Effect of vaccination against bovine respiratory disease (BRD) in a calf rearing unit in Finland.

Bovine Respiratory Disease (BRD) is the main health concern in calf-rearing units. It is a major cause of increased antibiotic use and the leading cause of morbidity and mortality in calves. Vaccination protocols against BRD for calf-rearing units are difficult to implement in practice. The aim of this study was to evaluate the effect of the vaccination protocol including intranasal and subcutaneous vaccinations on mortality, antibiotic treatment rate, and average daily gain (ADG). The vaccination protocol consisted of intranasal BRD vaccination when the calves arrived at the rearing unit at the age of two to four weeks and two subcutaneous BRD vaccinations at two and three months of age. Mortality, antibiotic treatments, and ADG were recorded and evaluated from arrival until six months of age. The batches that arrived at the rearing unit prior to the beginning of the trial were used as the historic control group. Altogether, 740 vaccinated and 914 unvaccinated calves were enrolled to the study. A total of 88 calves (5.3%) died or were euthanized during the study period, of which 29 (32.9%) were vaccinated and 59 (67.1%) unvaccinated. In the logistic mixed model, the vaccination protocol decreased mortality (odds ratio 0.57, P = 0.036). The deaths occurred mostly during the pre-weaning period and only six calves died after weaning. During the study period, 1592 (96.3%) of the calves were treated with antibiotics at least once. In 90% of the courses, respiratory infections were the cause of antibiotic therapy. The mean antibiotic treatment rate for vaccinated calves (2.3 courses/calf, standard deviation [SD] 1.2) was lower than unvaccinated calves (2.4 courses/calf, SD 1.3) (P = 0.003). The average daily weight gain during the entire study period did not differ between the groups (vaccinated calves 1.08 kg/d, SD 0.12; unvaccinated calves 1.09 kg/d, SD 0.13). The vaccination protocol used in this study decreased the odds ratio for mortality but did not affect ADG. The difference in number of antibiotic treatments used for BRD was clinically negligible. A limitation of the study design is the interpretation of the effect of the historical control group which may affect the results through seasonal variation.