title: The occurrence of Haemophilus somnus in feedlot calves and its control by postarrival prophylactic mass medication.
abstract: Three field trials were conducted in a large commercial feedlot in Saskatchewan to determine the prevalence of Haemophilus somnus in calves and to evaluate the efficacy of prophylactic mass medication with long-acting oxytetracycline on day 17 (1990, n = 1336), day 11 (1991, n = 4372), or day 8 (1992, n = 5632) postarrival. Hemophilosis accounted for > 40% of the mortality in feedlot calves each year. Haemophilus somnus was cultured from the blood of one febrile calf on day 1 (0.1%, n = 895), but it was not cultured from nasal swabs on day 1 or day 11 (n = 881) or from blood samples on day 11 (n = 883). Similarly, it was not cultured from nasal swabs or blood samples from sick calves first treated for bovine respiratory disease (BRD) (n = 219). Serological titers to H. somnus increased (p < 0.05) in unvaccinated calves from day 1 (Geometric mean titer = 11,846) to day 96 (Geometric mean titer = 63,712), indicating natural infection following feedlot entry. Calves that relapsed twice with BRD or died from BRD +/- hemophilosis had significantly (p < 0.06) lower titers to H. somnus on days 1 and 96 than those that did not relapse twice or die. Postarrival mass medication with long-acting oxytetracycline did not reduce (p > 0.05) the risk of hemophilosis mortality. However, it reduced (p < 0.05) the risk of BRD treatment by 14% and the risk of BRD mortality by 71%. Additional epidemiological studies of H. somnus are needed so that we can develop strategic medication and vaccination programs to reduce losses from hemophilosis.
authors.affiliation: Veterinary Infectious Disease Orgainzation, Saskatoon, Saskatchewan.
authors.forename: J
authors.initials: J
authors.lastname: Van Donkersgoed
authors.forename: E D
authors.initials: ED
authors.lastname: Janzen
authors.forename: A A
authors.initials: AA
authors.lastname: Potter
authors.forename: R J
authors.initials: RJ
authors.lastname: Harland
mesh_terms: D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000382:microbiology / Q000401:mortality / Q000517:prevention & control*; D006192:Haemophilus Infections / Q000382:microbiology / Q000401:mortality / Q000517:prevention & control / Q000662:veterinary*; D007273:Injections, Intramuscular / Q000662:veterinary; D008171:Lung Diseases / Q000382:microbiology / Q000401:mortality / Q000517:prevention & control / Q000662:veterinary; D008297:Male; D010118:Oxytetracycline / Q000008:administration & dosage*; D010865:Pilot Projects; D015995:Prevalence; D012525:Saskatchewan / Q000453:epidemiology
chemical_list: D010118:Oxytetracycline