Equid herpesvirus 1 (EHV-1) can cause a wide spectrum of diseases ranging from inapparent respiratory infection to the induction of abortion and, in extreme cases, neurological disease resulting in paralysis and ultimately death. It has been suggested that distinct strains of EHV-1 that differ in pathogenic capacity circulate in the field. In order to investigate this hypothesis, it was necessary to identify genetic markers that allow subgroups of related strains to be identified. We have determined all of the genetic differences between a neuropathogenic strain (Ab4) and a nonneuropathogenic strain (V592) of EHV-1 and developed PCR/ sequencing procedures enabling differentiation of EHV-1 strains circulating in the field. The results indicate the occurrence of several major genetic subgroups of EHV-1 among isolates recovered from outbreaks over the course of 30 years, consistent with the proposal that distinct strains of EHV-1 circulate in the field. Moreover, there is evidence that certain strain groups are geographically restricted, being recovered predominantly from outbreaks occurring in either North America or Europe. Significantly, variation of a single amino acid of the DNA polymerase is strongly associated with neurological versus nonneurological disease outbreaks. Strikingly, this variant amino acid occurs at a highly conserved position for herpesvirus DNA polymerases, suggesting an important functional role.Equid herpesvirus 1 (EHV-1), a member of the subfamily Alphaherpesvirinae, is a highly prevalent equine pathogen that can cause a range of clinical signs, from respiratory distress to the induction of abortion, neonatal foal death, and occasionally neurological damage resulting in paralysis (9,14,19,33,37,61). The severity of disease resulting from EHV-1 infection is likely to be influenced by a number of factors, including the age and physical condition of the host; whether the infection is primary, a reinfection, or a reactivation of latent virus; the immune status of the host; and the pathogenic potential of the strain involved. In order to assess the relative importance of EHV-1 strain variation regarding disease outcome, it is necessary to develop methods enabling precise discrimination between genetic subgroups of interrelated strains. Previous studies have utilized DNA restriction fragment length polymorphism (RFLP) to separate field isolates of EHV-1 into subgroups according to characteristic restriction enzyme site changes and the presence of variable numbers of copies of short sequence repeats. These studies demonstrated a relatively low frequency of genetic polymorphism for EHV-1 and suggested that distinct strains of EHV-1 do exist in the field (3,4,8,25,32,41,54,57). However, the relative lack of variation of EHV-1 sequences between strains has resulted in too few RFLP variants to be identified for detailed epidemiological studies. Furthermore, although such analyses may be used for tracing the genetic relatedness of strains, they allow identification only of those genetic changes resul...

Summary Reasons for performing study: A detailed review of laboratory records for equine abortion is fundamental in establishing current disease trends and suggesting problems important for further research. Objectives: To review the causes of abortion and neonatal death in equine diagnostic submissions to the Animal Health Trust over a 10 year period. Methods: The diagnoses in 1252 equine fetuses and neonatal foals were reviewed and analysed into categories. Results: Problems associated with the umbilical cord, comprising umbilical cord torsion and the long cord/cervical pole ischaemia disorder, were the most common diagnoses (38.8%: 35.7% umbilical cord torsion and 3.1% long cord/cervical pole ischaemia disorder). Other noninfective causes of abortion or neonatal death included twinning (6.0%), intrapartum stillbirth (13.7%) and placentitis, associated with infection (9.8%). E. coli and Streptococcus zooepidemicus were the most common bacteria isolated. Neonatal infections not associated with placentitis accounted for 3.2% of incidents; and infections with EHV‐1 or EHV‐4 for 6.5%. Conclusions: Definitive diagnosis of equine abortion is possible in the majority of cases where the whole fetus and placenta are submitted for examination. Potential relevance: Given the high incidence of umbilical cord torsion and related problems as causes of abortion in UK broodmares, more research on factors determining umbilical cord length and risk of torsion is essential.

Summary Twelve adult ponies and 2 conventional foals were exposed intranasal to EHV‐1, strain Ab4 (TCID50 10−6.6) and samples of respiratory tract associated lymphoid tissues were recovered between 12 h and 13 days after infection. Infectious virus was detected in tissue homogenates using susceptible cell monolayers and expression of viral antigens was monitored using indirect immunoperoxidase histochemistry on paraffin sections. The results showed both infectious EHV‐1 and viral antigens in respiratory tract associated lymph nodes 12 h after exposure. Infected leucocytes were identified morphologically as lymphocytes, monocytes, macrophages and plasma cells. The rapid intracellular localisation of EHV‐1 in lymph nodes implies that cell mediated immunity is an important aspect of the equine response to this virus.

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Summary From 1988 to 1991, 51 pregnant pony mares were challenged intranasally or by aerosol with an isolate of EHV‐1 (AB4) originally recovered from a quadriplegic mare. This resulted in 32 abortions, occurring from 9 to 29 days after infection. In 14 of the early abortions (Days 9–14), EHV‐1 was not demonstrated in the foetal tissues by virus isolation or immunostaining despite no other non‐viral cause for the abortion being evident. Application of the polymerase chain reaction to foetal tissues from 9 of these cases also proved negative. One of the 14 mares was destroyed immediately after abortion, and post‐mortem examination revealed severe and widespread vasculitis, thrombosis and secondary ischaemic damage in the endometrium with replication of EHV‐1 in endothelial cells. These findings suggest that EHV‐1 abortion can occur due to endometrial damage without the establishment of a foetal infection.