The term zoonosis (plural zoonoses) means a disease that can be transmitted from animals to humans. The name is derived from the Greek “zoi” or “zoe,” meaning life, plus “nosos,” meaning disease. An equine zoonosis is a disease that humans can contract from members of the equid family.
While there are a number of zoonoses, the good news is that horses spread relatively few diseases to humans. The majority of zoonoses involve other domestic species or wildlife. The bad news is that one or two of the diseases that we can contract from horses can be serious, even life-threatening.
An infectious disease is caused by a living organism or pathogen that invades and colonizes the host. Equine zoonoses are caused by a variety of pathogens–viral, bacterial, rickettsial, fungal, or parasitic (see “Equine Zoonoses on page 58).
Equine diseases are usually solely of veterinary importance. When equine diseases affect humans, those diseases also become a public health concern. From a public health standpoint, “significance” has a particular meaning. It reflects a combination of the clinical importance of a disease with the ease and extent to which it can be spread. For example, acute equine respiratory syndrome (caused by Hendra virus), first diagnosed in Brisbane, Australia, in 1994 has “considerable zoonotic significance.” Although it is rarely transmitted to man, the case fatality rate is very high in humans who get the disease.
The Office International des Epizooties (www.oie.int), the animal health equivalent of the World Health Organization, categorizes animal diseases as List A or List B, depending on their significance. List A diseases are “Transmissible diseases that have the potential for very serious and rapid spread, irrespective of national borders; that are of serious socio-economic or public health consequence; and that are of major importance with respect to the international trade of animals and animal products.”
List B diseases are “Transmissible diseases that are considered to be of socio-economic or public health importance within countries, and that are significant to the international trade of animals and animal products.” In other words, they are serious but not as potentially economically devastating as List A diseases.
OIE List A diseases include such illnesses as vesicular stomatitis, African horse sickness, and foot and mouth disease (FMD). Examples of List B diseases are anthrax and Eastern and Western equine encephalomyelitis (EEE and WEE).
Equine zoonoses either primarily affect members of the equine family or affect more than one domestic species, including horses. In the above example, African horse sickness and EEE/WEE are equid diseases, while vesicular stomatitis and anthrax are multiple-species diseases. Prevention and management of specific diseases from a public health viewpoint depends on knowing which animal species are susceptible to them and could therefore be a source of infection for humans.
Methods of Transmission
One method of exposure to infection is by inoculation. In this context, inoculation doesn’t refer to introduction of a pathogen into the body using a syringe and needle. Instead, it is when infective material–blood, saliva, pus, or other bodily secretions or excretions–from a sick horse comes into contact with a person’s system through a break in the skin, either via a pre-existing wound or introduced by a bite or scratch.
Most people are aware of the dangers of the furious form of rabies–mad dogs out in the midday sun. On the other hand, horses and other species might exhibit a dumb form of rabies. This can present a problem for humans. Since one of the initial signs of the dumb form of rabies is esophageal paralysis leading to excessive drooling, a horse in the early stages of rabies can be mistaken for a choking horse. Sticking a hand in such an animal’s mouth to check for an obstruction is very likely to expose a person to the rabies virus. The case fatality rate in human rabies is nearly 100% once symptoms appear. There is a means of treatment for the person accidentally
exposed to the virus if applied very shortly after exposure to a suspected rabid animal. There is none for the horse, however, even if exposure is known to have occurred.
Three other diseases that can be contracted by humans through inoculation on entry through broken skin are glanders, brucellosis, and leptospirosis. Glanders causes nodules and ulcerations in the upper respiratory tract and lungs, and is one of the oldest equine diseases known to be transmissible to man. It is primarily an equine disease, but it can also occur in carnivores, rodents, cattle, sheep, goats, and pigs. Humans are not highly susceptible to infection, but if they contract the disease and are not treated, the case fatality rate can be as high as 95%.
While the usual method of transmission is inoculation, other possible routes of exposure are inhalation or ingestion. Horses exhale droplets of infective respiratory secretions, which can be inhaled into the respiratory tract of in-contact humans. It is similar to having a person with influenza or a cold sneeze beside you.
Other diseases that humans can pick up through inhalation include anthrax (List B, multi-species), brucellosis, and melioidosis, an undesignated OIE multi-species disease.
Brucellosis is not primarily an equid disease, but horses are susceptible to the causal bacterium. The bacterium is a common cause of fistulous withers and “poll evil,” and it has also been shown to cause lameness, synovitis, and tendonitis. Infection is picked up through exposure to infective fluids or discharges, frequently from infected cattle. In humans, brucellosis can cause undulating fever, sweating, weakness, malaise, weight loss, and depression.
Leptospirosis can occur in many species, including horses, and it is especially common in regions with a warm, humid climate and alkaline soil. Transmission is by direct contact with infective urine, uterine contents, or tissues–putting farmers, veterinarians, and slaughter house workers at special risk of exposure. In humans, leptospirosis can range in severity from a mild infection to a fatal disease.
The final means of direct transmission of disease is through ingestion, or eating infected horse meat. One disease that a person can contract in this way is trichinellosis, and another is anthrax. Trichinellosis is a disease resulting from a parasite that is encysted in muscle tissue. Most people associate trichinellosis with undercooked pork. In the last 25 years, however, consumption of infective horse meat has been the main source of human trichinellosis outbreaks in Western Europe. The horsemeat implicated in the major European outbreaks since 1975 originated from countries outside the European Union: Romania, Poland, the former Yugoslavia, Mexico, Canada, or the United States. Trichinellosis usually occurs in wild animal species such as wild boar, bear, foxes, etc. It is unclear how horses as herbivores acquire the parasite.
Among the most important equine zoonoses are those that are transmitted directly by vectors, primarily mosquitoes or ticks. Horses contract these diseases when the vector, such as a mosquito, feeds on an infected animal, such as a bird, and then bites a susceptible horse. With the exception of Venezuelan equine encephalomyelitis (VEE), horses do not develop a high enough level of viremia (virus circulating in the blood) nor does the viremia last long enough for them to serve as a source of infection for mosquitoes. Because of this, the horse is called a dead-end or tangential host of the virus. Horses are considered dead-end hosts for EEE, WEE, and West Nile viruses. VEE, on the other hand, is transmissible by an infected vector from horse to horse or horse to human.
VEE is widely thought to be the most important equine infectious disease that is transmissible to man. Periodic outbreaks/ epidemics occurred in South and Central America until the early 1970s, when the “epidemic strains” of the virus seemed to disappear. However, in 1995, the disease reoccurred in epidemic form in Colombia and Venezuela, resulting in tens of thousands of equine cases and an estimated 75,000 human cases. Mortality in equine cases of VEE has been estimated at 19-83%, and the human case fatality rate at 3-4%. Humans become exposed after the disease has been established in horses; mosquitoes and possibly other types of biting flies are responsible for transmitting the disease. Outbreaks cease when the number of susceptible horses declines through mortality or immunization, or when mosquito populations decline.
EEE and WEE are also mosquito-borne equine zoonotic diseases. Both horses and humans are infected from birds via different species of mosquitoes. Human infections usually occur at least two weeks after the appearance of the disease in equines. Horses and humans are considered dead-end hosts of both viruses; neither species develops sufficient levels of virus in the bloodstream–nor for a long enough period of time–to infect mosquitoes. However, humans can catch either disease from infective brain or other tissue/fluids from equine cases. While many cases of human infection are mild, encephalitis can occur, usually in the very young or the very old. EEE can cause severe disease in any age group, with a fatality rate as high as 65%. WEE is usually seen in young children, with a fatality rate of 3-14%.
Japanese encephalitis occurs as a significant zoonosis throughout much of Asia, extending as far south as Papua New Guinea and the northeastern tip of Australia. The number of human cases of this disease worldwide has been estimated at 50,000 per year and rising, with a case fatality rate as high as 25%. Although usually considered a dead-end host, the occasional horse might develop a viremia of sufficient magnitude to infect mosquitoes.
West Nile virus is a close relative of Japanese encephalitis and is a recent newcomer to the United States, causing disease and deaths in humans and horses since 1999. (See www.TheHorse.com/wnv for more information on West Nile virus.)
Multiple Species Diseases Spread By Various Routes
Some diseases, such as vesicular stomatitis (VS) and anthrax, can be spread by several routes. VS is a viral disease primarily of horses, cattle, and swine. It is of particular importance because the clinical signs are indistinguishable from those of foot and mouth disease (FMD), one of the most highly contagious and economically devasting diseases of any species. The primary route of infection for VS is through the bite of virus-infected, small biting flies. Exposure also can occur through inhalation or inoculation, or contact with the saliva, vesicular fluid, or epithelium of infected animals. Infection in humans and susceptible animals is often sub-clinical. In other words, an individual can develop antibodies but no clinical evidence of infection.
Anthrax is potentially such a lethal infection that it has been used as a biological warfare weapon. It is an acute infectious disease of nearly all warm-blooded animals, including horses and humans. In horses, death usually follows two to three days after exposure. In developing countries, anthrax continues to account for livestock losses and human illness and death. Horses are less responsible for spreading the disease than other domestic species such as cattle, sheep, and goats, but cases of human disease have been contracted through the ingestion of infected horse meat.
Some pathogens lie dormant in the soil for years. Exposure can occur through stepping on a nail or scratching one’s hand on a barb-wire fence; the spores can thus gain entry to the body, multiply, and produce specific toxins. Tetanus and melioidosis can both be spread in this way. Horses are a major source of the anaerobic bacterium that causes tetanus.
Finally, the route of transmission for some diseases is unknown or has “not yet been fully established.” Acute equine respiratory syndrome caused by Hendra virus is an example. The reservoirs (sources) of this virus are species of fruit bats that can be infected with the virus. However, no one has yet confirmed how the virus gets from the bats to horses or humans. In humans, this virus causes severe disease of the lower respiratory tract or encephalitis. Since there is a very high fatality rate in humans, it is considered a highly significant zoonotic disease even though there have only been a few recorded cases of human infection.
Regardless of the route(s) of transmission, certain equine diseases can be spread from horses to humans. Horse owners need to be aware that a sick horse might be infected with a contagious disease that the owner, farm worker, or vet could contract. A small number of such equine zoonoses have the potential to cause significant human illness and even death. So observing caution and good hygiene around sick horses is advisable.