Long-term pathological and micro-biological progress in sheep of experimental disease resembling atypical pneumonia

Abstract

Fifteen lambs aged 76 to 116 days were injected intra-tracheally with a lung suspension containing Mycoplasma ovipneumoniae and Pasteurella haemolytica serotype A2. Coughing, dyspnoea and temperature rise followed, and a chest radiograph at 3 weeks post-inoculation (wpi) demonstrated opacity in the right apical lobe of the lung in all animals. Groups of 2 or 3 sheep were killed at approximately 4-week intervals from 3 to 28 wpi. The extent of lung lesions was significantly lower in lambs killed from 15 wpi onwards than in those at earlier stages. The lesion consisted of a proliferative exudative pneumonia indistinguishable from atypical pneumonia. Of the component changes, the neutrophil exudate was absent from 15 wpi, bronchiolar epithelial hyperplasia and the macrophage exudate were reduced in severity from 15 to 18 wpi onwards, hyaline scars diminished in frequency from 15 wpi, but well-developed lymphoid hyperplasia persisted from 7 wpi throughout. Though virus and chlamydia were sought, the only organisms recovered from lungs were M. ovipneumoniae, which was present in every animal, and P. haemolytica, which was recovered only up to 11 wpi. Sera collected pre-inoculation and at necropsy and examined by a micro enzyme-linked immunosorbent assay, demonstrated an increase in antibody to both organisms in all animals except one killed at 3 wpi in which P. haemolytica had probably not become established. We conclude that the lesions of this experimental pneumonia, and thus of atypical pneumonia, may decrease in severity after 2 or 3 months. Nevertheless, consolidation may persist for at least 7 months following infection, and this may have implications for a reduced performance in fattening lambs. A cause and effect relationship is suggested by the correlation with P. haemolytica and the neutrophil exudate, but the evidence is less definite for a similar relationship between M. ovipneumoniae and other components of the lesion. It is probable that the 2 types of lesion described in atypical pneumonia by Stamp and Nisbet (1963) separately reflect early and late stages of the disease.