Leptospirosis is an eye-threatening bacterial disease associated with uveitis/blindness and abortion.

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What is Leptospirosis?

Leptospirosis is a zoonotic bacterial infection, meaning it can be transmitted from person to animal and vice versa. There are many species of the bacteria found throughout the world, but Leptospira pomona is most often associated with leptospirosis infections in horses in North America.

How is it transmitted?

The bacteria require a maintenance host, which becomes infected by direct transmission via urine or body fluids. Maintenance hosts shed the organism in urine and other body fluids for months to years. Common maintenance hosts include rodents, raccoons, skunks, deer, and opossums.

Infection of an incidental host (horses, people) occurs when a mammal comes in contact with contaminated feed, soil, water, or bedding. Horses either graze moist, infected grass or drink infected standing water around the farm. Infection occurs by contact with mucus membranes or through damaged skin. The incubation period, the time before symptoms show, is 4-20 days. A list of clinical signs of infection are shown in the column to the right.

How to protect against it?

An equine vaccine to protect against L. pomona is available. It is suitable for all horses who are not currently experiencing a uveitis flare. There is no evidence at this time that it can hurt horses who have uveitis, but it will not protect them from future uveitis episodes in the affected eye. It may protect the unaffected eye. It is recommended for all horses of Warmblood or Appaloosa breeding, any mare that may become pregnant, and all horses on premises with a history of Leptospirosis cases or horses with uveitis.

Vaccination in previously unvaccinated horses requires administration of an initial vaccine followed by a booster approximately 4 weeks later. After which, the vaccine is administered annually.

How is it diagnosed?

Leptospirosis is an often undiagnosed disease and diagnosis is complicated. Serum titers can be used for screening, but should be interpreted with caution. Paired titers taken at 7 to 10 days are more accurate. A single titer must be interpreted with much caution, as up to 75% of horses in the United States have been exposed and are seropositive for Leptospirosis.


Risk Factors:

  • Standing water on premises
  • Wildlife on premises
  • Feeding hay or grain on ground
  • Horses with uveitis or abortions on premises
  • Warmbloods (and crosses)
  • Appaloosas (and crosses)
  • Broodmares
  • Horses who have lost an eye (protect the remaining eye)
  • Other horses on premises with uveitis
  • Eye tearing or discharge
  • Eye swelling or cloudiness
  • Redness surrounding the eye
  • Chronic uveitis, also known as "moon blindness"
  • Calcification of the cornea
  • Permanent blindness
  • Mid- to late-term abortion
  • Acute renal failure (excess drinking, excessive or lack of urination)
  • Fever
  • Loss of appetite
  • Lethargy

The prevalence of Leptospirosis seropositive horses throughout the U.S.

Additional Resource


Equine Leptospirosis: Disease Overview and the Risks and Economic Ramifications of Leptospira-Associated Recurrent Uveitis and Leptospiral Abortion

Read it here

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