Equine leptospirosis (Proceedings)
Leptospirosis is caused by a highly invasive, spiral bacteria in the genus Leptospira. The infectious agent is capable of infecting man and animals. There is less known about leptospirosis in horses than any common domestic animal except the cat. The DNA-based genus classification of Leptospira includes 8 pathogenic species and 5 non-pathogenic species. Serovars, which are based upon the older phenotype classification, are sometimes classified as host adapted infection or incidental host infection. Host adapted strains seldom cause clinical disease in their maintenance host, infection and shedding are prolonged, and the serologic response following infection is relatively low. Conversely, incidental host serovars are more likely to cause clinical disease in a non-maintenance host, a marked serologic response occurs following infection, and there is only a brief period of shedding. In North American horses, Leptospira pomona kennewicki is the prominent incidental (pathologic) serovar and the skunk is the most common maintenance host of this serovar. In Europe, important equine strains are L. kirschneri, serovar gripphotyphosa, strains duster (western Europe) and moskva (eastern Europe). Leptospira bratislava is considered by most researchers to be the host adapted serovar of the horse. This belief is met with some controversy, as horses may have high equine serum titers to bratislava and some researchers believe bratislava is pathogenic in the horse.
Pathogenic Leptospira infections in the horse appear to have organ trophism for either the reproductive tract of the female, kidney or eye. Infection may result in placentitis and abortion, acute renal failure, or hematuria and most importantly, uveitis.Reproductive Tract Infection
Leptospira pomona abortions account for approximately 13% of bacterial abortions in mares in endemic regions. Serovar Pomona is responsible for most of the Leptospira abortions in North America, but Grippotyphosa and Harjo have also been reported. Most abortions are late term (> 9 months) and rarely a live foal may be born ill due to leptospirosis. Leptospira organisms are commonly found in the placenta, umbilical cord, and kidney and liver of the infected fetus. Pathology is present in the placenta, a placentitis not involving the cervical star. Macroscopic lesions are edema and areas of necrosis in the chorion. Microscopic lesions include necrosis and calcification of the placenta. Macroscopically, the liver may have a yellow discoloration. Liver disease is caused by a combination of multifocal necrosis and giant cell hepatopathy. Tubulonephrosis and interstitial nephritis may be present in the kidney of the aborted fetus. Inflammation of the umbilical cord, funisitis, may be recognized by a diffuse yellowish discoloration. It is unknown if the abortion occurs because of the placentitis, funiculitis or fetal infection, or all 3. Although more than 1 mare on a farm may abort due to Leptospira infection, it is rare for an epidemic of abortions to occur. Some mares that are infected do not abort. Aborting mares and other recently infected horses are believed to shed L. pomona in the urine for approximately 2-3 months. A small number of horses on the farm experiencing 1 or more Leptospira abortions may develop uveitis weeks later.
Acute renal Failure
Occasionally, L. pomona may cause fever and acute renal failure in the horse. The kidneys are swollen due to tubulointerstitial nephritis and the urine may have pyuria without visible bacteria. On rare occasion, multiple horses may be affected with fever and acute renal failure following Leptospira infection.