Infection with equine herpesvirus 1 (EHV-1) strain HVS25A in pregnant mice

The abortigenic effects of equine herpesvirus 1 (EHV-1) strain HVS25A, given intranasally, were assessed in pregnant BALB/c, C57BL/6J and Quakenbush mice at day 16 of pregnancy. All EHV-1-infected BALB/c mice showed clinical signs typical of EHV-1-induced disease, together with evidence of abortion. However, although there were fetal and neonatal deaths in some C57BL/6J and Quakenbush litters, the respiratory and systemic effects of EHV-1 infection in the dams were inconsistent. BALB/c dams were then inoculated at day 15 of pregnancy with either EHV-1 or rabbit kidney (RK) cell lysate (controls) and animals were killed at days 1-5 post-inoculation (pi), i.e., before the occurrence of abortions. EHV-1-infected mice showed a significant fall in rectal temperature between days 1 and 2 pi and lost weight during the first 4 days pi, demonstrating a significant mean difference in weight gain from the control group at days 2, 3, 4 and 5 pi. Death in utero was seen in five of 90 fetuses of EHV-1-infected mice, but in no fetuses from RK-inoculated mice. On days 1, 2, 3, 4 and 5 pi, the fetuses from EHV-1-infected dams were significantly smaller than those from RK-inoculated dams. Congestion and necrosis of the middle layer of trophoblast and chorionic necrosis were observed in the placentae from EHV-1-infected dams and assessed by a scoring system. Virus was isolated rarely from the fetuses (1/73), placentae (3/72) and uteri (1/16) of EHV-1-infected dams, and only from those killed on day 1 or 2 pi. This indicates that, as in the horse, abortion caused by EHV-1 infection in mice is not necessarily a consequence of fetal infection but may be due to fetal compromise due to vascular effects on the placenta.