Tick Diseases in Horses

Ticks can serve as vectors for important and harmful equine diseases.
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Ticks can carry diseases to you and your horses.

Living in the great outdoors exposes horses to a number of pesky insects. As your horse stamps his feet and twitches his skin to be rid of flies and mosquitoes, there aren’t many indications that he might also be carrying ticks on board. Ticks burrow beneath the hair coat, mane or tail or into the softer tissues of the belly, armpits, scrotum, udder, or around the anus where they can persist undetected for a long period of time.

Ticks themselves aren’t a particular problem other than potentially causing some itching and inflammation, but their significance is based on how they serve as vectors for important and harmful equine diseases.

Lyme Disease

Lyme disease is becoming more prevalent in the USA, especially in the Northeast and the northern Midwest states. The blacklegged tick (Ixodes scapularis, also called the deer tick) is known to carry a bacterium, Borrelia burgdorferi, which causes Lyme disease. Infection with B. burgdorferi elicits a number of non-specific clinical signs: low-grade fever, shifting limb lameness, muscle tenderness, muscle wasting and weight loss, stiff gait, lethargy, behavioral changes, increased sensitivity of skin to touch, and uveitis (inflammation of the eye tissues surrounding the pupil). Joint swelling can occur, but that is less common in horses than in dogs and people. Rarely, an infected horse might experience neurologic signs, referred to as neuroborreliosis.

Not all horses infected with this bacteria develop clinical signs, and those that do might take up to six weeks to exhibit signs.

Anaplasmosis

Ticks also carry an illness referred to as equine granulocytic ehrlichiosis or anaplasmosis caused by Anaplasma phagocytophilum. (This bacteria was previously called Erlichia equi, but it has been renamed.) Blacklegged ticks (Ixodes pacificus or scapularis, aka deer ticks) and the Lone Star tick (Amblyomma americanum) are common vectors for disease transmission. Once bitten by an infected tick, a horse’s white blood cells are invaded by these bacteria, which cause destruction of red and white blood cells and platelets.

Incubation takes 1-2 weeks, leading to specific clinical signs of high fever over 104 degrees Fahrenheit, depressed appetite, depression, limb edema (swelling), petechial hemorrhages (tiny blood spots on mucous membranes such as the gums and inner tissue of the nose), icterus (jaundice), and reluctance to move due to muscle soreness and/or ataxia (incoordination).

Horses infected with anaplasmosis often require some form of tetracycline-based antibiotic treatment and supportive care, whereas those with a mild infection might achieve resolution without treatment.

Piroplasmosis

Two different parasites—Babesia caballi and Theileria equi—can be transmitted to a horse’s blood when an infected tick feeds on the horse or through blood-contaminated equipment, needles or blood transfusions. Either of these parasitic infections is referred to as equine piroplasmosis (EP) or babesiosis.

Ticks capable of transmitting piroplasmosis are the tropical horse tick, Southern cattle tick, the Cayenne tick, and the American dog tick (Dermacentor variabilis). Incubation in an infected horse takes 1-2 weeks. Clinical signs can include mild to general weakness, depression, loss of appetite, fever, anemia, limb selling, respiratory difficulty, jaundice and dark-colored urine. A pregnant mare might abort her fetus.

Equine piroplasmosis is a reportable disease. One significant problem, other than the potential to be a fatal disease, is that a horse remains a carrier despite treatment and resolution of clinical signs. This affects the horse through restriction of co-mingling and export. Euthanasia is an option considered for carriers.

Spinose Ear Ticks

Soft spinose ear ticks occasionally induce muscle cramping or a neurologic condition that resolves as soon as the ticks are removed from the ears.

In two accompanying articles in this series, you can learn how to prevent ticks on your property and to prevent them on your horses.