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Abstract Background Equine herpes virus type 1 (EHV‐1) infection in horses is associated with upper respiratory disease, neurological disease, abortions, and neonatal death. Review Question Does pharmacological therapy decrease either the incidence or severity of disease or infection caused by EHV‐1 in domesticated horses? Methods A systematic review was preformed searching AGRICOLA, CAB Abstracts, Cochrane, PubMed, Web of Science, and WHO Global Health Index Medicus Regional Databases to identify articles published before February 15, 2021. Selection criteria were original research reports published in peer reviewed journals, and studies investigating in vivo use of therapeutic agents for prevention or treatment of EHV‐1 in horses. Outcomes assessed included measures related to clinical outcomes that reflect symptomatic EHV‐1 infection or virus infection. We evaluated risk of bias and performed a GRADE evaluation of the quality of evidence for interventions. Results A total of 7009 unique studies were identified, of which 9 met the inclusion criteria. Two studies evaluated valacyclovir or small interfering RNAs, and single studies evaluated the use of a Parapoxvirus ovis‐based immunomodulator, human alpha interferon, an herbal supplement, a cytosine analog, and heparin. The level of evidence ranged between randomized controlled studies and observational trials. The risk of bias was moderate to high and sample sizes were small. Most studies reported either no benefit or minimal efficacy of the intervention tested. Conclusions and Clinical Importance Our review indicates minimal or limited benefit either as a prophylactic or post‐exposure treatment for any of the studied interventions in the mitigation of EHV‐1‐associated disease outcome.

Abstract Background Equine herpes virus type 1 (EHV‐1) infection in horses is associated with respiratory and neurologic disease, abortion, and neonatal death. Hypothesis Vaccines decrease the occurrence of clinical disease in EHV‐1‐infected horses. Methods A systematic review was performed searching multiple databases to identify relevant studies. Selection criteria were original peer‐reviewed research reports that investigated the in vivo use of vaccines for the prevention of disease caused by EHV‐1 in domesticated horses. Main outcomes of interest included pyrexia, abortion, neurologic disease, viremia, and nasal shedding. We evaluated risk of bias, conducted exploratory meta‐analyses of incidence data for the main outcomes, and performed a GRADE evaluation of the quality of evidence for each vaccine subtype. Results A total of 1018 unique studies were identified, of which 35 met the inclusion criteria. Experimental studies accounted for 31/35 studies, with the remainder being observational studies. Eight vaccine subclasses were identified including commercial (modified‐live, inactivated, mixed) and experimental (modified‐live, inactivated, deletion mutant, DNA, recombinant). Risk of bias was generally moderate, often because of underreporting of research methods, and sample sizes were small leading to imprecision in the estimate of the effect size. Several studies reported either no benefit or minimal vaccine efficacy for the primary outcomes of interest. Meta‐analyses revealed significant heterogeneity was present, and our confidence in the quality of evidence for most outcomes was low to moderate. Conclusions and Clinical Importance Our review indicates that commercial and experimental vaccines minimally reduce the incidence of clinical disease associated with EHV‐1 infection.

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.