Tuberculosis

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Worldwide, half of all individuals infected with HIV are also infected with TB.

Tuberculosis (TB) and AIDS, the most important infectious causes of death in the world, share many similarities and contrasts in demography, transmission and pathogenesis.

The huge research interest in HIV has meant that more is known about this virus, its dynamics and pathogenesis than any other.

Incontrast, although TB has been a scourge to humanity for thousands of years, much less is known about the basic pathogenetic mechanisms which cause the disease.

One-third of all HIV deaths are attributable to TB, and the lifetime risk of an HIV positive person developing TB is 50% compared with 5–10% for an uninfected individual.

In an individual previously infected with TB (Mantoux positive), the lifetime risk of reactivation is 5–7%, in contrast to an annual 10% risk in patients co-infected with HIV.

The main reason for the strong association between the two diseases has been considered to be the high rate of reactivation.

However, since TB and HIV flourish in similar poor socio-economic circumstances, it is likely that another important cause is recent infection, with the development of active disease as a result of immunosuppression.

In Africa, TB tends to occur in HIV infected individuals with high CD4 counts, and is likely to be a reflection of progressive primary disease.

In contrast, in the USA, the median CD4 count at presentation with TB is low (<200 cells/mm3) and even lower for disseminated disease (<100 cells/mm3), probably reflecting the reactivation of quiescent primary infection in severely immunosuppressed patients.

TB in an HIV positive individual is associated with falls in the CD4 count and rises in the viral load. These changes are not specific to TB – any infection causing activation of HIV via cellular mechanisms, including Nkb and changes in the cytokine milieu which favour HIV replication, will produce similar effects. There is no epidemiological evidence that TB worsens the prognosis of HIV.

Similarly, TB prevention in the form of chemoprophylaxis may not improve the overall mortality of HIV. Nevertheless, control measures directed against TB would have a marked benefit on the health of the HIV population worldwide.

Key Points

***The HIV epidemic has been associated with an enormous increase in the pandemic of tuberculosis (TB)

***In a minority of patients with HIV, a particular genetic constitution allows long-term control of infection. This is likely to be true of TB

***Mycobacterium tuberculosis is an organism that grows in macrophages. The induced cellular and cytokine responses are responsible not only for disease control or progression but also for the pathogenesis of the infection

***Successful treatment of HIV and TB infection requires long term adherence to multi drug regimens in order to avoid drug resistance

Atypical TB