Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

At some point, the US is going to have to admit that piroplasmosis, the bloodborne parasitic infection caused by Theileria equi, is endemic in some regions of the country. It's a declaration that will have major impacts on horse movement to some areas but, it's better for everyone to know what's going on. Piroplasmosis is technically still considered an exotic disease in the US, but there have been many cases identified over the past year and a clear source for the individual outbreaks in lacking, indicating there must be a reservoir in some part(s) of the country.

The latest incident involves the diagnosis of piroplasmosis in three race horses in New Mexico, which were picked up as part of routine screening. There were only three positives out of about 1200 horses tested, so the disease is still rare, but the fact that it was there and none of the positive horses had any link with previous outbreaks is definitely a concern. The OIE report states that transmission is suspected to have been from "artificial" means like sharing needles between horses, not natural tick transmission. This could account for the multiple horses affected but doesn't explain where the disease came from it the first place, and it's unclear how solid that hypothesis really is.

It's quite interesting (surprising, frustrating...) that few comments are put forth in any of these outbreaks indicating where the infections may have originated and why we are seeing recurrent, unrelated infections. Is increased testing in different areas helping to pick up cases that would otherwise have been missed (i.e. were already there), or is piroplasmosis in the US an emerging problem? How confident are they that there are no ticks capable of transmitting T. equi in some of these areas? Is wider screening of horses required to determine the extent of the problem and to determine whether it can be controlled? Is broader screening of ticks in the affected areas needed to see if there are ticks known to be able to transmit T. equi? Are studies needed of other tick species in areas where unexplained cases have occurred to determine if some tick species that are not currently known to be able to spread the parasite can actually do so? Lots of questions... hopefully someone's trying to find some answers.

Click image for source.

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Three barns at Calder Race Course in Florida have been quarantined after a horse was diagnosed with equine herpesvirus (EHV-1) infection. Horses from this barn are quarantined for 3 weeks and are not allowed to race. Other horses will not be allowed to enter the grounds for the next 2 weeks, but racing will continue with horses that are currently on the grounds and not under quarantine (1800 horses are present at the track). Track personnel believe the infection has been restricted to one horse but are taking these measures proactively.

Equine herpesvirus can cause a range of clinical signs, including fever and respiratory disease, severe neurological disease and abortion. The affected filly in this case had neurological disease and was euthanized. Outbreaks of abortions or neurological disease are the greatest concern, and a specific type of EHV-1 has been implicated as the main cause of neurological disease.

It's always hard to say what the most appropriate response is to a scenario like this. Equine herpesvirus is an endemic virus that is present throughout the world and lies dormant within a large percentage of healthy horses. Most infections are sporadic and only involve a single horse or small number of horses, but outbreaks can occur and that's why aggressive measures are sometimes taken. With only a single diagnosed case and no apparent evidence of transmission to other horses in this case, it's uncertain whether such an aggressive approach is required. However, it's worse to be too lax initially than too aggressive, and a logical approach is to implement aggressive measures, and then reassess them as more information becomes available. If no other horses develop signs consistent with EHV-1 infection, then loosening of the restrictions would be reasonable. If there is evidence of transmission and disease in other horses on the property, continuation with aggressive measures makes the most sense.

A great resource regarding EHV-1 is the ACVIM Consensus Statement on this subject, which is available on the equIDblog Resources page.

Image source: www.calderracecourse.com

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After a rather lengthy process, Hoosier Park (Anderson, Indiana) has lifted its strangles quarantine. A quarantine was implemented on September 12th after a horse on the premises began exhibiting signs of strangles. Fifty-four horses were placed under quarantine. When S. equi, the bacterium that causes strangles, was identified in a quarantined horses, officials decided to move all quarantined horses out of the track facility. Presumably, once it was clear that the quarantine was not just a matter of waiting for confirmation that all horses were actually negative, they decided that the risk of having potentially infectious horses living at the track was unacceptable (pretty logical thought process). Now that those horses have been removed and no other cases have been found in the approximately 1000 other horses housed in the other track barns, they are back to business as usual. The quarantined barn is being disinfected and will not be used for the rest of the 2009 racing season.

This incident demonstrated a very aggressive but apparently effective response to strangles. They have presumably ended this latest outbreak and hopefully won't have to deal with it again. However, infectious diseases and outbreaks are inherent risks in the racing industry (as well as other competitive horse industries). The way we manage race horses, moving them around, mixing them, having various (and sometimes minimal) preventive medicine programs, having minimal measures to keep sick horses off the track, and a financial disincentive (i.e. people lose money) to keep horses away, means that infectious disease risks are not going to go away. It's not a question of whether there will be another strangles outbreak on a racetrack. It's a question of when and where.

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