Page 197 - fourth year book

COMMUNICABLE DISEASES (T.B)

Risk factors of TB disease

The epidemiologic triad of disease causal factors explain agent,
host and environmental factors interaction for TB disease occurrence as
follows: -

1- Agent factors

 Bacillus is hardy-against weather, chemicals but can be killed in
sunlight.
 Highly infective bacilli, even 1-5 bacilli can infect.
 A smear positive case with high bacillary load
 Exposure to bacilli infects 30-60% of exposed people.

 Tend to stay dormant in human body.

2-Host factors

Factors that increase the progression of infection to TB disease are
primarily endogenous (host related): -

a. Young age especially those > 2 years

b. Male gender

c. Immunosuppressive conditions: -

- HIV coinfection, diabetes Malnutrition, Malignancy, silicosis, renal
disease, celiac disease, gastrectomy, transplantation

- Prolonged use of immunosuppressive drugs (corticosteroids and
tumor necrosis factor inhibitors)

- People who were not treated correctly for TB in the past

d. Socioeconomic and Behavioral Factors

- Low socioeconomic status

- Tobacco Smoke

- Alcohol and drug abuse

3- Environmental factors

a. Indoor air pollution
b. Overcrowding and poor ventilation as proximity and long
contact of infectious TB cases is a great issue
c. Settings with increased chances of social mixing (hospitals,
homeless shelters and nursing homes)

166