Influence of continued asbestos exposure on the outcome of asbestosis in sheep.

Clinical recognition of asbestosis is generally followed by exclusion of the worker from the active workforce and appropriate compensation. Given the better industrial hygiene conditions of the 1990s, and a recent report suggesting that low-level asbestos exposure may not affect progression of asbestosis, the medical rationale to pension asbestos workers with early asbestosis has been questioned in the legal courts. To study the influence of asbestos exposure after established asbestosis, we enrolled 25 sheep. Ten were normal controls (group A) and 15 had asbestosis, after 2 years of repeated 10-day interval, 100-mg chrysotile exposure (Eur Respir J 3:81-90, 1990). The asbestotic sheep were divided into a group of five without further asbestos exposure (group B), a group of five with continued exposure at induction doses (group C), and a group of five with continued exposure of 10% induction doses (group D). All were followed by chest radiography, lung function, and lung lavage at 3-month intervals and histopathology when appropriate. During the 12-month study, mortality occurred only in groups C and D (40% in both, P less than .02). Chest radiograph and lung function were worse in groups C and D. The enhanced macrophagic and neutrophilic alveolitis observed on lung lavage and the pathological findings in the deceased sheep suggested progression of asbestosis and bacterial infections as the major contributors to the higher mortality of sheep in groups C and D. In conclusion, asbestos exposure after disease recognition was associated with increased mortality in our sheep and this was largely associated with progression of asbestosis and added lung infection.