THE LESSER KNOWN PARASITE
Although Sarcocystis neurona is the most common cause of equine protozoal myeloencephalitis (EPM) in American horses, a different protozoal organism,o Neospora hughesi,h is responsibleresponsib for a smaller but significant num number of cases.
The life cy cycle of N. hughesi is not well underst understood, but it appear appears that horses canc become infec infected without consu consuming contaminated feed or water—a topicto Nicola Pu Pusterla, DVM DVM, PhD, DACVIM,DA hash been investigating at the University of California– Davis. “A closely related organism called Neospora caninum causes abortion in cattle and has a huge economic impact on the livestock industry,” he says. “In cattle there are two routes of transmission. One is horizontal—going to cattle from the definitive host [dogs or wild canines, who pass the oocysts in their feces]. But the most efficient way the organism is transmitted in cattle is vertically, from an infected dam to the offspring during gestation.” N. caninum reproduces in the cow’s intestine, then passes from the bloodstream through the placenta into the fetus.
“There can be different outcomes, depending on the stage of gestation when the fetus is exposed,” Pusterla says. “Infection of the fetus can cause abortion, stillbirth or birth of a persistently infected animal, depending on the immune stage of the fetus when infection occurs.” If a female calf is born with the infection, she in turn can pass the parasite on to her own offspring when she matures.
“About 95 percent of the infections in cattle occur vertically—from dam to fetus—and this mode of transmission also occurs in horses,” says Pusterla. “I worked with a herd that had two mares who tested serologically positive to Neospora hughesi. Every one of their offspring that we tested showed evidence of vertical transmission. All their foals were healthy but had evidence of transplacental transmission; they were born with high antibody levels to N. hughesi before they even ingested colostrum. This is a very effective way for this organism to be retained in horse populations.”
This finding has serious implications for the potential range of EPM as horses are moved about the country. “When reports about N. hughesi first came out in the mid-1980s, it was thought that this parasite occurred only on the West Coast,”
says Pusterla. “But we found that this organism is more widespread, based on serological data from a few years ago. That data showed that there were seropositive animals with high antibody titers to N. hughesi in horses from about 25 states. Infection with N. hughesi could be found in horses anywhere.”
That means that EPM is a possible diagnosis for horses with neurological signs even if they live in regions where S. neurona typically isn’t found. “S. neurona iss present only in areas where the definitive host is present, which is the opossum,” says Pusterla. “The farther north you go, the e less frequently we find opossums or EPM.” However, he adds, “N. hughesi is widespread ad across the U.S. and it is a pathogen that is probably spread farther than S. neurona.”
Nevertheless, says Amy Johnson, DVM, DACVIM, of the University of Pennsylvania, at least for now, the preponderance of EPM cases occur only in certain regions: “The Western states see far more clinical cases of N. hughesi than we do here on the East Coast. It has been found nearly everywhere in the United States, but in terms of a causative agent it is not as common here. In the seven years I’ve been here at the New Bolton Center I’ve seen only three N. hughesi cases, compared to well over a hundred caused by Sarcocystis. So there are some regional differences.” horses who develop EPM, and particularly the horses who develop recurrent infections. Now and then we get a horse who responds to treatment but then has a substantial relapse or frequent relapses.”
Another possibility is that some strains of these parasites are more virulent than others. “Siobhan Ellison [DVM, PhD], in Florida, presented her hypothesis that certain protozoa have surface antigens that might be more likely to be infectious,” Reed says. “This is certainly a possibility.”
Advances in diagnostics and treatments have come a long way toward reducing the number of horses who succumb to EPM. Finding more effective ways to prevent and treat this disease is a challenging goal---but one that researchers have real hopes of achieving in the years to come.