Once thought to be exclusively caused by Borrelia burgdorferi, it is now postulated that other strains of bacteria as well as many species of ticks may harbor and transmit Lyme disease
or other similar variations of this condition. Lyme disease in horses can manifest as a generalized body stiffness or soreness;
reluctance to move, vague, transitory lameness; or transitory joint swelling. Infected horses have also exhibited nervous
system disorders including blindness, head pressing, circling and seizures.
Ixodes species ticks carry these bacteria and are commonly found on deer but will also feed on other species such as dogs, humans
and horses, which is where the zoonotic potential, as vectors, exists. Lyme disease symptoms in humans vary dramatically among
patients, so it can be difficult to diagnose. A slowly expanding skin rash after a tick bite is the classic sign for Lyme
disease and is seen in 60 to 80 percent of human cases, but many cases are more subtle and include abnormalities of the musculoskeletal,
nervous and cardiovascular systems including arthritis-like symptoms, irregular heartbeat, and central nervous system or spinal
Lyme disease has not been considered especially relevant in the southern United States, but a recent variation of this condition,
southern tick-associated rash illness (STARI), has been attracting attention. It is attributed to infection with an as-yet-uncultured
spirochete tentatively referred to as Borrelia lonestari. The Lonestar tick has been implicated as the principal vector.
Diagnostic testing for Lyme disease, or any other new variants, is currently difficult because blood tests do not differentiate
between exposure and infection. Substantial research is ongoing in this area, and newer, more helpful diagnostics for both
horses and people should be forthcoming.
EGE and HGE
Two tick-associated diseases that mimic Lyme disease are equine granulocytic ehrlichiosis (EGE) and human granulocytic ehrlichiosis
(HGE), both of which are caused by Ehrlichia equi. EGE causes elevated temperature, depression, jaundice, limb swelling, ataxia and blood abnormalities. HGE produces flu-like
symptoms in people including fever, headache, chills and nausea. Both infections can, if not recognized and treated with antibiotics,
become systemic and result in death. Blood tests can identify E. equi in white blood cells and should be used whenever clinical signs in horses and people, along with any type of tick bite or
tick exposure history, make clinicians suspect these diseases.
Mosquito-borne diseases pose a zoonotic threat for veterinarians, though the horse, in these cases, acts as a reservoir or
vector. Eastern, Western, St. Louis and some subtypes of Venezuelan equine encephalomyelitis can affect humans. Clinical signs
in people vary from mild flu-like symptoms to severe central nervous system signs. Human deaths occur primarily in children
and the elderly. Postmortem examination of infected horses puts veterinarians at risk of direct disease transmission through
infected blood and cerebrospinal fluid, so always take appropriate protective measures.
Diarrhea commonly occurs in horses, and many diseases in this category can be transmitted to humans via the fecal-oral route.
Good hygiene control measures are crucial when veterinarians are working on cases involving diarrhea in horses. Also keep
in mind that any human, or other horse, that is receiving antibiotics (especially orally) is at increased risk for developing
an enteric infection from a horse with diarrhea.
Salmonellosis is a common form of enteric infection in both horses and humans. Stress-induced diarrhea (transport, training,
competition, hospitalization) is common, and Salmonella species are often reported after fecal culture in these cases. Fecal-oral transmission is the prominent route of zoonotic
Horses also shed Giardia species in their feces and, though it is unclear if Giardia intestinalis is a pathogenic organism in horses, it is parasitic in humans.
Crytoporidium parvum is another protozoal organism with the potential of horse-to-human spread. Research has shown higher levels of C. parvum in foals, so exercise caution when handling young horses with diarrhea — wear gloves, change clothes after handling affected
individuals, wash hands and use foot baths when entering and leaving affected areas.
These hygiene principles also apply in cases of dermatologic diseases of zoonotic potential in horses. Ringworm can be easily
transmitted to people through direct contact. Always keep in mind that many equine skin lesions may be potential sources of
Hendra virus is a paramyxovirus first isolated in 1994 from an outbreak of respiratory and neurologic disease in horses and
humans in Hendra, a suburb of Brisbane, Australia. This virus is thought to be carried by bats of the genus Pteropus. Horses become infected through contact with bats and their droppings or secretions.
Hendra virus infection in horses produces an initial respiratory infection and can progress to neurologic signs and total
systemic failure. These clinical signs are mirrored in human Hendra infection. The three cases reported in humans to date
include two veterinarians and a trainer, two of which died. Humans caring for infected horses are exposed to body fluids and
excretions and can easily become infected. Severe flu-like symptoms quickly develop. The globalization of the equine market
makes disease transmission across continents, including the Hendra virus, a more serious threat.
Infection with methicillin-resistant Staphylococcus aureus (MRSA) can be transmitted from horses to humans and vice versa. This bacterium can be found in skin wounds and various locations
in the respiratory tract. Many studies indicate that MRSA is becoming increasingly prominent, especially in veterinarians,
farm workers and others who have increased contact with animals.
In a study at the University of Utrecht in the Netherlands from 2006 to 2008, 43 percent of all horses that entered the hospital
clinic, for whatever reason, cultured positive for MRSA at some point during their hospital stay. Over 9 percent of horses
were carriers based on positive cultures done when they first arrived at the clinic, and 15 percent of hospital employees
who handled equine patients were also identified as MRSA carriers, though less than 1 percent of the general population of
the Netherlands is MRSA-positive. Clearly horses and those who work around them are at higher risk of cross-transmitting this
potentially serious infection.