• Infectious Diseases
• Working Animals & Livestock
• Veterinary Medicine

• Infectious Diseases
• Working Animals & Livestock

Clinical recognition of tick-borne diseases can be a challenge, because tick-horse interactions are not as well studied as tick interactions with humans and dogs. In addition, clinical signs of equine tick-borne diseases are nonspecific. Julia H. Wilson, DVM, Turner Wilson Equine Consulting, Stillwater, Minnesota, USA, discussed tick identification, characteristics of equine tick-borne diseases, and strategies for prevention and tick avoidance.

Ticks can carry the agents of 3 important equine infectious diseases in North America: anaplasmosis, borreliosis (Lyme disease), and piroplasmosis. The most common ticks found on horses are the black-legged tick (Ixodes scapularis) and Western black-legged tick (Ixodes pacificus), also known as deer ticks, and the American dog tick (Dermacentor variabilis). Both Ixodes species transmit anaplasmosis and Lyme disease, but D. variabilis is not an important transmitter of disease in horses. Dermacentor nitens and Amblyomma cajennense have been implicated in equine piroplasmosis transmission.

Tick-borne diseases are transmitted when nymphs and adults attach to a host and feed on blood. A tick needs to stay attached for 12 hours to transmit disease. Nonspecific signs that might suggest a tick-borne disease include fever, lethargy, weakness, limb edema, edema under the abdomen, poor appetite, and the presence of ticks. Making a differential diagnosis involves a physical examination; tick identification; a review of associated factors, including the herd history, vectors, exposure, age, and breed; and laboratory tests.

Equine anaplasmosis, also known as ehrlichiosis, is caused by Anaplasma phagocytophilum, primarily transmitted by Ixodes. Reservoirs include rodents and deer. A. phagocytophilum also infects humans and other animals. Incubation takes from 1 to 9 days. The diagnosis is made on the clinical signs and identification of the organism (Table 1).

Most horses recover from anaplasmosis in 1 to 4 weeks with antibiotic treatment. The most effective is intravenous oxytetracycline. Oral doxycycline is another option. Supportive care includes limb bandages, easily chewed food, and nonsteroidal anti-inflammatory drugs (NSAIDs) for fever.

Lyme disease is caused by the spirochete bacteria, Borrelia burgdorferi, transmitted primarily by Ixodes species. Clinical signs are widely variable (Table 1). Diseases with similar symptoms include anaplasmosis, immune-mediated disease, and leptospirosis. Diagnosis is based on identification of the organism and serum antibodies. Oxytetracycline is the first choice for treatment. Alternatives include high-dose penicillin, oral doxycycline, and minocycline. NSAIDs are used to reduce inflammation. The blood antibody level falls if treatment is successful.

Piroplasmosis is a tropical protozoal disease caused by Babesia caballi or Theileria equi. It is common worldwide but not in the United States, Canada, Australia, the United Kingdom, Ireland, and Japan. Testing is required before importation into the United States. In 2009, however, an outbreak occurred in Texas in illegally imported horses, which led to ticks becoming infected and subsequently transmitting the disease to horses in at least 15 states. Diagnosis is made by identifying the organism (Table 1). Piroplasmosis is treated with intramuscular imidocarb for 2 days.

Tick-borne diseases can be prevented by daily inspection and removal of ticks from horses. Pastures should be mowed, horses kept out of woodlands, exposure to leaf litter reduced, and deer excluded from the pasture. Mice can be treated with fipronil, and their nests with permethrin. Tick repellents such as permethrin and DEET (N,N-diethyl-meta-toluamide) should be used on the horses and their riders.

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