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Archived Comments for: Is the current decline in malaria burden in sub-Saharan Africa due to a decrease in vector population?

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  1. Could mosquitoes be dying from malaria?

    Frank Ringsted, Dept. International Heath, ISIM, University Cph,

    29 August 2011

    The observed decline in malaria transmission and vector population occurs simultaneously with the spread of SP drug resistance. Could it be considered if the changes in the genetic profile of the parasite has become maladaptive for the parasite vector symbiosis, with altered parasite virulence eventually extinguishing the vector?

    Competing interests

    none declared

  2. A possible explanation for the decline: ivermectin from MDAs for LF

    Edward Walker, Michigan State University

    29 August 2011

    This interesting paper reveals a long term, downward trend in relative densities of Anopheles gambiae s.l. and Anopheles funestus in a region of northeastern Tanzania where vector control measures specifically focusing on malaria control have not previously been implemented intensively. The authors conclude that the decline is therefore not due to any specific chemical intervention against the malaria vector populations and must have some other cause.

    Ironically, this same research team is studying control of lymphatic filariasis at the same study sites, and have elsewhere documented significant declines in prevalence of microfilaremia, due to Wuchereria bancrofti infection, through well done longitudinal studies. Importantly, that process has been effected by mass distribution of a combination of two drugs: ivermectin and albendazole. Recently, several studies have shown that ivermectin has lethal and sublethal effects, mediated through the blood meal and in the ppb concentration, on Anopheles vectors of malaria in Africa; and also reduces vector survival and therefore vectorial capacity (see references below). It is indeed surprising that the authors here did not attribute the vector population decline to the effects of constant exposure to ivermectin in human blood meals, a decidedly strong chemical intervention whose effects would be predicted to achieve long term population declines such as were observed. That the Culex quinquefasciatus population under study did not decline in the same manner is likely because females of this species have considerably greater host breadth than either Anopheles funestus or Anopheles gambiae and so would have less overall exposure to ivermectin.

    Fritz ML, Siegert PY, Walker ED, Bayoh MN, Vulule JR, Miller JR. Toxicity of bloodmeals from ivermectin-treated cattle to Anopheles gambiae s.l. Annals Trop Med Parasitol 2009; 103: 539-547.
    Kobylinski KC, Sylla M, Chapman PL, Sarr MD, Foy BD. Ivermectin mass drug administration to humans disrupts malaria parasite ransmission in Senegalese villages. American Journal of Tropical Medicine and Hygiene 2011; 85: 3-5.
    Foy BD, Kobylinski KC, Marques de Silva I, Rasgon JL, Sylla M. Endectocides for malaria control. Trends in Parasitology 2011; (article in press).

    Competing interests

    No competing interests

  3. Possible effects of MDA

    Dan Wolf Meyrowitsch, University of Copenhagen, Denmark

    18 October 2011

    Dear Dr. Walker,

    Thank you for the comment to our recent paper, and for the interesting suggestion that ongoing Mass Drug Administration (MDA) with ivermectin applied in the study area for control of lymphatic filariasis might be the course of the observed downward trend in the Anopheles populations.

    We acknowledge that ivermectin treatment may have contributed to the observed decline, but find it highly unlikely that a single annual MDA can be the only/major course of this dramatic reduction. First, the vectors were not exposed constantly to ivermectin, as indicated in your comment, but only during a few days each year. Second, the treatment coverage during each MDA was generally well below 80% of the eligible population (i.e. below 60% of the total population). Third, major declines were also observed in the years before the start of MDAs (late 2004). We do find the studies of Kobylinski et al. (2011) on the effect of ivermectin treatment on malaria transmission very interesting, but as also indicated in the paper of Foy et al. (in press) such treatment most likely needs to be given with short intervals to exert a significant effect on the Anopheles population (by the way, both of these papers became available after our paper was submitted to Malaria Journal). However, it is definitely an important issue that needs to be looked further into, and we are currently breaking down our time series of observations in order to analyse for a possible relationship between the timing of MDAs and the decline in Anopheles population.

    Finally we would like to emphasize that if MDAs with ivermectin contributed to the decline in Anopheles population there is reason to be worried for the consequences after stop of the lymphatic filariasis or onchocerciasis MDA campaigns. Will the Anopheles vectors return, and will this result in dramatic malaria epidemics with many fatalities?

    Best wishes,

    Paul Erik Simonsen and Dan Meyrowitsch

    Competing interests

    None declared