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Clinical, bacteriological and serological examination of 35 calves from the age of 5 to 26 days was performed in a Holstein-Friesian dairy herd endemically infected with Mycoplasma bovis. M. bovis was isolated from 48.6% of nasal swabs taken from the calves at the age of 5 days, and from 91.4% of the same calves at the age of 26 days, indicating the gradual spread of infection. 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 viruses (including bovine viral diarrhoea virus, infectious bovine rhinotracheitis virus, bovine adenoviruses, parainfluenza-3 virus, and bovine respiratory syncytial virus) as a large proportion of the sera of newborn calves contained colostral antibodies against these viruses. In most of the newborn calves severe clinical signs (fever, depression, inappetence, hyperventilation, dyspnoea, nasal discharge and coughing) due to M. bovis infection developed. The clinical signs appeared already on the fifth day of life, and their 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 calves showed very poor weight gain (ranging from 1.5 to 3.5 kg) during the first two weeks of life.

In a blind trial, alternate calves in six consecutive production batches of calves (total 70), on a farm with a high incidence of respiratory and reproductive disease, were allocated to treatment with either valnemulin or a placebo premix added to the milk from four days of age. The calves were weighed at the beginning and end of a 21-day period of medication. Blood samples and nasal swabs were taken and examined for the presence of Mycoplasma and Pasteurella species, and antibodies to viral agents. Clinical condition, rectal temperature, respiratory and other signs and refusals of milk were recorded daily. Dead calves were examined postmortem. The calves medicated with valnemulin gained weight more quickly, had fewer cases of Mycoplasma infection and fewer respiratory signs, and required fewer treatments with antibiotics than those in the placebo group.

The minimum inhibitory concentrations (MICs) for valnemulin, tiamulin, enrofloxacin, tylosin, and lincomycin/spectinomycin were determined for a virulent strain of Mycoplasma gallispeticum (MG). At the initial reading, the lowest MICs were seen with valnemulin and tiamulin, followed by tylosin, enrofloxacin, and a relatively high MIC for lincomycin/spectinomycin. At the final reading, at 14 days, a similar pattern was obtained, with valnemulin giving the lowest MIC (< 0.008 mg/ml). The same strain of MG was used to infect groups of 20 2-day-old chicks in two separate experiments. In both, several concentrations of valnemulin and tiamulin and one each of tylosin and enrofloxacin were administered to separate groups in the drinking water. In the second experiment, one group of chicks was given lincomycin/spectinomycin. Each experiment had one infected unmedicated group and an uninfected unmedicated group. Mortality, clinical signs, and gross lesions, in both experiments, were significantly less (P < 0.001) in the uninfected and infected medicated groups (except for the two lowest dosages of valnemulin, lincomycin, and spectinomycin) than in the infected unmedicated groups. Also, the mean body weight gain was greater in the uninfected and infected medicated groups. Among the infected birds, MG was recovered from fewer chicks in the infected medicated groups except for the lowest two dosages of valnemulin. Serologic results were negative for the uninfected groups, and there were fewer positive reactors for the infected medicated groups except for the group treated with lincomycin/spectinomycin. Valnemulin should prove to be a useful addition to the antimicrobials in the control of MG infection in chickens.