Now, let’s say a mare slips her foal. And then another. And yet another. Maybe the entire herd aborts late-term. Just one loss would wrench your heart and derail your breeding program; multiple losses in the same season would devastate you.

This scenario is one of several that could result from a horse contracting leptospirosis. The zoonotic disease is found worldwide and is prevalent in developing countries, but horses in any wet climate (think flooding and standing water) or exposed to wetlands, ponds, or streams are susceptible to contracting the bacterium. “Lepto,” as it’s sometimes called for brevity, can manifest in several ways—from loss of eyesight to loss of life.

Leptospires most commonly live and multiply in the renal tubules (where urine collects in the kidneys) of reservoir or carrier hosts such as rodents, wildlife, and domestic animals. In addition to spreading in urine, leptospires can be transmitted via infected blood or tissues or by infected urine splashing into eyes or the mouth, says Dr. Craig Carter.

When an infected reservoir host urinates, the leptospires pass out of its body in the urine and contaminate the surrounding ground and/or water. In fact, exposure to standing water, such as ponds or floodwaters, is the biggest risk factor for leptospirosis infection. Occasionally, says Carter, animals (horses, livestock, etc.) inhale leptospires, ingest them with feed, or transmit them via wounds or bites.

Researchers classify leptospires into many subgroups and serovars (distinct variations), several of which more commonly affect horses: Leptospirosa pomona (the major cause of leptospirosis-induced abortions), L. grippotyphosa, L. hardjo, L. bratislava, L. canicola, and L. icterohaemorrhagiae.

Leptospirosis Risk Factors

Veterinarians can use a variety of tests to isolate the causative organism or its antigens or nucleic acids (DNA/RNA evidence of its presence) in horses’ blood, urine, or milk. Other diagnostic options run the gamut from eye exams to tissue cultures to DNA tests.

• Eye inflammation;
• Cloudy, red, painful eyes;
• Light sensitivity and squinting;
• Tearing or discharge;
• Corneal swelling that could make the colored eye area appear bluish; and
• Prominent vessels in the whites of the eye.

Long-term, says veterinary ophthalmologist Dr. Amber Labelle, ERU can damage the eyes by causing cataracts, pupil scarring, retinal detachment, glaucoma, and blindness. And, for reasons not yet understood, uveitis might become chronic (known as equine recurrent uveitis, or moon blindness).

Labelle says that if, from an ophthalmological exam, she were to suspect leptospirosis as the cause of a horse’s uveitis, she’d usually concentrate on short-term management using eye and oral medications. “Eye medications would usually be a combination of anti-inflammatories that help stop the horse’s immune system from attacking the inside of the eye and pain-relievers that can help dilate the pupil,” she says, explaining that pupil dilation helps prevent scar tissue formation that could interfere with normal pupil function and, thereby, vision.

She’d also deliver oral anti-inflammatory medications. “We’d attack the inflammation from multiple routes and with multiple drugs to get it under control quickly,” she adds. “There’s good scientific evidence that when leptospirosis is involved, a horse’s (uveitis) prognosis is worse.”

To prevent other horses stabled near infected ones from contracting leptospirosis, Labelle says she would give them oral antibiotics—probably for several weeks—in addition to the eye medications mentioned. “I’d recommend that all the horses have complete eye exams, and if they were showing any other signs of lepto (e.g., abortions or kidney problems), depending on specifics, I’d probably perform a different type of diagnostic work-up: blood tests or, if a mare miscarried, a reproductive work-up.”

Dr. John Timoney, of UK’s Gluck Equine Research Center, suggests isolating affected mares and treating them with antibiotics to knock down the bacterial infection and then watching the mares and neonates for any further signs.

Veterinarians sometimes perform pre-emptive blood tests in the fall on broodmares at breeding facilities where leptospirosis is a threat, either because of past cases, prevalence in the region, or an environment (i.e., ponds or wetlands) or climate (wet) that’s leptospire-friendly. However, says Timoney, a positive test doesn’t necessarily mean a mare is infected. “It may mean, instead, that the horse has developed antibodies (from previous exposure),” he adds. “Antibodies will remain in a horse’s system longer than the organism. A recent diagnosis of Leptospira abortion in a broodmare band ideally should trigger antibody screening to identify those mares to be treated and so prevent further losses.”

• Control rodents and other wildlife in and around your horses’ living areas;
• Prevent horses from drinking standing water that could be contaminated with urine;
• Recognize the risk of commingling horses with livestock that might also contract and spread the infection;
• Quarantine (for the three-week incubation period) and test horses new to your herd;
• Administer preventive antibiotics to horses if nearby animals are infected; and
• Vaccinate high-risk horses, including pregnant mares.

Leptospira live in the environment for up to three weeks and, says Carter, are killed by drying, freezing, and temperatures of 122°F or higher. “But even if the organisms die,” he adds, “the opossums, raccoons, rats, and other critters are continually shedding leptospires into the environment. If these rodents are running around in your barn and feed, and you’re feeding off the top of that bin, even if the organism only lives for a day it can enter the horse’s mouth, penetrate the oral mucosa, and lead to possible infection.”

The vaccine was developed based on work done by scientists at the Gluck Center. The team’s challenge had been finding a vaccine that produced immunity against each of Leptospira’s various serovars; the scientists overcame that challenge by targeting proteins common to all the serovars.

“We identified several proteins only found during infection, and two were found to have immunizing properties that are protective in experimental animals—mice and hamsters,” says Timoney. “We’ve been testing this in horses to be sure the formulation is safe, and we had a very good immune response.”

Field studies found the vaccine was 99.8% reaction-free in horses and is safe for use in healthy foals 3 months old and older and broodmares in the second trimester of their pregnancies.

“Our team at (the UK Veterinary Diagnostic Laboratory) worked to build a case for a vaccine by conducting a national sero-epidemiological study,” Carter further explains. “We’ve found that horses are seroconverting to lepto in all regions of the U.S. and in Ontario, Canada. This means that exposure to leptospires is likely ubiquitous. Exposure can lead to disease; one of my graduate students, Gloria Gellin, demonstrated seroconversion in equine veterinarians and horse farm workers, pointing to some zoonotic risk. It’s a big awareness campaign that you’d think would be simpler than the science.”

Diane E. Rice earned a bachelor’s degree in agricultural journalism from the University of Wisconsin, then melded her education and her lifelong passion for horses in an editorial position at Appaloosa Journal. She currently works as a freelance writer, editor, proofreader, and photographer and has served on American Horse Publications’ board of directors. Rice spends her spare time gardening, reading, serving in her church, and with her daughters, grandchildren, and pets.

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