Equis ISSN 2398-2977

Equine herpesvirus

Synonym(s): EHV, equine rhinopneumonitis virus, EHV-1, EHV-2, EHV-3, EHV-4, EHV-5

Contributor(s): Sarah Binns, Rachael Conwell, Susan Dawson, Melissa Kennedy, Vetstream Ltd

Introduction

Classification

Taxonomy

  • Family: Herpesviridae.
  • Subfamily: Alpha herpesviridae and Gamma herpesviridae.

Etymology

  • Gk: herpein - creeping.

Active Forms

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Clinical Effects

Epidemiology

Habitat

  • Equine herpesviruses are maintained in infected horses as latent infections.

Lifecycle

  • Herpesvirus enters susceptible cells by binding to receptors and initiating fusion at the cell membrane, or by endocytosis/phagocytosis → viral nucleocapsid is released into cytoplasm → viral DNA enters nucleus → transcription and replication take place → new viral DNA is produced and released.

Transmission

  • EHV-1, 2 and 4 transmitted by aerosol and direct contact.
  • EHV-3 transmitted venereally, by fomites, or indirect contact.
  • EHV-5 transmission not yet established.

Pathological effects

  • EHV-1 infects circulating white blood cells, resulting in a leukopenia.
  • Complement-fixing and virus-neutralizing antibodies develop; 50% of horses are seropositive to EHV-1.
  • 90-100% of adult horses have antibodies to EHV-2.

EHV-1

  • Associated with abortion Abortion: overview Abortion: EHV-1, respiratory tract disease and occasionally neurologic disease (myeloencephalitis CNS: myeloencephalopathy - EHV with ataxia and posterior paresis).
  • Primary virus replication occurs in respiratory epithelium → viremia → secondary localization.
  • Abortion usually occurs in the 7th-8th month of gestation, without previous clinical signs. May occur in the presence of neutralizing antibody.
  • Aborted fetuses show jaundice, petechia, edema, splenomegaly and focal hepatic necrosis.
  • Foals may be born alive but infected and die within a few days.

EHV-2

EHV-3

EHV-4

  • Previously EHV-1 subtype 2.
  • Associated with equine rhinopneumonitis:
    • Primary replication in respiratory epithelium → transient viremia → malaise and pyrexia → relocalization in respiratory tract → nasal discharge which may become mucopurulent due to secondary bacterial infection and conjunctivitis Conjunctivitis: overview or secondary pneumonia.
    • May infect lymphocytes → necrosis of germinal centers of lymph nodes with reactive hyperplasia → submandibular lymphadenopathy.

EHV-5

  • Interstitial pneumonia, presenting with cough, tachypnea, increased respiratory effort, fever, weight loss.
  • Loss of functional pulmonary parenchyma due to extensive nodular to coalescing interstitial fibrosis.

Other Host Effects

  • Latent infections may result in carrier status.
  • Subclinical infections are probably common, especially with EHV-2.

Control

Control via animal

  • In the UK, the HBLB included guidance in their Codes of Practice on  Equine Herpesvirus on how to control EVA and how to protect the horse population from this disease.
  • Complete rest for respiratory disease cases.
  • Antibiotics Therapeutics: antimicrobials to control secondary bacterial infection in respiratory disease.
  • Minimize stress, especially in pregnant mares.
  • Topical ointments for EHV-3 infection.

Control via environment

  • Removal of infective material such as aborted fetus and fetal membranes.
  • Isolation of infected horses.
  • Disinfection.

Vaccination

  • Inactivated vaccines available for EHV-1 and EHV-4.
  • Vaccinate pregnant mares at the beginning of the 5th, 7th and 9th months of gestation.
  • Other horses: primary course of 2 doses 3-4 weeks apart, 3rd dose 6 months later, then 6 monthly boosters.
  • No protection versus the neuropathogenic strain of EHV-1.

Diagnosis

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Kydd J H, Slater J, Osterrieder N, Antezak D F & Lunn D P (2010) Report of the second Havermeyer EHV-1 workshop, Steamboat Springs, Colorado, USA, September 2008. Equine Vet J 42 (6), 572-575 PubMed.
  • Pusterla N et al (2010) Prevalence of equine herpesvirus type 1 in trigeminal ganglia and submandibular lymph nodes of equids examined postmortem. Vet Rec 167 (10), 376-379 PubMed.
  • Wong D M, Maxwell L K & Wilkins P A (2010) Use of antiviral medications against equine herpes virus associated disorders. Equine Vet Educ 22 (5), 244-252 VetMedResource.
  • Paillot R, Case R, Ross J, Newton R & Nugent J (2008) Equine herpes virus-1: virus, immunity and vaccine. Equine Vet J 2, 68-91 VetMedResource.
  • Luce R et al (2007) Equine herpesvirus-1-specific interferon gamma (IFNy) synthesis by peripheral blood mononuclear cells in Thoroughbred horses. Equine Vet J 39 (3), 202-209 PubMed.
  • Smith D, Hamblin A & Edington N (2001) Infection of endothelial cells with Equine herpesvirus-1 (EHV-1) occurs where there is activation of putative adhesion molecules - a mechanism for transfer of virus. Equine Vet J 33 (2), 138-142 PubMed.
  • Van Maanen C et al (2001) Neurological disease associated with EHV-1 infection in a riding school - clinical and virological characteristics. Equine Vet J 33 (2), 191-196 PubMed.
  • Friday P A et al (2000) Ataxia and paresis with equine herpesvirus type 1 infection in a herd of riding school horses. J Vet Intern Med 14 (2), 197-201 PubMed.
  • Perkins G, Ainsworth D M, Erb H N et al (2000) Clinical, hemotological and biochemical findings in foals with neonatal Equine herpesvirus- 1 infection compared with septic and premature foals. Equine Vet J 31 (5), 422-426.
  • Murray M J, Eichorn E S, Dubovi E J, Ley W B & Cavey D M (1996) Equine herpesvirus type-2 - prevalence and seroepidemioloy in foals. Equine Vet J 28 (6), 432-436 PubMed.
  • Drummer H E, Reynolds A, Studdert M J, Macpherson C M & Crabb B S (1995) Application of an equine herpesvirus-1 type-specific ELISA to the management of an outbreak of EHV1 abortion. Vet Rec 136 (23), 579-581 PubMed.
  • Sharma P C, Cullinane A A, Onions D E & Nicolson L (1992) Diagnosis of equid herpesviruses-1 and -4 by polymerase chain reaction. Equine Vet J 24 (1), 20-25 PubMed.

Other sources of information

  • Horserace Betting Levy Board (2019) Codes of Practice. 5th Floor, 21 Bloomsbury Street, London WC1B 3HF, UK. Tel: +44 (0)207 333 0043; Fax: +44 (0)207 333 0041; Email: enquiries@hblb.org.uk; Website: http://codes.hblb.org.uk.

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