At the time, EPM caseloads were steady and practitioners perceived they were dropping or staying the same in their practices. Seven years, more specific diagnostics, and an additional EPM treatment product later, what’s the picture? The research team wanted to find out.
“What we wanted to do in this particular study was to calculate the proportional morbidity rate (PMR), which is an indicator of incidence in a population of horses for EPM … that were presented to the veterinary medical teaching hospital over a period of time,” he said. Incidence is the number of new cases of a specific diagnosis in a specific population during a specific period, divided by the total number of all diagnosed cases in that same population. So, in this study, it would be the number of EPM cases among all the veterinary teaching hospitals 1990-2015 divided by the total caseload (excluding field service cases and well-animal checks).
Eighteen and nine veterinary colleges reported their EPM numbers to the Veterinary Medical Data Base (VMDB) from 1990-2006 and 2007 to 2016, respectively, and total caseload counts over that time. Andrews reported those teaching hospitals made 2,111 diagnoses of EPM from 263,862 accessions. So, the overall PMR was 0.8% in 25 years, he said, and probably underestimated based on population.
He reflected on the legacy data, noting factors that might have contributed to trends.
“The incidence or the PMR was fairly low initially in 1990 up until about 1994,” he said, at which time the data shows a spike in cases 1995-1999, likely due to the new Western blot diagnostic tests, which had recently become available.
“So likely EPM was at a higher incidence (before then),” he said, “but we really didn’t have a good diagnostic test and, so, most of those cases went undiagnosed and we didn’t particularly have treatment.
“The other thing that was happening during this time, we also had the approval studies FDA trials for the major drugs that were being produced [Marquis (ponazuril) and ReBalance (sulfadiazine/pyrimethamine)] and although Protazil (diclazuril) was approved in 2007, the trials were actually done from 1999 to 2001.
“As we looked at the most recent data from 2010 to 2015, there was a reduction in … the incidence of EPM,” he said. The lowest the PMR has dropped since 1990 was to 0.2% in 2013. They also noticed an increase in PMR among Standardbreds, Tennessee Walking Horses, Thoroughbreds, Warmbloods, and stallions. Quarter Horses, other breeds, ponies, and drafts had a lower incidence rate.
“Age is 3 to about 15 years, so it’s a middle-aged disease,” he said. They saw “very low incidence in the young horse up to two years of age; Andrews theory was that these horses tested young are either sport horses not yet competing or racehorses and, then, later in life when they’re competing as sport horses and their soundness is scrutinized, they’re diagnosed with EPM. Also, it might take months to years for the offending bug to cause disease once infecting the horse.
The practitioner perception survey, which the team conducted January-August 2016 revealed that the majority (63%) of practitioners thought the incidence of EPM hadn’t changed (44%) or had increased (19%) in the previous two to four years. (He explained the total adds up to more than 100% because some of the private practices represented had both ambulatory and in-clinic groups.) From the survey Andrews and his colleagues estimated that 97 practices saw 157,000 horses—625 horses per year per practice and 4.5 practitioners per practice. Overall, the surveyed practitioners saw 1,769 EPM cases in 126 practices, yielding a 1.2% PMR.
Ninety-seven percent of those surveyed practitioners performed a neurologic exam in the EPM cases, many performed blood testing, but the second-most-common way of diagnosing EPM was treatment response. Veterinarians used ponazuril as their primary treatment.
As far as perception, surveyed veterinarians felt that EPM cases were increasing or had not changed.
“The data suggest that practitioners are treating EPM in the field and fewer cases are being referred to veterinary teaching hospitals,” Andrews and his fellow authors wrote. “However, the data should be interpreted with caution, as reporting of EPM cases through the VMDB is dependent on the veterinary teaching hospitals.”
In response to audience questions, Andrews pointed out that the PMR might be higher among the veterinary teaching hospitals because the population is a hospitalized one and that the more conservative estimates of the National Animal Health Monitoring System (1998) study of 0.14% in an assumedly healthy population of horses makes sense.