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LEPTOSPIROSIS is a bacterial infection that can result in abortion, chronic uveitis, and/or kidney failure in horses and can also infect you, your pets, and other livestock. Learn how to reduce your risk.

A band of broad-backed, pregnant broodmares grazes contentedly in the pasture outside your window, representing your farm’s future. You’ve studied bloodlines, evaluated conformation and performance records, and assembled this herd at no small effort or cost—financial, physical, and emotional. Just a few months from now they’ll deliver their long-legged, adorably awkward babies that will feed and frolic in your pasture before growing up to excel in the roles for which you’ve bred them.

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.

Zoonotic disease:

One that can be transmitted from animals to people or, more specifically, a disease that normally exists in animals but that can infect humans.

Leptospirosis: Clinical Signs

Some horses with leptospirosis appear completely normal. Others might show generalized flulike signs. More serious cases present as mid- to late-term abortions, chronic uveitis (an eye disease that’s the leading cause of blindness in horses), or renal (kidney) disease. Foals from infected dams born alive might suffer from malnutrition, jaundice, pulmonary hemorrhage, or severe respiratory distress—all of which can be fatal.

If you can start treatment immediately—before the infection damages eyes or organs—horses with leptospirosis generally have good prognoses. And, you can protect the rest of your herd (and yourself and other animals) by isolating infected horses, treating your other horses with preventive antibiotics or, depending on the species involved, vaccinating.

How Does Leptospirosis Spread?

Leptospirosis is caused by spiral-shaped bacteria called spirochetes, specifically leptospires, that enter an animal’s body through mucous membranes in areas such as the nostrils, lips, eyes, trachea, stomach, genitals, or anus, or through broken skin. In addition to mammals (horses, humans, squirrels, voles, and scores more), amphibians (such as frogs) and reptiles (including snakes) can become infected with and pass on the disease-causing bacteria. In horses, foals can become infected in utero.

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

Infected wildlife and domestic animals are the primary source for disease spread, as they can shed leptospires in their urine directly into water sources. You can reduce your horse’s (along with your own and any other animals’) chances of infection by eliminating ways an animal’s mucous membranes or wounds could come in contact with these sources.


Motile (able to move), Gram-negative, single-cell organisms that thrive in moist, temperate environments with a neutral to slightly alkaline pH. They require oxygen to survive.

Craig Carter, DVM, PhD, Dipl. ACVIM

is a board-certified diplomate in the American College of Veterinary Preventive Medicine. He’s also the director of the Livestock Disease Diagnostic Center and a professor in the Department of Epidemiology at the University of Kentucky’s Veterinary Diagnostic Laboratory in Lexington. His research interests include infectious disease epidemiology, near-real-time syndromic surveillance, clinical decision support, and medical information systems.

Yes, You Can Get Lepto, Too!

Humans who work with animals or have frequent exposure to them are at a higher risk of contracting leptospirosis.

Marta Guerra, DVM, MPH, PhD, Dipl. ACVPM, is an leptospirosis expert with the Centers for Disease Control’s (CDC) Bacterial Special Pathogens Branch. She says the leptospirosis is now classified as a re-emerging disease and was reinstated as a nationally notifiable disease as of January 2013. “Worldwide, the incidence is increasing, which may be due to increased episodes of flooding,” she says.

According to the CDC, most human leptospirosis symptoms are flulike and include:

  • High fever;
  • Headache;
  • Chills;
  • Muscle aches;
  • Vomiting;
  • Jaundice (yellow skin and eyes);
  • Red eyes; and
  • Abdominal pain.

“Treatment is fairly straightforward,” Guerra adds. “The emphasis now is on rapid diagnostics so the disease can be diagnosed quickly and treatment started.

“Prevention is characterized differently for different risk groups,” she adds. “For occupational exposure (for example, veterinarians and animal caretakers that might be exposed to the disease), appropriate PPE (personal protective equipment, including protective gloves and glasses) is important. For recreational exposure, preventing contact of mucous membranes and broken skin with contaminated water is advised.”

How do Veterinarians Diagnose Leptospirosis?

One of the leptospire’s unique characteristics is that it tends to cluster in certain body areas. Once infected, it can take up to three weeks for clinical signs to develop in a horse. For example:

But veterinarians don’t clearly understand the route the bacterium takes to these organs. “It’s thought to travel through the bloodstream to the eye, the reproductive tract, or the kidneys or liver,” says Carter. “These are the places where these particular organisms like to live. At this point, just why is one of the mysteries of nature.”

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.

Recognizing and Treating Localized Leptospirosis

Eyes: Uveitis (Moon Blindness)

Recent research shows that up to 70% of equine recurrent uveitis (ERU) cases might be related to leptospirosis. (Other uveitis causes include toxoplasma; Brucellaand Streptococcus bacteria; viruses; parasites; and trauma.) The condition’s clinical signs include:

  • 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.”

Amber Labelle, DVM, MS, Dipl. ACVO

is a board-certified veterinary ophthalmologist practicing at WestVet, in Garden City, Idaho. Prior to that, she was an assistant professor in comparative ophthalmology at The Ohio State University in Columbus and assistant professor at the University of Illinois at Urbana-Champaign. Her research interests include equine ophthalmology and ocular neoplasia, and pigmentary keratinopathy in pugs.

Reproductive Tract

Bacteria can travel via the bloodstream and cluster in the placenta, leading to abortion or to the birth of emaciated foals with short life expectancies. Unfortunately, the first sign of leptospirosis affecting the reproductive tract is often miscarriage. A University of Kentucky (UK) study estimates that, from 1993 to 2012, reproductive leptospirosis infection caused economic losses topping $103 million in the state, which relies heavily on Thoroughbred racehorse breeding.

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.”

John F. Timoney, MVB, MRCVS, MS, PhD

is a professor in the Department of Veterinary Science at the Maxwell H. Gluck Equine Research Center at the University of Kentucky, in Lexington. His leptospirosis research targets a better understanding of the organism in the pregnant mare, fetus, and eyes of horses affected with moon blindness, including discovery of prevention methods by immunization, rapid diagnosis, and serovar identification in clinical specimens.

Kidneys and Liver

Leptospires that travel through the bloodstream and collect in the kidneys and/or liver can cause jaundice. In these cases, your veterinarian would generally administer antibiotics to wipe out the Leptospira, and your horse might require hospitalization and/or intensive care. Left untreated, the infection could lead to organ failure, resulting in death.

How Can I Prevent Leptospirosis?

In short, you can break the cycle of infection through good management practices:

  • 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.”

A New Vaccine for Horses

Leptospirosis vaccines have been widely available for pigs, cattle, and dogs, but at this time a human vaccine is not available in the United States. Historically, veterinarians sometimes used a cattle vaccine off-label to try to control equine outbreaks, but the practice posed risks. As of October 2015, however, a new vaccine designed specifically to prevent leptospirosis in horses is available. The vaccine is labeled to help protect against L. pomona (the serovar most frequently associated with clinical disease in horses in North America).

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.”

Take Home Message

Leptospirosis is the most common zoonotic disease worldwide, yet its effects on the U.S. horse industry are just now coming into focus. In horses, it can cause eyesight loss, abortion, and kidney/liver failure. You can help prevent the disease in your horses, yourself, and in pets and other livestock by using good management practices and, if nearby cases pose a threat, with preventive antibiotics and other treatments your veterinarian recommends. A new USDA-approved leptospirosis vaccine specifically for horses is also now available.

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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