Histopathology

Histologically the lesions can vary from typical tubercle formation to poorly organised lesions composed mainly of necrotic tissue.

Lake (1974) describes the typical areas of nodular consolidation as being composed of granulomatous tissue, with no distinctive fibrous capsule and with varying amounts of amorphous eosinophilic debris centrally. Reactive infiltrating cells included lymphocytes, plasma cells, an occasional eosinophil, macrophages, giant cells and many neutrophils. Neutrophils were in greatest concentration around the area of caseous material. No mineralisation was observed. Acid-fast bacteria were often present in large numbers; both free and phagocytosed in macrophages. Smith (1972) described the reaction as being mainly suppurative and estimated that approximately 5 × 10⁹ organisms were present in 1 gm of tissue.

Typical tubercles with epithelioid cells, Langhans giant cells and capsule formation have been seen on rare occasions.

Preliminary observations from experimental work (O'Hara et al. 1976) have suggested that the earliest lesions appear to be small foci of necrosis of the invaded tissue infiltrated by neutrophils. These foci are invaded by macrophages which accumulate in considerable numbers but appear to be randomly arranged and do not organize as epithelioid cells as seen in other species. At this stage small to moderate numbers of bacilli are present. Coagulative necrosis occurs at the centre of the macrophage nodules, the necrotic tissue becomes caseous and bacilli are usually more numerous. While caseation does occur, there is usually a substantial amount of coagulative necrosis indicating that necrosis is proceeding at a rapid rate. The granuloma does not acquire a significant capsule and the formation of satellite granulomas is common. The large lesions found in the lungs and lymph nodes consist of large necrotic centres surrounded by a narrow rim of macrophages and neutrophils. Liquefaction is common in these large lesions and at this stage the number of bacilli present is reduced.

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The limited cellular response, lack of organisation of the response, extensive tissue necrosis and high rate of multiplication of bacilli are the histological hallmarks of a susceptible host.