The infectiveness of bovine tuberculosis

Abstract

Sir, I must make a brief response to Tumbull's suggestion (1995 Animal Welfare 3:340) that I am misinformed in my belief that cattle are infectious immediately after exposure to the tuberculosis organism. There seem to be two schools of thought on this issue, and yet it is one of critical importance in tackling the final stages of tuberculosis eradication schemes. On the one hand it is claimed that only cattle with gross visible lesions at abattoir inspection are capable of passing on tuberculosis (TB) to other cattle (Gallagher 1980; Dunnetet al1986; Tumbull 1994; Wilesmith & Williams 1986). On the other hand this view is not substantiated by other studies on the aetiology and pathogenesis of cattle TB. For most adult cattle, tuberculosis seems to start as a lung infection following inhalation of infected aerosolised sputum or dust. Primary lesions may heal, but more usually they remain open, and may persist as the subclinically latent condition, or progress slowly or rapidly to chronic, or fatal and acute bronchopneumonia. And even where an apparent sealed tubercle develops, it would seem that intracanalicular bronchiolar spread continues, such that intermittent or continuous bacterial shedding occurs in the sputum. Therefore cattle with micro-lesions that would be missed at gross abattoir inspection could be infectious to other cattle despite being non-visibly-lesioned in the lungs or visibly-lesioned only in bronchomediastinal lymph nodes. Such cattle may also be producing infectious faeces via swallowed sputum (Bloodet al1979; Francis 1947; Jubb & Kennedy 1970; Neillet al1994; Richards 1972).