I read with interest the paper by Michel Cot and colleagues (see Treating parasitic worm could increase malaria risk) but do not feel that the results reported are in "complete disagreement" with other studies, as stated by Cot. The situation is as follows:

1. Eight of nine distinct studies performed by us and others indicated a worsening of clinical malaria by worm co-infection. A further four studies performed more recently, and presented at the Multilateral Initiative on Malaria in Yaoundé, Cameroon (November 2005), also showed a worsening of malaria by worms.

2. Based on a single study, Cot and colleagues found a different outcome (a marginal benefit against malaria in wormy individuals) and only in older but not in younger children.

We do not feel there is a contradiction since we had stressed that the interaction will surely prove more complex than initially foreseen.

Indeed, our study on schistosomiasis indicated that worms had an opposite effect on malaria infection between very low and medium to high worm loads. Hence the possibility of opposite effects is not new. The influence of the worm load is very likely (as this is how children differ most from adults) but has not been addressed by other groups to date.

Cot and colleagues followed individuals during a single year, whereas in the only other intervention study the treatment of worms became beneficial only during the second year after de-worming. Moreover, Cot's team did not measure worm loads. The occurrence of a beneficial effect in older children may be an indication that the latter carried fewer worms.

Finally, their study relies on a parasitological criterion (parasite densities) not on a clinical one (malaria attacks).

These observations merely stress the need for further studies, which was actually the main goal of our report in Trends in Parasitology. There remains certainly a lot more to do.

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