Investigation and management of an outbreak of abortion related to equine herpesvirus type 1 in unvaccinated ponies

ABORTION outbreaks (storms) caused by equine herpesvirus type 1 (EHV-1) infection are rare in vaccinated brood mares (Mumford and others 1987, Carrigan and others 1991). Abortion generally occurs in late pregnancy (Smith and others 1995) and is preceded by leucocyte-associated viraemia (Bryans 1969). Subsequent viral infection of endothelial cells in the uterus results in varying degrees of thromboischaemic necrosis, which is critical in determining the outcome of the pregnancy (Allen and others 1999). In some abortions, the viral infection is restricted to the placenta, without detectable infection of the fetus (Smith and others 2004). Such atypical abortions have previously occurred as sporadic events. This short communication describes an atypical abortion in the early stages of a large abortion epizootic, highlighting the importance of such cases and illustrating the value of placental examination in EHV-1 abortion diagnosis. During February and March 2004 an EHV-1 abortion outbreak occurred in a group of unvaccinated Welsh mountain ponies. The animals were kept for pleasure and show, and the majority were home-bred. The breeding herd had been closed for two years, but several brood mares and juvenile ponies had visited shows in the summer of 2003. There were 140 ponies on the property, 37 of which were breeding mares that were kept separately in two herds; 21 section A mares and 16 section B/C mares. Each group was pasturebred with a different stallion, and there had been no direct contact between the two groups since November 2003. Pony stallions were run with the mares for natural service from February 2004. A veterinarian confirmed pregnancy, within three months of covering, in 18 of the 21 section A mares. Of these mares, 11 (61 per cent) aborted between February 15 and April 10, one had early fetal resorption and the remaining six produced healthy foals (Table 1). No samples were submitted from the first three abortions, but the following four abortions were all submitted for complete postmortem examination. Management practices according to the 2004 Horserace Betting Levy Board Code of Practice (Anon 2004) were implemented on March 10, following advice from the referring veterinarian and the Animal Health Trust (AHT) Epidemiology Unit, and were confirmed after the first diagnosis of EHV-1 abortion had been made. These measures comprised dividing the mares into groups of five to six animals at a similar stage of gestation and moving each animal to an individual stable a few days before its anticipated foaling date. Separate facilities and clothing were used for different groups of animals, and the mares that had aborted were attended to after the other ponies on any given day, then kept together as a group in an isolation paddock. The Veterinary Record (2007) 160, 378-380