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The effect of route and dosage of administration on the serological response to a vaccine containing genetically attenuated leukotoxin of Pasteurella haemolytica combined with bacterial extracts of P. haemolytica and Haemophilus somnus (Somnu-Star Ph, Biostar Inc., Saskatoon, Saskatchewan) was evaluated in a controlled field trial in 301 feedlot calves. Vaccination of calves on arrival at the feedlot with Somnu-Star Ph significantly (p0.05) increased P. haemolytica and H. somnus serum antibody titers and reduced bovine respiratory disease (BRD) morbidity. A single subcutaneous vaccination with Somnu-Star Ph was as effective in stimulating a humoral antibody response and in reducing BRD morbidity as double vaccination by the intramuscular or the subcutaneous route. Furthermore, there were no swellings or adverse reactions observed with either subcutaneous or intramuscular administration of Somnu-Star Ph.These results suggest that feedlot calves can be immunized subcutaneously once on arrival with Somnu-Star Ph. Double vaccination was of no added value in this trial, because the majority of BRD morbidity occurred prior to revaccination fourteen days postarrival. Additional larger-sized field trials are needed to monitor the duration of immunity following vaccination and to test the effect of route and dosage of vaccination on mortality.

Hemophilosis was the most significant cause of mortality in fall-placed calves in a large feedlot in Saskatchewan, despite routine single immunization of calves with a commercial Haemophilus somnus bacterin on arrival. Common manifestations of fatal H. somnus infection were myocarditis and pleuritis; occasionally, thrombotic meningoencephalomyelitis (TME), peracute septicemia, and pneumonia were observed. Circumstantial evidence suggested that H. somnus may be a pathogen in polyarthritis. Death from pneumonia mainly occurred during the first five weeks in the feedlot. Death from myocarditis, pleuritis, TME, and septicemia, and euthanasia because of polyarthritis, occurred mainly after the third week in the feedlot.The median fatal disease onset (FDO) for pneumonia was day 12; for septicemia, day 17; for polyarthritis, day 18; for myocarditis and pleuritis, day 22; and for TME, day 29. Calves that died from myocarditis frequently were found dead in their "home" pen; however, 88% of these animals had been treated previously. Fifty-seven percent of the calves that died from pleuritis were never treated, and those that died from TME or septicemia were either never treated or died shortly after initial treatment.