Bit & Bridle

Equine Leptospirosis: Horse Disease Guide

Other Names

  • Lepto
  • Leptospiral infection
  • Weil’s disease (in severe human cases — also applicable to horses)

What is Equine Leptospirosis?

Leptospirosis is a bacterial disease caused by spiral-shaped bacteria of the genus Leptospira. It is a zoonotic disease — meaning it can be transmitted from animals to humans — and affects a wide range of mammalian species including horses, cattle, dogs, pigs, and wildlife such as rats and foxes, which serve as important reservoir hosts.

In horses, leptospirosis most commonly manifests as recurrent uveitis (moon blindness), which can lead to permanent vision loss. It may also cause abortion in pregnant mares, kidney disease, and occasionally a more acute systemic illness. Many horses that are exposed to Leptospira seroconvert (develop antibodies) without showing obvious disease, making the true prevalence difficult to establish.

The disease is found worldwide and is particularly prevalent in areas with surface water, mild climates, and significant wildlife populations. In the UK, Leptospira exposure in horses is well documented, with studies indicating that a significant proportion of horses have antibodies to various serovars.

Causes

Leptospirosis is caused by pathogenic serovars of Leptospira interrogans and related species. Different serovars tend to be associated with different host species and clinical presentations:

  • Leptospira pomona: Strongly associated with equine recurrent uveitis, particularly in parts of Europe and North America. Cattle and pigs are common reservoir hosts.
  • Leptospira grippotyphosa and L. bratislava: Also implicated in equine uveitis and reproductive failure.
  • Rodent reservoirs: Rats and other small mammals are the primary maintenance hosts for many pathogenic serovars. Horses are typically incidental (dead-end) hosts.

Transmission routes include:

  • Direct contact with infected urine: The most common route. Infected animals shed bacteria in urine, contaminating water troughs, pasture, and feed.
  • Contaminated water sources: Leptospira bacteria survive well in warm, moist environments — particularly surface water, ponds, and flooded pastures. Risk increases after flooding events.
  • Mucous membranes or abraded skin: Bacteria enter through the conjunctiva, nasal mucosa, or skin wounds.
  • Vertical transmission: Infected mares can transmit the bacteria across the placenta to the foal.
  • Eating infected material: Rarely, via ingestion of infected tissue.

Symptoms and Signs

The clinical picture of leptospirosis in horses varies widely:

  • Equine recurrent uveitis (ERU): The most important consequence of leptospiral infection in horses. Characterised by repeated episodes of painful intraocular inflammation — squinting, tearing, cloudiness of the eye, sensitivity to light, and eventually progressive vision loss. ERU caused by Leptospira tends to be more severe and carries a higher risk of blindness than non-infectious ERU. See the equine uveitis guide for full detail.
  • Reproductive failure: Abortion in pregnant mares, usually in the middle to late stages of pregnancy; stillbirths; or delivery of a weak foal. Leptospiral abortion can affect multiple mares in a herd simultaneously.
  • Acute systemic illness: Less common but can include fever, jaundice (icterus), haemolytic anaemia, kidney failure, and haemoglobinuria (red-brown urine). More commonly seen in foals than adult horses.
  • Subclinical infection: Many horses show no signs at all but develop antibodies — these animals may still shed bacteria and spread the disease.

Diagnosis

Diagnosis requires a combination of clinical signs and laboratory testing:

  • Microscopic agglutination test (MAT): The gold standard serological test. Measures antibodies against specific Leptospira serovars in the blood. Requires acute and convalescent paired samples (taken 2–3 weeks apart) to confirm active infection — a fourfold rise in titre is significant. A single high titre suggests exposure but does not always confirm active disease.
  • PCR testing: Can detect Leptospira DNA in blood, urine, or aqueous humour (fluid from the eye). Particularly useful early in infection before antibodies have risen.
  • Culture: Growing the bacteria from urine or tissues is possible but slow and technically demanding.
  • Ocular testing: Aqueous humour PCR and antibody testing in uveitis cases can confirm Leptospira as the cause.
  • Blood work: Haematology and biochemistry may reveal anaemia, elevated liver enzymes, or elevated creatinine in acute cases.

Treatment

Treatment depends on the clinical presentation:

  • Antibiotics: Doxycycline is the antibiotic of choice in horses; penicillin can also be used. Early antibiotic treatment can reduce the severity of acute illness and may limit the duration of leptospiral shedding. In the context of ERU, systemic antibiotics are used alongside ocular management.
  • Supportive care: For acute systemic illness — intravenous fluids, NSAIDs for fever and inflammation, and supportive treatment for kidney or liver involvement.
  • Uveitis management: Topical and systemic anti-inflammatory drugs (corticosteroids, NSAIDs) to reduce ocular inflammation. Atropine eye drops to prevent iris adhesions. In some chronic cases, surgical implantation of a cyclosporin intravitreal device or enucleation (removal of the eye) may be considered to manage or eliminate persistent infection.
  • Abortion management: Aborting mares should be isolated, and strict hygiene protocols observed due to the zoonotic risk. The aborted foetus and foetal membranes should be handled with gloves and submitted for post-mortem examination.

Prevention

Reducing exposure to Leptospira is the most effective preventive measure:

  • Water and feed hygiene: Protect feed stores and water troughs from contamination by rodents and wildlife. Regularly drain and clean water troughs. Avoid allowing horses to drink from ponds, streams, or standing water — particularly after flooding.
  • Rodent control: Reduce rat and mouse populations around stables and feed stores, as rodents are the primary reservoir host.
  • Biosecurity: Isolate new arrivals and aborting mares. Handle aborted material with gloves.
  • Vaccination: A licensed Leptospira vaccine is available in some countries for horses aged 6 months and older. In the UK, the availability of equine-specific vaccines varies — your vet can advise on current options. Vaccination of pregnant mares may reduce the risk of leptospiral abortion.
  • Wildlife management: Deer, foxes, and other wildlife can carry pathogenic serovars — minimising contact between horses and wildlife reduces exposure risk.

Prognosis

The prognosis depends significantly on the clinical manifestation:

  • Subclinical infection: Excellent — the horse recovers without any lasting effects.
  • Acute systemic illness: Variable; with prompt treatment, most horses recover, though severe kidney or liver involvement carries a more guarded outlook.
  • Leptospiral ERU: Guarded to poor for long-term vision preservation. Progressive and recurring uveitis leads to blindness in a significant proportion of affected eyes, even with treatment. The fellow eye is at risk if the horse remains exposed or carries persistent infection.
  • Abortion: The aborting mare typically recovers well, though the loss of the pregnancy is the immediate outcome. Future reproductive success is generally not compromised.

When to Call the Vet

Contact your vet if you observe:

  • A horse with a squinting, watering, or cloudy eye — particularly if it has occurred before
  • Abortion in a mare, especially if multiple mares are affected
  • Fever, jaundice, or red-brown urine in any horse
  • A weak or jaundiced newborn foal
  • Multiple animals on the property showing similar signs — which may indicate a point source of contamination

Remember that leptospirosis is a zoonotic disease. If you suspect the disease on your premises, take precautions — wear gloves when handling potentially contaminated water, urine, or aborted material — and inform your vet so they can advise on appropriate human health precautions. BEVA and Public Health England have guidance on equine zoonoses.

Frequently Asked Questions

Can I catch leptospirosis from my horse?

Yes — leptospirosis is a zoonotic disease, meaning it can be transmitted from horses to humans. Infection in people typically occurs through contact with infected urine or urine-contaminated water via mucous membranes or skin abrasions. In humans, it can range from a mild flu-like illness to severe, life-threatening disease (Weil’s disease). Always wear gloves when handling potentially contaminated material, and wash hands thoroughly. If you develop a fever or flu-like symptoms after handling a horse suspected of having leptospirosis, consult your GP promptly.

What is the link between leptospirosis and moon blindness in horses?

Equine recurrent uveitis (ERU), historically known as moon blindness, is the most important clinical consequence of leptospiral infection in horses. Leptospira bacteria can enter the eye and trigger an ongoing immune-mediated inflammatory response, leading to recurrent painful flare-ups that progressively damage the eye’s structures. Studies in Germany and elsewhere have found Leptospira DNA in the eyes of a high proportion of horses with ERU. See the equine uveitis entry for more detail.

How long can Leptospira survive in the environment?

Under ideal conditions — warm, moist, neutral or slightly alkaline environments — pathogenic Leptospira bacteria can survive in surface water and soil for weeks to months. Cold, dry, or acidic conditions reduce survival time considerably. This is why flooded pastures and stagnant ponds pose a particularly significant risk, especially in temperate climates during warm, wet periods.

Is there a vaccine for equine leptospirosis in the UK?

Vaccination options for equine leptospirosis in the UK have historically been limited, though the situation is evolving. Some European countries have had licensed equine vaccines available for longer than the UK market. Your vet will be able to advise on current availability and whether vaccination is appropriate for your horse’s risk profile — particularly if you are in a high-risk area or have had previous cases.