Block 5: TB and occupation in South Africa - Radiology Of Silica, Silicosis And Tuberculosis:

RADIOLOGY OF SILICA, SILICOSIS AND TUBERCULOSIS:

The combined effects of silica and tuberculosis will be described, beginning with their pathology and moving on to comparing the classic radiological features of tuberculosis and pneumoconiosis, emphasizing the similarities and the differences. Complicated silicosis and tuberculosis will further be discussed.

This discussion is inspired and draws illustration from the work of:

• Prof Albert Solomon.
• Prof Ray Glyn Thomas.
• Dr Jill Murray.
• Dr Gavin Churchyard.

Complex relationships between the lung and environmental factors are played out in the radiology of lung disease in the current South African context. These include the nature of silica, its retention by the lung and its interaction with lung tissue. This interaction not only impairs the lung’s defences against M. tuberculosis, but results in modification of the natural history of tuberculosis (higher mortality, even with treatment) and silicosis (greater risk of P.M.F.).

Interaction between silica and tuberculosis

Silica exposure alone (without overt radiological silicosis) increases the risk of PTB (3 - 6 times). This risk persists after exposure ceases. Silica exposure can influence the radiological appearance of tuberculosis (? more likely to be nodular)

Interaction between silicosis and tuberculosis:

Silicosis increases the risk of tuberculosis, relative to non-exposed and silica exposed individuals. Risk increases with silicotic profusion. In silicotics tuberculosis can induce new changes that mimic the natural history of silicosis:

• Aggregation;
• Coalesence;
• Progressive Massive Fibrosis (PMF).

Interaction between tuberculosis and silicosis:

An episode of tuberculosis can alter the natural history of silicosis:

• Silicosis first becomes apparent during or after an episode of tuberculosis.
• PMF more likely after an episode of tuberculosis.

Silicotic fibrosis may provide a "haven" for M.Tb to commence replicating.

Interaction between HIV, TB and silicosis:

• HIV and silicosis are synergistic for TB risk.
• Silica exposure and HIV increase TB risk.
• HIV increases the variety of PTB appearances (nodes, effusions, infiltrates).
• HIV increases the proportion of smear negative PTB cases as well as extra-pulmonary TB.
• HIV and silicosis both increase mortality of people being treated for TB.