Frequency of latent equine herpesvirus 1 (EHV-1) in New Zealand horses : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies at Massey University, Palmerston North, New Zealand
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Date
2019
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Massey University
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Abstract
AIM: To estimate the frequency of infection with equine herpesvirus 1 among a selected population of horses from the central North Island of New Zealand, including determination of the open reading frame (ORF) 30 N/D752 genotype.
METHODS: Fresh heads were collected from horses euthanised for unrelated reasons between March and November 2015. Small pieces of retropharyngeal lymph nodes (RLN) and submandibular lymph nodes (SLN) were dissected from the heads and transported to the laboratory in RNA later solution. DNA extracted from these tissues was subjected to enrichment for EHV-1 sequences by hybridisation with biotin-labelled EHV-1 specific probe, followed by recovery of EHV-1 sequences on streptavidin-coated magnetic beads. The enriched samples were tested for the presence of EHV-1 using nested PCR. The EHV-1 amplicons were sequenced to determine the ORF30 genotype of the virus.
RESULTS: Overall, EHV-1 DNA was detected in RLN samples from 6/63 (9.5%) horses. Of those, three were also positive for EHV-1 DNA in SLN samples. There was no association between EHV-1 positivity and age, sex, or breed of the animals sampled. All EHV-1 positive horses harboured ORF30 N752 genotype. The D752 genotype, which has been linked to increased neurovirulence, has not been detected in any of the samples.
CONCLUSION: Equine herpesvirus 1 continues to circulate among horses in New Zealand. The RLN appear to be the sample of choice for detection of EHV-1 DNA in a recently euthanised horse. The frequency of latent EHV-1 infection among sampled horses may have been higher than detected, as some of latently infected horses may have harboured EHV-1 DNA at the levels beyond the sensitivity limit of the assay or at anatomical sites not sampled in the study. Lack of detection of EHV-1 with ORF30 D752 genotype, together with detection of only one horse positive for that genotype in the previous South Island based study (Dunowska et al., 2015) suggest that infection with this genotype is not common in New Zealand.
CLINICAL RELEVANCE: If live animals are tested using SLN biopsy it should be kept in mind that negative results do not rule-out the presence of latent EHV-1 at other sites inaccessible for testing. While EHV-1 with ORF30 D752 genotype was not detected in this study, the importance of this genotype should not be over-interpreted because the markers for EHV-1 neurovirulence are most likely more complex than this single amino acid substitution. Viruses with either genotype have been recovered from equine herpesvirus encephalopathy cases worldwide. The data presented provided baseline information on the frequency of EHV-1 infection among horses in New Zealand. These can provide useful information during any future outbreaks of EHV-1 associated diseases and for the development of control measures to minimise the impact of such viral disease for horses and their owners.
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Equine herpesvirus diseases, New Zealand, Horses, Virus diseases, Latent virus diseases