The current thought is that EPM is caused by horses ingesting possum feces which contains the EPM protozoa that they got from eating infected birds. This theory has yet to be proven in a controlled study. A study at U of Florida failed to produce EPM in healthy horses. Now there is the thought that horses who are unable to fight off EPM protozoa have a problem with their immune system. But nobody really knows why some horses get it and others who are treated identically don’t.
On the EPM horse list, many horse owners report that their horses became symptomatic after being wormed or receiving vaccinations. I know that vaccines and wormers are around for a reason, to protect horses from deadly diseases. And I also know that correlation does not prove causation. The timing could be coincidental. For myself, however, I am rethinking my approach to vaccines and wormers; perhaps more is not better.
I am going to worm my horses only as necessary from now on depending on fecal counts. I used to worm every 7 weeks. I also plan to do some serious thinking about vaccines and narrow them down to the bare necessities. When I do use vaccines, I will avoid the 3 or 4 in one type and I will space them out generously. I had spaced them out before but I had one, possibly serious error. My horse always reacted badly to the strangles shot so I decided to try the intranasal vaccine. I didn’t know it was a two part series the first time so the second in the series was given one week before the FEC 50. I can’t help but wonder if dealing with that vaccine combined with the stress of the ride compromised her immune system’s ability to fight the protozoa.
If the possum is the main carrier of EPM, the trick is prevent the horse from ingesting possum feces. This is not so easy to do because it can be in the hay you buy or out in your pasture. One simple strategy would be to contain your feed in metal containers or trash cans with lids. That reduces the temptation for possums to arrive at your barn. Also, keep your hay in some sort of enclosed area. Dogs and cats can serve as deterrents but don’t count on that because possums can be mighty bold. Finally consider using fencing that makes it tough for possums to get inside, like wire mesh, and regularly check for holes underneath it.
I sure do wish that I had had health insurance on my horse but I didn’t because “my horses never get sick”--famous last words. Having insurance gives you freedom from the financial stress of treatment and provides you with more options if you aren’t always having to check your bank account or reach for the credit card before you decide to try something new.
In the last of this series, I will share some resources for more information about EPM.