Leptospirosis in Horses: Causes, Symptoms, Treatment, and Prevention

Leptospirosis is a bacterial infection that affects horses and many other animal species, including humans. Caused by spiral-shaped bacteria of the genus Leptospira, it can lead to a wide range of clinical problems in horses, from mild illness to serious conditions such as abortion, kidney disease, and recurrent uveitis (also known as moon blindness).

Because leptospirosis can spread through the environment and poses risks to both horses and humans, understanding its causes, symptoms, and control measures is vital for every horse owner and breeder.

What Is Leptospirosis

Leptospirosis is a zoonotic disease, meaning it can be transmitted from animals to people. The infection is caused by Leptospira interrogans and related species, which thrive in moist environments such as puddles, ponds, or damp soil. Horses usually become infected when the bacteria enter through mucous membranes (eyes, nose, mouth) or cuts in the skin after exposure to contaminated water, urine, or feed.

Once inside the body, the bacteria spread through the bloodstream, affecting organs such as the liver, kidneys, and reproductive system. The infection may clear on its own or become chronic, leading to serious long-term complications.

How Horses Become Infected

Leptospirosis is commonly spread by contact with water or soil contaminated by the urine of infected animals. Wildlife species such as rodents, raccoons, deer, and cattle are common carriers. The bacteria can survive for weeks in damp conditions, making pastures, ponds, and feed storage areas potential sources of infection.

Routes of Transmission

• Drinking or wading in contaminated water sources
• Eating feed or hay contaminated by infected urine
• Exposure to infected urine through wounds or mucous membranes
• Direct contact with aborted foetuses or placentas from infected mares
• Human exposure during handling of contaminated material (a zoonotic risk)

Symptoms of Leptospirosis in Horses

The signs of leptospirosis can vary widely depending on the organs affected and the severity of infection. Many horses may have mild or no symptoms, while others develop severe illness.

Common Clinical Signs

• Fever
• Lethargy and loss of appetite
• Jaundice (yellowing of the gums and eyes)
• Muscle tenderness and stiffness
• Increased drinking and urination (signs of kidney involvement)
• Nasal discharge or mild respiratory signs

In more advanced or chronic cases, leptospirosis can lead to two major complications: abortion in pregnant mares and recurrent uveitis.

Abortion in Pregnant Mares

Leptospirosis is a recognised cause of abortion, usually in mid to late pregnancy (between 6 and 10 months of gestation). Mares often show no signs of illness before aborting. The placenta and aborted foetus typically contain high levels of Leptospira, and the mare’s uterus may remain infected afterward. Infected mares may also give birth to weak or premature foals.

Equine Recurrent Uveitis (Moon Blindness)

One of the most serious long-term effects of leptospirosis is equine recurrent uveitis (ERU), a painful and progressive eye condition that can lead to blindness. It is thought that infection triggers an immune-mediated response, where the horse’s own immune system attacks the eye tissues. Signs include:

• Tearing and squinting
• Cloudiness or bluish discolouration of the eye
• Redness around the iris
• Sensitivity to light
• Recurrent flare-ups leading to scarring or vision loss

ERU can occur weeks, months, or even years after the initial infection.

Diagnosis

Diagnosing leptospirosis can be challenging because the symptoms overlap with other diseases. Laboratory testing is essential for confirmation.

Diagnostic Methods

• Blood tests (serology): Detect antibodies to Leptospira bacteria, indicating current or past infection.
• PCR testing: Identifies bacterial DNA in blood, urine, or tissue samples, providing a definitive diagnosis.
• Culture: Isolation of the bacteria from urine, foetal tissue, or placenta (though this is difficult and time-consuming).
• Eye fluid analysis: Used in cases of uveitis to confirm Leptospira as the cause.

For mares that abort, both the foetus and placenta should be submitted for laboratory testing to confirm the diagnosis and prevent further spread.

Treatment

Leptospirosis can often be treated successfully if diagnosed early. The main goals are to eliminate the infection and manage any complications.

1. Antibiotic Therapy

Antibiotics such as penicillin, oxytetracycline, or doxycycline are effective against Leptospira bacteria. They help reduce bacterial shedding in urine and limit the spread of infection to other horses.

2. Supportive Care

Horses with fever, dehydration, or kidney involvement may require:

• Intravenous fluids
• Anti-inflammatory drugs to reduce pain and fever
• Rest and close monitoring

3. Management of Recurrent Uveitis

Treatment of equine recurrent uveitis focuses on reducing inflammation and preventing flare-ups:

• Corticosteroid eye drops or systemic anti-inflammatories
• Atropine to dilate the pupil and relieve pain
• Antibiotics if active infection is still present
• Surgical options (such as vitrectomy) in severe or recurring cases

Unfortunately, ERU is a lifelong condition, and while treatment can control symptoms, it cannot always prevent progressive vision loss.

Prevention and Control

Prevention is the most effective way to manage leptospirosis, as the bacteria are widespread in the environment and difficult to eliminate completely.

1. Vaccination

A licensed vaccine for leptospirosis is available in some regions, particularly targeting Leptospira interrogans serovar Pomona, the strain most often associated with abortion and uveitis in horses.

• Mares should be vaccinated before breeding and again during pregnancy as advised by a veterinarian.
• Stallions and geldings can also be vaccinated to reduce shedding and environmental contamination.

2. Environmental Management

• Control access to standing or stagnant water where the bacteria may survive.
• Minimise contact with wildlife or livestock that can carry the disease.
• Store feed and hay in dry, elevated areas protected from rodents.
• Maintain good stable and pasture hygiene.

3. Biosecurity Measures

• Isolate horses showing signs of fever or jaundice until a diagnosis is confirmed.
• Wear gloves when handling aborted material or urine-contaminated bedding.
• Disinfect equipment, buckets, and surfaces with appropriate antibacterial agents.
• Remove and properly dispose of contaminated bedding and manure.

4. Breeding Management

• Test mares that abort and disinfect foaling and breeding areas thoroughly.
• Avoid reusing contaminated paddocks for pregnant mares.
• Monitor mares closely during mid to late gestation for early signs of illness.

Zoonotic Risk

Leptospirosis is a zoonotic disease, meaning humans can contract it from infected animals or contaminated environments. Farm workers, veterinarians, and stable hands are at particular risk.

To protect yourself:

• Always wear gloves when handling foetal material, urine, or potentially contaminated water.
• Wash hands thoroughly after handling horses or cleaning stables.
• Cover cuts and abrasions before working with horses or in wet environments.

Prognosis

Most horses recover from leptospirosis if treated early, though the prognosis varies depending on the complications.

• Horses that abort usually recover fully but may remain carriers for a short period.
• Horses with kidney or liver damage require longer treatment and careful management.
• Horses with recurrent uveitis may experience permanent vision loss if not treated promptly.

With proper care, prevention, and monitoring, long-term outcomes are generally favourable.

Conclusion

Leptospirosis is a significant disease in horses that can lead to abortion, eye disease, and organ damage, as well as posing a risk to humans. Because the bacteria thrive in wet environments and are spread by wildlife, total elimination from farms is difficult, but the disease can be controlled through vaccination, good hygiene, and careful management.

By recognising the symptoms early and working closely with a veterinarian, horse owners can protect both their horses and themselves from this potentially serious infection, ensuring safer breeding operations and healthier equine populations.