EPM; in defense of possums
In recent months, I’ve seen multiple cases of confirmed or suspected EPM in both my life outside of tumblr, as well as tumblr users. The common thread to all of these cases has been misinformation being spread in the comments (by well meaning, but ill informed friends, family and followers of the affected) regarding the disease, the treatment and definitely the method of spread. As I am set to graduate in a few months, I felt like I wanted to get back to using my platform to attempt to spread correct and accurate veterinary information within the community - and to bring up some undervalued points.
EPM - equine protozoal myeloencephalitis
EPM is a neurological disease caused by a protozoan life form known as Sarcocystis neurona. This is a condition unique to horses, though other forms of Sarcocystis are able to infect cattle, some sheep, pigs, very occasionally alpacas and some birds. Human infection is very rare. (The term myeloencephalitis refers to the condition caused by the parasite, not the parasite itself and references inflammation of the spinal cord).
The majority of horses that are infected with EPM will never show clinical signs, as their immune system will mount a sufficient response to the protozoa. This is one of the main reasons that serological testing (blood draws) are not considered to be definitive tests for EPM, as horses can be exposed to the protozoa without developing any neurological signs. It’s also possible for EPM positive animals to test negative to blood tests (20% of cases). The standard for diagnostics is a combination of clinical signs and a positive blood test, though the only definitive means of diagnosis is to test cerebrospinal fluid.
Symptoms of EPM are largely nonspecific; ataxia, generalized weakness, muscle spasticity, focal muscle wastage, atypical lameness with occasional presentations involving nerves of the face, head and neck. Most commonly the hind limbs are affected and generally unilateral - both sides may be affected, but there is often a side that is considered worse or more dramatic.
There are currently three treatments authorized and supported by the FDA: Ponazuril (Marquis) - 28 day treatment of oral paste, Pyrimethamine and Sulfadiazine (Rebalance) - up to 120 day treatment of oral suspension, Diclazuril (Protazil) - 28 day treatment of alfalfa based, pelleted food top dressing.
These are the ONLY approved and proven methods of treating EPM. This certifies that they have undergone rigorous testing to prove efficacy, safety and long term benefit. Anything else lacks scientific evidence and is based entirely around anecdotal evidence at this point. While ongoing investigation into further treatments is active, at this point, these should be considered the only option.
Now, one of the hotbed issues with regards to EPM is the lifecycle. Basically all small mammals (cats, raccoons, squirrels, skunks, sea otters) can act as intermediate hosts, which provides a body for protozoan sporocysts to develop into sarcocysts, which are then ingested by possums, which are considered the definitive host. Infection of horses is through ingestion of contaminated feed or water sources with fecal material from infected possums.
This has lead to a very knee jerk response regarding the possibility of possum culling. This is, in short, nothing short of a reckless and ill thought plan. While I empathize with those who have lost horses to EPM, possums are an incredibly important component of local ecosystems and help protect us from another very nasty neurological condition known as Lyme Disease.
Unlike EPM, Lyme Disease does not really have a preference for species. It can infect humans, dogs, cats, horses (though less commonly) and other animals. Annually, 365,000 people are infected, so this is no small feat. If left untreated, Lyme can cause debilitating lifelong problems that will flare up when infected individuals are stressed or ill or under other conditions. Full recovery from Lyme Disease, even when caught early, is considered to be between 64 and 71%.
So, what do possums have to do with Lyme Disease? Well, possums are one of the only creatures that have a natural immunity to Lyme Disease. They can be bitten by as many ticks carrying the disease, without ever developing the condition. Not only that, but possums LOVE to eat ticks. They consider them a gourmet snack. Possums can eat up to 5,000 ticks in a single season! That’s potentially up to 5,000 (or more, if a single tick is able to bite multiple creatures) cases of Lyme Disease thwarted by a single possum! If you have multiple living in one area, then that number can increase even more.
The reasonable recommendations are to ensure that your food supply for your horses is considered safe; lifting bales of hay off the ground by placing them on pallets, or storing them in areas that can be fitted to prevent possums getting inside. Water supplies should be checked to ensure that where the water is drawn from, is not somewhere that possums are going to defecating or getting access to. Plus, remember, not all horses exposed will ever show symptoms. They might very well pick up the protozoa, kick its ass and go on with life.
So please, this is your local (very soon to be) equine veterinarian, begging you to be nice to possums because they protect us from Lyme Disease and are an important part in the food chain and local ecological system!