Leptospirosis is a contagious bacterial infection that is spread through the urine of infected animals, usually rodents and skunks. Unlike many bacterial types, Leptospirosis may be transmitted between mammalian species. Horses contract this disease by ingesting infected water or coming into contact with contaminated soil.

The organism can infect a horse through the mucous membranes of the eyes or mouth or broken skin by contact with infected urine, blood, or tissues. Horses can become infected by eating feed or drinking water that is contaminated by infected urine.

Leptospirosis organisms are called Spirochetes. They are spiral in shape (and unlike most bacteria, use this shape to move) and cause disease in ways that are different from the more familiar bacterial species. There are many Lepto species and subspecies, further complicating our understanding of their role in disease.

This disease can cause flu-like symptoms including fever. In horses, Leptospirosis most importantly can affect the eye, causing inflammation (uveitis), and irritation to the clear, corneal surface of the eye (keratits). It is thought that up to 50% of cases of ERU relate to chronic inflammation caused by this organism.

Less commonly, Leptospirosis causes liver or kidney failure, or causes red blood cell rupture and anemia (hemolysis).

Body-wide disease is more common in foals, but sometimes affects adults. Leptospirosis is a well known and important cause of late-term abortion and stillbirth. 

In cases where Leptospirosis is suspected, the horse should be quarantined (isolated from other horses), and given a separate water source. 

Diagnosis requires laboratory testing, looking for the organism itself with culture or PCR, or changing antibody levels in blood (titers).

The diagnostic approach might depend on the syndrome the horse is displaying. For aborted fetuses, vets usually try to directly identify the organism. In cases of kidney infection, isolation is sought from urine. Diagnosis is difficult to obtain from blood samples. Antibody titers might be helpful in cases of uveitis. It can be difficult to differentiate active infection from prior exposure. 

Leptospirosis is a zoonotic disease, meaning that it can be transferred from an infected horse to humans. If you have been exposed to a horse with (or suspected of having) Leptospirosis you should contact your doctor immediately.

Leptospirosis is a reportable disease, meaning that if a horse has or is suspected of having this disease, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other states and the USDA APHIS in preventing the spread of illness or disease on a national and international level.