be necessary to continually maintain protective antibody levels for the vaccine to be effective.
The CDC lists veterinarians and animal caretakers as being at an occupational risk for leptospirosis, and it recommends decreasing or avoiding contact with the urine of infected animals.
The zoonotic potential of Leptospira Pomona type kennewicki is unknown but it should be considered a potential zoonotic serovar of unknown human importance.
Key Points
Leptospira infection is common in horses in North America.
Leptospira interrogans Pomona type kennewicki and Leptospira kirschneri Grippotyphosa are responsible for most clinical disease in horses.
Clinical signs of disease affecting the reproductive tract, kidney, and eye are most common.
