Out of sight but not gone: Sero-surveillance for cattle grubs, Hypoderma spp., in western Canada between 2008 and 2010

Despite the lack of evidence of clinically infected cattle in western Canada since the late 1990s there is persistent evidence from serological sampling that warble fly infections remain endemic. As part of a general survey of cattle parasites conducted on calves entering a feedlot in southern Alberta sera were tested for the presence of anti-Hypoderma antibodies using an established ELISA. Calves for the study were drawn from auction markets in Manitoba, Saskatchewan, Alberta and British Columbia and serum samples were collected, according to a randomization scheme, as the calves entered a single feedlot in October through December (Survey) of three consecutive years (2008-2010). In addition, a sub-sample of calves originating on a ranch in southeastern Alberta (Ranch 1) that had been the subject of previous surveillance was tested. Sero-prevalence of anti-Hypoderma antibodies of the Survey calves ranged from 0.0% to 100.0% depending on origin and overall varied among the three years with the sero-prevalence being 45.6%, 26.9%, 28.5% in 2008, 2009 and 2010 respectively. Sero-prevalence in Ranch 1 calves varied among years, with one year being much higher and was different from overall Survey sero-prevalence in two of three years (31.7%, 64.1%, 38.5% in 2008, 2009, 2010 respectively). In the Survey population there was a trend to lower sero-prevalence over the three years while on Ranch 1 there was no clear trend. In the samples with ELISA adjusted absorbance values above the positive cut-off an examination of the cumulative relative frequency showed that in all years approximately 90% of the samples were in the lower 3 of 14 bins in both populations and in two of three years in Ranch 1 population. The clustering of the positive ELISA data at the lower end of the range is suggestive of low level grub infestations. The increased frequency of high positive ELISA results from Ranch 1 in 2009 potentially indicate an 'outbreak year' in which pupal and adult fly survival was higher than for other years of this study resulting in more infested animals with higher burdens. Examination of precipitation and temperature profiles from the subject ranch support this suggestion.