Sarcocystis falcatula from passerine and psittacine birds: synonymy with Sarcocystis neurona, agent of equine protozoal myeloencephalitis.

Equine protozoal myeloencephalitis (EPM) is a neurologic disease of horses caused by Sarcocystis neurona. The horse is a dead-end host for S. neurona and the definitive and intermediate hosts have not previously been identified. We hypothesized that S. neurona is actually Sarcocystis falcatula, a parasite that cycles in nature between Virginia opossums (Didelphis virginiana) and any of a variety of avian intermediate hosts. We extracted DNA from S. falcatula sarcocysts in the muscle of a brown-headed cowbird (Molothrus ater) and from schizonts in a fixed specimen of lung from a Moluccan cockatoo (Cacatua moluccensis). Three segments of the small subunit ribosomal RNA (SSURNA) gene, containing a total of 742 nucleotides, were amplified by the polymerase chain reaction, sequenced, and compared with the SSURNA sequence from two isolates of S. neurona. The S. falcatula sequence was identical to the sequence of the S. neurona isolate UCD-1 and differed in only 3 positions from isolate SN5. Recent evidence, also based on SSURNA sequences, implicates the opossum as the definitive host of S. neurona. Based on the SSURNA gene sequences S. falcatula and S. neurona are synonymous, thus the parasite cycles between opossums and birds maintaining a reservoir of the organism from which horses can be infected.