Author: J Teun Bousema
Position: MSc/Epidemiology
Institution: Department of Medical Microbiology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands
E-mail: t.bousema@ncmls.ru.nl
Additional Authors: Robert W Sauerwein
Submitted Date: October 11, 2005
Published Date: October 11, 2005
This comment was originally posted as a “Reader Response” on the publication date indicated above. All Reader Responses are now available as comments.

We read the article "Malaria Infection Increases Attractiveness of Humans to Mosquitoes" [1] with great interest. Factors associated with mosquito behaviour can lead to promising interventions to reduce malaria transmission. Despite the elegant approach of comparing children before and after clearance of parasites, we feel that this study does not complete the picture of the influence of the malaria parasite on human attractiveness and mosquito biting behaviour, as the authors state. Several shortcomings in study design and recent findings on the prevalence of low density gametocyte carriage undermine the authors' conclusions.

Since the proportion of attracted mosquitoes and the absolute number of mosquitoes are low in all experiments, one should seriously consider all potential confounding factors. The information about some of these is lacking:

i) although some efforts were made, positioning of study subjects in the tents should be controlled for by repeating each individual experiment after re-arranging subjects in different tents [2].
ii) matching of gametocyte carriers and controls on body size, sex and age. Considering the wide age range (3-15 years), and the well-established negative age-dependency of gametocyte prevalence, this could seriously bias study results.
iii) the remarkable choice of Fansidar to clear parasites. Treatment with Fansidar results in poor parasite clearance in the area [3] and it is well established that Fansidar increases gametocyte prevalence and density after treatment. We have recently conducted a study in the same area and found that 80% of the children harbour (low density) gametocytes two weeks after Fansidar treatment for asexual parasitaemia. This will certainly have affected the results.
iv) information on the sensitivity of parasite detection. Parasite densities of the enrolled volunteers could support the hypothesized relation between gametocytes and attractiveness if gametocyte density was positively associated with attractiveness to mosquitoes.

Recent findings with molecular gametocyte detection techniques revealed that the vast majority of gametocyte carriers harbour gametocytes below the microscopic detection limit [4]. The authors conclude correctly that this means that they are in fact comparing individuals with no parasites or a low level of parasitaemia with individuals with more intense infections. Our own unpublished results from the same study area indicate that 80-90% of the children with asexual parasites also harbour (low-density) gametocytes in their peripheral blood. We therefore assume that the majority of children with asexual parasites but no microscopical gametocytes in fact harbour gametocytes below the detection limit. If gametocytes increase the attractiveness of the human host to mosquitoes, this should have also resulted in an increased attractiveness of asexual parasite carriers compared to parasite free children. This is not observed and reduces the likelihood of a true effect of gametocytes on the attractiveness to mosquitoes.

In conclusion, the work by Lacroix and others provides valuable input for the discussion on the influence of gametocytaemia on the attractiveness to mosquitoes but it seems to early to draw firm conclusions on the presented data.

References:
[1] Lacroix R et al. (2005) PLoS Biol 3(9): e298

[2] Mukabana WR et al. (2004) Malar J 3: 1.

[3] Bousema JT et al. (2003) Trop Med Int Health 8: 427-430.

[4] Nassir E et al. (2005)Int J Parasitol 35: 49-55.