When a horse is treated for a bacterial problem, like a wound infection, you can expect improvement within a day or so and an eventual cure. This isn’t necessarily the case when treating protozoal infections like EPM. EPM, or equine protozoal myeloencephalitis, is a serious neurological/central nervous system disease that can debilitate your horse. It is caused by the horse ingesting the protozoa, which are shed mainly by opossums and possibly other animals.
When veterinarians and drug manufacturers talk about success rates with EPM treatment, they’re not talking about cures. A treatment is considered a success if a horse improves at least one degree on the modified Mayhew neurological assessment scale:
0 = no gait deficits
1 = deficits barely perceptible, worsening with head elevation
2 = deficits noted at walk
3 = deficits noted at rest and walking, nearly falls with head elevation
4 = falls/nearly falls at normal gait
5 = recumbent
For example, if the horse improves from falling or nearly falling when he tries to move to still severely ataxic but not actually falling unless you elevate his head, the treatment is considered a “success.” You and your horse certainly won’t feel that way, but it’s important you understand this definition going in.
There’s also a big difference between curing the infection and eliminating the symptoms. Even horses that show improvement or elimination of symptoms should be rechecked by a Western Blot (antibody test) on the cerebrospinal fluid (CSF) to make certain the infection is eliminated.
Horses treated with the sulfadiazine/pyrimethamine combination until their CSF test was negative had only about a 3% relapse rate, while failure to do this had a relapse rate of as high as 95%.
Horses that have treatment continued for an additional month after symptoms have stabilized or disappeared, without having CSF testing, are reported to have a 10 to 30% relapse rate. The relapse rate may be even lower with the new drugs but only if treatment continues for an adequate period of time.
Solid figures on how many horses show complete resolution of their symptoms with drug treatment are hard to come by but it’s around 50%. However, if the horse showed improvement and has been confirmed by CSF testing to be cured of the infection, there’s a chance the horse will return to normal over the next six to 12 months with a carefully designed exercise/rehab program. Only time will tell if damage will be permanent. Horses that show no symptomatic improvement with treatment have a poor prognosis for recovery.
To maximize your horse’s chance of recovery, we suggest you:
• Use sulfadiazine/pyrimethamine or an FDA-approved drug.
• Confirm elimination of the infection by CSF testing.
• With the drug sulfadiazine/pyrimethamine, treat the horse at least three to four months, then retest CSF and continue the treatment if the CSF is still positive. Alternatively, if you’ve opted not to do CSF testing, treat the horse for at least a month past the point the symptoms disappear or residual symptoms have stabilized.
• With the new drugs (Marquis or Navigator), 28 days may not be a sufficient length of treatment for all horses. Treat for two to four weeks past the point that symptoms disappear or have stabilized. Retest CSF four months after treatment was started. If the CSF remains positive, continue treatment or try switching to a different drug regimen.
• If the horse does not respond fully but CSF is negative, implement a careful exercise program.
Relapse Or Reinfection
A relapse refers to worsening or reappearance of symptoms in a horse that has been incompletely treated. This can be largely avoided by continuing therapy for two to four weeks after symptoms have disappeared or improvement plateaus.
A relapse can also occur with appropriate treatment times if the organism is not particularly sensitive to the drugs or if resistance develops. Factors that contribute to resistance include stopping treatment too early, inadequate dosing, and interruptions in treatment.
A reinfection occurs when a horse has been cleared of the infection, as documented by a negative CSF tap, but symptoms return and CSF is again positive. Since most horses exposed to EPM never develop the disease, the causes of reinfection are likely to be the dose of organism consumed, health of the horse’s immune system and digestive tract, and possibly an immune defect in highly susceptible horses. Horses that have had EPM probably should not be allowed to graze. Muzzles that prevent close grazing will help limit exposure but not eliminate it. Be sure you:
• Realize that natural water sources attract all wildlife, including opossums, which are the primary carriers of the EPM organism.
• Never leave spilled or uneaten pet or horse feed, uncovered garbage/trash or dead animals in close proximity to the barn overnight, as these attract opossums.
• Deworm regularly, treat as needed for gastric ulcers, and consider therapeutic daily use of probiotics in horses prone to intestinal upsets.
• Feed a balanced diet with generous optimal levels of important minerals and vitamins (see antioxidants article in this issue).
Stick to your vet’s advice when treating EPM and don’t be tempted by non-drug therapies as a sole treatment. While the new FDA-approved drugs work more quickly because they kill more effectively, a single 28-day treatment course still may not be enough, and the cost is high. Navigator appears to be the most effective but also has the highest risk of side effects. Sulfadiazine/pyrimethamine treatment is still a viable option. (For additional information on EPM, see December 2002.)