Horse Diseases and Humans
If you work around horses long enough, you'll be handling ones that have lameness, diarrhea, abortions, skin diseases, and even neurologic signs. Our first concern is generally for the horse's well-being, and the last for our own health, since, after all, who gets sick from a horse?
Zoonotic diseases are those that are transmissible between animals and people. This article is going to address only selected diseases that humans can obtain from horses by direct or indirect contact (see sidebar "Infectious or Contagious?" below).
Rabies is a viral disease transmitted via the saliva of a rabid animal and usually passed on through a bite or scratch. Infected saliva contacting mucous membranes or a break in the skin constitutes an exposure, as does direct contact with nervous system tissue (necropsy or laboratory exposure). The disease is virtually 100% fatal in domestic animals and people (only two people have been documented to have partially recovered from the disease).
While annually about 40 to 50 equine rabies cases are documented in the United States and Puerto Rico, it is a disease that warrants precaution since the clinical signs can vary from colic to lameness to neurologic disease, all which progress to death. It is always prudent to consider any horse with neurological signs as being possibly rabid (isolate and avoid all contact, especially with his mouth and saliva), and consult a veterinarian promptly.
In 2006, a horse housed at the Tennessee Walking Horse National Celebration developed neurological signs and was diagnosed with rabies after euthanasia (TheHorse.com article #7589). This resulted in the Tennessee Department of Health, with the Centers for Disease Control and Prevention (CDC), attempting to contact 150,000 people who were at the event about possible rabies exposure (article #7611 on TheHorse.com). Just one rabid horse can cause an enormous response (see sidebar, "Rabies!").
Out of more than 2,400 different types of zoonotic Salmonella bacteria, just one is transmitted only between people; the rest have animals involved. S. typhimurium is a serovar commonly detected in people, and it also is commonly found in horses.
Salmonellosis in horses causes diarrhea, septicemia, and localized infections (joints, eyes, etc.). People get exposed by fecal-oral contact (generally due to ineffective hand washing) or oral contact with contaminated materials. Any horse or foal with diarrhea should be isolated, and proper protective clothing should be worn when handling the animal, with further precautions assessed by a veterinarian.
Cryptosporidium parvum is a protozoal intestinal parasite that can cause significant diarrhea in humans and young calves. Foals have been shown to shed C. parvum in feces, but generally they don't appear sick, and people in contact with affected foals have developed cryptosporidiosis (the disease associated with this parasite). Therefore, good hygiene, such as thorough hand washing, should be practiced when handling foals.
The sudden death of a horse gets the owner's attention immediately, and so do colic and severe diarrhea leading to death. If blood comes from the nose, mouth, or other orifices of the dead animal, the owner should prevent other animals from accessing the carcass, avoid all contact with the carcass, and call a veterinarian or state health official. The cause of death might be the spore-forming bacteria Bacillus anthracis that causes illness and death in livestock in several U.S. areas. However, unusual weather conditions, floods, and alkaline soils can potentially stir up spores deep in the soil and cause anthrax cases in other geographic areas. Anthrax is usually rapidly fatal in horses, so it's unlikely there would be time to treat horses with clinical signs, but you could potentially treat suspected cases very early on or prophylactically treat exposed horses. Special diagnostic sampling techniques and disposal methods are necessary for suspected anthrax cases; in some jurisdictions it might be a reportable disease.
People can become ill from dead animals by dermal exposure (through the skin, called cutaneous anthrax) to infected animals, tissues, and soils; inhalation anthrax is possible if spores are aerosolized, which can happen while processing livestock hides or wool.
Rhodococcus equi is commonly found in the soil and in feces of horses and other herbivores. This Gram-positive bacterium has long been recognized as a pulmonary pathogen of foals and a cause of other localized infections in foals and occasionally adults. R. equi is now considered a pathogen of importance to immunocompromised people, especially those with human immunodeficiency virus (HIV) infection. The majority of human patients with R. equi infections are immunocompromised, with approximately half of those being HIV patients (see sidebar "Immunosuppressed Humans and Horses" on page 32). Researchers are studying the epidemiology of human R. equi infection, as less than half of infected patients have a history of exposure to horses or pigs (another source of R. equi).
Testing and eradication efforts have nearly eliminated brucellosis from U.S. livestock herds. However, brucellosis still exists in wildlife that can be a source of infection to livestock. Many species of Brucellosis bacteria exist, but it's Brucella abortus and B. suis that affect horses. These Gram-negative bacteria can cause fistulous withers and poll evil. The former is an inflammation and infection of a bursa (fluid-filled sac) over the withers; poll evil is an inflammation and infection of the bursa over the first and second cervical vertebrae, close to the horse's poll. Swelling can occur in these areas, and when bursae burst, purulent material drains from the tracts. Brucella is only one of several bacteria that can cause these conditions, but it should be first on the minds of handlers because of its zoonotic potential.
Direct contact with discharges and possible aerosol exposure in enclosed spaces can serve as routes of transmission to people. While documented cases of human brucellosis are rare, the severity of the disease in people (called undulant fever) and the risk of transmission warrant prompt veterinary services for your horse if you notice an abscesslike swelling occurring in the poll or withers area. Strict hygiene measures should be applied.
Foreign Animal Diseases
Of the zoonotic encephalitic diseases of horses (Eastern and Western equine encephalitis and West Nile virus), none is transmitted via mosquito directly from the horse to human. However, high enough levels of Venezuelan equine encephalomyelitis (VEE) in horses' blood can be a source of infection to humans via mosquito bites. Direct transmission from horses to humans does not occur. VEE has been detected in South America and Central America; the last U. S. outbreak was 1969-1972, when it crossed over from Mexico. Other foreign animal diseases that can affect humans are glanders and Hendra virus.
This is not an exhaustive list of horse-to-human zoonotic diseases. Also consider vesicular stomatitis virus, campylobacterosis, leptospirosis, ringworm (see article #5531 at TheHorse.com) and other fungal skin diseases, and methicillin-resistant Staphylococcus aureus (MRSA) infection (see article #11078 at TheHorse.com).
A few simple guidelines can reduce the risk of obtaining an infection from a horse. Here are the key points:
- Obtain appropriate vaccinations for horses and for people!
- Isolate horses with neurologic signs and have a veterinarian out to see the animal; avoid contact with the horse's saliva.
- Caretakers should use protective clothing and gloves when handling horses or foals with diarrhea.
- Use mosquito and insect control strategies to reduce vector-borne diseases.
- Immunocompromised individuals should consult with a health care professional about specific concerns with any horses and other animals in their care.
- And, like your mother said, "Wash your hands before eating or drinking!" or after any contact with horses.
- Equine Infectious Diseases, Ed. Debra C. Sellon and Maureen T. Long. St. Louis: Saunders 2007.
- Bender, J.B.; Tsukayama, D.T., Horses and the risk of zoonotic infections. Veterinary Clinics of North America: Equine Practice. Vol 20, 643-453, 2004.
- Control of Communicable Diseases Manual, 18th edition, Ed. David Heyman. Washington, D.C.: American Public Health Association, 2004.
About the Author
Roberta Dwyer, DVM, MS, Dipl. ACVPM, is a professor within the University of Kentucky's Maxwell H. Gluck Equine Research Center and editor of Equine Disease Quarterly.