When it comes to horses, two things are immediately clear: Everyone loves them and no one likes it when they get sick. That’s especially true if the offending disease is hard to diagnose.

When it comes to tick-borne illnesses in horses, two have become the focus of Canadian veterinarians: Lyme disease (LD), which is caused by infection with Borrelia burgdorferi and equine granulocytic anaplasmosis (EGA), which is caused by Anaplasma phagocytophilum. Both pathogens are transmitted by blacklegged ticks (Ixodes scapularis) and both have turned up in Canadian horses going back many years. However, the number of cases have been low.

LD has been reported in Canadian horses as far back as 1988 and there continues to be considerable debate amongst veterinarians over what the key risk factors for horses are, how best to prevent and treat equine cases, and how many of the horses that contract Borrelia burgdorferi go on the develop full-blown LD. 

That LD can make horses ill is not in dispute. The most common symptoms are lameness, stiffness and chronic weight loss. Other symptoms include a low-grade fever, muscle tenderness, poor performance, swollen joints, hypersensitive skin, depression, head tilt, and difficulty eating or swallowing.

A 2016 study undertaken by the University of Guelph examined 564 blood samples from Ontario horses with no history of travel outside the province looking for evidence of exposure to LD. That study found the average rate of exposure was 14% but the rate was not uniform across all regions. The highest percentages were recorded in eastern (24%) and southern Ontario (15%) with central and northern parts of the province posting much lower rates (10% and 6% respectively). A later study would determine that 64% of horses remained positive for LD 12 months after initially testing positive.

A 2021 study would confirm earlier findings, reporting that slightly more that 15% of Ontario horses involved in that study tested positive for LD with a spatial cluster being identified in eastern Ontario. Risk factors for LD included the presence of oak trees near pastures. Horses that were regularly checked for ticks were at lower risk for contracting LD.

Ontario is not the only province in which equine cases of LD have been identified. They have also been recorded in British Columbia, Manitoba, Saskatchewan and Nova Scotia. A study is currently underway at Mount Allison University trying to put some numbers on infection rates in New Brunswick.

EGA, on the other hand, has been reported in Canadian horses as far back as 1996. However, the number of cases were low and didn’t start to pick up until 2011. EGA has been identified in British Columbia, Manitoba, Ontario, Nova Scotia, New Brunswick and Saskatchewan. (Since blacklegged ticks are not endemic to Saskatchewan, researchers hypothesize these infections were the result of bites from adventitious ticks carried into the province by migrating birds.) In general, it’s felt the geographic range of EGA correlates to those regions where Anaplasma phagocytophilum has been identified in dogs.

Symptoms of EGA include depression, anorexia, fevers, coughs, lethargy, jaundice, and swollen lymph nodes.

Like equine LD, EGA is diagnosed largely based on exposure probability, clinical signs and geographical location. As of 2018, only one horse had officially been diagnosed with EGA in Ontario. However, a study published that same year showed that 1% of horses in that province tested positive for exposure to the pathogen suggesting that either infected horses weren’t falling ill or their symptoms were attributed to other causes.

EGA tends to be more severe in older horses and often runs its course in under a month. Antibiotics can cause symptoms to resolve much quicker. And there is some evidence that horses can develop immunity to EGA for up to 12 months following initial exposure.

The reliability of testing for tick-borne illnesses is as much an issue for horses as it is for humans. A 2015 study evaluating how well existing blood tests detect antibodies to LD and EGA in horses reported what can best be described as variable results. In that study, blood samples from 50 horses with no known exposure to either pathogen were examined. Researchers ultimately concluded that existing diagnostic tests could not reliably determine whether a horse had been exposed to LD or EGA and should not be used to screen asymptomatic horses in low-risk regions where false-positive results could lead to unnecessary treatment.

It’s intriguing how frequently LD and EGA co-infections popped up in research papers. In the 2015 University of Guelph study, 50% of the horses infected with EGA were co-infected with LD. Other studies showed a similar pattern of anaplasma and borrelia occurring together. That may or may not be meaningful. So few Canadian cases of either equine LD or EGA have been studied to date that the high co-infection rates currently being noticed may ultimately turn out to be a statistical anomaly once the pathogens become more prevalent and there are more cases to assess. Time will tell.

As an interesting aside, one of these studies noted that while no vaccines are currently approved for horses, veterinarians practising in high-risk regions have been going off-label and vaccinating horses with canine vaccines. It doesn’t take much to guess that regulatory boards would frown on the practice, but those regulatory boards aren’t on the front lines dealing with concerned owners whose horses are at highest risk for exposure.

Originally published in The Lyme Report, Issue 16. Updated on June 13, 2023.