Epidemiology of Bovine Respiratory Disease Complex in California Dairy Calves

Bovine respiratory disease complex (BRDC) is a major cause of illness, lost productivity and welfare concern for the US dairy industry. Research from the last 25 years has produced new treatments, vaccines, and diagnostics, yet BRDC remains a major source of economic loss. The current research is the foundation of a risk assessment tool for BRDC in California dairies. A new clinical scoring system was developed to rapidly and accurately detect BRDC using data from 2,030 Holstein dairy calves enrolled in a pair-matched case-control study. The novel scoring system, labeled the "CA BRD system", uses nasal discharge, ocular discharge, spontaneous coughing, rectal temperature, respiratory character, and head and ear position to detect BRDC. The CA BRD system was validated in a nested-case control study of 536 calves from 3 dairies and 2 calf ranches in the central San Joaquin Valley. Auscultation, thoracic ultrasound, nasal culture and viral PCR were used to diagnose BRDC. The CA BRD system's estimated screening sensitivity was 46.8% for all prevalent BRDC cases. The system's estimated sensitivity was 72.6% for clinically apparent BRDC cases. Overall specificity was 87.4%. Finally, a questionnaire of calf-raising practices was distributed to all Grade A milk-producing dairies in California to describe current calf-raising practices. A total of 224 usable responses were collected (response rate 14.7%). Northern California dairies significantly differed from dairies in other parts of the state in terms of herd size, percent Holstein breed in the herd, time calves spent with their dams after birth, and proportion of certified organic dairies. Dairies in the Northern San Joaquin Valley had significantly smaller average herd sizes than Southern California herds. Principal component analysis of the questionnaire results identified 11 components that described 66.5% of the variance in the data. The results of these studies will be used to estimate the prevalence of BRDC on California dairies and collect information about husbandry practices to develop a risk assessment tool for BRDC in pre-weaned dairy calves.