EPIDEMIOLOGY AND IMPACT OF GASTROINTESTINAL NEMATODE INFECTION IN YOUNG BEEF CATTLE IN WESTERN CANADA

A series of studies were conducted to meet the overarching objective of determining the epidemiology and the impact of gastrointestinal nematode (GIN) infection in young beef cattle in western Canada. The first study was conducted with 844 calf fecal samples from 43 cow-calf operations to determine the GIN fecal egg count (FEC) intensity, prevalence, herd-level relative species abundance, and risk factors related to the GIN FEC intensity and prevalence. The predicted mean strongyle-type FEC was 18.6 eggs per gram of feces, and the prevalence was 92.3%. Rotational grazing increased the risk of Nematodirus spp. prevalence compared to continuous grazing. An unusually high relative abundance of Cooperia punctata was detected in Manitoba beef herds compared to Saskatchewan and Alberta herds. Overall, Ostertagia ostertagi and C. oncophora were the predominant GIN in most herds across western Canada. The second study aimed to investigate the effects of FEC intensity and serum O. ostertagi ELISA antibody titers on the antibody response of feedlot steers (n = 240) to non-parasitic vaccine antigens using bovine viral diarrhea virus type 1 (BVDV type 1) as a candidate antigen. Neither FEC intensity nor serum O. ostertagi antibody titers affected the antibody fold change and seroconversion to BVDV type 1 antigens in the steers. The objective of the third study was to determine if anthelmintic resistance was present in western Canadian beef operations. Feedlot steers (n = 234) were randomly allocated to three treatment groups, each with six replicated pens and 13 steers per pen: untreated control, injectable ivermectin only, and a combination of injectable ivermectin and oral fenbendazole. A FEC reduction test was integrated with ITS-2 rDNA nemabiome metabarcoding to determine the resistance status. Ivermectin resistance of C. oncophora was confirmed, and that of H. placei and C. punctata was strongly suggested in these steers. The re-emergence of O. ostertagi between three to six months post-ivermectin treatment following a significant reduction between pre- and day 14 post-treatment suggested ivermectin resistance in hypobiotic larvae. The fourth study intended to determine the effects of currently used anthelmintic treatments, FEC intensity and serum O. ostertagi antibody titers on growth, production performance and carcass quality in feedlot steers. The same treatment groups as in Study 3 were enrolled in Study 4. Overall, anthelmintic treatment significantly increased or tended to increase the percentage of carcasses with quality grade AAA, AA, yield grade 2, and modest marbling category compared to no treatment. Although the combination treatment had more positive effects on carcass quality than ivermectin treatment, this was not reflected in the commercial grid- and carcass quality-based economic assessment of net income related to anthelmintic treatments. At the individual steer level, negative relationships were evident between the FEC and finishing body weights, average daily gain, hot carcass weight, and quality grade AAA. Both the FEC and the serum O. ostertagi titers had a positive relationship with yield grade 1. The fifth and final study aimed to determine the salivary anti-CarLA IgA response in beef heifers over a grazing season and its relationships with growth (body weight, average daily gain) and other parasitological indicators (FEC, serum O. ostertagi antibody titers) in order to determine its potential application for targeted selective treatment of cattle. Forty-four yearling beef heifers used in the study mounted a detectable anti-CarLA IgA response, which gradually increased and peaked towards the end of the grazing season. However, there were no obvious phenotypic correlations with other parasitological indicators. In conclusion, the overall low FEC intensity identified throughout all studies suggests that young cattle in western Canada do not tend to get heavy GIN infections. Such a low level of GIN infection may not adversely affect their ability to mount an antibody response to vaccination; however, it may still compromise their carcass quality. Therefore, effective GIN treatment during the early feedlot phase is important. The identified ivermectin resistance is likely widespread in western Canadian beef operations and may compromise economic production in the future. The suggested resistance of C. punctata to ivermectin, which could also compromise cattle production, could be a risk factor for its high regional abundance, but additional investigations are required. Particularly in the presence of anthelmintic resistance, evidence-based GIN control, such as targeted treatment and targeted selective treatment, are necessary to reduce the selection pressure for resistance; western Canadian producers must be informed of available options and encouraged to apply them. Although the salivary anti-CarLA IgA response cannot currently be recommended for routine application in a GIN management program, it should be further evaluated, focusing on specific age groups and determining phenotypic correlations