THE EFFECT OF ROUTE AND DOSAGE OF IMMUNIZATION ON THE SEROLOGICAL RESPONSE TO A PASTEURELLA-HAEMOLYTICA AND HAEMOPHILUS-SOMNUS VACCINE IN FEEDLOT CALVES

The effect of route and dosage of administration on the serological re sponse to a vaccine containing genetically attenuated leukotoxin of Pa steurella haemolytica combined with bacterial extracts of P. haemolyti ca and Haemophilus somnus (Somnu-Star Ph, Biostar Inc., Saskatoon, Sas katchewan) was evaluated in a controlled field trial in 301 feedlot ca lves. Vaccination of calves on arrival at the feedlot with Somnu-Star Ph significantly (p < 0.05) increased P. haemolytica and H. somnus ser um antibody titers and reduced bovine respiratory disease (BRD) morbid ity. A single subcutaneous vaccination with Somnu-Star Ph was as effec tive in stimulating a humoral antibody response and in reducing BRD mo rbidity as double vaccination by the intramuscular or the subcutaneous route. Furthermore, there were no swellings or adverse reactions obse rved with either subcutaneous or intramuscular administration of Somnu -Star Ph. These results suggest that feedlot calves can be immunized s ubcutaneously once on arrival with Somnu-Star Ph. Double vaccination w as of no added value in this trial, because the majority of BRD morbid ity occurred prior to revaccination fourteen days postarrival. Additio nal larger-sized field trials are needed to monitor the duration of im munity following vaccination and to test the effect of route and dosag e of vaccination on mortality.