Clinical study of the disease of calves associated with Mycoplasma bovis infection

Clinical, bacteriological and serological examination of 35 calves from the age of 5 to 26 days was performed in a Holstein-Friesian dairy herd endemi cally infected with Mycoplasma bovis. M. bovis was isolated from 48.6% of n asal swabs taken from the calves at the age of 5 days, and from 91.4% of th e same calves at the age of 26 days, indicating the gradual spread of infec tion. The isolation rate of Pasteurella multocida did not change much, and varied from 28.6 to 25.7%. No P. haemolytica could be detected. In addition to M. bovis and P. multocida, the herd was also infected with different vi ruses (including bovine viral diarrhoea virus, infectious bovine rhinotrach eitis virus, bovine adenoviruses, parainfluenza-3 virus, and bovine respira tory syncytial virus) as a large proportion of the sera of newborn calves c ontained colostral antibodies against these viruses. In most of the newborn calves severe clinical signs (fever, depression, inappetence, hyperventila tion, dyspnoea, nasal discharge and coughing) due to M. bovis infection dev eloped. The clinical signs appeared already on the fifth day of life, and t heir incidence was the highest at the age of 10 to 15 days. Three calves (8 .6%) died as a result of severe serofibrinous pneumonia. The surviving calv es showed very poor weight gain (ranging from 1.5 to 3.5 kg) during the fir st two weeks of life.