The NAIS and Horses: Why Horses Should be Included

The National Animal Identification System (NAIS) is a tool developed to ensure the health of the national livestock herds by facilitating the traceback and traceforward of animals associated with a significant disease outbreak. Often, when the topic of the NAIS is raised a familiar refrain is heard: “why are horses included? They don’t carry diseases that affect humans or other livestock.” This paper is intended to examine that question and determine whether its accuracy.

Although rarely experienced in the U.S., horses are in fact susceptible to numerous diseases that can also affect people. In most cases, horses do not play a role in spreading these diseases to humans. Lyme disease is an example of a disease that affects horses and humans but horses do not give Lyme disease to humans. In some cases, horses serve as sentinels for human disease surveillance. For example, West Nile Fever and Eastern and Western Equine Encephalomyelitis are diseases that frequently appear in horses before cases are seen in humans. However, horses can also contract infectious diseases that they can pass on or transmit to humans. Examples of zoonotic diseases of horses include Rabies, Salmonella, Ringworm, Leptospirosis, Brucellosis, and Anthrax.

The following is a listing of some of the zoonotic diseases that horses can share with humans and other livestock species. In addition, there are several diseases that are common to horses and to other livestock species as well as to humans, which are considered multispecies diseases.

· Anthrax - Anthrax is an acute infectious disease caused by the spore-forming bacterium Bacillus anthracis. Anthrax most commonly occurs in wild and domestic lower vertebrates (cattle, sheep, goats, horses, camels, antelopes, and other herbivores), but it can also be transmitted to humans exposed to infected animals or tissues from infected animals.

· Borna Disease - Borna disease (BD), first described more than 200 years ago in southern Germany as a fatal neurologic disease of horses and sheep, owes its name to the town of Borna in Saxony, Germany, where a large number of horses died during an epidemic in 1885. Infection results in movement and behavioral disturbances akin to some neuropsychiatric syndromes. (Some have suggested that the virus is linked to selected neuropsychiatric disorders in humans but the evidence for this is not conclusive).

· Brucellosis - On infrequent occasions, horses have been known to contract brucellosis caused by Brucella abortus and have, on even less frequent occasions, been a source of human infection.

· Encephalomyelidities (West Nile Fever, Eastern, Western) – horses do not play a role in transmission of these diseases to humans. The same infection occurs in horses and in humans. All three diseases occur in the U.S. However, horses do play a role in the transmission of another mosquito-borne disease, Venezuelan Equine Encephalomyelitis (VEE), to humans (see separate heading below). It should be emphasized that this disease has been exotic to the USA since the early 1970s.

· Glanders – Glanders is one of the oldest known equine diseases that is of important biosecurity concern. It is a disease of horses, mules and donkeys. Glanders is not currently found in the U.S. It is a disease that can be spread from horses to humans and it was used as a biological warfare weapon by the German army in World War I. The disease continues to exist in various parts of the world, including both eastern and western hemispheres. The U.S. requires that all horses imported into the U.S., including those temporarily exported for competition purposes be tested negative for Glanders before being permitted entry (or re-entry as the case may be) .

· Hendra Virus Disease - (Acute Equine Respiratory Syndrome caused by Hendra Virus, first considered an Equine Morbillivirus) is a relatively new and emerging disease. It causes a severe respiratory illness in horses which is very frequently fatal. Humans having direct contact with blood or saliva of an infected horse are in danger of contracting the disease. Hendra Virus has only been reported in Australia, first appearing in Queensland, Australia in 1994. During the first recorded outbreak of the disease, 14 horses died, 7 more euthanized and 3 humans became ill, one of which died. The virus was re-isolated five years later in Australia in January of 1999.

· Japanese Encephalitis - Japanese encephalitis (JE) is a vector-borne virus capable of causing serious infection of the central nervous system (CNS) of humans. Swine are very susceptible to the infection and are also amplifiers of the virus. Less frequently, horses become infected with the disease. From an epidemiologic standpoint, people and horses are considered dead-end hosts of the virus. Under experimental conditions, however, horse-to-horse transmission has been demonstrated.

· Leptospirosis - Leptospirosis is a bacterial disease that affects humans and many animal species, both domestic and wildlife. Outbreaks of leptospirosis usually result from exposure to water contaminated with the urine of infected animals. Many species of animals can carry the bacterium; they may become sick but sometimes develop asymptomatic infection. Leptospira organisms have been found in cattle, pigs, horses, dogs, rodents, and in a diversity of wildlife species.

· Rabies – like other mammals, horses can be infected with rabies virus and be a source of infection for humans.

· Ringworm - Ringworm is a skin disease which can be caused by several different types of dermatophilic fungi. Numerous species of animals can transmit ringworm to people: dogs and cats, especially kittens or puppies, cows, goats, pigs, and horses can transmit ringworm to people. Humans contract ringworm through direct contact with an infected animal's skin or hair.

· Salmonellosis – This bacterial disease is frequently associated with poultry. However, strains of salmonella can infect horses and they constitute a zoonotic risk for humans. Furthermore, there are strains of this bacterium that exhibit resistance to multiple antibiotics. These represent a significant health risk to horses as well as to humans.

· Screwworm - The scientific name for Screwworm is Chochliomyia hominivorax, which literally means "eater of man", although Screwworm infestation in humans is rare, it can, and does occur. Horses infested with Screwworm, like other livestock, could be a source of infection for humans.

· Tetanus - Tetanus is an uncommon but often fatal disease that affects the central nervous system and which causes painful muscular contractions. Tetanus bacteria gain entry to the body, usually through a wound or cut exposed to contaminated soil. Tetanus spores are widely distributed, usually in soil, dust, and manure. Horses and humans are the most susceptible of all the animal species to tetanus.

· Venezuelan Equine Encephalomyelitis - VEE is a disease that is fatal to both horses and humans. In this case, horses do play a role in the transmission of the disease. The U.S. is currently free of VEE; the last outbreak occurred in 1971. At the time, the disease had spread from South America up through Central America and Mexico into Brown County, Texas, killing tens of thousands of horses and humans during that epidemic. The disease remains an ever prevailing threat in several South American countries. It is sporadically reported from Mexico, the last confirmed case in horses being reported in 1991. VEE is considered a very serious biosecurity threat because of its highly infectious nature and its significant human health impact.

· Vesicular Stomatitis (VSV) - Another disease that is common to cattle, sheep, swine and horses as wall as humans is Vesicular Stomatitis. The clinical features of this disease in ruminant species cannot be differentiated from Foot and Mouth Disease. This requires laboratory confirmation of the virus. Direct contact with VS lesions reportedly can spread the virus from animals to humans.

It is directly attributable to the high quality of equine management and care in this country and the extensive effort to eradicate these diseases from the U.S. horse population or prevent their occurrence that horse owners in the U.S. are unlikely to contract any of these diseases. Nonetheless, horse owners should be aware of the zoonotic significance of each of these diseases and the potential for their occurrence in horses.

Imported Diseases

Import quarantine and post-entry testing are important components in thwarting the introduction of foreign animal diseases, such as Glanders, but it does not entirely guarantee that these diseases won't appear here, either through natural or intentional introduction. Vector-borne diseases such as VEE and VSV can be introduced without necessarily importing infected animals.

Emerging Diseases

Emerging diseases are always a concern. Although it is rare for viruses to jump from one species to another, it does happen. Recently, equine influenza virus was isolated from greyhound racing dogs in Florida after the animals began to show signs of respiratory illness. This has not been seen before and indicates that, in this case, the equine flu virus may have mutated enough to jump species.

Conclusion

Undoubtedly, this list is not fully inclusive of all diseases that may be common between humans or that can be passed from horses to humans. Clearly, some equine diseases do have public health impact. Horse owners must be knowledgeable and aware of the diseases that are common to horses and other livestock species and to humans. A severe outbreak of any of these diseases would have a substantial veterinary and economic impact on the U.S., and on the horse industry. It is for this reason that the horse industry must be included in the National Animal Identification System.

We wish to acknowledge the kind assistance of Dr. Peter J. Timoney, Chairman and Director of the Department of Veterinary Science at the University of Kentucky, in the development of this paper.