Divergent strains of EHV-1 in Swedish outbreaks during 2012 to 2021.
Authors: Öhrmalm Johan, Cholleti Harindranath, Theelke Anna-Karin, Berg Mikael, Gröndahl Gittan
Journal: BMC veterinary research
Summary
# Editorial Summary: Genetic Diversity of EHV-1 in Swedish Outbreaks (2012–2021) Between 2012 and 2021, Swedish researchers sequenced partial genomes from 55 EHV-1 outbreaks to establish which viral strains were circulating and how they related to disease severity, particularly in cases of equine herpes myeloencephalopathy (EHM). Genotyping focused on three regions of the viral genome (ORF11, ORF30, and ORF34) revealed 14 distinct genovariants, with a single dominant strain accounting for nearly half of all outbreaks; the work also documented seven previously unreported mutations, including four in ORF34 that resulted in amino acid changes. Strikingly, five of six EHM outbreaks involved different genovariants, yet each outbreak showed clonal spreading—suggesting that multiple strains can cause severe neurological disease but that transmission within an outbreak typically stems from a single introduction. Perhaps most concerning for biosecurity protocols was the detection of the Valencia 2021 EHM-associated viral clone in apparently healthy returned horses despite weeks of quarantine, demonstrating that recovered animals can remain infectious and transmit pathogenic strains internationally. These findings underscore the need for more refined epidemiological tracking, careful movement controls during EHM incidents, and recognition that clinical recovery does not guarantee viral clearance or reduced transmission risk.
Read the full abstract on PubMed
Practical Takeaways
- •EHV-1 genetic diversity in Sweden is higher than previously documented, with multiple circulating strains requiring surveillance to track disease patterns and outbreak sources
- •Quarantine alone may be insufficient to eliminate viral shedding in recovered horses; additional biosecurity measures and testing protocols should be implemented before returning horses from international events with EHV-1 exposure
- •Different genovariants are associated with EHM outbreaks, suggesting strain-specific virulence factors that warrant investigation for better disease management and prevention strategies
Key Findings
- •Analysis of 55 EHV-1 outbreaks (2012-2021) identified 14 different genovariants, with one dominant variant found in 49% of outbreaks
- •Seven previously undescribed mutations were identified: three synonymous mutations in ORF11 and four mutations in ORF34 (two synonymous, two non-synonymous)
- •Five of six EHM outbreaks involved different EHV-1 genovariants, but clonal spreading occurred within individual outbreaks
- •Healthy horses returning from a 2021 Valencia EHM outbreak remained positive for the outbreak virus strain despite weeks of quarantine, indicating persistent viral shedding