Zoonotic Diseases and the Horse Owner

Stephen M. Angelos, DVM, DACVIM

Board Certified Large Animal Internal Medicine

Large Animal Medical Associates

Essex Junction, VT 05452

802-879-4288

Can My Horse and I Share Diseases?

            A zoonotic disease is an animal disease that can also infect humans.  In some zoonotic diseases, the disease can, under the right circumstances, go directly from animal to human.  In other cases, the organism (virus, bacteria, or fungi) causing the disease cannot go directly from animal to human.  There are multiple diseases horse owners should be aware of that have the potential to be transmitted to them from their horse.  Although this is not a frequent occurrence, and one that usually requires special circumstances (such as immunosuppression), transmission of disease from horse to human is a concern in the veterinary field.  One of the duties of a veterinarian is to be aware of which diseases are potentially transmissible to humans, and to alert owners about a possible risk.  Veterinarians cannot diagnose or treat human diseases, but they can alert an owner to a potential risk so the owner may bring this to the attention of their physician, should the need arise.  This article will provide a brief overview of some of the common zoonotic diseases that are present, and what individuals involved with equine care and husbandry can do to try to reduce the potential for infection.

Which individuals should be most careful about getting a zoonotic disease?
Specifically, any human in contact with an infected animal or disease vector has the potential to contract a zoonotic disease.  A vector is a disease carrier (i.e. insect, rodent, etc.) that has the potential to spread a disease from an infected animal to an uninfected human or other animal.  As stated above special circumstances are frequently (but not always) present when a human develops a zoonotic disease.  The following individuals may be at higher risk:

• infants and small children (immature immune systems, poor hygiene - hands in mouth, etc.)
• pregnant women (immune systems are more susceptible and there are additional fetal hazards)
• elderly (immune systems may be impaired)
• immunocompromised people (undergoing cancer therapy, HIV/AIDS patients, etc.)
• individuals receiving oral or intravenous antibiotics
• veterinarians, zoo/wildlife/primate and other animal health care workers.

There are many sources of good information on zoonotic diseases.  Information should be available through your veterinarian as well as through the local health department.  The internet is also an excellent source of information.  The following is a list of some of the diseases with the potential to infect both horses and humans.  The list was compiled using a variety of sources including the following website made by the Veterinary School at the University of Wisconsin-Madison.  Visit this site for more in-depth information regarding zoonotic diseases: http://www.vetmed.wisc.edu/pbs/zoonoses/.

Transmission prevention

The key to avoiding spreading and/or contracting zoonotic disease is preventative measures.  These measures include keeping sick horses isolated if they have a disease that is transmittable directly to other animals.  Horse owners should wear protective clothing when handling these animals, and then remove the clothing prior to handling any other horse.  Gloves are important protective items, as well.  A box of disposable vinyl or latex gloves should be part of any equine first aid kit.  If no gloves are worn, an owner should avoid contact with their face, and immediately wash their hands with disinfectant before moving onto another horse.  It is also recommended that blankets, saddlepads, and all other grooming items that are used on isolated horses do not come in contact with any other animal until they have been properly washed and disinfected.  While these recommendations are for diseases that are easily transmitted, these measures are encouraged even for those diseases with a low likelihood of infecting humans or other horses.  Many of the diseases discussed in this article are not entirely understood, and it is always better to err on the side of caution than to potentially open the door to illness and the spread of disease.  If a horse owner has any doubts or concerns over a condition their horse has, the most important recommendation that can be made is to consult their veterinarian.  A veterinarian will work with an owner to devise a plan to keep not only other horses on the property safe, but to also protect the humans who handle the affected horse.

NOTE: THE FOLLOWING LIST IS NOT A COMPLETE LISTING OF ALL ZOONOTIC DISEASES AFFECTING HUMANS AND HORSES.  FOR MORE INFORMATION, PLEASE CONSULT WITH YOUR VETERINARIAN, PHYSICIAN, OR PUBLIC HEALTH DEPARTMENT.

Rabies

This is a highly fatal disease, although human cases are rare.  The risk of exposure to rabies through horses is low, but there is still the potential for infection.  An equine infected with rabies can exhibit a variety of symptoms, making it difficult to diagnose.  Some horses may initially rub or bite the area of inoculation intensely.  An infected horse may also colic, or show signs of lameness.  It may be difficult to conclude that a horse is infected with rabies because there may be an absence of visible wounds (a possible route of infection), and some of the neurological signs of rabies can be present with other neurologic diseases.  Because it can be unclear during treatment of a horse what is causing their illness, it is important to consider rabies a possibility and handle the sick animal according to directions provided by your veterinarian.  Rabies can be transmitted from horse to human via saliva; a small abrasion or cut on your hand can be an open door in which the rabies virus can enter your body.  Rabies would be considered directly transmissible from one mammal to another without requirement for an in-between host.  Because the presenting signs of rabies can be so variable, any horse with neurologic signs must be initially considered a rabies suspect, and should be handled accordingly.

Encephalitis

Another group of diseases with zoonotic potential are diseases classified as mosquito-borne arboviruses.  Mosquitoes are obligatory biological vectors for transmission of these viruses. These diseases include eastern encephalitis, western encephalitis, venezuelan encephalitis, and West Nile virus.  These diseases most likely exhibit many of the same symptoms in horses as rabies, therefore they are to be considered when a veterinarian is working on a horse with acute neurologic disesase.  It is important to note that none of these arboviruses are transmittable directly from horses to humans under normal circumstances.  However, if the horse dies from their illness, and a necropsy is performed, there is a risk present for disease transmission from handling infected blood and cerebrospinal fluid.  If a post-mortem examination is performed, it should be done very carefully under protected conditions to lower the risk of infection.  West Nile virus infection has received a great deal of publicity since its introduction to the United States.  Through effective vaccination programs, the incidence of the encephalitis diseases has been reduced and controlled.  Contact your veterinarian to discuss a vaccination program.

Prevention of encephalitis through vector control:

• drainage of standing water to reduce mosquito replication
• use of insecticides during times of high mosquito populations ( usually via local government)
• sentinel pheasants (pheasants are susceptible to infection with EEEV) or light traps (and subsequent virus isolation from mosquitoes that are caught) to alert public health officials to the presence of virus in a given geographic area
• vaccination of horses
  • This is particularly critical from a public health point-of-view for VEEV because of the role of horses as reservoir hosts for the virus.

Diseases causing Diarrhea

There are numerous diseases that can cause diarrhea in horses.  Some of these can be transmitted directly to humans via the fecal-oral route.  The following is not a complete list.  It is especially important to use good hygiene control mechanisms when working with a horse that has diarrhea.  Other horses are also potentially at risk of contracting illness from the affected horse.  Any horse or human receiving antibiotics, especially oral antibiotics, is at greater risk for getting diarrhea or acquiring an enteric (gastro-intestinal) infection from a horse with diarrhea.

Salmonellosis

Salmonellosis is a very common gastrointestinal problem in horses and humans caused by the genus Salmonella, a bacteria.  Sudden diarrhea is usually the typical sign of infection, although there are many other symptoms that can occur.  Salmonellosis usually occurs after antimicrobial therapy or after the animal has been in a stressful situation (shipping, training, or hospitalization).  Humans can become infected with salmonella bacteria from their horses directly through the fecal-oral route.  A relatively high level of salmonella organisms need to be ingested in order to cause a healthy adult to become sick, but if that individual has a condition (immunosuppression) or therapy (antibiotics) that causes their immune system to be compromised or suppressed, the risk of infection from a lower number of organisms is much greater.  There are multiple organisms that cause acute or chronic diarrhea, so it is important to be aware that your horse can pass along an illness to you even before a diagnosis can be reached.  Arriving at a diagnosis includes submitting fecal samples to a laboratory for culture or other testing.  It is important to note that even with testing, it can still be difficult to arrive at a conclusive result, and a definitive diagnosis may not occur.  If your horse develops diarrhea, you should separate it from other horses, and use proper hygiene around the horse.  Contact your veterinarian for advice.

Clostridium difficile 

Clostridium difficile is another bacterial infection of the gastrointestinal tract that can cause diarrhea and colitis in horses and humans.  Like salmonellosis, C. difficile infection can be diagnosed by submitting fecal samples to a laboratory, but those samples may not guarantee a definitive answer.  Infection of C. difficile usually occurs after antibiotic administration and/or hospitalization, although random cases do occur.  There has never been a reported case of transmission of C. difficile infection from horse to human, but there is also very little research or reporting that has been done on this potential for transmission.  Therefore, because it is uncertain if C. difficile is a zoonotic organism, it is best to err on the side of caution and assume that it is until there is more information available. 

Giardia

Giardiasis is a parasitic intestinal disease that is the most common disease of its kind to infect people in North America.  It is unclear if Giardia intestinalis plays a role in gastrointestinal disease in horses, but horses do shed Giardia cysts in their feces, posing a potential theoretical threat of infection to humans.  Giardiasis commonly causes transient diarrhea in humans, and the most likely route of transmission from horses to humans is via the fecal-oral route.

Crytosporidiosis

Crytoporidium parvum is another protozoal pathogen that has the potential to be transmitted from horses to humans.  Some studies show high levels shedding of c. parvum in foals, so good hygiene should always be followed when working with horses.

NOTE:  WHENEVER YOUR HORSE HAS DIARRHEA USE COMMON SENSE TO PREVENT DISEASE FROM SPREADING TO HANDLERS OR OTHER HORSES:  WEAR GLOVES, CHANGE CLOTHES, WASH HANDS, USE FOOTBATHS WITH APPROPRIATE DISINFECTANT BEFORE ENTERING, AND WHEN EXITING THE AFFECTED HORSE’S STALL.

Anthrax

Anthrax is caused by the bacteria Bacillus anthracis.  This spore-forming bacterium can infect many mammals.  If sudden death of a horse occurs, anthrax should be a consideration as to the cause.  In the U.S., there are higher incidences of anthrax occurrences in Arkansas, South Dakota, Louisiana, Missouri, and California.  A warm, dry climate tends to favor suitable conditions for infection to occur.  Infection from horses to humans is not direct, but rather through the formation of spores that occurs when the vegetative form of B. anthracis is exposed to the air.  If a horse dies, and anthrax is a possible cause, it is crucial that the body of the horse not be opened up for post-mortem examination.  This will allow spore formation and spore release into the air, causing infection via inhalation, or directly through skin.  The body of an animal that has the potential to have anthrax should be burned, along with all instruments and articles that were used in its treatment.  The anthrax spores are very resistant to chemical disinfectants, so the best course of action is to burn any items that may pose a threat.

Leptospirosis

A bacterial disease that is considered to be the most widespread zoonosis in the world is caused by Leptospira interrogans and its serovars.  Leptospirosis poses an occupational hazard to those involved in cattle and pork production.  In horses, the most common clinical manifestations of leptospirosis is uveitis; abortion, renal disease, and stillborn foals can also occur.  Diagnosis of leptospirosis in any species can be difficult, and may require many methods of testing to reach a definitive result.  Leptospirosis can be transmitted between species, including horse to human exchange.  This can occur through infected body fluids including urine, as well as soil and water that may be contaminated.  The actual threat of transmission from horses to humans is minimal, but it can still occur.

Diseases involving the Skin

Ringworm

Most people involved in the equine industry will, at one time or another, be confronted with a skin problem in their horse.  One is Dermatophytosis, better known as ringworm.  This is a fungal disease of the skin.  It can be transmitted through direct or indirect routes to humans.  Annually, it is estimated that 2 million cases of zoonotic transmission of ringworm occur.  It is important to recognize ringworm, and to handle infected animals appropriately.  Direct contact with a horse that has ringworm may introduce the fungus to human skin, but it can also be contracted through contact with grooming tools or blankets used by the horse.  Proper precautions should be taken to ensure that not only do other members of the herd not contract dermatophytosis, but that the people who care for the horses on a daily basis also do not develop ringworm.

In immunocompetent humans, infection with these fungi is limited to the outer layers of the skin and hair. However, in immunocompromised hosts, there is mounting evidence for deep tissue, even systemic, infections.  Although morst cases of zoonotic ringworm in people occur through small animal (cat and dog) contact, an infected horse can, under appropriate conditions, infect a handler.  Please contact your veterinarian for additional information regarding ringworm treatment and contol.

Dermatophilosis

Dermatophilosis, also known as “rain rot” is a common ailment in horses, with transmission between animals occurring by release of spores from the lesions when they get wet.  The organism Dermatophilus congolensis is a bacterium. Transmission may be either direct or via insect vectors. The skin lesions appear as oozing, crusted areas in which the hairs are cemented together into characteristic "paint-brush" clumps. Dermatophilus is transmitted to humans by direct contact with lesions on animals. Humans develop a crusting, oozing dermatitis similar to that seen in animals.  Although a chronic wet environment can set up the correct circumstances for infection and transmission, many cases have occurred without the link to “rain”.

Brucellosis     

Brucellosis has a low occurrence in horses because they are relatively resistant to infection by the bacteria Brucella abortus.  Disease can occur, however, and transmission to humans is a possibility.  The contact of a skin abrasion with an infected animal or material is the typical route of infection.  Again, while rare, there is always the potential for transmission, so it must be mentioned.  Commonly, brucellosis infection has higher potential for those who work in direct contact with high volumes of livestock (butchers, slaughterhouse workers, and veterinarians), and

Brucellosis in animals is generally typified by late-term abortions and inflammatory lesions in the male reproductive tract.  In horses, Brucella abortus is one cause of the bursitis conditions called "fistulous withers" and "poll evil".

            As previously stated, the list described here is not inclusive.  Other zoonotic diseases that occur in both humans and horses include: Ehrlichiosis, Lyme Disease, Rhodoscoccus and systemic fungal infections, among others.  Please see the previous reference for additional information.

PLEASE CONTACT YOUR VETERINARIAN IF YOU SUSPECT THAT YOUR HORSE HAS ACQUIRED A ZOONOTIC, OR OTHER, DISEASE.