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][5]

Young foals (1-3 months) infected by Leptospira are at a higher risk for developing severe infections. Rarely, foals can develop fatal pneumonia, abnormal blood clotting disorders, and renal failure. [1][10]

Appaloosas and warmbloods have a higher risk of developing equine recurrent uveitis secondary to leptospirosis. [5]

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Clinical Syndromes

In horses, leptospirosis can manifest in various clinical syndromes, reflecting the diverse effects the Leptospira bacteria can have on different body systems.

Some of the key clinical syndromes associated with leptospirosis in horses include acute infection, kidney infection, reproductive disease, and equine recurrent uveitis.

Acute Infection

In some cases, leptospirosis can cause systemic illness characterized by fever, lethargy, anorexia (loss of appetite), and muscle stiffness. This form of the disease can be challenging to diagnose due to its non-specific symptoms.

Clinical Signs

While most horses infected by Leptospira don’t show symptoms during the initial acute infection, some horses do become symptomatic. [5]

Occasionally, more harmful bacterial serovars can lead to the development of fever and dullness as the infection spreads through the body and damages tissues and organs. [1][2][4][6][9]

Additional symptoms that may occur in these more severe cases include: [2][3][6]

  • Yellow skin (jaundice)
  • Red spots on the gums
  • Swelling and redness of tissues around the eyes

Disease outcomes depend on the bacterial strain with which the horse is infected. Harmful bacterial types may necessitate diagnostic tests and treatment, while other subtypes may be eliminated by the immune system without the need for treatment. [6][7]

Kidney Infection

Certain Leptospira serovars can cause inflammation and injury to the kidneys. Horses with kidney disease due to leptospirosis can shed the bacteria in the urine for months, potentially passing it to others. [1][2]

Clinical Signs

Kidney infections caused by Leptospira can result in symptoms such as: [1][3]

Diagnosis

While special laboratory testing is necessary to diagnose leptospirosis, various diagnostic tests may be conducted in horses exhibiting symptoms of urinary tract infection. [4]

Bloodwork

Bloodwork may be performed to help identify potential causes of urinary-related signs. Horses with kidney damage often exhibit azotemia, characterized by increased blood levels of urea and creatinine, waste products typically excreted by the kidneys.

Additionally, horses with leptospirosis may develop anemia (low red blood cell counts) linked to kidney injury. [2][6]

Specialized Blood Tests

The gold standard for diagnosing leptospirosis is called the microscopic agglutination test (MAT). This test identifies antibodies against the bacteria in the blood. This test must be interpreted carefully as many horses can be exposed to Leptospira without being affected. [8]

To properly diagnose an active infection of leptospirosis, a single blood sample must show a very high titer (greater than 800) when tested using the MAT.

Another method involves collecting two blood samples from the horse two weeks apart. A Leptospira infection is confirmed if there is a fourfold increase in antibody levels between the two samples. [1][2][8]

Specialized blood tests are unable to differentiate an active infection versus recent vaccination. Therefore, it is important to disclose vaccination history to your veterinarian if leptospirosis diagnostic testing is being performed. [8]

Urinalysis

Urine samples are usually collected in horses displaying symptoms of urinary tract infection. Horses with leptospirosis may exhibit red and white blood cells in their urine, even if no bacteria are visible. [1][5]

A special staining technique, called darkfield staining, can be applied to the urine to reveal visible bacteria for diagnosis. This technique can also be used on blood, cerebrospinal fluid, and milk. [5][8]

Urine samples can also be cultured for diagnostic purposes. However, Leptospira is a slow-growing bacterium, which makes blood culture less practical. Other tests such as a Polymerase Chain Reaction (PCR) test are quicker and are also frequently conducted on both urine and blood samples. [8]

Diagnostic Imaging

An ultrasound may be used to examine the kidneys in horses displaying urinary-related symptoms. The kidneys of affected horses may be swollen from inflammation. [5]

Post-Mortem Examination

Horses that die or are euthanized from leptospirosis may undergo post-mortem examination and tissue sample collection. On microscopic examination, the kidneys can have evidence of inflammation, referred to as interstitial nephritis.

The bacteria may also be identified in the kidneys, liver, eyes, and the placenta of pregnant mares. [1]

Treatment

Treatment of kidney infection due to leptospirosis involves a combination of antibiotics to eliminate the bacteria and supportive care to maintain kidney function.

Antibiotics

Antibiotics are the treatment of choice for horses with signs of kidney injury. Common antibiotics used to treat leptospirosis include: [1][2][3][5]

  • Streptomycin (10 mg/kg bw)
  • Penicillin (10,000-15,000 IU/kg bw)
  • Oxytetracycline (5-10 mg/kg bw)

Antibiotics may also be used to help reduce bacterial shedding, minimizing the risk of spreading the disease to other horses. [1][6]

Supportive Care

Intravenous fluids are often administered to horses with kidney damage from leptospirosis. [1][6] Administering fluid supports kidney function by ensuring adequate hydration.

Prognosis

Most horses with kidney injury have a good prognosis if they are treated appropriately. In cases of more severe injury, the prognosis may be poorer, especially in young foals. [1][3]

Reproductive Disease

In pregnant mares infected with Leptospira, the bacteria can target the placenta and developing fetus, potentially leading to abortion, stillbirth, or premature birth of the foal. [1][3]

Approximately 3-4% of equine abortions each year are caused by leptospirosis infection. However, this number can vary anywhere from 0.3-95% depending on geographic location. Leptospirosis can also lead to abortion outbreaks during environmental events such as flooding. [1][8][11][12][13][14]

Mares can also contribute to the further spread of the bacterium on the farm, as they can continuously shed the bacteria for up to 14 weeks after they abort. [2]

Clinical Signs

Abortions caused by leptospirosis typically occur late in gestation, around 8 months. Mares may develop a fever before aborting the fetus, but otherwise, they typically do not show any symptoms. [1][2][5]

Fetuses with leptospirosis from abortive mares can have a yellow to red umbilical cord due to infection and inflammation. [1][5] Fetuses that undergo veterinary post-mortem examination may display the following: [1][5][8]

  • Large, swollen liver
  • Pale to yellow liver
  • Irregular streaks or blotches on the liver
  • Red spots on the lungs
  • Swollen kidneys
  • White streaks in the kidneys

The placenta can also be examined and may show the following signs: [2][8]

  • Thickened
  • Wet with fluid
  • Dull
  • Have raised masses

Rarely, foals can be born premature (or full term) with leptospirosis. These foals are often weak, extremely thin, have yellowed eyes, skin, and gums (jaundice), and have bloody urine. [1][2][3]

Diagnosis

Diagnosis of reproductive disease due to leptospirosis usually occurs after the mare aborts her fetus. The placental and fetal tissues can be examined to confirm a diagnosis.

Microscopic Examination

If the placenta undergoes examination following abortion, there can be microscopic evidence of inflammation and tissue death. [2]

Tissues from aborted fetuses with leptospirosis can have inflammation of the kidneys, brain, heart, lungs, and liver. Special stains can also be applied to these tissues to help highlight the bacteria. [1][2][5]

Microscopic examination is less accurate than specialized tests. [5]

Specialized Tests

Polymerase Chain Reaction (PCR) is the test of choice for identifying Leptospira bacteria in fetal tissues. Fluorescent antibody testing (FAT) can also be used.

Common tissues collected for testing include placenta, umbilical cord, fetal liver, and/or fetal kidneys. [1][2][5]

Treatment

Antibiotics can be proactively administered to pregnant mares to help protect against fetal infection. Penicillin G (20 million units) can be administered twice per day to prevent intra-uterine infection and abortion. [2]

Antibiotics may also be used to help reduce bacterial shedding, with the aim of minimizing the spread of bacteria to other horses. [1][6]

Prognosis

Mares with leptospirosis often recover fully with no long-term reproductive consequences. [3]

Equine Recurrent Uveitis

One of the most common conditions horses can develop from leptospirosis is inflammation in the eye, potentially leading to vision problems. [4]

Equine recurrent uveitis (ERU) is a disorder where the immune system mistakenly attacks eye tissues due to a cross-reaction between the bacteria and the lens, cornea, and retina.

This reaction occurs months to years after the initial leptospirosis infection, with the most common onset occurring 2-8 months post-infection. [1][4]

The onset of ERU involves a complex process that isn’t exclusively tied to Leptospira infection. Most horses with uveitis do not have a history of acute leptospirosis infection. Genetics also likely play a role, as not all horses develop ERU following leptospirosis infection. [6]

Inflammation in the eye over time can lead to permanent damage. Diseases that can develop secondary to uveitis include: [5]

  • Cataracts
  • Vision loss from breakdown of the retina, the vision center of the eye
  • Lens luxation or movement of the lens within the eye
  • Glaucoma from increased pressure within the eye

Clinical Signs

The Leptospira bacterium can infect various tissues within the eye, resulting in a range of clinical symptoms. These symptoms are not specific for ERU and may occur with other eye conditions as well.

Common signs of leptospirosis causing ERU include: [2][3][5][6][8]

  • Cloudiness of the cornea (the front, clear part of the eye)
  • Squinting
  • An abnormally small pupil
  • Excessive blinking
  • Shying away from light
  • Watering of the eye
  • White fluid or material in the eye
  • Swelling of the tissues around the eye
  • Redness in the iris
  • Color change in the iris
  • Vision loss

Diagnosis

Horses with ocular-related symptoms often undergo a thorough eye examination to identify the cause of symptoms. Tests such as swabs from the eye may be performed to rule out other bacterial causes of inflammation. Fluorescence staining is another common test to detect ulcers on the eye’s surface, which has several overlapping symptoms of ERU. [3]

In horses with recurrent bouts of inflammation in the eye that respond poorly to medical management, the eye is often surgically removed. This procedure is referred to as enucleation.

Eyes that are surgically removed should be examined microscopically by a veterinary specialist. The presence of protein in the ciliary body, a portion of the vascular uveal tissue in the eye, is specific for ERU but is not specific to leptospirosis. [2]

If ERU is diagnosed in an enucleated eye, further testing is required to support leptospirosis as the potential underlying cause. [2][8]

Specialized tests

Diagnosing leptospirosis as the cause of ERU is challenging, as the syndrome occurs after the infection is cleared from the body. Blood tests are often unreliable to correlate a previous leptospirosis infection with ERU for this reason. [8]

A combination of bacterial culture, PCR, and antibody testing can help identify leptospirosis as the underlying cause of ERU.

Live bacteria can occasionally be identified in swabs or samples from the eye. However, a PCR test can also be performed on the fluid in the eye, such as the aqueous and vitreous humor. [1][5]

Depending on what diagnostic tests are available in your location, samples of eye fluid may be alternatively tested for antibodies against Leptospira. Antibody levels in the eye that are four times higher than in the blood support a diagnosis of leptospirosis-associated uveitis. [1][3]

Treatment

Treatment for ERU in horses with leptospirosis typically involves antibiotics and supportive care to reduce inflammation in the eye.

Antibiotics

Antibiotics are not effective in treating ERU caused by Leptospira. However, a single 4 mg dose of gentamicin injected into the eye can reduce the frequency and severity of inflammation in many affected horses. [1][5][6]

Supportive Care

For secondary conditions such as uveitis, medications are commonly used to decrease discomfort and prevent irreversible injury to the eye. Medications used for this purpose include topical eye drops that dilate the eye (atropine HCl 1%), in addition to topical or systemic anti-inflammatory medications.

Topical immunosuppressive drugs have also been used to reduce inflammation in the eye. [4][5][15]

Topical anti-inflammatory medications used to control ERU in horses may include: [15]

  • Prednisolone acetate 1%
  • 03% Flurbiprofen
  • Dexamethasone HCl 0.5-1%
  • Cyclosporin A 0.02-2%

Systemic anti-inflammatory medications used in the treatment of ERU consist of: [15]

  • Flunixin meglumine
  • Phenylbutazone
  • Prednisolone
  • Dexamethasone
  • Subconjunctival triamcinolone
  • Doxycycline
  • Intravitreal gentamycin

A newer treatment for controlling ERU is a surgically-implanted device releasing cyclosporine A. This devise is implanted into the eye during an anesthetic procedure. [2][15]

Administration of these medications can potentially predispose horses to other eye infections. [15] Consult with your veterinarian to determine an appropriate treatment plan.

Prognosis

Horses that develop uveitis from leptospirosis have a guarded long-term prognosis. These horses often suffer from recurrent bouts of eye pain from inflammation which can eventually lead to blindness. [1]

Prevention

Preventing leptospirosis in horses involves a combination of management practices, vaccination, and awareness of environmental factors that contribute to the risk of infection.

Prevention should focus on reducing exposure to bacteria and minimizing spread of infectious diseases between horses.

Biosecurity

Isolating infected horses is recommended to prevent transmission of leptospirosis to other horses on the farm. A quarantine period of 3-4 weeks is typically recommended, along with obtaining a Leptospira-negative urine result, prior to reintroducing a horse into the herd. [1]

Many common disinfectants can kill Leptospira bacteria. Cleaning and disinfecting isolation areas is another important biosecurity measure to reduce bacterial spread. [1][6]

Pasture Management

Good pasture management strategies also play a role in reducing exposure to the Leptospira bacteria. Maintaining a dry pasture and restricting access to stagnant water is crucial in reducing the risk of transmission. [6]

Rodents and wildlife are reservoir species for Leptospira. Adequate rodent control and minimizing wildlife on pasture can reduce the risk of bacteria on the farm. [6]

Vaccines

A vaccine is available to protect horses against Leptospira interrogans serovars Pomona. [1][16] The vaccine has been safely used in pregnant mares of all gestational ages, and foals as young as 3 months old. [1][6][16]

Vaccination protocols for horses involve the following: [16]

  • Healthy, adult horses: Administer two doses, 3-4 weeks apart. Revaccinate annually thereafter.
  • Pregnant mares: Vaccinate according to adult horses.
  • Foals: Vaccinate at 6 months and again 3-4 weeks later. Revaccinate annually thereafter.

In addition to preventing disease in your horse, vaccines can also minimize the spread of bacteria by infected horses. [9] However, vaccination is contraindicated in horses with ERU as it may further stimulate the immune system, leading to inflammation in the eye. [6]

The leptospirosis vaccine is not a core vaccination, meaning that your veterinarian will recommend it depending on the perceived risk of infection in your area. Consult with your veterinarian to determine whether the leptospirosis vaccination is necessary for your horse. [17]

Risk to Human Handlers

Some species of leptospirosis can infect humans. [1][4][5] While symptoms in humans vary, 5-10% of infected individuals can develop a severe infection, potentially leading to death from acute kidney failure, organ failure, and/or blood in the lungs and brain. [2]

Veterinarians or other personnel handling infected horses or fetal tissues are recommended to wear personal protective equipment (PPE) such as: [1]

  • Gloves
  • Goggles and/or face shields
  • Coveralls or gowns
  • Boot covers

Summary

  • Leptospirosis is cause by a bacterial infection in horses that can have severe consequences such as abortions, kidney disease, and equine recurrent uveitis.
  • Early identification and treatment helps combat the infection and prevents further spread of the disease on the farm.
  • Horses that develop uveitis have a poorer prognosis and may require lifelong management.
  • To prevent leptospirosis on your farm, implement a combination of vaccinations, biosecurity practices, and pasture management.

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References

  1. Divers T.J. and DeNotta S., AAEP Infectious Disease Guidelines: Leptospirosis. AAEP Infectious Disease Guidelines: Leptospirosis. American Association of Equine Practitioners, 2022.
  2. Verma A. et al., Leptospirosis in Horses. Veterinary Microbiology. 2013.
  3. Lavoie J.-P. and Hinchcliff K.W., Blackwell’s five-minute veterinary consult: equine. 3nd ed. Wiley-Blackwell. 2019.
  4. Thomas J. Divers, Leptospirosis in Horses – Horse Owners. Merck Veterinary Manual. Accessed Jan. 15, 2024.
  5. Thomas J. Divers, Leptospirosis in Horses – Generalized Conditions. Merck Veterinary Manual. Accessed Jan. 15, 2024.
  6. Divers T.J. et al., Leptospirosis: An Important Infectious Disease in North American Horses. Equine Veterinary Journal. 2019.
  7. Bernard W.V., Leptospirosis. Veterinary Clinics of North America: Equine Practice. 1993.
  8. Sellon D.C. and Long M.T., Equine infectious diseases. Second edition. Saunders/Elsevier. 2013.
  9. Adler B. and de la Peña Moctezuma A., Leptospira and Leptospirosis. Veterinary Microbiology. 2010.
  10. Broux B. et al., Acute Respiratory Failure Caused by Leptospira Spp. in 5 Foals. Journal of Veterinary Internal Medicine. 2012.
  11. Ricard R.M. et al., A 13-Year Retrospective Study of Equine Abortions in Canada. Can Vet J. 2022.
  12. Ellis W.A. et al., Leptospiral Infection in Horses in Northern Ireland: Serological and Microbiological Findings. Equine Veterinary Journal. 1983.
  13. Williams D.M. et al., Serological and Microbiological Findings on 3 Farms with Equine Leptospiral Abortions. Equine Veterinary Journal. 1994.
  14. Hill W.K. and Weenink-Van Loon C.D., Review of the incidence of antibodies to various serological groups of the species Leptospira interrogans in a number of farm animals in the Netherlands (author’s transl). Tijdschrift voor diergeneeskunde. 1976.
  15. Gilger B.C. and Michau T.M., Equine Recurrent Uveitis: New Methods of Management. Veterinary Clinics of North America: Equine Practice. 2004.
  16. American Association of Equine Practitioners, Leptospirosis | AAEP. 2019. Accessed Jan. 15, 2024.
  17. American Association of Equine Practitioners (AAEP), Vaccinations for Adult Horses. 2020.