Chronic non-progressive pneumonia of sheep in New Zealand - a review of the role of Mycoplasma ovipneumoniae

Chronic non-progressive pneumonia (CNP) is a common disease which affects l ambs in New Zealand during late summer and autumn. Mycoplasma ovipneumoniae can be recovered from a high proportion of lesions but it is also present in some normal lungs. Bacteria, especially Pasteurella haemolytica, can als o be recovered from more than half the lungs of affected animals. Isolates of M. ovipneumoniae are genetically heterogeneous, as demonstrated by examination of their DNA or total cellular proteins, and are serologica lly heterogeneous as shown by metabolic inhibition tests. The number of str ains present in New Zealand is large and several distinguishable strains ca n be recovered from each affected lung. Mycoplasma ovipneumoniae has pathog enic potential as indicated by its ability to produce hydrogen peroxide, ca use ciliostasis and by its possession of a capsule. Chronic non-progressive pneumonia can be transmitted consistently to over 5 0% of lambs by inoculation of pooled pneumonic lung homogenate and transmis sion can be suppressed by broad spectrum antibiotics. In contrast, penicill in does not prevent the development of lesions but diminishes their severit y. Pooled lung homogenate treated with digitonin, which inactivates mycopla smas, has failed to transmit CNP. Pure cultures of M. ovipneumoniae produce only mild lesions in some animals, whereas inoculation with pooled lung ho mogenate (from which no viruses were isolated) containing mixed strains of M. ovipneumoniae and free from bacteria, is more effective in producing les ions. Research work to date suggests that CNP may be initiated by colonisation of the lung by M. ovipneumoniae which causes ciliostasis and elicits an exuda te allowing colonisation of the lungs by bacteria especially P. haemolytica and by other strains of M. ovipneumoniae. The immune response to the initi al strain of M. ovipneumoniae may inhibit its replication but would be less effective in inhibiting heterologous strains of the organism allowing thei r sequential replication. Eventually production of a broad immune response to M. ovipneumoniae would lead to its elimination which in turn would facil itate the elimination of other microorganisms and the resolution of lesions . As natural immunity to CNP occurs within the first year, it may be possib le to develop an effective and useful vaccine. Such a vaccine may need to i nclude multiple strains of M. ovipneumoniae.