# Presentation: 232713

## Advanced Course on AFB Microscopy17 – 28 November 2003Nairobi, KenyaEpidemiology of Tuberculosis

*Dr. John. R. Mansoer, MD, MPH*
*Technical Advisor TB/Leprosy Control*
*CDC/NLTP Kenya*

## Slide 2

## Exposure to tubercle bacilli

## Slide 4

| Risk factors for exposure to
M. tuberculosis |  |
| --- | --- |
| o | Incidence of infectious tuberculosis |
| o | Duration of infectiousness |
| o | Number of cases/contacts interactions per unit of time of infectiousness

-population density
-family size
-climatic conditions
-age of sources of infection
-gender

 |

## Infection with tubercle bacilli

- Transmitted by airborne route
- (M.Bovis: contaminated milk)

## Risk of Infection Given Exposure:

**Risk of Infection Given Exposure:**
**Largely Exogenous Factors**

- Particles
- Volume

- x

- Exposure time

- Particles:
- Volume:
- Exposure time:

- Production of infectious droplet nuclei
- Volume of air and ventilation
- Time of inhaling air with droplet nuclei

## Slide 7

## Infection with tubercle bacilli

## Characteristics of an infectious patient

- 5,000 bacilli/1mL: positive smear
- 10,000 bacilli/1mL: 95% probability positive
- Sputum smear positive patients are the major source of infection in the community
- Sputum smear negative patients are responsible for 15-20% of transmission

## Slide 10

## Clearance of Air

- o Dispersion of bacilli
- o Survival of bacilli

## Slide 12

## Extent of Contact

- o Proximity
- o Length of contact

## Slide 14

## Incidence and prevalence of infection

- Tuberculin survey: imperfect measuring tool?
- Low-income countries: 50% or more of productive population infected
- Industrialized countries: 10% or less of productive population infected
- Globally: approximately 30% of population infected

## Slide 16

| Estimate Disease Burden from Risk of Infection? |  |  |
| --- | --- | --- |
| It has been stated: |  |  |
| 1% risk of infection | <=> | 50 incident sputum smear-positive cases / 100,000 |
| 
Underlying assumptions: |  |  |
| o  10 transmissions / case / year |  |  |
| o  2 years of infectiousness / case |  |  |
| 
Thus: |  |  |
| 100 cases / 100,000 |  | 2,000 transmissions / 100,000 |
|   50 cases / 100,000 |  | 1% risk of infection |

## Tuberculosis Disease

- Major risk factors:
- Time since infection
- HIV infection
- Untreated/poorly treated previous TB
- Other factors:
- 		-age
- 		-gender
- 		-malnutrition
- 		-diabetes
- 		-silicosis
- 		-genetic factors

## Slide 18

## Slide 19

## Slide 20

## Slide 21

## Slide 22

## Slide 23

## Slide 24

| Public Health Importance of a Risk Factor |
| --- |
|   There are two elements that make a risk factor to be important for public health: the prevalence of the factor in the population and the strength of the factor |
|   It is a bit of a judgment call, but we consider recent infection to be the most important on this list.  Everywhere in the world, whenever a new case of infectious tuberculosis emerges, the diagnosis and commencement of treatment are usually only made after that case has already infected one or more other persons. |
| HIV infection is certainly the most important risk factor in many countries of sub-Saharan Africa, but it is not (yet) the case in India and China which account together for about one third of the world population.
 |
|  |

## HIV infection and tuberculosis disease

- TB infected/HIV-: 5-10% life time risk to progress to TB disease
- TB infected/HIV+: 5% annual risk to progress to TB disease

## Determinants for the Frequency of HIV-Associated Tuberculosis in a Community

- Total population

- Infected with
*M. tuberculosis*

- Infected with HIV

- Prevalence and incidence of        infection with *M. tuberculosis*
- Prevalence and incidence
- of HIV infection
- Overlap of the two respective
- population segments

## Slide 27

| Impact of HIV on the Epidemiology of Tuberculosis |  |  |
| --- | --- | --- |
| Direct: | o | Reactivation of tuberculous infection acquired before HIV infection |
|  | o | Progression of tuberculous infection acquired after HIV infection |
| Indirect: | o | Transmission to the population not infected with HIV |
| Sutherland I.  Br Comm Dis Rep 1990:10 |  |  |

## Slide 28

## Slide 29

## TB incidence and HIV prevalence in Kenya

**Notes:**
The above shows one of the scenarios considered for Kenya, where HIV remains constant at its 1999 level.  We also considered scenarios in which HIV was reduced to 50% of its 1999 level, in the long term and increased to 150% of its 1999 level in the long term.  Here, the paler blue curve shows the HIV, and the darker blue curve the TB incidence, with associated error bars and data.

## Slide 31

## Slide 32

| Risk Factors for Death from Tuberculosis |  |
| --- | --- |
| o | Site and form of tuberculosis |
| o | Patient’s and doctor’s delay |

## Slide 33

## Sources

- Epidemiologic basis of tuberculosis control –
- 	*Hans Rieder/IUATLD* *(1-st Edition 1999)*
- NLTP Kenya: Annual Reports
- NLTP Kenya/WHO: research data *(not yet published)*