Defining EPM Relapses: Introduction

Occasionally horses that have been treated for EPM will relapse days or weeks later. Recurrence of signs of EPM can be due to S. neurona, S. fayeri, or polyneuritis equi. This blog introduces the experiment to investigate disease conditions in relapsing horses.

Clinical disease is associated with Sarcocystis fayeri and Sarcocystis neurona infections in horses. Both of these infections will produce inflammation that can produce a disease called polyneuritis equi.  Equine protozoal myeloencephalitis is a  tough disease to understand and the following question was posed by the EPM society as an area of high interest: a) can S. neurona establish a persistent but unapparent infection in the horse? We are investigating the seroprevalence of antibodies to S. neurona, S. fayeri, and polyneuritis equi in these relapsing horses in an effort to answer that question.

The Trojan horse model taught us that any horse can get disease, at least those we challenged showed signs. But some horses were more susceptible than others. We observed that organisms could more easily enter the cells of some horses.  Does that mean that genetics plays a part? We’re not sure yet.  The model taught us that antibodies horsegroupwere produced by day 17 of the infection, clinical signs could be as early as day 10, and in every case the signs were sequential.  That means each horse showed the same signs in a very predictable way.

In field cases of suspected EPM we start by measuring antibodies. In most of these cases, the disease is not acute, but has progressed to a chronic state.  A statistically significant finding in acute cases was that antibodies against S. neurona increased as the infection progressed, yet the clinical signs varied with the horse.  That means some horses which had experience with the organism would have more pronounced signs than those which were infected for the first time.  We learned about the immune response and protection against S. neurona by using our challenge model.  The immune responses were similar in the experimentally challenged horses and horses that were naturally infected.

What we couldn’t do with the model was create a chronic condition that mimicked what we see in field cases. Field cases offer various disease manifestations however we don’t know when the horse was infected or the immune background (experience) of the animal.  Sorting this out takes many observations over a long period.  Observations and measurements require objective tests, run many times, with the data evaluated statistically.

Our long term objective was to gather data from a lot of horses, over a year, and produce meaningful analysis.  We analyzed over 17,000 blood samples and saw a pattern in a hundred or so horses.  These horses were chronic and their signs reoccurred after treatment.  It could take a few weeks. or happen after years of being healthy.  Some horses responded to treatments we suggested but they would eventually show signs again.

After manikins-puzzle-four-orange-cartoon-characters-colored-pieces-white-background-34676756developing tools and identifying horses, we designed an experiment that could answer  the question posed by the EPM society.  The experiment is finishing up after 1 year.  Enrollment was staggered and new horses are enrolling, some horses are still completing 3, 6, 9 and 12 month evaluations.  The exciting, novel, and interesting findings are that we were able to define 3 disease conditions associated with EPM.  The data from the experiment is available from the International Journal of Applied Research in Veterinary Medicine, Vol 15, No 1, 2017.  The paper will show up on their web page. The next 3 blogs will define the three diseases and explain what it means to the horse and potential treatment.