Leptospirosis In Horses: A Widespread But Underreported Threat

Most people who have heard of leptospirosis often think about it affecting dogs or cattle. But did you know that leptospirosis can affect both humans and horses, too?

Leptospirosis in horses can be the cause of serious health problems, such as abortion and chronic uveitis (or moon blindness as it is often called). You might think this is a rare disease, but studies have shown that it is more widespread than previously thought.

Leptospirosis: What Is It?

Leptospirosis is a bacterial disease that can affect any mammal, including humans, livestock (cattle, sheep, goats), horses, dogs, wildlife (deer, raccoons, foxes, skunks, possums), and rodents. Leptospires are a distinctive type of bacteria called spirochetes, which are spiral-shaped and motile (capable of motion). This allows the leptospires to move and populate into several organs, such as the eye, liver, kidneys, and reproductive tract.

In the past, leptospirosis had been considered a relatively uncommon infection, but recent data suggests that infection ranges from 2% to 70% worldwide and from 5% to 67% in the United States. There are several serovars and subgroups that can cause infections in a variety of animals, but the most common cause of leptospirosis in horses in the U.S. and Canada are Leptospira interrogans serovar Pomona type kennewicki and Leptospira interrogans serovar grippotyphosa. Anitbodies to Leptospira interrogans serovar bratislava have been reported in horses, but horses are thought to be a maintenance host for this particular serovar and it has not been associated with clinical disease.

Data on file, Study Report No. Restricted Grant-FTLEPTO13 (v1.0) TI-01366, Zoetis Inc. (Photo Source: Zoetis, Inc.)

Data on file, Study Report No. Restricted Grant-FTLEPTO13 (v1.0) TI-01366, Zoetis Inc. (Photo Source: Zoetis, Inc.)

How Does a Horse Contract Leptospirosis?

Horses can become infected with leptospirosis through the mucous membranes of the eyes or mouth, and sometimes through broken skin that is in contact with infected urine, blood, or tissues (e.g., mares will sniff the contaminated aborted fetus and/or placenta and become infected). They can pick up the disease by eating hay or grain that has been contaminated with urine from an infected and shedding animal or by drinking from standing water that has become contaminated as well. There are some cases of horses becoming infected by direct contact with urine from infected animals splashing into the eyes or mouth.

Divers, TJ.  Leptospirosis.  In Sprayberry KA, ed. Current Therapy in Equine Medicine.  7th ed.  St. Louis, MO: Saunders Elsevier, 2015: 175 (Photo Source: Zoetis, Inc.)

Divers, TJ.  Leptospirosis.  In Sprayberry KA, ed. Current Therapy in Equine Medicine.  7th ed.  St. Louis, MO: Saunders Elsevier, 2015: 175 (Photo Source: Zoetis, Inc.)

What Are the Clinical Signs of Leptospirosis?

The incubation period for leptospirosis in horses is one to three weeks. Infection can produce no symptoms or cause a variety of clinical signs that include fever, anorexia, painful swelling of the eyes, light sensitivity, excessive tear production, ocular discharge, cloudiness and/or redness of the eyes, lethargy, renal failure, and mid- to late-term abortion in pregnant mares. The rare foals that are born alive and infected with leptospirosis will be weak, have liver and/or kidney disease, fevers, hematuria (blood in urine), and may be born premature or be carried to term.

The primary conditions that are associated with leptospirosis in horses are chronic Equine Recurrent Uveitis (ERU), or moon blindness, and late-term abortion. ERU occurs when the leptospires migrate to the eye, causing a local immune reaction with ocular tissues. Signs of ERU include persistent ocular inflammation, redness and/or cloudiness of the eye, light sensitivity, and strong muscle spasms that close the eye. In severe cases, calcification of the cornea, cataracts, permanent blindness, atrophy (degeneration) of the eye, and glaucoma (increased pressure in the eye) can occur. This condition is extremely painful for the affected horse. Genetic factors are likely involved in the disease process, which is why only some horses will develop ERU. Appaloosas are believed to be genetically predisposed to ERU.

Leptospiral abortions usually occur around or after nine months gestation. Lesions commonly found are edema and/or areas of necrosis of the placenta, inflammation of the umbilical cord (funisitis), and placentitis that does not involve the cervical star. Microscopic calcification of the placenta can also be found. The placental lesions can sometimes cause the mare to develop hydroallantois, a condition that causes extremely excessive fluid within the uterus.

The aborted fetus will have liver and kidney lesions. Aborting mares will typically have very high leptospiral antibody titers at the time of abortion and can shed the organism in their urine for two to three months after abortion. This can cause great economic impact to breeders if an outbreak occurs, especially in large breeding populations such as areas in Kentucky, Texas, Florida, New York, and California.

Occasionally, leptospirosis presents as a fever and acute renal failure in horses. The kidneys will become swollen, and urinalysis will show blood (hematuria) or white blood cells (pyuria) with or without visible bacteria.

How Is Leptospirosis Diagnosed?

Leptospira abortion is best diagnosed by fluorescent antibody testing (FAT) or immunohistochemical (IHC) evaluation of the placenta, umbilical cord, and fetal kidney and liver. Sensitivity and specificity of the FAT in these tissues (not urine) are close to 100%. Polymerase Chain Reaction (PCR) testing is best for fluid samples, such as ocular fluids, urine, and blood. Increased serum titers in blood can usually be found in horses that have had Leptospira abortion and/or acute renal failure, but titers may be low in horses with ERU because of the localized nature of the infection.

L. interrogans serovar Pomona can also cause high antibody titers to several different serovars (especially icterohemorrhagiae and bratislava) due to cross-reactivity, but non-infecting titers will decline much more quickly over several weeks than the titers of the actual infecting serovar. A combination of serology, culture, and PCR testing of ocular fluid may be the only way to confirm Leptospira-induced ERU. The leptospires are most commonly found in fluid in the back chamber of the eye (vitreous) than the front chamber (aqueous), which limits the practical application of testing ocular fluid due to vitreous collection being much more difficult and invasive.

Can Leptospirosis Be Treated?

In acute disease, systemic antibiotics such as enrofloxacin (Baytril®), penicillin, tetracyclines, or aminoglycosides can be used for treatment. This is not the case with ERU. Uveitis treatment varies, but it usually consists of symptomatic and local treatment of the eye, which includes steroids topically or systemically to reduce inflammation, and atropine to dilate the iris, helping to relieve ocular muscle spasms, photosensitivity, and tearing.

How can Leptospirosis Be Prevented?

Until recently, prevention was the best option to keep animals from becoming infected with leptospirosis. Good management techniques, such as keeping wildlife away from feed sources and not allowing standing water to accumulate (or keeping horses from drinking from stagnant water sources) are still good to practice. Practicing good biosecurity such as frequently cleaning water and feed buckets or troughs, and avoiding direct contact with contaminated uterine fluids, placental or fetal tissue, and urine by wearing gloves and/or other protective garments is recommended, especially since humans can become infected.

In November 2015, Zoetis announced its development of the first and only licensed leptospirosis vaccine for horses, Lepto EQ Innovator®. The vaccine is approved for horses age six months or older and aids in the prevention of leptospirosis caused by Leptospira interrogans serovar Pomona. The vaccine has been shown to be clinically safe for use in foals three months of age or older and healthy pregnant mares in the second trimester. The vaccine is initially administered as a 1 ml dose intramuscularly, followed by a second 1 ml booster dose three to four weeks after the initial dose. Horses are then vaccinated once annually. Zoetis reports a reaction rate of 0.002%.

Time will tell if the vaccination has a substantial impact on the widespread infection rate of leptospirosis in horses, but prevention is a much better option than treatment of this rampant, clinically variable, and often underreported disease.

Talk to our Badger Equine team about leptospirosis in horses and work with us on collecting samples if you think it is indicated.