Leptospirosis is a bacterial infection that is commonly spread to horses through direct contact with contaminated food and water sources. While there are many different species of the Leptospira bacteria, symptoms in horses are most commonly caused by Leptospira interrogans. [1][2][3]

Infection with leptospirosis can cause a series of health conditions in horses, including equine recurrent uveitis (moon blindness), abortion in pregnant mares, kidney disease, and liver issues. [2][3][4]

Diagnosing leptospirosis in horses is typically done through specialized blood tests or by testing tissue samples from aborted fetuses. [1][2]

Many affected horses exhibit no symptoms and do not require treatment. Antibiotics and supportive care are the primary approaches for treating symptomatic horses. However, horses that develop equine recurrent uveitis often require lifelong management. [4][5]

Leptospirosis can be prevented in horses with a vaccine. Additional preventative measures include minimizing contact with wildlife and preventing access to stagnant water to reduce bacteria exposure. [1][6]

Leptospirosis in Horses

Leptospirosis in horses is caused by infection with the gram-negative bacteria Leptospira. This spiral-shaped organism, known as a spirochete, comprises over 35 different species that can infect animals, with several of these species capable of affecting horses. [1][4][5][7]

Leptospira species are categorized into serovars based on proteins they display on their bacterial surface. Serovars are broken into three groups based on their likelihood to cause disease:

  • Pathogenic
  • Intermediately pathogenic
  • Non-pathogenic

Pathogenic serovars of Leptospira are responsible for causing severe diseases in animals. [1][4][6] Leptospirosis is also considered a zoonotic disease, meaning it can be transmitted from animals to humans.

Prevalence

The exact prevalence of leptospirosis in the horse population is unknown. Rates of infection vary with geographic location and are likely underestimated since many horses may become infected without displaying clinical signs. [2][5]

Most cases of leptospirosis in horses are caused by Leptospira interrogans serovars Pomona, which has widespread prevalence in wildlife. In North America, serovars Grippotyphosa and Hardjo can also cause symptoms in horses. [1][2][3][5]

Transmission

Horses contract leptospirosis from direct contact with infected urine or reproductive fluids. Feed, water, and soil can become contaminated by these fluids, serving as common sources of infection. Less frequently, horses can become infected through bite wounds, ingestion of infected tissue, or during birth. [1][2][4][6]

Racoons, skunks, opossum, foxes, and rodents are reservoir species, meaning they carry the bacteria without signs of infection. These wildlife species shed Leptospira into the environment, where it can survive in the soil for up to 10 months, serving as an environmental reservoir for potential future infections. [1][8]

Pathogenesis

Leptospira can enter the horse’s body through direct contact with mucous membranes and wet skin. It may infect horses through contact with the eyes, mouth, nose, genitals, and directly through moist skin. [5][6][7]

Once the bacteria is in the body, it spreads through the bloodstream and lymphatic system. This phase can last anywhere from 2 to 20 days, during which horses often show no clinical signs. [1][4]

As the bacteria travels through the body, it multiplies in various tissues and organs. Commonly affected organs include the placenta, kidneys, and eyes, leading to the development of clinical symptoms. [1][8]

Bacterial infection of the kidneys is a significant manifestation of the disease as it can result in the persistent shedding of bacteria in the urine. [1][4][8][9]

Risk Factors

Horses living in endemic areas, where the Leptospira bacteria is highly prevalent, face an increased risk of developing leptospirosis. Additionally, horses in close proximity to wildlife and rodent species also have a higher risk of contracting leptospirosis. [1]

Outbreaks of infection can occur in horses with access to stagnant water. This is often observed after flooding events or heavy rainfall, or in geographical regions with milder, wetter fall/winter months. [1]