This week an article on TheHorse.com discussed the current situation in the western provinces of Canada with regard to equine infectious anemia (EIA). The prairies are seeing the highest number of EIA cases in years, with more than 70 horses affected on 22 different properties.
Also commonly called "swamp fever," EIA is caused by a retrovirus, similar to the human HIV. There is currently no vaccine against virus, and once infected a horse carries the virus for the rest of its life. Although EIA can cause severe clinical signs (including high fever, weakness, swelling of the lower limbs and along the ventral abdomen, and even sudden death) most horses that are diagnosed are not showing any signs of illness at the time, or may show milder, non-specific signs such as exercise intolerance and intermittent fever. Affected horse of course also have varying degrees of anemia as the name suggests. Episodes of more severe signs can occur even years after the initial infection, and during these episodes an infected animal poses the greatest threat to other horses because the viral load in the bloodstream is very high. The virus cannot be transmitted directly between horses though – it is transmitted by blood, typically via blood-sucking insects like like deer flies, horse flies and stable flies (hence the association of the disease with swamps) or by reuse of needles for injections. Fortunately EIA does not affect humans or any other animal species. It has also not been shown to be transmitted by mosquitoes.
Because infection is life-long, in order to control the disease the only options for positive horses are euthanasia or life-long quarantine in a building with rigorous insect control to prevent spread.
The question is, why the sudden spike in the number of cases out west? There are a few possibilities:
- Possibility #1: There’s been one, or a few, small local outbreaks that were initially caused by a very small number of positive horses that likely brought the virus back with them from somewhere to which they had traveled. Hopefully this is the case, and testing has identified all the horses that were subsequently infected so that the virus won’t spread further. EIA testing is required prior to travel to many places and prior to participating in many competitions or shows. Regular testing of animals that travel frequently helps to identify infected animals more quickly.
- Possibility #2: For whatever reason, there are a bunch of horses being tested this year that have not been tested in the past, and they’re coming up positive. This would be much more concerning, because there’s no way to tell how long a horse has been infected if it has not been tested in the past. The longer an infected horse is around outside of a fly-proof quarantine, the greater the chance that an insect (or a needle) will transmit the virus to another horse.
- Possibility #3: There are number of infected horses across the prairies that have not been tested, and the virus has been slowly spreading from these index cases and has finally reached part of the horse population that does get tested regularly. This would be the worst case, as it would mean that there’s a reservoir of infected horses that is still not being identified, and could continue to perpetuate the infection.
In the end, only time will tell which of these scenarios (or a combination of them) is playing out in western Canada. Although a relatively low population density (of horses as well as people) in provinces like Saskatchewan helps to decrease the transmission pressure (simply by making it harder for an insect carrying the virus in its mouth parts from one horse to find another horse to bite), gatherings of horses for shows, trail rides and the like (particularly when EIA testing is not required) still create prime opportunities for transmission of the virus.
There are also a few things you can do to help decrease the chances of your horse contracting EIA:
- Protect your horse from biting insects that transmit EIA (which will simultaneously help protect your horse from insects like mosquitoes that can transmit other viruses). Use fly repellants, fly sheets, and avoid turnout during times of peak insect activity (dusk and dawn).
- Never reuse needles, especially between different horses. Also ensure that any other equipment that may be used on your horse (e.g. mouth gags) are always properly cleaned between animals and free from any blood contamination.
- Avoid proximity to horses of unknown EIA status. This can be tough to do if your horse goes to shows where EIA testing is not mandatory, but particularly with the current problems out west (or anywhere else that EIA may be circulating) testing for EIA prior to any event where there will be a gathering of horses should be strongly promoted.
Testing for EIA can be done with a simple blood test. This used to be called a Coggins test, but now a more accurate ELISA-type test is used to test for the disease instead.
Image: Cross-section of the EIA retrovirus (source: USDA Animal and Plant Health Inspection Service)