INFECTIOUS DISEASES
PCR Testing Could Improve EPM Diagnosis By Adam Marcus The use of real-time polymerase chain reaction (rPCR) testing could help veterinarians more accurately diagnose horses with equine protozoal myeloencephalitis (EPM), particularly in animals that have not previously received treatment for the infection, researchers found. The study, by Pedro N. Bernardino, DVM, MS, and his colleagues The study, done by Pedro N. Bernardino, DVM, MS, and his colleagues at the University of California, Davis, School of Veterinary Medicine, at the University of California, Davis, School of Veterinary Medicine, showed that DNA from Sarcocystis neurona—a leading cause of damage to the central nervous system in horses—was detectable in cerebrospinal fluid (CSF) using real-time PCR. The test appeared to be effective even in animals whose fluid samples lacked intrathecally derived antibodies to S. neurona. However, PCR testing was less accurate in horses previously treated for protozoal infection, according to the researchers. Caused by the consumption of contaminated opossum feces, EPM is a widespread but uncommon condition in horses, affecting an estimated 1% of animals in the United States that are exposed to the organism that causes the infection, according to the American Association of Equine Practitioners.
Tricky Diagnosis
Symptoms of EPM include loss of coordination, gait issues, paralysis of the muscles in the head, seizures and other clinical signs that may also characterize other disorders. What’s more, no specific test exists for the disease, making diagnosis complicated. In animals suspected of having EPM, veterinarians often order a spinal tap, an invasive procedure that carries a risk of infection and other complications and which also is prone to contamination by blood. Antibody testing of the blood also can reveal both active infection and past exposure to S. neuroma. For the study, Dr. Bernardino’s group assessed the accuracy of rPCR testing in 210 samples of CSF taken from horses with a suspected diagnosis of EPM. Of those, 25 animals tested positive for S. neuroma
DNA using rPCR alone and 30 horses had intrathecally derived antibodies to S. neurona only, the researchers reported. Thirteen horses with a positive rPCR test of their spinal fluid also had antibodies in their intrathecal fluid, they said. “These results suggest that previous treatment with antiprotozoal drugs eliminates S. neurona in the [cerebrospinal fluid] or makes its DNA undetectable. …,” the researchers reported. “An alternative explanation could be that horses with no history of antiprotozoal treatment were more likely to be acutely infected with S. neurona compared to horses experiencing chronic disease, suggesting that disease chronicity can play a role in the usefulness of [real-time] PCR as a diagnostic tool.” Nicola Pusterla, DVM, PhD, a professor of medicine and epidemiology at UC Davis, and the senior author of the study, said rPCR will not replace serological testing but rather would be best used as an adjunct test in specific cases of neurologic illness in horses. These include animals with clinical symptoms associated with EPM, those with an early onset of disease and those that have not received antiprotozoal drugs and that do not have antibodies in their CSF. “In such cases it is sometimes possible to detect S. neurona DNA,” he told Modern Equine Vet. “While we mostly rely on immunodiagnostics (antibody detection) to support a diagnosis of EPM using CSF antibody titers alone or serum/CSF antibody ratio, these tests are not 100% accurate,” Dr. Pusterla added. “On the other hand, CSF PCR is a very unreliable way to diagnose EPM, mostly because of the high rate of falsenegative results. The idea is to use a 2-tiered approach: collect serum and CSF and test for S. neurona. If the horse has signs of EPM and the serum/CSF ratio is in the diagnostic range, a diagnosis of EPM can be reached. However, if the same horse with clinical signs compatible with EPM has no evidence of antibodies in the CSF, it is warranted to test the CSF for S. neurona by PCR or to test for other protozoal parasites such as Neospora hughesia and Toxoplasma gondii. I must reinforce the concept that PCR is not a replacement for antibody detection in the field of EPM, it is just another tool that in some cases can shed light onto the diagnosis.” MeV
For more information: Bernardino P, et al. Molecular detection of Sarcocystis neurona in cerebrospinal fluid from 210 horses with suspected neurologic disease. Vet Parasitol. 2021;291:109372. https://www.sciencedirect.com/science/article/abs/pii/S0304401721000327?via%3Dihub
ModernEquineVet.com | Issue 9/2021
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