Strains of TB are similar in HIV-positive and HIV-negative individuals

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Strains of tuberculosis (TB) in HIV-positive patients are no more virulent than those in HIV-negative individuals and are genetically similar, according to a study published in the January 11th edition of AIDS. It has been speculated that the immune suppression caused by HIV would allow HIV-positive individuals to become infected with unusual strains of TB, but investigators in Burkina Faso found that this was not the case and are calling for further research into this question.

The incidence of tuberculosis has been increasing in Africa since 1990, and in 2003 it was estimated that there were two million new infections with the disease and 600,000 tuberculosis deaths. In South Africa, 30-40% of all HIV-related deaths are thought to be caused by tuberculosis.

Investigators have hypothesised that immune dysfunction caused by HIV would lead to infection with “peculiar” genotypes of tuberculosis. To test this hypothesis, doctors in Burkina Faso conducted a cross-sectional study involving 120 patients with tuberculosis, 43 of whom had HIV. The study was conducted in 2003.

Glossary

strain

A variant characterised by a specific genotype.

 

virulence

The power of bacteria or viruses to cause a disease. Different strains of the same micro-organism can vary in virulence.

 

hypothesis

A tentative explanation for an observation, phenomenon, or scientific problem. The purpose of a research study is to test whether the hypothesis is true or not.

cross-sectional study

A ‘snapshot’ study in which information is collected on people at one point in time. See also ‘longitudinal’.

immune system

The body's mechanisms for fighting infections and eradicating dysfunctional cells.

They found that tuberculosis genotypes did not differ significantly between HIV-positive and HIV-negative patients.

“It has been speculated that HIV/AIDS patients constitute a specific niche for tuberculosis, in which less virulent strains could multiply without selection pressure provided by a competent immune system. The present results do not agree with these hypotheses”, write the investigators.

A number of possible reasons are offered by the investigators for the lack of genetic difference in tuberculosis strains between HIV-positive and HIV-negative patients. In particular, they suggest that the HIV patients could have had enough immune function intact to effectively select infecting tuberculosis strains. An alternative explanation offered by the investigators is that the HIV-positive patients were experiencing an activation of tuberculosis acquired before their infection with HIV when they were still immune competent.

They conclude, “studies such as this should be replicated or extended before one can conclude that HIV-1-infected patients should be considered as a niche for tuberculosis strains with particular virulence or viral characteristics.”

References

Godreuil S et al. Genetic diversity and population structure of mycobacterium tuberculosis in HIV-1-infected compared with uninfected individuals in Burkina Faso. AIDS 21: 248 - 250, 2007.