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BMC veterinary research2024; 20(1); 270; doi: 10.1186/s12917-024-04096-7

Divergent strains of EHV-1 in Swedish outbreaks during 2012 to 2021.

Abstract: Equid alphaherpesvirus 1 (EHV-1) is a ubiquitous and significant viral pathogen in horses worldwide, causing a range of conditions, including fever, respiratory disease, abortion in pregnant mares and the severe neurological disease called equine herpes myeloencephalopathy (EHM). Despite that EHV-1 is a notifiable animal disease in Sweden, there is limited knowledge about the circulating strains. This study aimed to analyze the genetic diversity of EHV-1 strains in equine samples from different Swedish outbreaks by partial genome sequencing. Genotyping based on three selected open reading frames ORF11, ORF30, and ORF34 in the viral genome was conducted for 55 outbreaks of EHV-1 spanning from the years 2012 to 2021. The analysis revealed 14 different genovariants, with one prominent genovariant identified in 49% of the outbreaks. Additionally, the study identified seven mutations not previously described. Three new mutations were demonstrated in ORF11, all synonymous, and four new mutations in ORF34, two synonymous, and two non-synonymous. Notably, different EHV-1 genovariants were found in five out of six studied EHM outbreaks, but clonal spreading was shown within the outbreaks. Moreover, the study demonstrated that healthy (recovered) horses that returned from an EHM outbreak at an international meeting in Valencia, Spain (2021), were positive for the virus clone responsible for the severe disease outbreak despite several weeks of quarantine. These findings shed light on the genetic diversity and transmission dynamics of the virus and significantly contribute to better understanding of the epidemiology of EHV-1 in Sweden and globally.
© 2024. The Author(s).
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Publication Date: 2024-06-22 PubMed ID: 38909196PubMed Central: PMC11193201DOI: 10.1186/s12917-024-04096-7Google Scholar: Lookup
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  • Journal Article

Summary

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Overview

  • This research article investigates the genetic diversity of Equid alphaherpesvirus 1 (EHV-1) strains involved in outbreaks in Sweden from 2012 to 2021.
  • The study identifies various genetic variants of EHV-1, including new mutations, and examines their implications for disease spread and epidemiology.

Background

  • Equid alphaherpesvirus 1 (EHV-1) is a common and important virus affecting horses globally.
  • It causes diverse clinical manifestations such as:
    • Fever
    • Respiratory disease
    • Abortion in pregnant mares
    • Equine herpes myeloencephalopathy (EHM) — a serious neurological disorder
  • In Sweden, EHV-1 is a notifiable disease, meaning outbreaks must be reported, but detailed information on the genetic types of circulating virus strains was limited prior to this study.

Objectives

  • To analyze the genetic diversity of EHV-1 strains from different outbreaks across Sweden between 2012 and 2021.
  • To conduct genotyping by sequencing three specific regions (open reading frames) of the virus genome:
    • ORF11
    • ORF30
    • ORF34
  • To investigate the presence of new mutations and understand the relationship between viral genotypes and outbreaks of disease, especially neurological disease (EHM).

Methods

  • Samples were collected from 55 outbreaks of EHV-1 in Sweden over a 10-year period.
  • Partial genome sequencing targeted the three selected viral regions (ORF11, ORF30, ORF34).
  • Genotyping was conducted to identify different “genovariants” — distinct genetic variants of the virus.
  • Mutations were characterized as either synonymous (not altering the amino acid sequence) or non-synonymous (altering the amino acid sequence), to assess possible functional impacts.

Findings

  • Genetic Diversity: 14 distinct genovariants of EHV-1 were identified in the studied outbreaks.
  • Nearly half (49%) of all outbreaks were caused by one predominant genovariant.
  • New Mutations: Seven previously undescribed mutations were discovered:
    • 3 new synonymous mutations in ORF11
    • 4 new mutations in ORF34, including 2 synonymous and 2 non-synonymous
  • Diversity in Neurological Disease Outbreaks: Among six outbreaks associated with EHM, five showed different EHV-1 genovariants, indicating multiple viral strains can cause neurological disease.
  • Within outbreaks, the virus displayed clonal spreading, meaning a single virus clone typically spreads among affected horses.
  • Virus Persistence: Horses that recovered from EHM during an international event in Valencia, Spain (2021) remained positive for the virus strain that caused the severe outbreak, even after several weeks of quarantine.

Significance and Implications

  • The study enhances understanding of how genetically diverse EHV-1 strains spread and persist within horse populations in Sweden.
  • Identifying multiple genovariants in neurological disease outbreaks reveals complexity in how EHV-1 causes disease, which can inform clinical management and outbreak control.
  • The finding that infected horses can remain carriers despite quarantine highlights challenges for controlling virus transmission in equestrian events.
  • Discovering new mutations contributes to the global knowledge of EHV-1 genetics, potentially aiding vaccine and diagnostic development.
  • This research helps build a genetic epidemiology framework for EHV-1, useful for both national veterinary authorities and the international equine community.

Cite This Article

APA
Öhrmalm J, Cholleti H, Theelke AK, Berg M, Gröndahl G. (2024). Divergent strains of EHV-1 in Swedish outbreaks during 2012 to 2021. BMC Vet Res, 20(1), 270. https://doi.org/10.1186/s12917-024-04096-7

Publication

BMC veterinary research
ISSN: 1746-6148
NlmUniqueID: 101249759
Country: England
Language: English
Volume: 20
Issue: 1
Pages: 270
PII: 270

Researcher Affiliations

Öhrmalm, Johan
  • Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden. jnoh0002@stud.slu.se.
Cholleti, Harindranath
  • Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.
Theelke, Anna-Karin
  • Swedish Veterinary Agency, Uppsala, Sweden.
Berg, Mikael
  • Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.
Gröndahl, Gittan
  • Swedish Veterinary Agency, Uppsala, Sweden.

MeSH Terms

  • Animals
  • Horses
  • Sweden / epidemiology
  • Herpesvirus 1, Equid / genetics
  • Herpesvirus 1, Equid / isolation & purification
  • Horse Diseases / virology
  • Horse Diseases / epidemiology
  • Disease Outbreaks / veterinary
  • Herpesviridae Infections / veterinary
  • Herpesviridae Infections / epidemiology
  • Herpesviridae Infections / virology
  • Genetic Variation
  • Genome, Viral
  • Genotype
  • Open Reading Frames

Conflict of Interest Statement

The authors declare no competing interests.

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Citations

This article has been cited 2 times.
  1. Liu D, Zhao X, Wang X. The Genomic Characterization of Equid Alphaherpesviruses: Structure, Function, and Genetic Similarity. Vet Sci 2025 Mar 3;12(3).
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  2. Wagner B, Schnabel CL, Rollins A. Increase in Virus-Specific Mucosal Antibodies in the Upper Respiratory Tract Following Intramuscular Vaccination of Previously Exposed Horses Against Equine Herpesvirus Type-1/4. Vaccines (Basel) 2025 Mar 10;13(3).
    doi: 10.3390/vaccines13030290pubmed: 40266191google scholar: lookup
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