Learn how to recognize the symptoms and protect your horse, plus what researchers are doing to help identify and prevent the spread of disease stemming from ticks.
Glendening Avis, a flashy buckskin Canadian Sport Horse mare ridden at intermediate-level horse trials by Callie Evans of Lindsay, Ontario, was at the top of her game in 2012. Then, at the Fair Hill CIC** that fall, the mare appeared stiff and did not pass the first jog. She was diagnosed with arthritis in her neck and treated. Despite winning two intermediate events in Florida in early 2013, Avis lost weight and remained uncharacteristically lethargic. At the suggestion that she might have Lyme disease, a subsequent blood test revealed that Avis was indeed infected. She underwent aggressive IV and oral antibiotic treatment, and has since returned to high-level competition.
Lyme disease and equine granulocytic anaplasmosis (EGA) are two tick-transmitted diseases that affect equines. Lyme disease is caused by Borellia burgdorferi, corkscrew-shaped bacteria known as a spirochete that invades warm-blooded animals including humans, dogs, cats and horses via the bite of the black-legged or deer tick (Ixodes scapularis). The Public Health Agency of Canada reports that black-legged tick populations currently occur in small pockets, but their geographic scope is spreading in much of southern Canada, in part due to being carried in by migratory birds. Lyme disease can be tough to diagnose, because it mimics other neurologic disorders, with the animal displaying an abnormal gait, as well as transient lameness, muscle pain, vision problems, weight loss, sensitivity to touch, irritability or behavioural changes and lethargy.
EGA, formerly referred to as “equine ehrlichiosis,” is caused by the bacterial organism Anaplasma phagocytophilum and is also spread by the black-legged tick. Clinical signs will vary, based on factors such as the horse’s age. Horses older than four can develop fever (39.4-41.6°C), loss of appetite, lethargy, swelling of the limbs, ataxia (uncoordinated muscle movements), body soreness and tiny red lesions on and/or yellow discolouration of the mucous membranes. Symptoms in young horses may be similar, but milder. Blood tests can reveal decreased white blood cell, platelet, and red blood cell counts, as well as bacterial capsules within the white blood cells.
Several research projects involving tick-borne illnesses in equines recently concluded at the Western College of Veterinary Medicine, University of Saskatchewan. Conducted by Drs. Katharina Lohmann, Gili Schvartz, Hilary Burgess and Tasha Epp, Neil Chilton and David Pearl (ON), one project involved a survey where veterinarians and horse owners in Saskatchewan were asked to submit ticks found on horses. Between January 2012 and September 2013, more than 800 ticks were submitted for the study. “It raised a lot of interest; I was very surprised by the response,” said Dr. Lohmann. While the submission program did not produce any black-legged ticks, just dog ticks, Rocky Mountain ticks and winter ticks, Dr. Lohmann was not terribly surprised, as black-legged ticks are considerably smaller than the Dermacentor varieties, and thus harder to see. “We were hoping to get some black-legged ticks and would have tested them to see if they actually had the disease organisms in them.”
In another study, researchers from the WCVM and the U of S Department of Biology compared the results of blood samples obtained by the portable screening test SNAP ELISA with traditional lab results for determining disease exposure. A large number of samples were tested without knowing beforehand if they were negative or positive for Lyme disease or EGA. “We certainly did see variability between the different tests. We can only say they differed – we can’t necessarily say which was more accurate than the other,” said Dr. Lohmann.
“There are some limitations to the tests; for example, there probably is cross-reactivity – when we run a test for EGA, for instance, we look for antibodies against that specific organism, not the actual organism. Sometimes an animal makes antibodies once it has “seen” the organism – like we may have antibodies against the flu because we were exposed to the virus, or because we were vaccinated, but we didn’t necessarily get sick.”
This is especially true of Lyme disease. “Of the animals that are exposed, only a small percentage actually gets sick.” As a result, Dr. Lohmann cautions against annual Lyme disease screening. “If people started running these tests “just to see” without necessarily having a suspicion that the horse is sick, it may be very confusing as to what to do with the result. The big take-home point is that in the absence of additional information about the history of the horse, the clinical status of the horse, or if there’s any suspicion of the horse even being sick, those tests may be very difficult to interpret.” When there is any doubt, a follow-up blood test a couple of weeks later is recommended.
The most effective current treatment for Lyme disease is a barrage of antibiotics – especially intravenous tetracycline – over the course of several weeks. Supporting treatments include anti-inflammatory medicine to combat joint pain and prebiotics/probiotics to support the digestive tract against the detrimental effects of antibiotics.
For EGA, treatment may involve intravenous oxytetracycline, a broad-spectrum antibiotic, once daily for several days, or a single IV dose of oxytetracycline followed by a week of oral doxycycline, a semisynthetic tetracycline. Banamine may be administered to regulate high fever, against which other supportive therapies such as IV fluids and laminitis prevention may also be necessary.
There have been cases of horses recovering without any treatment. “Spontaneous recoveries are reported, but, as a clinician, I would be hard-pressed if I had made the diagnosis to then say ‘let’s just see,’” said Dr. Lohmann. “It may depend on the individual situation, but, generally, I think it would require antibiotic treatment.”
Vaccination against Lyme disease, while currently available for dogs, is not yet available for horses. There are anecdotal reports of people using dog vaccine on horses, but there have been no studies done to determine its efficacy. There is also currently no vaccine for EGA.
“It really comes down to prevention of tick bites,” said Dr. Lohmann. Ticks can be difficult to spot, and can attach themselves anywhere on a horse’s body. In the tick submission study, owners reported finding ticks “in the mane, in the tail and between the legs where the skin is thinner. Some horses seem to be predisposed to having lots of ticks; in groups of horses, some individuals will have lots of ticks, while others may have none – or at least none that the owners will easily find.” Even if the tick has already dropped off, you will observe a welt.
Tick repellents are helpful, but some must be applied several times a day for maximum effect. Dr. Lohmann has seen good results with dusting powder (five per cent carbaryl), which only needs to be applied once or twice a week (wear gloves and avoid inhaling). Avoid tick-killing products that are not specifically formulated for horses. “Pesticides are not supposed to be used off-label,” warned Dr. Lohmann.
Keep your grass short and remove brush piles to make your property less attractive to ticks, which tend to be a seasonal pest. “Black-legged ticks are most active in the fall, so the risk is highest in the fall,” said Dr. Lohmann, who added that most horses that test positive to tick-borne diseases had access to autumn pasture. Extra-diligent inspection of your horse can be your best defence against these tiny parasites.