Risk Factors

Risk Factors - The rates at which individuals move along the various arrows from one box to another in determine the burden and distribution of disease. Factors which affect those rates, or which affect the proportion of individuals who take each path when an arrow branches, will in turn affect the size and dynamics of the epidemic. These ‘risk factors’ result from inherent characteristics of the biology of the human host and of the mycobacterial pathogen and from characteristics of the environment. Some key risk factors and their effects are shown in . 

such as crowded living conditions

malnutrition

While the association between certain risk factors and TB disease is clear, it is in practice difficult to determine which part of the life cycle of TB is affected by a particular risk factor (e.g. whether it is the risk of infection or the risk of breakdown to disease which is affected). The problem of confounding further complicates the study of risk factors; factors such as crowded living conditions, malnutrition, and exposure to indoor air pollution from cooking fires are clearly all linked to poverty, and difficult to study independently.

The likelihood of transmission depends largely on the proximity and duration of contact with an infectious TB case, and is therefore increased by poorly ventilated, overcrowded housing. People in close contact with infectious cases (family members, health workers, prisoners) are at elevated risk of infection. Susceptibility to tuberculosis may be affected by factors such as tobacco smoking, silicosis, exposure to smoke from cooking fires and excessive alcohol use as well as by HIV infection, but it is difficult in practice to distinguish between the effect of such factors on susceptibility to infection and the likelihood of progression to disease.

Malnutrition influences breakdown to disease,although to what extent requires careful investigation in order to distinguish pre-existing malnutrition from wasting resulting from TB. Nutrition is also to influence the likelihood of recovery from disease. HIV infection has a dramatic effect on the likelihood of breakdown to disease, increasing the likelihood of breakdown from a lifetime risk of between 10% and 20% to an annual risk of over 10%.

Both age and sex have biological and social effects which are difficult to distinguish. The risk of developing primary disease is lower in children than in adults, and children are more likely to develop severe forms of disease in organs other than the lungs (e.g. tuberculous meningitis). Young women (15–44 years) may be more likely than men to develop active TB following infection, but the socially driven effects of gender are generally more marked than the biological effects of sex. Men and women experience different environmental risk factors, and demonstrate different health-seeking behaviours and tendencies to adhere to treatment