Diseases with Horse-to-Human Transmission
Can you get a disease from your horse?
Source: UC Davis Center for Equine Health
Yes, but the good news is that direct horse-to-human disease transmission is rare. Here’s a quick refresher on some bacterial, fungal, parasitic, and viral diseases that can potentially be transmitted to us directly from the equines in our lives. This list does not include diseases like West Nile virus that can be passed indirectly from horses to humans, for example through an insect bite.
CONDITIONS COMMONLY TRANSMITTED
All those who spend time around horses should take precautions against the following conditions that are commonly transmitted from horses to humans:
Dermatophilosis is caused by the bacteria Dermatophilus congolensis, which enter the body through an open wound such as a skin abrasion, laceration, or insect bite site.
Horses: Dermatophilosis is common in horses and is often known as “rain rot,” “rain scald,” “mud rash,” or “mud fever.” It is most often associated with prolonged exposure to moisture in the form of rain or sweat under blankets or tack. The disease can result in painful lesions and swelling on the back, muzzle, and limbs that characteristically form dry, “nobbly” crusts.
Transmission: The bacteria can be transmitted from horses to humans through direct contact with infected animals.
Humans: In humans, lesions typically occur on the hands and arms.
Treatment and prevention: Topical and systemic antibiotics are used to treat dermatophilosis. For horses, it is also important to address management changes to keep animals dry. Although not life threatening, dermatophilosis can lead to secondary infections if left untreated.
Ringworm, or dermatophytosis, is an infection of the skin or hair caused primarily by the fungi Trichophyton equinum and Trichophyton mentagrophytes.
Horses: These fungi live in the soil and horses can be exposed when they dig, roll, or lie down in dirt. The fungi can be transmitted between horses by contact with infected individuals and contaminated objects such as grooming equipment and tack. The clinical signs of ringworm in horses usually consist of patches of crusty, dry skin with hair loss, and circular lesions that are itchy.
Transmission: The fungus can be transmitted from horses to humans by direct contact with an infected animal’s skin or hair or touching contaminated objects.
Humans: Similar to horses, humans often develop a scaly, reddened, circular rash.
Ringworm on a human leg, with mildly raised border and central clearing. Photo: Wikimedia/James Heilman MD
Treatment and prevention: Infections often clear up without treatment, but medicated shampoos are effective in some cases.
Salmonellosis is caused by the bacteria Salmonella enterica and causes diarrhea in horses and humans.
Horses: In horses, the bacteria are transmitted through contaminated feed or water. Transmission can also occur by direct contact with infected individuals or surfaces that have been contaminated with manure from an infected horse. Some horses are carriers that do not show any signs of disease but can actively transmit the organism. Carriers can become sick when under stress from surgery, transportation, changes, in feed, etc. Severe cases can be fatal if not treated.
Transmission: People that work around horses can get salmonella by touching contaminated surfaces and accidentally transferring the bacteria from their hands to their mouths.
Humans: Many people show no symptoms of salmonella infection. Others may develop fever, abdominal discomfort, and diarrhea. Clinical disease can be mild or severe and severe cases can be fatal if not treated.
Treatment and prevention: In humans, most cases resolve even without treatment. In horses, non-steroidal anti-inflammatory drugs (NSAIDs) may be used to alleviate pain, and gastrointestinal protectants such as bio-sponge or activated charcoal may bind the bacterial toxin. Severe cases may require treatment with IV fluids and electrolytes. Since the organism is prevalent in the environment, prevention of salmonellosis is difficult. Identification of infected horses and strict biosecurity protocols are the best approach to preventing the spread of the disease.
LESS COMMON CONDITIONS
Conditions that are less commonly passed from horses to humans, but still warrant appropriate biosecurity precautions are:
Bacterial species in the genus Campylobacter can cause irritation and inflammation of the stomach and intestines (gastroenteritis) in animals and humans. In fact, it is globally the most common bacterial cause of gastroenteritis.
Horses: Infections in horses cause inflammation of the small intestine (enteritis) which leads to diarrhea.
Transmission: The bacteria are typically transmitted through fecal contamination of the environment, and subsequent ingestion through food or water. People can become infected through horse-related activities, including cleaning stalls and grooming. However, you are more likely to be exposed to Campylobacter in uncooked or undercooked chicken or unpasteurized milk.
Campylobacter is the most common bacterial cause of gastroenteritis. People become infected through horse-related activities, including grooming and cleaning stalls. Photo: Clix Photography
Humans: Symptoms in humans include abdominal pain, diarrhea, nausea, fever, and vomiting.
Treatment and prevention: Antibiotics may be used to treat infected individuals. Campylobacter infection is rarely life threatening.
Parasites in the genus Cryptosporidium are an important source of gastrointestinal disease in humans and animals globally. These highly contagious parasites infect the intestine and cause diarrhea and weight loss.
Horses: Cryptosporidiosis is most commonly seen in foals, especially those that are immunocompromised or stressed. Horses become infected by ingesting the parasite in contaminated food or water.
Transmission: The parasite can be transmitted from horses to humans in contaminated water, soil, or surfaces, and can survive in the environment for long periods.
Horses become infected with cryptosporidiosis by ingesting the parasite in contaminated food or water. The infection is most commonly seen in foals. Photo: Canstock/Anakondasp
Humans: In humans, ingestion of the parasite can cause watery diarrhea, vomiting, stomach cramps, and weight loss. Symptoms may last up to two weeks.
Treatment and prevention: Symptoms often resolve without treatment in people with healthy immune systems. For those with compromised immune systems or severe infections, anti-parasitic drugs (such as nitazoxanide) and anti-motility agents (such as loperamide) can help relieve diarrhea.
Single-celled organisms of the genus Giardia, most commonly Giardia duodenalis, attach to the intestine, causing problems with absorption of vitamins and other nutrients, often resulting in diarrhea. Contaminated water and soil are the primary sources of infection.
Horses: Giardia can cause intermittent diarrhea in some infected horses, as well as poor hair coat and weight loss. Foals are frequently infected but often do not show clinical signs.
Transmission: Infected horses can transmit the parasite to humans indirectly through fecal contamination of water, or directly through handling or on contaminated surfaces.
Humans: Symptoms in humans include diarrhea, abdominal cramps, bloating, and weight loss.
Treatment and prevention: The antibiotic metronidazole is commonly used to treat giardiasis in both species.
Equine leptospirosis is often the result of infection with the bacteria Leptospira interrogans through contact with infected urine, often in contaminated feed or water. The bacteria spreads rapidly to tissues throughout the body via the bloodstream.
Leptospirosis can be associated with equine recurrent uveitis (moon blindness).
Leptospirosis, often the result of infection through contact with infected urine, can be associated with equine recurrent uveitis (moon blindness). Photo: Wikimedia/Christian Muellner
Horses: In horses, the disease is associated with abortions, stillbirths, kidney disease, and equine recurrent uveitis (moon blindness), although some infected horses show no clinical signs.
Transmission: Humans can become infected when contaminated water contacts broken skin or mucous membranes (eyes, nose, or mouth).
Humans: In humans, leptospirosis causes flu-like symptoms, including abdominal pain, high fever, chills, vomiting, and jaundice, but neurologic, respiratory, cardiac, and ocular issues can occur.
Treatment and prevention: Antibiotics and anti-inflammatories are used to treat leptospirosis. There are currently no human vaccines for leptospirosis, but one is available for horses.
Rabies, a highly fatal disease found worldwide, is caused by the rabies virus. All mammals are susceptible to rabies, but it is relatively rare in horses, with less than 100 cases reported annually.
Rabies infection manifests in two forms: furious (classical or encephalitic) and paralytic. Paralytic rabies accounts for approximately 20 percent of the total human cases; furious rabies accounts for the other 80 percent. This dog is in late-stage paralytic rabies, and appears depressed, lethargic, and uncoordinated, and will gradually become completely paralyzed. When their throat and jaw muscles are paralyzed, the animals will drool and have difficulty swallowing. Photo: Wikimedia
Horses: Clinical signs in horses include uncoordinated movements (ataxia), partial paralysis (paresis) of the hindquarters, a desire to lie down, lameness, difficulty breathing, colic, increased sensitivity to light and sound, and fever. Most horses die of cardio-respiratory failure within two to five days, but it can take up to two weeks in some cases.
Transmission: The virus is found in the saliva of animals. Rabies can be transmitted from horses to humans through a bite or contact of infected saliva with the eyes, nose, or mouth, or broken skin.
Humans: Symptoms in humans include fever, headache, itching at the site of infection, and pain. People who are infected can become hyperexcitable or suffer from generalized paralysis, and death occurs within two to ten days. In rare cases of survivors, severe neurological disorders may result.
Treatment and prevention: There is no treatment for rabies in horses. Humans who think they may have been bitten by a rabid animal are given a series of shots to prevent infection. There is no effective treatment once the infection has become established. Rabies vaccines are available for horses and humans. Rabies is a reportable disease in Canada and the United States; suspected cases must be reported immediately.
IMPORTING HORSES & INTERNATIONAL TRAVEL
In addition, those who import horses and/or travel to other countries for equine-related business and activities should consider precautions against:
Anthrax is caused by the Bacillus anthracis bacterium, which can survive in the soil for decades. The bacteria produce a toxin that breaks down cells in infected tissues. Bacterial spores have historically been weaponized (in aerosolized form) and are considered possible agents of biological attack or bioterrorism.
Horses: In horses, the clinical signs of anthrax vary depending on whether the horse ingested the bacteria, or it was transmitted through an insect bite. Fever, loss of appetite, inflammation of the small intestine (enteritis), colic, and bloody diarrhea can occur if the horse ingests the bacteria. Death typically results in two to four days. For cases in which anthrax is introduced via insect bite, clinical signs include swelling at the site of the bite, and on the neck, chest, belly, and genitals.
Transmission: The disease can be transmitted from horses to humans through handling of infected animals.
Humans: In humans, anthrax may be cutaneous, inhalational, or gastrointestinal. The cutaneous form is the most common (and the type that can be transmitted from horses). This form results when the bacteria enter the body through cuts or abrasions in the skin. Lesions with black centres appear on the skin of the arms, hands, face, and neck within two to three days. Death is rare with appropriate antibiotic treatment.
A skin lesion caused by anthrax, with the characteristic black centre. Photo: Wikimedia
Treatment and Prevention: Prompt treatment with antibiotics is essential to a successful outcome in both species. Although a human vaccine is available, vaccination is not recommended for the average person since the vaccine is not easily accessible and the risk of infection is considered low in the United States. A vaccine is available for horses but is generally only recommended for horses in areas that are known to be contaminated. Anthrax is a reportable disease in Canada and the United States; the appropriate agency must be contacted immediately.
The bacteria Burkholderia mallei causes glanders and infection and is fatal in most species.
Horses: Infection in horses can result in pus-forming lesions on the skin and respiratory infections. The source of infection is discharge from the nose and skin of infected equids. The primary route of infection is believed to be ingestion of feed or water contaminated by B. mallei, although inhalation and skin exposure may occur. Infected equipment, tack and clothing can also transmit the bacteria. Some infected horses show no symptoms but can serve as sources of infection by shedding B. mallei intermittently or constantly.
Transmission: People can get the disease through direct contact with infected animals.
Humans: Symptoms in humans include fever, muscle pain, chest pain, light sensitivity, and headache. If the bacteria get into the bloodstream, the infection can be fatal without treatment.
Treatment and prevention: Antibiotics are used to treat cases of glanders in people. Equine cases are not treated in the United States. Since it is contagious and can be fatal in humans, the bacterium is considered a potential agent of bioterrorism (Tier 1 biological agent). All horses imported to the United States are tested for glanders to prevent introduction of the disease. Glanders is a reportable disease in the United States; suspected cases must be reported immediately.
In Canada, the Canadian Food Inspection Agency (CFIA) imposes strict regulations on the import of animals and animal products from countries where glanders occurs, enforced through port of entry inspections. Glanders is an immediately notifiable disease in Canada. The CFIA website states that glanders has been eliminated in North America, Australia, Japan and Europe. It is thought to be endemic in parts of the Middle East, Asia, Africa, and South America.
This disease is caused by the vesicular stomatitis virus.
Horses: Vesicular stomatitis results in lesions and blisters on the tongue, mouth, nose, and lips of horses. Other clinical signs include drooling or frothing at the mouth, lethargy, fever, and loss of appetite. Mortality is rare. Horses typically recover within two weeks, but outbreaks can have significant financial impacts on the equine industry. Vesicular stomatitis has been reported around the world, with most equine outbreaks in the United States limited to western states.
Transmission: Transmission to humans can occur through direct contact with the lesions or saliva of infected horses.
Humans: Symptoms in humans resemble the flu and include fever, chills, nausea, vomiting, and headache. Lesions may form in the mouth and throat in adults and children may develop encephalitis, although it is rare.
Treatment and prevention: In horses, treatment is limited to supportive care and anti-inflammatory medications. Although vaccines are available for some livestock species in some countries, there are currently no equine or human vesicular stomatitis vaccines. Vesicular stomatitis is a reportable disease in Canada and the United States; suspected cases must be reported immediately. The CFIA website states that the disease was last diagnosed in Canada in 1949.
HORSES HOUSED WITH LIVESTOCK
Properties that house horses with livestock should also take appropriate precautions to avoid:
Most cases of brucellosis in horses are due to the Brucella abortus bacteria, which is also the cause of the disease in cattle. Perhaps not surprisingly, cases of brucellosis in horses usually occur in horses that are housed with cattle.
Horses: Horses become infected with the bacteria through ingestion, inhalation, or contact with skin abrasions. Termed “poll-evil” or “fistulous withers” in horses, brucellosis causes painful swelling at the base of the neck and sides of the withers, which can lead to abscesses. Joints may also become swollen and reproductive issues, including abortions, have been reported.
Brucellosis causes painful swelling at the base of the neck and sides of the withers, and eventually at the top of the animal’s head, traditionally called “poll evil.” Photo: Wikimedia
Transmission: B. abortus may be transmitted from horses to humans through contact with contaminated body fluids, tissues, or carcasses, although documented cases of transmission are rare.
Humans: In humans, the disease can mimic many other infectious and non-infectious diseases. Symptoms include fever, anorexia, headache, backache, joint pain, and weight loss. Even without treatment, most patients recover within three weeks.
Treatment and prevention: Treatment in horses consists of broad-spectrum antibiotics and draining of abscesses. Surgical intervention may be required in severe cases. Although reported worldwide, brucellosis has been eradicated from many countries, so infections are rare. Brucellosis is a reportable disease in Canada and the United States; suspected cases must be reported within 48 hours.
PRECAUTIONS FOR THOSE LIKELY TO CONTACT INFECTED ANIMALS
Veterinarians, veterinary technicians, breeding managers, and anyone more likely to come in contact with infected animals should take precautions to protect themselves from:
MRSA (methicillin-resistant strains of Staphylococcus aureus)
Although Staphylococcus aureus often does not cause any problems for humans or animals, antibiotic-resistant forms are difficult to treat. Methicillin-resistant strains of Staphylococcus aureus are resistant to antibiotics that are often used to treat Staphylococcus infections.
Horses: In horses, skin, soft tissue, joint, and surgical site infections can occur. Clinical signs may include wounds or surgical sites that discharge pus and fail to heal. Some horses carry MRSA without showing any clinical signs, but they can still transmit the bacteria to people and other horses.
Transmission: People can get MRSA from horses through direct contact with infected animals, especially direct hand-to-nose contact.
Humans: MRSA infections in humans often appear as a red rash or a skin boil. Symptoms may also include fever.
Treatment and prevention: Most MRSA infections can be effectively treated with antibiotics if diagnosed early. Healthy horses can carry MRSA, and it is known to be common in people who work with horses, such as veterinarians. This means that it is virtually impossible to eliminate exposure to MRSA in people or horses.
Anyone likely to come in contact with infected animals should take precautions to protect themselves from diseases that can be transmitted from horses to humans. Photo: Clix Photography
Rhodococcus equi infections
Rhodococcus equi is a bacterium that lives in the soil and can cause pneumonia in young (one to six months old) foals. In rare cases, immunodeficient adult horses and humans can also become infected. Disease prevalence varies, but R. equi can be detected on most horse farms around the world.
Horses: A foal can inhale airborne bacteria into their respiratory tract or ingest contaminated soil. Unfortunately, foals often do not begin to show signs until the disease has become severe. Clinical signs may include fever, laboured breathing, cough, and depression.
Foals can inhale R. equi or ingest contaminated soil. Photo: Canstock/PaulB
Transmission: Since it lives in the soil, R. equi can be inhaled in dust on farms or enter the body through broken skin. The bacteria can also be transmitted by handling infected foals.
Humans: In immunocompromised humans, R. equi infection can cause lung infection that results in the formation of abscesses. Mortality can be as high as 50 percent. Among people with healthy immune systems, infections may only affect a small part of the body and are commonly associated with wound sites.
Treatment and prevention: Antibiotics can be used to treat the infection in horses, but they can also result in potentially serious side effects. There are no available vaccines. Early detection in foals through bloodwork, ultrasound screening, and antibiotic treatment as needed is currently the best approach to ensure a successful outcome. In humans, a combination of antibiotics may be used to treat the infection, with surgical intervention to drain large abscesses in some cases.
Printed with the kind permission of the UC Davis Center for Equine Health. The UC Davis Center for Equine Health is dedicated to advancing the health, welfare, performance and veterinary care of horses through research, education and public service. https://ceh.vetmed.ucdavis.edu
BIOSECURITY BEST PRACTICES
- Wash your hands with soap and water or use hand sanitizer often.
- Practice good hygiene and sanitation.
- Avoid eating and drinking in barns, stables, and other areas where horses are housed.
- Isolate sick horses, use dedicated grooming supplies and equipment, and handle these horses last each day if possible.
- Vaccinate horses based on risk factors for your area and facility and as recommended by your veterinarian.
- Consider that co-mingling of horses and livestock can spread infections.
- Quarantine and test new horses before letting them come in contact with the resident herd.
- Control rodents and wildlife in areas where horses are housed.
- Use antibiotics as directed by your veterinarian and ensure that the full course of treatment is followed to minimize the spread of antibiotic resistant bacteria.
Main Photo: Shutterstock/Rolf Dannenberg
This article was originally published in the Spring 2020 Issue of Canadian Horse Journal