QUESTION: How concerned should I be about ticks biting my horse? How do I look for ticks on a horse?
DR. BILL GILSENAN: With the spring and summer just upon us, it is natural to consider some of the trouble horses might get into as they will be outside more and active more in the warmer weather. Increased time outside may well translate to increased exposure to ticks. Ticks are arachnids that obtain their nutrients by feeding on the blood of the larger animals that they can parasitize. As ticks advance through their life cycle, they require feedings from different species of animals to progress. The tick's reliance on blood-feeding has unfortunately allowed it to function as an effective transmitter of microscopic pathogens. There are over 900 species of ticks recognized worldwide. Different pathogens have adapted to different species of ticks. Consequently, a variety of diseases can be transmitted by ticks to mammals and birds.
A handful of these diseases are of importance in equine medicine. Arguably the most notable of them is Lyme Disease. Lyme Disease is an infectious disease caused by the bacterium Borrelia burgdorferi. B. burgdorferi is transmitted by deer ticks (Ixodes spp.). The disease is named for the town of Old Lyme, Connecticut where it was first diagnosed in a human in 1975. Since then, recognition of the disease has increased and is estimated to be diagnosed in 30,000 people in the United States each year. As awareness of the disease heightened, it was determined that deer ticks were also capable of infecting dogs and horses with B. burgdorferi.
Lyme Disease was initially primarily diagnosed in the northeastern United States, but there is strong evidence that the geographic range of the deer tick is expanding. A recent study determined that prevalence of antibodies to B. burgdorferi in dogs is highest in the states between Maine and Virginia, and also in Minnesota, Wisconsin, and northern Illinois. The prevalence of deer ticks is increasing in West Virginia and North Carolina. Large nationwide studies examining exposure of horses to B. burgdorferi are lacking, but smaller studies have estimated that greater than 50 percent of horses in the northeastern states have been exposed to the pathogen.
These studies have established that horses can become infected with B. burgdorferi. However, the majority of horses infected with B. burgdorferi in these studies were apparently healthy, so infection does not necessarily translate to disease. Several reports have confirmed, though, that B. burgdorferi is rarely capable of causing disease associated with the nervous system. Clinical signs in these horses include ataxia, hyperreactivity, difficulty swallowing, and muscle atrophy. These clinical signs overlap with several other common equine diseases. It can be extremely difficult for equine veterinarians to parse out which horses have developed disease due to B. burgdorferi in regions of the United States where most horses have been exposed to the bacterium and have developed antibodies against it. An added challenge is our incomplete understanding as to why the vast majority of horses that become infected with B. burgdorferi never develop disease and as to why a small minority can become quite sick. Notably, there are no reports proving B. burgdorferi as the cause of muscle stiffness or laminitis. Regardless, Lyme Disease has emerged as a disease of clinical importance in horses and is one that demands awareness of ticks.
Lyme Disease is not the only tick-borne disease of horses. Anaplasmosis, also transmitted by deer ticks, can result in acute fever, lethargy, and swollen limbs. Fortunately, anaplasmosis responds well to antimicrobial therapy and in some cases is even self-limiting. Piroplasmosis, considered to be a foreign animal disease in the United States, is a protozoal tick-borne disease of horses that can cause fever, anemia, decreased platelet count, and even death. Outbreaks of the disease occurred in Florida and Texas in 2008 and 2009.
Deer ticks actually feed most frequently in the early spring and fall, but it is worth checking your horse for ticks year-round. Use both your eyes and your hands when checking your horse. It is always best to examine your horse systematically, starting at the nose and working backward to the tail. For regions with longer hair, use your fingers to separate strands apart to enable exposure of the skin. For regions with shorter hair, rub your hands along the horse's skin and evaluate for irregularities. If a tick is identified, removal can be attempted. Because ticks feed on blood, they attach themselves quite well to the skin. It is important to not squeeze or crush the tick as this may promote the release of infectious contents into the horse. Instead, forceps can be used to carefully detach the tick. If the tick is in a precarious spot, or if you are not comfortable with removing the tick, it is probably most appropriate to enlist a veterinary professional to get the job done.
Dr. Bill Gilsenan received his veterinary degree from the University of Pennsylvania in 2008. Following an internship at Colorado State University, he completed a residency in large animal internal medicine at the New Bolton Center, University of Pennsylvania. He held a faculty position at the Virginia-Maryland College of Veterinary Medicine until joining the staff at Rood and Riddle Equine Hospital as an internal medicine specialist in 2015. Do you have a question you want to see featured in Ask Your Veterinarian? Email natalie at paulickreport.com.
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