Humans share a lot with their horses — even disease. A handful of infections can be passed between the two species — some are easily treatable, some can be fatal. In most of these cases, humans contract disease from horses; it is possible but less likely for an infected human to pass disease to a horse.
Ringworm, rabies, methicillin-resistant staphylococcus aureus (MRSA), salmonella, and anthrax occur worldwide and are among the diseases that can impact both horses and humans. Hendra, a usually fatal disease spread by bats, is endemic to Australia. Glanders, which attacks the internal organs and mucous membranes, is a global threat, although the last recorded case in the United States was in 1937. Glanders was weaponized during World War I, and anthrax has been used as bioterrorism.
Ringworm, a fungal skin infection, probably is the most common disease passed between horses and humans. It is characterized by a circle of itchy, flaky, hairless skin. Highly contagious, ringworm is spread by contact with the lesions or by using infected tack. Horsemen working with an infected horse can contract ringworm.
Disinfection of the horse and its environment followed by application of antifungal ointment and systemic fungicide kill the fungus within six weeks.
Rabies is spread by contact with saliva from an infected animal or person. In horses, symptoms include a drastic change in temperament, head-pressing, circling, neurologic signs, loss of appetite, and choking. Horsemen are warned always to wear gloves when reaching into a horse's mouth because exposure to the saliva of a rabid horse, or saliva that comes in contact with an open wound, can spread rabies.
With the exception of a single survivor in the history of the disease, rabies is 100 percent fatal in unvaccinated humans and animals.
MRSA is a so-called superbug because of its resistance to most antibiotics. In humans, the disease is widespread throughout the world, with nearly every hospital screening incoming patients for MRSA to curtail its spread. Contact with the skin of a person or animal infected with MRSA or an object contaminated with MRSA is enough to spread the disease.
A 2011 study by researchers at Hollins University in Virginia and the Minnesota Racing Commission found approximately one-third of grooms, trainers, and veterinary personnel at the racetrack are infected with MRSA. They cited lack of barn cleanliness, poor hygiene, and overuse of antibiotics as factors in its spread.
Dr. Scott Weese, assistant professor of clinical studies at Ontario Veterinary College, believes MRSA in humans originally jumped species to infect horses. About 5 percent of healthy horses in the U.S. have MRSA in their nostrils.
Evidence of MRSA infection begins with skin lesions resembling spider bites. The infection may invade the bloodstream and progress to deeper tissues, bones, joints, heart, and lungs, where it can become life-threatening. Treatment is by the limited number of drugs to which MRSA remains sensitive.
Cryptosporidium, giardiasis, and salmonella can cause intestinal distress in horses and humans. When an infected horse defecates, the pathogen in its diarrhea can spread the disease across its surroundings. Horses and other animals, as well as humans, can become infected if they accidentally ingest the pathogen. Horses could eat contaminated hay or bedding; animals could lick a contaminated surface; and humans could become infected by touching their mouths with hands that came into contact with a contaminated surface, including the horse itself.
A 2011 Italian study found an association between foal-heat diarrhea and Cryptosporidium parvum. Ten out of a dozen foals in the study that developed foal-heat diarrhea were positive for the parasite. The researchers warned caretakers of foals with diarrhea to be vigilant about biosecurity, especially disinfecting hands often, when handling them.
Giardiasis is caused by parasites that invade the intestines. In 2005, the World Health Organization identified horses as a potential source of human infection, either through direct contact or by contact with contaminated water. Swimming in a farm pond containing contaminated runoff or handling a contaminated water trough can expose humans to the disease, as well as contact with an infected horse or contaminated surfaces.
Salmonella bacteria can overrun and attack the digestive system and then invade the bloodstream through damaged intestinal walls. Usually, horses ingest the bacteria in feed, hay, or water contaminated by the feces of infected birds, reptiles, amphibians, or other animals. While a relatively high-dose ingestion of salmonella is necessary to cause disease in healthy animals, those in a stressed or weakened condition can develop the disease by exposure to an otherwise harmless level of pathogens.
Contamination goes both ways. Infected humans can expose horses to these diseases by contact with fecal matter. If you have diarrhea, don't use a stall for a bathroom.
Same disease, no threat
Just because humans and horses are susceptible to the same diseases doesn't necessarily mean they can infect each other. Horses and humans are considered dead-end hosts for some diseases, meaning they don't have high enough levels of the virus in their bodies to transmit it.
West Nile Encephalitis (WNE), Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), and Venezuelan Equine Encephalitis can infect horses that have not been properly vaccinated. These diseases are known to infect humans also, especially those compromised by other health problems, but WEE in humans rarely occurs, with a single fatal case documented in Uruguay in 2009.
Humans and horses become infected by the bite of a mosquito previously infected by sucking blood from a bird with encephalitis. With the exception of VEE, horses and humans can't transmit the disease.
“The viremia levels in horses infected with EEE, WEE and West Nile viruses do not reach high enough levels to reach or exceed the threshold necessary to infect competent species of mosquitoes for each of these three viruses; hence transmission to humans cannot occur,” said Dr. Peter Timoney, Frederick Van Lennep Chair in Equine Veterinary Science at the University of Kentucky.
VEE, uncommon in the U. S. except in the southwest, could be transmissible between humans and horses, but the subtypes capable of transmission emerge only occasionally, Timoney said.
“VEE is the only one of the four that can give rise to high enough viremic levels in the infected horse sufficient to infect mosquitoes, and these in turn can transmit the virus to humans…
“It is known that humans infected with this virus can develop viremias of sufficient magnitude to infect a mosquito. Under such circumstances, it is conceivable that such a mosquito could potentially infect a naïve or susceptible equid with the virus.”
Can your horse give you the flu?
Dr. Thomas Chambers, a faculty member at the University of Kentucky's Gluck Equine Research Center and one of the world's experts on influenza, said, “I would never say that it is impossible for influenza to jump from horses to humans or vice-versa. But it must be an exceedingly rare event or we would have seen more evidence of it by now.”
Chambers explained that humans with horse exposure sometimes develop antibodies against equine influenza virus, but experiments done in the late 1960s found those infections were mild or even subclinical (cause no symptoms). He believes similar experiments done today would show an even milder infection, perhaps undetectable, because widespread exposure to H3 influenza has made humans more resistant to that strain than when it was an emerging subtype 50 years ago.
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