Serum antibodies to Sarcocystis neurona–half the horses in the United States have them!

This research article investigates the prevalence of serum antibodies to Sarcocystis neurona, a causal agent of equine protozoal myeloencephalitis (EPM), in horses across different regions in the United States. The article suggests that horses are an incidental, cul-de-sac host for Sarcocystis neurona, and it provides trends in seroprevalence in relation to geographic location and environmental factors.

Understanding Equine Protozoal Myeloencephalitis (EPM)

• This research focuses on equine protozoal myeloencephalitis (EPM), a neurological disease in horses caused by Sarcocystis neurona, a type of apicomplexan protozoa. The existence and understanding of this disease have developed since the 1970s, with Sarcocystis neurona confirmed as a primary causative organism in the following decades.
• All Sarcocystis species have a two-host predator-prey lifecycle. Currently, it’s hypothesized that for Sarcocystis neurona, the definitive hosts are opossums and the intermediate hosts are brown-headed cowbirds, based on DNA comparisons.
• In this lifecycle, horses are considered aberrant or dead-end hosts, meaning they become infected with the organism, but it doesn’t complete its normal lifecycle within them, rather, it causes the disease EPM.

Investigation of Serum Antibodies

• The study involved collecting blood samples from horses across Oregon, Chester County (Pennsylvania), and Ohio. These were systematically tested using immunoblot to detect serum IgG against Sarcocystis neurona.
• The results revealed that a large proportion of horses carried these antibodies, with figures of 45% in Oregon and Chester County, and 53% in Ohio. No significant effects of breed or gender were found, suggesting that the organism infects indiscriminately.

Geographic and Environmental Factors

• Seroprevalence, or the level of individuals in a population who test positive for a specific disease, was observed to significantly vary among different geographic regions in Oregon and Ohio.
• This variation suggests that environmental factors may play a role in the infection, with more elevated levels noted in wetter coastal regions (60% – 65%) and lower levels in drier, colder regions.

Implications of Seroprevalence

• It is important to note that the vast majority of horses carrying these antibodies did not show any symptoms of EPM suggesting that Sarcocystis neurona infection is usually subclinical in horses.
• However, why some horses develop clinically evident neurological disease while others remain unaffected remains a mystery and is not addressed in these reports.