The equine herpesvirus EHV-1 outbreak affecting horses throughout the United States initially went undetected by testing facilities because it is comprised of a novel genotype and one not traditionally tested for in labs. Named H-752, the strain doesn't fit into the other two recognized categories: D-752, which often causes neurologic disease, and N-752, which causes neurologic issues less frequently.
Most labs utilize a test that screens for the two common EHV-1 strains using allelic discrimination assays; the new genotype that causes both respiratory and neurologic signs was not detected by these tests, reports The Horse.
Dr. Nicola Pusterla, professor of equine medicine at the University of California, Davis, School of Veterinary Medicine, suggests that screening for a universal gene which is independent of genotype would allow labs to find EHV-1 variants more successfully. This, in turn, would lead to quicker isolation of the affected horse and better containment of the disease.
This EHV-1 variant was found in March 2021 when Pennsylvania vets pulled blood on 10 horses that were exhibiting fevers, but were otherwise healthy. The potential sickness was caught early as regular temperature taking of horses was part of the farm's wellness protocol. They tested for equine flu, rhino A and B, EHV-1, and EHV-4 using a quantitative polymerase chain reaction (qPCR), even though the horses appeared “normal.”
Eight of the qPCR tests came back positive for EHV-1 using the more-sensitive gB gene target assay (gB is the universal gene in EHV-1); the more generalized tests showed all eight tests as negative for EHV-1. Additional horses at the farm fell ill; Pusterla and veterinarians testing the blood for EHV-1 received the same results, with the broader test showing no EHV infection, though some of the horses were ill.
When genotyped, results showed the EHV-1 strain was new in the United States, but the strain had been seen in France in 2018. Additional research showed that the gene mutation occurred in the same spot that determines whether the virus will cause the horse to exhibit neurologic symptoms.
Pusterla notes that it is impossible to know how prevalent the H-752 variant is since labs were not routinely screening for it. He recommends that labs utilize diagnostic techniques that are not limited to detecting known variants.
Read more at The Horse.
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