Evidence that inflammation associated with Sarcocystis neurona infection plays a role in clinical signs associated with equine protozoal myeloencephalitis

Abstract

Sarcocystis neurona is a protozoal pathogen that causes equine protozoal myeloencephalitis (EPM). The commonly recognized neurologic signs that EPM causes are due to the pathological effects of the disease and can be due to infections of somatic and neural tissues by the asexual stages of S. neurona that cause cell dysfunction and death. A second component of EPM is the inflammatory host response to the parasitic insult. Inflammation may be as deleterious to the host as the initial insult. Anticoccidial drugs are necessary to treat the infectious aspect of EPM. An objective evaluation of treatment effect on protozoa is measured by a reduction in SnSAG 1, 5, and 6 antibodies. Recurrences of neurologic signs in horses that have diminishing antibody titers was controlled by levamisole HCl and indicate inflammation is a significant component of disease. There are no specific markers of inflammation due to S. neurona, therefore clinical exam was used to determine effectiveness of treatment. In a clinical field trial horses with a presumptive diagnosis of EPM based on veterinary clinical exam and the presence of serum antibodies against recombinant SAG’s 1, 5, and 6 by ELISA (> 1:16) were treated with 0.5 mg/kg decoquinate and 1 mg/ kg levamisole HCl PO for 10 days. A post-treatment clinical examination and antibody titer was determined. Horses that improved and then experienced recurrence of neurologic signs were treated with levamisole HCl PO until the horse resolved the clinical signs. Ninety-four percent of the EPM suspect horses resolved neurological signs within the 10 day treatment period based on clinical exam. Eighty-nine percent showed a reduction in antibody titers at 4-5 weeks and 97% showed a reduction in antibody titers by 12 weeks post treatment. Eleven percent of horses responded incompletely to decoquinate/ levamisole or responded completely but experienced recurrence from 7 to 14 days following treatment. Horses that experienced recurrence of signs responded to levamisole HCl and maintained the improvement after