Influence of an equine viral arteritis (EVA) outbreak on embryo recovery, pregnancy and live foal rates at an insemination and embryo transfer center in Northern Germany 2020

Abstract

Equine viral arteritis (EVA) is a contagious disease caused by the equine arteritis virus (EAV). Although EVA outbreaks occur infrequently, EAV is present in different horse populations in many countries and can cause high economic loss. EVA is of special concern to horse breeders because it can cause abortion in pregnant mares, death in very young foals and infected stallions might become permanent carriers of the virus. Most horses exposed to EAV will develop no signs of the disease. If clinical illness does occur, EVA can be difficult to diagnose because it is clinically similar to several other equine diseases. Incubation period varies between 2 – 14 days, depending on the route of exposure. The interval can be 2 – 3 days after respiratory exposure and 6 – 8 days after venereal transmission of the virus. (Balasuriya.et.al. Equine Infectious Diseases (Second Edition). 2014; 169-181). In total, 72 horses (64 mares, 8 foals) were based at the stud at the time of confirmed outbreak. The first clinical signs of fever of unknown origin began on 1st June 2020. Blood samples of all horses were taken on 5th June 2020 and showed a 42.0% seroconversion via ELISA and VNT. PCR testing was performed on nasal swabs and viral DNA was detected in 8.0% of horses. A consecutive blood sample two weeks later showed88.0% seroconversion. Embryo collection was carried out using a closed system. An experienced team of veterinarians transferred all embryos non-surgically using the Wilsher-technique (Wilsher.et.al. Equine Vet Edu 2004; 16:1:39-44). Retrospective data analysis of embryo recovery rate, 7-day post embryo transfer pregnancy rate- and live foal rate were calculated for Period One (P1, before possible incubation period, 13th February 2020 – 30th April 2020) and Period Two (P2, incubation period and outbreak, 1st May 2020 – 8th June 2020). Statistical analysis was performed using Chi-Square-Test. A total of 100 embryo flushes were performed in P1 and 116 in P2. Embryo recovery rates were 66.0% for P1 and 63.8% for P2 (n.s.), pregnancy rates after transfer were 90.9% for P1 and 71.6% for P2 (n.s.). Live foal rates after a positive 7-day-scan were 93.3% for P1 and 77.4% for P2 (p<0.05). Foal survival rate was 69.8% in P2 compared to 91.7% in P1, when comparing the number of live foals at 14 days post-partum with the number of positive pregnancies (p<0.01). Apart from the possible economic impact, EVA seems to have a great effect on the live foal rate and foal survival, but not on the actual positive embryo recovery rate and initial 7-day-post-embryo transfer pregnancy rate.