11 Impact of a Cache Valley Fever Outbreak on Lambing Performance in a Landrace Hair Sheep Flock: A Case Study

Abstract

Abstract Cache Valley Fever (CVF) is a mosquito-born, viral disease endemic to North America. While generally not associated with clinical symptoms in ewes, it affects fetal development during the first trimester of pregnancy resulting in malformations and pregnancy loss. A suspected outbreak of CVF was recorded in a flock of Barbados Blackbelly and St. Croix hair sheep managed under an 8-mo, accelerated mating system with ewes bred in March, July and November. The outbreak was associated with a July breeding, in a summer with unusually high rainfall and standing water in pastures. There had been no previous occurrence of CVF or other abortion storms in the flock (20+ years). Lambing outcome associated with the suspected outbreak of CVF was compared with that of the three prior July matings (CON). Ewes (n = 32-35) were managed in a pasture-based system under rotational grazing. Ewes were mated in two single sire groups of 16 to 18 ewes to like-breed sires each breeding season. Pregnancy was determined by transrectal ultrasound at end and 21 d after the 25-day breeding season. Ewes lambed on pasture, and lambs were tagged, and birth weights recorded within 24 hours. Blood samples were collected from all ewes following the CVF lambing and submitted for CVF antibody testing. All ewes showed CVF antibody titers. Pregnancy rate for the CVF lambing was not different from CON (85.7 vs. 89.8%; P = 0.512), whereas lambing rate was less in CVF for both ewes exposed (74.3 vs. 88.8%; P = 0.039) and ewes lambing (86.7 vs. 98.9%; P = 0.004). Incidence of stillborn and neonatal death of lambs was greater in CVF than CON (37.0 vs.5.6%; P = 0.002), as was the number of malformed lambs (13.0 vs. 0%; P < 0.001). Malformed lambs were associated with breech presentations. Litter size was not different in the CVF lambing (1.77 vs 1.86; P = 0.471), but lamb birth weight tended to be less (2.80 vs. 3.04 kg; P = 0.055) while ewe postpartum weight was not different (45.3 vs. 47.0 kg; P = 0.384). The outbreak of CVF reduced the lambing rate and increased the incidence of neonatal death and malformations. The outbreak occurred during a summer with unusually high rainfall and may be associated with changing weather patterns linked to climate change. Delaying summer breeding may be considered a mitigation strategy if feasible.