Epidemiologic and pathologic characteristics of respiratory tract disease in dairy heifers during the first three months of life.
410 dairy calves were examined weekly to study the development of disease during the first 3 months of life. Blood samples were collected for serological testing at the first visit for each calf and during acute and convalescent periods for calves with pneumonia. Transtracheal wash samples were obtained during the acute period from cases and controls. Incidence and case-fatality risk for 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. From post-colostral to acute-phase, M. dispar titres increased 1.3-fold among cases, compared with 0.7-fold among controls; from acute- to convalescent-phase, M. dispar titres increased 2.4-fold among cases, compared with 5.6-fold among controls. A synergistic effect between Mycoplasma sp. and P. multocida is suggested and a possible initiative role of M. dispar in the development of respiratory tract disease. Post-colostral total IgG values and antibody titres were not significantly different between cases and controls, implying that other factors have an important role in the development of respiratory tract disease.