Epidemiology of Gastrointestinal Nematodes in Canadian Breeding-Age Dairy Heifers

Gastrointestinal nematodes (GIN) are a threat to the health and welfare of cattle worldwide and have substantial detrimental effects on cattle productivity. There is a lack of information regarding the GIN epidemiology in Canadian dairy heifers. Considering the potential for significant production impacts and the threat of anthelmintic resistance, the aims of this research were to determine the GIN prevalence, fecal egg count intensity and predominant species in breeding-age dairy heifers in Canada. In chapter 2, fecal samples were collected from breeding-age heifers on 306 dairy farms from all 10 Canadian provinces. In chapter 3, fecal samples were collected from six dairy farms in Saskatchewan (SK), Canada, three times over a grazing season as a pilot study. A producer questionnaire on the use of anthelmintics was also conducted as part of the pilot study. Eggs per gram of feces (EPG) were determined using the Modified Wisconsin Sugar Flotation Technique. Predominant GIN species at the farm-level were identified by deep amplicon nemabiome sequencing of the ITS-2 DNA locus of nematode larvae. The Canadian heifer prevalence and population-averaged strongylid EPG, accounting for clustering on farms, were 20.9% (95% CI: 17.2 – 24.6%) and 1.1 EPG (95% CI: 0.6 – 1.6 EPG), respectively (chapter 2). In SK, the population-averaged mean strongylid egg counts were 1.7 (95% CI: 0.3-3.1), 4.3 (95% CI: 1.9-6.8) and 9.3 (95% CI: 2.3-16.3) EPG for June, July and August, respectively, and many producers reported using avermectin anthelmintics for their heifers (chapter 3). The predominant species in both studies were Cooperia oncophora and Ostertagia ostertagi. The results found here are consistent with the literature for young cattle in temperate climates and provide much needed epidemiological data on GIN in Canadian dairy heifers