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Understanding EPM
& Its Causes
Nicola Pusterla, DVM, PhD,
DACVIM, DAVDC-Equine is
a professor of equine internal
medicine and dentistry at
the University of California,
Davis, School of Veterinary
Medicine. His research focus is
on equine infectious diseases
with an emphasis on molecular
epidemiology. Dr. Pusterla also
leads the UC Davis Equine
Infectious Disease Research
Laboratory.
Equine protozoal myeloencephalitis (EPM) was first recognized as a condition in 1976 and it
remains one of the most common infectious neurologic diseases of horses in North
America.
1
A progressive disease, EPM can impact any horse and cause irreversible
damage to the brain or spinal cord if le unchecked.
Unlike most infectious diseases, EPM is not black and white, which is cause for
much frustration among equine practitioners. e "gray area" around this disease has
prevailed since it was originally discovered. As one mystery is solved another arises,
beginning with the most basic question: What causes EPM in the horse?
e Main Culprits
Originally thought to be caused by Toxoplasma gondii, it took nearly 20 years aer the
disease was recognized to identify its primary causative agent as Sarcocystis neurona.
Today, we know EPM can be caused by both S. neurona and Neospora hughesi. It
may come as a surprise, however, that T. gondii is back in the mix as a possible cause
(though less is known). Table 1 depicts the common characteristics of each parasite that can
cause EPM.
by Nicola Pusterla, DVM, PhD, DACVIM
If immunodiagnostics on serum
and CSF come back negative for
S. neurona on a case presenting
with classic signs of EPM,
practitioners should be open to
investigating other pathogens.
