Epidemiologic and pathologic characteristics of respiratory tract disease in dairy heifers during the first three months of life

Abstract

Objective - To describe the incidence of respiratory tract disease in dairy calves and to compare antibody titers and microbial isolates from transtracheal wash samples between calves with and without respiratory tract disease (cases and controls, respectively). Design - Prospective observational cohort study, with matched case-control substudy. Animals - 410 dairy heifers; in substudy, 105 cases and 59 controls from the same population. Procedure - Calves were examined weekly by a veterinarian during the first 3 months of life. Blood samples were collected for serologic testing at the first visit for each calf and during acute and convalescent periods for cases. Transtracheal wash samples also were obtained during the acute period from cases and controls. Results - Incidence and case-fatality risk for clinician-diagnosed pneumonia were 25.6 and 2.2%, respectively. Mycoplasma sp and Pasteurella multocida together were isolated from 29% of cases and 11% of controls, and Mycoplasma sp alone from 7% of cases and 30% of controls (both P ≤ 0.05). From postcolostral to acute-phase serum samples, Mycoplasma dispar titers increased 1.3-fold among cases, compared with 0.7-fold among controls; from acute- to convalescent-phase samples, M dispar titers increased 2.4-fold among cases, compared with 5.6-fold among controls (both P ≤ 0.005). Clinical Implications - Results of this study suggested a synergistic effect between Mycoplasma sp and P multocida and a possible initiative role of M dispar in the development of respiratory tract disease. Postcolostral total IgG values and antibody titers were not significantly different between cases and controls, implying that other factors have an important role in the development of respiratory tract disease.