If your horse is standing in its pasture looking rather dejected, isn’t willing to move and won’t come in to eat, something’s obviously wrong. After looking over the horse to check for injuries and finding no gaping wounds or obvious lameness, a call to the vet is in order.
Clinical signs such as lethargy, lack of appetite and a high fever might seem rather vague, but at certain times of the year, those signs are a good start in helping the veterinarian make an accurate diagnosis. Veterinarians in the northeast are seeing an increase in cases of tick borne illnesses in horses, and the incidence of one particular disease — anaplasmosis — is highest in late fall and early winter; sometimes into early spring depending on location and temperatures of the previous winter.
Anaplasmosis is an infectious, tick borne disease, and veterinarian Dr. Sarah Link says diagnoses typically correspond with the heaviest tick season, which can range from late fall to early spring. With the abundance of deer in the northeast, anaplasmosis is being diagnosed more frequently than in the past. Link, who has been in a large animal and equine practice for 25 years, has seen an increasing number of tick-borne illnesses in horses over the past five years.
“Horses present with a fever of unknown origin, lethargy and they aren’t eating,” said Link, describing the signs horse owners first notice in a horse with anaplasmosis. “We sometimes see vasculitis, or swelling of blood vessels, which leads to leaking of vessels that causes edema in the legs.” Petechia, which is bruising resulting from capillary leakage, and icterus, a yellowing of the sclera, can occur in advanced cases.
The initial clinical signs of a horse suffering from anaplasmosis and Potomac horse fever (PHF) are quite similar, so the veterinarian uses additional clues to make a diagnosis. While one of the distinguishing clinical signs horse owners typically watched for with PHF was profuse diarrhea, Link isn’t seeing that now.
“Potomac typically hits more during mid-summer to early fall,” said Link, “but I’ve seen cases of Potomac later in the fall as well. We don’t see diarrhea in horses with Potomac like we used to — we see more of a high fever and they can get very toxic and very sick with it, despite not having the diarrhea. The toxemia is what causes the secondary issues such as laminitis, colic and severe immunodepression. It’s my personal experience that horses with Potomac have a more toxic appearance to their mucous membranes, especially when they’re severely ill with it. The horse with Potomac has a low white blood cell count, but so does a horse with anaplasmosis. Potomac is more toxic and more likely to lead to laminitis because bacteria are releasing toxins into the bloodstream.”
The process of a horse acquiring anaplasmosis from a deer begins when an infected deer tick falls from a deer. “When an infected tick attaches to the horse, it releases bacteria into the horse’s bloodstream,” Link explained. “The incubation period is about one to three weeks. The bacteria is in the bloodstream of the horse and moves into the white blood cells. When it gets into the white blood cells, that’s when we see clinical signs like fever. Depending on the severity — how many blood cells are affected — that’s what causes swelling of the blood vessels that leads to vasculitis and edema. That would be in a very severe infection.”
The veterinarian uses several diagnostic tools to evaluate a horse that presents with the clinical signs typical of a tick-borne disease. In addition to a thorough physical examination, a blood draw for PCR (polymerase chain reaction) testing for anaplasmosis is appropriate. The PCR can identify the specific DNA of the Anaplasma phagocytophilum bacterium and is the gold standard test for anaplasmosis. The veterinarian may include a CBC (complete blood count) as part of the testing. Supportive care usually includes a fever-reducer and vitamin B supplementation for appetite stimulation.
Although the veterinarian cannot definitively diagnose anaplasmosis or other tick-borne illnesses without a blood test, he or she will likely begin a course of antibiotics, usually a tetracycline, while waiting for the results. Link’s experience in treating horses with anaplasmosis is that they respond to antibiotic treatment quickly, typically within 24 to 48 hours.
While some cases of anaplasmosis go unnoticed and resolve on their own, most owners notice their horses’ changed demeanor and seek veterinary care. Horses that recover from anaplasmosis have a degree of immunity from the disease for around two years.
Link is seeing more tick borne diseases in horses in general, most likely due to higher deer numbers. When deer ticks are found on horses, they’re most often under the jaw, through the throatlatch and down the neck toward the chest. While some horse owners choose to apply tick repellants, such products probably don’t work for longer than the day of application. “They might be a good idea if you’re riding in an area where there are tall grasses or areas that aren’t maintained,” said Link. “After you’ve been on a ride, you should probably go over the horse for the next couple of days to check for ticks.”
It’s a nearly impossible task to find every tick on a horse, but it’s worth running your hands over the horse to check for ticks. Link says a deer tick bite can result in a significant bump that’s easy to feel, but by the time the bump can be detected, the tick has probably already fed and fallen off.
Pasture maintenance can impact tick population, so it’s worth mowing pastures. “Keep horses out of dark, wooded, overgrown and moist areas,” said Link.
Anaplasmosis is not contagious from one horse to another, but conditions that favor high tick populations mean that horses that share pastures or are taken on rides in tall grassy areas where tick populations are high are all at risk. And since the tick itself must be infected with the bacterium, only infected ticks can transmit the disease to a horse.
Although it’s been cold throughout the Northeast this winter, Link believes that a prolonged period of severe cold would be more likely to make a dent in the tick population. “They’re very adaptable and they just don’t die that easily,” she said.