Equine Protozoal Myeloencephalitis (EPM) Panel

Test codes:
P0017 - Qualitative detection and differentiation of Sarcocystis neurona, Neospora hughesi, Neospora caninum and Toxoplasma gondii by polymerase chain reaction
P0014 - Equine neurological panel (includes EPM and other pathogens)

Equine Protozoal Myeloencephalitis (EPM) is one of the most challenging and exasperating diseases in horses, not only for veterinary scientists but for horse owners as well. EPM is the most commonly diagnosed neurologic disease of horses in North America (MacKay, 1997). It occurs when protozoal parasites infect and invade the central nervous system. EPM infection results in characteristic lesions in the brain and spinal cord that are evident during necropsy. The presence of these lesions correlates well with the clinical signs generally attributed to EPM (ataxia, muscle atrophy, etc).

Until the availability of molecular testing, it was almost impossible to definitively identify the causative agent and infection status of an affected horse. If a horse showed signs of neurologic problems, the veterinarian began a process of elimination to determine what was NOT causing the symptoms. Traditional EPM tests were only effective at determining that the horse did not have EPM. If traditional EPM test results were positive, that only definitively revealed that the horse had been exposed in the past to the parasites that cause EPM. Testing could not show whether the horse had an active infection by those parasites.

Treatment of EPM is a long process that should begin as soon after clinical presentation as possible. Accurate identification of the causative agent is critical to successful treatment. Various anti-parasitic drugs are used and generally must be administered for several weeks to months. With proper, quick and aggressive treatment, 60% to 70% of horses make a significant or complete recovery. Recently an EPM vaccine has become commercially available under a conditional license from the USDA. Studies are underway to determine the efficacy of the vaccine.

S. neurona, N. caninum, N. hughesi and T. gondii are related coccidian parasites associated with EPM. The source of infection for N. hughesi is unknown, whereas opossums, dogs, and cats are the definitive hosts for S. neurona, N. caninum and T. gondii, respectively.

The parasitic organism S. neurona is carried by a number of hosts at different stages of its life cycle. The opossum is the major end host and feces from infected opossums can transmit the disease to horses. The horse is a dead end host for S. neurona -- i.e. the protozoa are unable to complete their life cycle in the horse. However, they can cause severe neurological damage during their development within a horse’s central nervous system. Because >50% of horses show serological reactivity to S. neurona, antibody testing is not very effective for detecting current infections.

Until recently S. neurona was thought to be the sole cause of EPM. However, a newly identified parasite, Neospora hughesi, has now been recognized as another cause of this disease. Both species are challenging to treat due to the concealment of cysts in tissue, which can result in recrudescence of infection even after treatment. Because infections from N. hughesi, their lesions and the actual parasites are so similar to S. neurona, it is likely that some N. hughesi infections have been mistaken in the past for Sarcocystis infections. However, Serological prevalence of N. hughesi is relatively low at 6.5%

Neospora caninum is also a recently discovered, apicomplexan, coccidial protozoan that causes abortion in many mammals including cattle, goats, horses and sheep. Some evidence also indicates association of this organism with neonatal neurological and neuromuscular disease in mammalian species including horses. 31.1% of horses show serological reactivity to N. caninum.

N. hughesi and N. caninum are very similar in their genomic organization and biochemical characteristics, making clinical differentiation of the two species very difficult. Clinical differentiation of S. neurona from N. hughesi/caninum is also difficult, as the range of symptoms overlap broadly. Although N. caninum seems to have wide serological prevalence, EPM cases are most often attributed to S. neurona.

Infection by Toxoplasma gondii is prevalent in many species of domestic and wild animals (Dubey, 1993). However, a recent serological study of 276 horses in central Wyoming (Dubey et al., 2003) indicated that the serological prevalence of T. gondii was less than 1%, suggesting that this parasite is a relatively infrequent cause of EPM in horses.

PCR is the most specific and sensitive method available for detection and differentiation of Sarcocystis neurona, Neospora hughesi, Neospora caninum and Toxoplasma gondii. This testing technique is vital to correctly identifying these EPM pathogens in affected horses.

Utilities:

• Confirm the disease causing agent
• Shorten the time required to confirm a clinical diagnosis of EPM.
• Determine proper EPM treatment regimen
• Ensure that animal populations are free of organisms causing EPM
• Early prevention of spread of these organisms
• Minimize personnel exposure to these organisms
• Safety monitoring of biological products that derive from horses and other host animals

References:
Dubey, J.P. (1993) Toxoplasma, Neoplasma, Sarcocystis, and other tissue cyst-forming coccidian of human and animals. pp1-56. In: Parasitic protozoa (Kreier, P.J. ed), vol. 6, 2nd ed., Academic Press, Inc., San Diego, California.
Dubey, J.P. and Lindsay, D.S. (1998) Isolation in immunodeficient mice of Sarcocystis neurona from opossum (Didelphis virginiana) faeces, and its differentiation from Sarcocystis falcatula. Int. J. Parasitol. 28:1823–1828..
Dubey, J.P., Lindsay, D.S., Saville, W.J.A., Reed, S.M., Granstrom, D.E. and Speer, C.A. (2001) A review of Sarcocystis neurona and equine protozoal myeloencephalitis (EPM). Vet. Parasitol. 95: 89–131.
Fenger, C.K., Granstrom, D.E., Langemeier, J.L. and Stamper, S. (1997) Epizootic of equine protozoal myeloencephalitis on a farm. J. Am. Vet. Med. Assoc 210: 923–927.
NAHMS (2001). Equine Protozoal Myeloencephalitis (EPM) in the US. APHIS:VS, CEAH, National Animal Health Monitoring System. USDA, Fort Collins, CO.
Saville, W.J.A., Reed, S.M., Granstrom, D.E., Hinchcliff, K.W., Kohn, C.W., Wittum, T.E. and Stamper, S. (1997) Prevalence of serum antibodies to Sarcocystis neurona in horses residing in Ohio. J. Am. Vet. Med. Assoc. 210:519–524.
Stanek, J.F., Stich, R.W., Dubey, J.P., Reed, S.M., Njoku, C.J., Lindsay, D.S., Schmall, L.M., Johnson, G.K., LaFave, B.M. and Saville, W.J. (2003) Epidemiology of Sarcocystis neurona infections in domestic cats (Felis domesticus) and its association with equine protozoal myeloencephalitis (EPM) case farms and feral cats from a mobile spay and neuter clinic. Vet Parasitol. 117:239-49.

Specimen requirements: 1 ml whole blood in EDTA (purple top) or ACD (yellow top) tube, or 1 ml cerebrospinal fluid or brain tissue, shipped overnight at room temperature; or frozen CSF or tissue, shipped frozen.

For specimen types other than those listed here, please call to confirm specimen acceptability and shipping instructions.

For all specimen types, if there will be a delay in shipping, or during very warm weather, refrigerate specimens until shipped and ship with a cold pack unless more stringent shipping requirements are specified. Frozen specimens should be shipped so as to remain frozen in transit. See shipping instructions for more information.

Turnaround time: 3 business days

Methodologies: Qualitative PCR and qualitative real time PCR

Normal range: Nondetected