Relationship between equine herpesvirus‐1 viremia and abortion or equine herpesvirus myeloencephalopathy in domesticated horses: A systematic review

Abstract Background Equine herpes virus type 1 (EHV‐1) infection in horses is associated with upper respiratory disease, neurological disease, abortions, and neonatal death. Objective To determine if there is an association between the level and duration of EHV‐1 viremia and either abortion or equine herpesvirus myeloencephalopathy (EHM) in domesticated horses? Methods A systematic review was performed searching numerous databases to identify peer reviewed reports that evaluated viremia and EHM, or viremia and abortion published before January 19, 2021. Randomized controlled trials and observational studies were assessed for risk of bias or publication quality. Results A total of 189 unique studies were identified, of which 34 met the inclusion criteria. Thirty studies evaluated viremia and neurologic outcomes including 4 observational studies. Eight experimental studies examined viremia and abortion, which used the Ab4 and OH03 virus strains or recombinant Ab4 derivatives. Incidence rates for both EHM and abortion in experimental studies varied among the studies as did the level of evidence. Viremia was generally detectable before the onset of either EHM or abortion. Risk of bias was generally low to moderate, sample sizes were small, and multiple studies reported negative outcome data. Conclusions and Clinical Importance The results of this study support that viremia is regularly present before EHM or abortion occurs. However, no inferences could be made about the relationship between the occurrence of either neurological signs or abortion and the magnitude or duration of viremia.