‘Age has a significant effect on the immune system in childhood TB’

Peter R. Donald - R. Prasad

Prof. Peter Donald

Prof Peter R. Donald, Emeritus Professor in the Department of Paediatrics and Child Health of the Faculty of Health Sciences at Stellenbosch University, South Africa was awarded the highest honour, the Union Medal, of the International Union Against Tuberculosis and Lung Disease, at the 2010 General Assembly in Berlin.

This medal is given to people who have made outstanding contributions to tuberculosis and non-tuberculosis lung disease. Prof. Donald was recognised for his contributions to child lung health and especially childhood tuberculosis

Prof. Donald explained to me by email why childhood TB is a family disease in those aged less than five years and not every child in the same age band gets infected.  Excerpts: 

You note that children in the 0-1 age group ran the highest risk of dying followed by children aged 1-3 years when TB medicines were not available. How can you be sure that death was caused by TB?

The clinicians working at that time had available chest radiography, tuberculin skin testing and cultures for Mycobacterium tuberculosis. This in addition to their clinical findings would have enabled them to make a fairly accurate diagnosis of TB.

Considering that children older than five years have greater contact with the community, can childhood TB still be considered as a family disease?

The older the child, the less likely it becomes that the family is the source of the tuberculosis. However, in young children, who have a very high incidence of disease after infection, it is very important to look for a history of contact in the family or household who will most often turn out to be the source of infection.

Your 2006 paper indicates that unlike the 0-1 age group, the 4-5 age group is less vulnerable. But the latter age group has greater contact outside the family and hence have greater chances of getting infected. Could you explain?

Tuberculosis infection in older children 4-10 years is much less likely to be followed by disease than in the younger children or adolescents.

Unlike smear-negative pulmonary TB, why is the risk of transmission considered to be higher when an adult has sputum smear-positive pulmonary TB?

This is most likely due to the numbers of bacilli in the sputum when it is smear-positive. Smear negative sputum may contain 103 bacilli while smear positive sputum may contain 105 bacilli.

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LISTEN NOW: Prof. Peter Donald talks about why children under the age of five years run 30-40 per cent risk of progressing to a diseased state.

Are there studies to show that young children are at greater risk when the mother rather than the father has active pulmonary TB?

There is little doubt about this and many studies confirm the risk of infection and subsequent disease in young close contacts

Why is a breastfeeding infant considered to be at higher risk of getting infected and progressing to a diseased state when the mother has smear-positive pulmonary TB?

I think this probably has to do with the closeness of contact with the mother as much as anything else

Not every child less than five years of age gets infected when exposed to an adult in the same household with sputum smear-positive pulmonary TB. What is the reason for this?

There is no sound answer to this; it may be chance or it may be genetics, but we do not know at present.

Is it true that in some cases primary childhood TB can “cure” itself or convert from active TB into a state of latent infection even without treatment? What’s the reason for this?

Yes, it is true. Even in adult-type TB, many adults survived pulmonary tuberculosis before treatment was available. If an adult had smear positive tuberculosis before treatment was available approximately 50 per cent of individuals died, 25 per cent recovered completely and 25 per cent became chronic cases.

If that is the case, is it prudent to put every infected child aged under five years on prophylactic treatment?

This will depend on what you consider to be an acceptable risk of developing a possibly serious form of tuberculosis. It will also depend upon where you work.

A five per cent to 10 per cent risk of disease will probably make it worthwhile to give prophylaxis to a child. In a developed country even a positive tuberculin test might be considered an indication for prophylaxis at any age.

In a developing country as children get older it is more and more difficult to know who is recently infected so that we tend to draw a line regarding using chemoprophylaxis at five years of age and we emphasise that the younger the child the more urgent chemoprophylaxis is.

Not only is the risk of disease after infection higher the younger the child, but infection will also be more recent and we also know that the greatest risk of disease developing is soon after infection. In children with HIV-infection contact with TB at any age might be an indication for chemoprophylaxis.

Your 2004 paper “Childhood tuberculosis: the hidden epidemic” mentions that TB in children between 5-10 years of age could be considered as a “benign disease.” Could you explain?

Like much else in tuberculosis, and other diseases, age has a significant effect on the immune system, the consequence of which is the variable response to infection seen very clearly in tuberculosis. Chicken pox, for example, in children is a very mild disease usually, but can be very severe in adults.

Why does the risk of infection shoot up at puberty? Considering that a large percentage of children <5 years progress from the infected to a diseased state within two years, what causes this peaking at puberty?

Again, all we know is that this does happen and that it is more likely in females than males. This must be linked to the endocrinological changes of adolescence, but what these are precisely remain unknown.

(The Correspondent is a recipient of the 2013 REACH Lilly MDR-TB Partnership National Media Fellowship for Reporting on TB.)

Published in The Hindu on November 14, 2013

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