An outbreak of West Nile virus-associated disease in domestic geese (Anser anser domesticus) upon initial introduction to a geographic region, with evidence of bird to bird transmission

West Nile virus activity in Manitoba was documented for the first time by the collection of an infected crow found on July 8, 2002, in Winnipeg. West Nile virus was identified as the cause of death for a large number of domestic geese at a single farm in southern Manitoba in August. Of the 5 differently aged cohorts on the affected farm, which included 2 breeding flocks and 3 growing flocks, the 6-week-old cohort was most severely affected with 692 of 2731 goslings dying within a 10-day period. Seroprevalence of West Nile virus in 2 clinically affected and recovered juvenile cohorts was 98% and 100%. In breeding geese without clinical disease, seroprevalence was 90% for 15-month-old birds and 10% for 5-year-old birds. Seroreaction in 3 of 4 cohorts tested exceeded what would be expected by mosquito transmission alone.