Common Complaints: Diseases Horses and Humans Share
The titles of numerous news stories in Florida papers during a few weeks in October shared some common words--Eastern equine encephalomyelitis. The disease has been responsible for several human deaths recently in Florida, and while in Orlando for the American College of Veterinary Surgeons' annual meeting, the hotel at which I was staying was closing the pool early at night and taking extra precautions against the mosquito population. Eastern equine encephalomyelitis is one of many diseases to which horses and humans share susceptibility.
Horses and humans are considered "dead end hosts" for the enchepalomyelitis disease as blood levels in them generally are not high enough to infect a feeding mosquito.
An important point is that although humans are susceptible to the virus that causes Eastern equine encephalomyelitis, the disease is not directly contagious to humans from horses. So, these diseases to which horses and people share susceptibility can be divided into two groups. The first group is diseases that can be transmitted to humans directly from an infected horse, and the second is diseases from which both people and horses can suffer, but the disease is not directly contagious from horse to human.
An infection or infectious disease transmissible under natural conditions from vertebrate animals to people is called a "zoonotic disease." Other diseases, such as influenza ("the flu"), are caused by a similar agent in both animals and people, but the specific disease agent usually can only cause disease in a specific species--the disease agent is said to be species specific.
Equine encephalomyelitis can be broken down into the following parts: encephala=brain, myelo= spinal cord, and itis=inflammation. Hence, this is an inflammation of the brain and spinal cord. This disease not only affects horses, but also is a deadly disease of humans. After a major outbreak in horses in the 1930s, the virus was isolated, characterized, and vaccines were produced. The two forms currently active in the United States are Eastern equine encephalomyelitis (EEE) and Western equine encephalomyelitis (WEE). The viruses responsible for causing these diseases both are members of a family of viruses called the alphavirus.
WEE is most active in the western United States and parts of the Midwest. The first documented cases were in Kansas and adjacent states in 1912. WEE was responsible for the death of 180,000 horses between 1931 and 1938 in the San Joaquin Valley, Calif. There have been isolated cases in every state west of the Appalachians, parts of western Canada, and in Central and South America.
EEE has an eastern United States distribution, and also was seen in 1931. The virus was isolated from a horse's brain and characterized in 1933. There have been isolated cases in Michigan and Wisconsin. There also have been outbreaks in the Caribbean, Dominican Republic, Haiti, Panama, and Central and South America.
A disease "reservoir" generally is an animal or human that has been infected with a disease (and might or might not have been sick). This "reservoir" acts as a source of the disease. The reservoirs for EEE and WEE are bird and rodent populations that remain rather clandestine, since many of the avian species can become infected and carry high blood titers, but do not become diseased--they are "silent" carriers. There usually is a 15-20 day lag phase between the infection of a given bird population and the outbreak in a juxtaposed equine population. If a horse in a particular geographic area is diagnosed, it becomes a sentinel that there is a reservoir of the disease in the area that might be of concern to humans.
A vector is defined as "an organism that carries pathogens from one host to another." The diseases are well maintained in nature with their prevalence being proportional to the size and state of the vector, reservoir, and the target hosts. The mosquito--Culex tarsalis for WEE and Aedes sollicitans for EEE--is the required vector necessary for transmission of the disease from reservoir to host. The prime time for vector existence is mid- to late summer, since the warm, humid weather is good for mosquito breeding. The vector can be a key control point--by controlling the availability of swamps used by mosquitoes for breeding, the chain of infection can be broken.
Horses and humans are considered "dead end hosts" for the disease, as blood levels in them generally are not high enough to infect a feeding mosquito. Again, transmission of EEE and WEE is not horse to human, but reservoir (bird/rodent) to human via the mosquito vector.
In contrast, Venezuelan equine encephalomyelitis can be transmitted from an infected horse to a human via a mosquito vector. In countries where Venezuelan equine encephalomyelitis occurs, the epidemics are considered explosive. One such epidemic started near Ecuador in 1969 and caused 31,000 human cases. The last known outbreak of Venezuelan equine encephalomyelitis in the United States occurred in Texas in 1971.
Approximately two days after equine infection with encephalomyelitis, there is an inapparent infection and low-grade fever. The first apparent signs are at four to five days. At that time, the animal generally is febrile (having a fever), showing anorexia, depression, tachycardia (rapid heart rate), and variable other neurologic signs.
Early Central Nervous System (CNS) signs are of a diffuse and multi-focal cerebral cortical disease with brainstem and cord involvement as the illness progresses. This yields ataxia and paresis (muscle weakness) of the trunk and limbs. With the onset of neurological symptoms, there are behavioral changes and dementia. The most notable symptoms are aggression, head pressing, wall leaning, compulsive circling, and blindness. Other neurologic signs might include nystagmus (uncontrolled twitching of the eye ball itself), and facial, tongue, and pharyngeal (swallowing) muscle paralysis.
As the disease progresses, a semi-comatose and convulsive state occurs before death. These symptoms can be followed by death two to three days later. If the animal survives, residual nervous system deficits are common. The reported mortality (death) rate for EEE in horses is between 75-90%, and between 20-50% for WEE.
Confirmation of the diagnosis is based on the history and symptoms, and by immunofluorescent assay of the serum or brain tissue (attaching a fluorescent compound to the virus in a sample, then looking for it under the illumination of a special light).
Humans are vulnerable to both EEE and WEE infections. After transmission to the host, via the mosquito vector, the onset is of an influenza-like syndrome. The progression in humans follows this pattern: sore throat, fever, headache, diffuse CNS signs, fulminate encephalitis, and death. Children and elderly adults are the most susceptible, with the mortality rate for humans being 75% for EEE and 20% for WEE.
Encephalomyelitis vaccines are available for horses from several pharmaceutical firms. They are packaged as single vaccines (EEE or WEE only), or as combination vaccines. The combination vaccines include one type that vaccinates for EEE, WEE, and tetanus, and another type for EEE, WEE, tetanus, and influenza. These preparations have a good history of being both safe to use and highly effective in controlling the disease.
On the East Coast, there are approximately 75 equine cases of EEE each year and five human cases. Since 1983, there have been nine equine cases and one human case in New York state. The virus reservoir is in birds and rodents, with the mosquito being the vector for both horses and humans. EEE has a higher mortality rate than WEE, but both can be fatal. Any unimmunized horse is at risk; therefore, immunization programs and control of the vector are the most available ways to protect the equine population. Mosquito control is necessary to protect the human population.
Death from rabies has been a horrid fear in both modern and ancient times. The law tablets of ancient Mesopotamia might recount the first existence of this disease: "If a dog is vicious and the authorities have brought the fact to the knowledge of its owner, he does not keep it in, it bites a man and causes death, then the owner of the dog shall pay two-thirds of a mina of silver." Rabies has been reported to occur in all continents of the world with the exception of Australia and Antarctica. Typically a disease of bats and carnivores, including the domestic dog and cat as well as many wild species, rabies also affects horses and humans.
Rabies is caused by a virus that is a member of the genus Lyssavirus. The word lyss is Greek for "madness" or "rage." In North America, skunks, foxes, raccoons, and bats are the most common reservoirs of the virus. The incidence of rabies-infected raccoons has increased in the northeastern United States due to the translocation of raccoons from the Southeast.
The horse usually is infected by the bite of a rabid wild animal, although that event, most of the time, is unobserved. One horse that I worked on, confirmed to have had rabies, had a history of having had a "dumb acting" skunk in his stall several months prior to illness. At the time, the skunk was chased off and was not thought about further.
The rabies virus is secreted in the saliva of infected animals, hence the ability of bite transmission of the disease. The clinical signs of rabies in the horse are variable and unpredictable. As one of my veterinary professors often warned us: "The only predictable thing about the clinical signs of rabies is that they are unpredictable."
After the bite, the virus enters the nerves in the immediate vicinity and travels up them to the spinal cord and eventually to the brain. The clinical manifestations of rabies generally are divided into either the "furious" form or the "dumb" or "stuporous" form. The furious form can manifest as rage and mania, with affected animals exhibiting aggressive signs such as attacking humans, animals, or inanimate objects. The aggressive or bizarre behavior can be intermittent, with severe depression occurring between periods of rage. The furious form actually is less common in horses, with many just being "distant" and exhibiting severe depression and stupor that progresses to recumbency, seizure, and death. Some animals with rabies show extreme sexual excitation or self-mutilation.
The incubation period for rabies in the horse is reported to average between two and nine weeks, but is thought to be as long as 15 months. In humans, the incubation period is reported to be from two to eight weeks, but can vary from 10 days to eight months or more. Once the incubation period is over, the disease generally progresses quite rapidly; death can occur as soon as 12 hours after the onset of clinical signs. The average time period observed between the onset of clinical signs and death is three to five days. Rabies has an invariably fatal outcome--there is no treatment. Rabies should be considered as a possibility in any case of rapidly progressing, unexplainable neurologic disease.
Although, to my knowledge, there has never been a reported case of transmission from rabid horse to human, the possibility does exist. Most human cases of rabies are related to dog bites or exposure to bats. The virus is excreted in an infected horse's saliva as in other affected animals. Some horses do demonstrate aggressive behavior, so biting is a possibility. Exposure of open cuts or scratches on human skin to infected saliva also could result in transmission.
Unfortunately, due to the unpredictable nature of the disease, by the time the diagnosis of rabies has become a possibility, there already has been significant human exposure. The virus is destroyed relatively easily by common disinfectants (70% isopropyl alcohol or povidone-iodine, to name two), so routine hand washing procedures could be helpful. Hand washing should be a routine practice after handling any horses suspected of being ill. If there is any chance of having been exposed to a rabid animal (horse or otherwise), your local state health officials should be consulted and their instructions for any post-exposure vaccination procedures strictly followed. The human post-exposure treatment is no longer the horrid series of injections into the abdominal muscles, but a shorter series of vaccinations in the arm and one big shot in the butt.
There are several products against rabies on the market for the vaccination of horses more than three months of age. If there is any rabies reported in the wildlife population in your area, veterinary consultation should be sought regarding vaccination procedures. In addition, there are many shows and events that now are requiring proof of rabies vaccination prior to horses being allowed on the grounds.
In September of 1994, an outbreak of severe respiratory disease affected 18 horses in Queensland, Australia--14 horses died. The infectious agent subsequently was identified as a morbillivirus, a member of the virus family Paramyxovirus. The virus that causes measles is also a member of this family. The outbreak had an unusual twist, as there were also people involved that were confirmed to have been suffering from the same virus--one person died. A 1995 edition of the Medical Journal of Australia reported that "five and six days after the index (first case) mare's death, a stable-hand and a horse trainer, both of whom had had close contact with the sick mare's mucous secretions, developed pneumonitis (inflammation of the lungs), respiratory failure, renal (kidney) failure, and arterial thrombosis (blood clots) and died from a cardiac arrest seven days after admission to the hospital." Another human was also stricken ill during this outbreak, but survived. No recent cases have been reported and the origin of the virus remains unknown.
Salmonella is a bacteria that is very common to the environment. There are more than 2,500 different strains of Salmonella species, with a large number of them able to cause disease in both people and animals. Salmonella is very common in people and is reported to cause 40% of the cases of food-borne disease, with poultry, meat, eggs, and dairy products being predominantly responsible. In addition, aquarium animals (small turtles) have been identified as sources of the bacteria. It has been reported that there are between 750,000 and 5,000,000 cases of salmonella in people each year.
In horses, infection with salmonella generally causes diarrhea in young foals, with immune-suppressed horses and horses receiving anti-microbial therapy being most susceptible. It has been reported that between 1-5% of horses are asymptomatic carriers that potentially could serve as sources of the organism.
As with most causes of diarrhea, the cause (be it viral or bacterial) is often shed in the diarrhea. Therefore, the diarrhea can act as a source of infection for other animals and people. When handling horses with diarrhea (even after the feces have become firm again), the manure should be treated as an infectious agent--protective latex gloves should be worn. The manure and contaminated bedding should be disposed of in an area of the farm where it will be unlikely for other horses to have exposure to it. In addition, any contaminated stalls, trailers, farm tools, or grooming equipment should be thoroughly washed with a detergent soap, rinsed with water, and sprayed with a disinfectant.
If you have young children they, just as young foals, are more susceptible to infection. With many of the organisms that cause diarrhea, it actually takes a certain quantity of infectious material (the feces) to cause disease. So, the simple act of hand-washing with a disinfectant soap can greatly reduce the chances of spreading infectious disease to both yourself and your horses.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: Horses Living With Livestock