Equine protozoal myeloencephalitis

Previous authors: J P DUBEY

Current authors:
J P DUBEY - Senior scientist, MVSc, PhD, DSc, Animal Parasitic Diseases Laboratory, U.S. Department of Agriculture, Building 1001, Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, Maryland, 20705, United States of America

Introduction

Equine protozoal myeloencephalitis (EPM) is a neurological disease of horses in the Americas caused by Sarcocystis neurona. The clinical syndrome was recognized in the USA by Rooney¹²⁵ in the 1960s and protozoa were first demonstrated in lesions in 1974.^{2, 19, 29} Initially, the protozoa were considered to be Toxoplasma gondii.^{2, 19, 29} Dubey et al. in 1974²⁹ reported lesions and protozoa in horses and distinguished the parasite from T. gondii. He also re-examined^{29, 30} the  original  cases of 1974^{2, 19} and concluded that the parasite that causes EPM was not T. gondii but probably a species of Sarcocystis, which was later confirmed by ultrastructural examination.¹⁴⁰ The protozoan was first cultured in 1991 from the spinal cord of an affected horse and was named Sarcocystis neurona.³⁹ The development of a serological test in 1993⁸³ based on antigen prepared from in vitro cultured S. neurona merozoites led to the discovery that S. neurona infections are common in horses in the Americas.^{3, 4, 7, 148} The opossum was proposed as its definitive host⁷⁴ and the life cycle was completed when the sarcocyst stage was identified.³¹ The development of an interferon gamma gene knock out (KO) mouse model for S. neurona made it possible to study its biology and chemotherapy.^{33, 50, 53, 54} Its genome was sequenced and annotated, potentially leading to development of  better diagnostic methods and therapies.⁶ The history of EPM and its biology have been fully reviewed.^{33, 37, 48}

Aetiology and life cycle

Sarcocystis neurona is a coccidian belonging to the family Sarcocystidae. Opossums of the genus Didelphis (D. virginiana and D. albiventris) are the definitive hosts (Figure 1).^{31, 50, 53, 55, 72, 74} Many other mammals are aberrant or intermediate hosts.⁴⁸ Aberrant hosts are those in which only part of the asexual cycle (only schizonts) is completed; horses are aberrant hosts. Intermediate hosts are those in which the full asexual cycle (schizonts and sarcocysts) takes place. Raccoons (Procyon lotor), skunks (Mephitis mephitis), armadillos, domestic cats, and sea otters (Enhydra lutris) are its proven intermediate hosts as evidenced by excretion of S. neurona sporocysts by laboratory-raised opossums fed infected tissues.^{16, 60, 62, 145, 150} Sarcocystis neurona-like sarcocysts have also been reported in two dogs,¹⁵² one horse,¹¹⁴ one mink (Neovison vison),¹²⁰ one bobcat (Lynx rufus),¹⁵³ and one fisher (Pekania pennanti). There is a report of brown-headed cowbirds (Molothrus afer) as intermediate host of S. neurona¹⁰² but it needs confirmation because S. neurona is usually not infectious to birds.⁵⁰

The sarcocysts are microscopic in size (about 700 μm long) with a 1 - 2 μm thick cyst wall. They have villar protrusions that are up to 2.8 µm long that do not reach bradyzoites.^{48, 52, 135} The bradyzoites are slender and tiny (about 5 μm long).  The sporocysts are about 10 × 8 μm.⁹³

A sexual cycle occurs in the lamina propria of the intestines of opossums. After gametogony, oocysts sporulate in the lamina propria of the small intestine and are excreted in the faeces. Sporocysts contain four sporozoites and a residual body.

Horses become infected by ingesting food and water contaminated with sporocysts. The only stages of S. neurona definitively identified in horses are schizonts and merozoites, which are confined to the brain and spinal cord.^{39, 48} (Figure 2) The precise time period and the route of dissemination of the parasite in horses are unknown.⁶⁹ After oral inoculation of ponies with S. neurona sporocysts, evidence of invasion of brain occurred as early as seven days post- inoculation. In an experimental aberrant host, the KO mouse, sporozoites excyst in the small intestine and can be found in peripheral blood within one day of feeding sporocysts. Sarcocystis neurona first multiplies in visceral tissues and eventually resides in the central nervous system (CNS).^{32, 33, 52-54, 76} In the natural intermediate host (raccoon) schizogony occurred 7 to 22 days post- inoculation and mature sarcocysts were detected by 77 days post- infection.¹⁴³ Sarcocystis bradyzoites are not infectious for intermediate hosts.⁶⁵

Figure 1  Life cycle of Sarcocystis neurona

Figure 2 Sarcocystis neurona in sections of the spinal cord of a horse. Haematoxylin and eosin stain

1. Perivascular infiltration and necrosis. Two infected neurons (arrows) but parasites are barely visible at this magnification
2. Neuron containing several schizonts (arrows)
3. Four schizonts (arrows) in various stages of development
4. Intra- and extracellular individual merozoites (arrow heads)

Attempts to reproduce EPM after oral inoculation of horses with sporocysts were largely unsuccessful, including in immune-deficient foals.^{20, 21, 72, 98, 113, 129, 130, 137-139, 141}

Sarcocystis neurona schizonts are found in neurons, various glial cells and resident macrophages in the brain and spinal cord.^{40, 48, 76, 142} They divide by endopolygeny:¹⁴² the  nucleus of the parasite becomes multilobed and eventually each nuclear lobe is incorporated into two budding merozoites (Figure 3