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Equine leptospirosis is an infectious bacterial disease caused by spirochetes belonging to Leptospira spp.

Horses across the country may be at risk.

Evidence indicates that exposure to Leptospira bacteria is common nationwide:

  • A 45% seroprevalence was revealed in an analysis of diagnostic laboratory samples from 29 states and one Canadian province.1

Video: Incidence, Prevalence & Serovars

  • 75% of 5,261 healthy horses tested positive for at least one leptospiral serovar in a study involving 53 veterinary clinics in 18 states.2


Primary pathogen: L. pomona

Horses are primarily affected by Leptospira interrogans serovar Pomona, or L. pomona, in North America.3 Although classified together, serovars are distinct variations within a species of bacteria. L. pomona is the leptospiral serovar most commonly associated with clinical disease in horses.

How are horses infected with leptospirosis?

The bacteria penetrate the mucous membranes of the eyes or mouth or enter through skin abrasions. Once in the bloodstream, leptospires can concentrate in the kidneys, be shed in the urine and cause serious medical problems.

How are horses infected?
Divers T. Leptospirosis. In Sprayberry KA, ed. Robinson’s Current Therapy in Equine Medicine. 7th ed. St. Louis, MO: Saunders Elsevier; 2015:179.

Horses can become infected when exposed to bacteria in urine from:

  • Contaminated soil, bedding, feed and drinking water4,5
  • Standing or slow-moving water3,6
  • Maintenance hosts such as skunks, white-tailed deer, raccoons and opossums

Video: Risk Factors & Disease Prevention

Genetics may play a role, as well. Appaloosas, Drafts and Warmblood breeds are more frequently and severely affected by Leptospira-associated uveitis than other breeds.7,8

Additionally, outbreaks of leptospirosis may be related to rainfall.6,9 Heavy rainfall can increase the risk of leptospiral abortions by as much as 3.7 times, with losses as high as $4.2 million for the Thoroughbred breed alone.1,9,10

A costly disease

$2.1 BILLION: the estimated economic impact of Leptospira-associated equine recurrent uveitis (ERU) in the U.S., including the cost of diagnosis, treatment and loss in horse value due to visual impairment or blindness.5,11-16

$102 MILLION: the estimated losses from Leptospira-associated abortions in Thoroughbred horses in Kentucky from 1993-2012.1

Leptospirosis is likely underdiagnosed.

Clinical signs associated with acute infection are general, such as fever, depression, anorexia and pain.17

Diagnostic testing can be challenging as the most commonly used diagnostic test is the Microscopic Agglutination Titer (MAT) test; however, a single titer does not differentiate between exposure and infection.18 Multiple titers taken at different times may be helpful in diagnosing disease, as well as other sophisticated tests.

“We only have the benefit of material that the veterinarians and farmers bring us. We call this our ‘iceberg phenomenon’ and what that means is if we confirm a diagnosis, there are generally X number of diagnoses out in the field that we’re missing because we’re not seeing that material. It’s anywhere from 10% to 50% of the cases we’re likely not seeing.”





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