Zoonotic diseases: Horses to humans

Zoonotic diseases: Horses to humans

All equine practitioners should be familiar with the many infections that can be passedto people by horses in order to protect themselves and their clients
Dec 01, 2009

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.
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.


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.