Zoonotic diseases: Horses to humans


Veterinarians are one of the most at-risk populations for equine-human disease transmission.

The average horse owner is highly unlikely to come down with a disease or infection passed on from horse to human. Veterinarians, however, are in daily contact with higher numbers of sick horses and are one of the most at-risk populations for equine-human disease transmission. Although equine practitioners are familiar with many zoonotic diseases in horses, they might not be as aware of the clinical picture of these maladies in humans. Equine veterinarians must understand the zoonotic potential of certain diseases and organisms affecting horses and both educate their clients about these risks and take appropriate precautions themselves.

Contact and transmission: Barn workers, stable personnel and equine competitiors are potentially at risk for various horse-to-human diseases. Gatherings of groups of horses from many different areas that have undergone shipping and competition stress are potential situations for disease transmission. People at shows should be especially careful and make hygiene protection procedures a priority.


Although the incidence of rabies in both horses (45 to 50 cases annually in the United States) and humans is low, it is highly fatal, and clinical signs in horses may be quite variable, making it difficult to diagnose. It seems that almost every year there is a report of a horse presenting with unusual clinical signs that is later found to have rabies. Affected horses may have colic (especially foals and younger horses) or show vague lameness. Often, no evidence of a bite or recent wound is present, and many neurologic diseases exhibit similar signs, further confusing a diagnosis. Rabies is transmitted from horses to people via saliva, and any small cut or abrasion can serve as an entry point. Veterinarians often include at least an examination of the oral mucosa as part of a diagnostic work-up and can easily become infected. Unusual clinical signs, especially if associated with any degree of neurologic abnormality, should be a warning for potential rabies risk, and appropriate precautions should always be taken.


Brucellosis occasionally occurs in horses. The bacteria usually localizes in muscles, tendons and joints, though it is most commonly seen in cases of infected withers in horses. Drainage from areas infected with this organism contains high numbers of bacteria and is very infectious. Brucellosis can cause abortion in mares, so transmission is possible for veterinarians handling fetal membranes. Because the exact nature of the infection in many of these cases is known only after obtaining culture results, veterinarians are urged to be cautious and to observe good protective technique when dealing with such cases.

Anthrax and glanders

Anthrax can infect virtually all animal species and can cause local carbuncles and pustules in humans from direct lesion contact along with pneumonia from inhalation of the infectious agent. Higher incidences of anthrax occur in Arkansas, South Dakota, Louisiana, Missouri and California, and sudden equine death in these areas should especially place this disease high on the differential list. Do not perform a postmortem examination of suspected anthrax cases as opening the body and exposing the organism to oxygen will cause spore formation. These spores are then released into the air, which significantly increases exposure potential.

Glanders, caused by Burkholderia mallei, occurs in horses, donkeys and mules, and it also has cutaneous and pulmonary forms that are usually fatal to both horses and humans. Use of a mask is commonly overlooked by practitioners examining horses presenting with a cough and an elevated temperature but could be the difference between making a diagnosis and needing one yourself.


Leptospirosis is considered to be the most widespread zoonosis in the world and is caused by highly invasive bacteria of the genus Leptospira. Leptospirosis in horses commonly causes uveitis and can also result in abortions and renal disease. This bacterium is transmitted between species by infected body fluids (commonly urine) as well as contaminated water and soil, and it can enter the body through even minor skin lesions. The disease in humans can range from mild to severe and can result in death.

Lyme disease

Once thought to be exclusively caused by Borrelia burgdorferi, it is now postulated that other strains of bacteria as well as many species of ticks may harbor and transmit Lyme disease or other similar variations of this condition. Lyme disease in horses can manifest as a generalized body stiffness or soreness; reluctance to move, vague, transitory lameness; or transitory joint swelling. Infected horses have also exhibited nervous system disorders including blindness, head pressing, circling and seizures.

Ixodes species ticks carry these bacteria and are commonly found on deer but will also feed on other species such as dogs, humans and horses, which is where the zoonotic potential, as vectors, exists. Lyme disease symptoms in humans vary dramatically among patients, so it can be difficult to diagnose. A slowly expanding skin rash after a tick bite is the classic sign for Lyme disease and is seen in 60 to 80 percent of human cases, but many cases are more subtle and include abnormalities of the musculoskeletal, nervous and cardiovascular systems including arthritis-like symptoms, irregular heartbeat, and central nervous system or spinal cord issues.

Lyme disease has not been considered especially relevant in the southern United States, but a recent variation of this condition, southern tick-associated rash illness (STARI), has been attracting attention. It is attributed to infection with an as-yet-uncultured spirochete tentatively referred to as Borrelia lonestari. The Lonestar tick has been implicated as the principal vector.

Diagnostic testing for Lyme disease, or any other new variants, is currently difficult because blood tests do not differentiate between exposure and infection. Substantial research is ongoing in this area, and newer, more helpful diagnostics for both horses and people should be forthcoming.


Two tick-associated diseases that mimic Lyme disease are equine granulocytic ehrlichiosis (EGE) and human granulocytic ehrlichiosis (HGE), both of which are caused by Ehrlichia equi. EGE causes elevated temperature, depression, jaundice, limb swelling, ataxia and blood abnormalities. HGE produces flu-like symptoms in people including fever, headache, chills and nausea. Both infections can, if not recognized and treated with antibiotics, become systemic and result in death. Blood tests can identify E. equi in white blood cells and should be used whenever clinical signs in horses and people, along with any type of tick bite or tick exposure history, make clinicians suspect these diseases.

Equine encephalomyelitis

Mosquito-borne diseases pose a zoonotic threat for veterinarians, though the horse, in these cases, acts as a reservoir or vector. Eastern, Western, St. Louis and some subtypes of Venezuelan equine encephalomyelitis can affect humans. Clinical signs in people vary from mild flu-like symptoms to severe central nervous system signs. Human deaths occur primarily in children and the elderly. Postmortem examination of infected horses puts veterinarians at risk of direct disease transmission through infected blood and cerebrospinal fluid, so always take appropriate protective measures.

Fecal-oral transmission

Diarrhea commonly occurs in horses, and many diseases in this category can be transmitted to humans via the fecal-oral route. Good hygiene control measures are crucial when veterinarians are working on cases involving diarrhea in horses. Also keep in mind that any human, or other horse, that is receiving antibiotics (especially orally) is at increased risk for developing an enteric infection from a horse with diarrhea.

Salmonellosis is a common form of enteric infection in both horses and humans. Stress-induced diarrhea (transport, training, competition, hospitalization) is common, and Salmonella species are often reported after fecal culture in these cases. Fecal-oral transmission is the prominent route of zoonotic spread.

Horses also shed Giardia species in their feces and, though it is unclear if Giardia intestinalis is a pathogenic organism in horses, it is parasitic in humans.

Crytoporidium parvum is another protozoal organism with the potential of horse-to-human spread. Research has shown higher levels of C. parvum in foals, so exercise caution when handling young horses with diarrhea — wear gloves, change clothes after handling affected individuals, wash hands and use foot baths when entering and leaving affected areas.

These hygiene principles also apply in cases of dermatologic diseases of zoonotic potential in horses. Ringworm can be easily transmitted to people through direct contact. Always keep in mind that many equine skin lesions may be potential sources of disease transfer.

Hendra virus

Hendra virus is a paramyxovirus first isolated in 1994 from an outbreak of respiratory and neurologic disease in horses and humans in Hendra, a suburb of Brisbane, Australia. This virus is thought to be carried by bats of the genus Pteropus. Horses become infected through contact with bats and their droppings or secretions.

Hendra virus infection in horses produces an initial respiratory infection and can progress to neurologic signs and total systemic failure. These clinical signs are mirrored in human Hendra infection. The three cases reported in humans to date include two veterinarians and a trainer, two of which died. Humans caring for infected horses are exposed to body fluids and excretions and can easily become infected. Severe flu-like symptoms quickly develop. The globalization of the equine market makes disease transmission across continents, including the Hendra virus, a more serious threat.


Infection with methicillin-resistant Staphylococcus aureus (MRSA) can be transmitted from horses to humans and vice versa. This bacterium can be found in skin wounds and various locations in the respiratory tract. Many studies indicate that MRSA is becoming increasingly prominent, especially in veterinarians, farm workers and others who have increased contact with animals.

In a study at the University of Utrecht in the Netherlands from 2006 to 2008, 43 percent of all horses that entered the hospital clinic, for whatever reason, cultured positive for MRSA at some point during their hospital stay. Over 9 percent of horses were carriers based on positive cultures done when they first arrived at the clinic, and 15 percent of hospital employees who handled equine patients were also identified as MRSA carriers, though less than 1 percent of the general population of the Netherlands is MRSA-positive. Clearly horses and those who work around them are at higher risk of cross-transmitting this potentially serious infection.

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