Minimal vertical transmission of Equid alphaherpesvirus 1 (EHV-1) in normal developing foals during a naturally occurring abortion outbreak

Equid alphaherpesvirus 1 (EHV-1) is a major cause of abortion or neonatal death in horses worldwide. Using an experimental EHV-1 abortion model, only a fraction of mares aborted and the remaining foals were delivered normally, with a normal neonatal development beyond day 10 (Gardiner et al. Vaccine 2012;30:6564-72). Interestingly, high EHV-1 (genome) abundance was identified in the fetal membranes of the normal foals. It was suggested that vertical viral transmission from dam to foal had occurred without apparent impairment of the foal. During a vast EHV-1 abortion outbreak among approximately 1000 broodmares on a single operation, approximately one quarter of the expected foal crop was lost. There were (late trimester) abortions or foals that died/were euthanized within 24 hours postpartum (pp) due to severe respiratory distress syndrome (group 1). There was a (for this farm) disproportionally large group of foals that died or were euthanized due to signs of maladjustment/placental insufficiency within 96 hours after parturition (group 2). Normal foals were also born which lived beyond 10 days without complications (group 3). Our aim was to test the following hypothesis, namely whether there is evidence of natural vertical EHV-1 transmission in foals with uncomplicated parturitions and normal pp development during an abortion outbreak. During this EHV-1abortion outbreak, we collected fetal membranes from any pregnancy outcome of groups 1 – 3. With pregnancies grouped according to gestational age in paddocks of 40 - 50 mares, unfavorable pregnancy outcome varied between paddocks. To avoid sampling bias, we matched for any group 1 or 2 pregnancy outcome within the same paddock with two group 3 outcome samples. Samples were fixed in formaldehyde and paraffin embedded. Tissue sections were prepared for DNA extraction, and EHV-1 genome abundance was determined by semiquantitative PCR assay (Hussey.et.al. J Vet Diagn Invest. 2006;18:335-42). Results were reported as ‘moderate abundance’ at cycle threshold (CT) <34; ‘low’ at CT (34-37), and negligible at CT>37-39. Samples were considered ‘undetectable’ if CT≥39. Viral genome copy numbers are reported as (moderate/low/negligible) abundance and the total number of samples per group. In group 1 abundance was 2/9/1 in 22 samples (54.4%); group 2: 0/3/0 in 15 samples (20.0%), and 0/2/4 in 85 samples (7.1%) in group 3. We conclude that there is minimal vertical EHV-1 carry-over in those foals that develop normally post-partum during an EHV-1 abortion outbreak. Whether carry-over is limited to the fetal membranes or whether EHV-1 reaches the respiratory tract for a productive period of shedding in the foals could now be investigated.