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Most outdoor malaria transmission by behaviourally-resistant Anopheles arabiensis is mediated by mosquitoes that have previously been inside houses.

Killeen GF, Govella NJ, Lwetoijera DW, Okumu FO - Malar. J. (2016)

Bottom Line: Life histories of a well-characterized An. arabiensis population were simulated with a simple but process-explicit deterministic model and relevance to other vectors examined through sensitivity analysis.The estimated proportion of vector blood meals ultimately obtained from humans indoors is dramatically attenuated by availability of alternative hosts, or partial ability to attack humans outdoors.While the exact numerical results predicted by such a simple deterministic model should be considered only approximate and illustrative, the derived conclusions are remarkably insensitive to substantive deviations from the input parameter values measured for this particular An. arabiensis population.

View Article: PubMed Central - PubMed

Affiliation: Environmental Health and Ecological Sciences Thematic Group, Ifakara Health Institute, Ifakara, Kilombero, Morogoro, United Republic of Tanzania. gkilleen@ihi.or.tz.

ABSTRACT

Background: Anopheles arabiensis is stereotypical of diverse vectors that mediate residual malaria transmission globally, because it can feed outdoors upon humans or cattle, or enter but then rapidly exit houses without fatal exposure to insecticidal nets or sprays.

Methods: Life histories of a well-characterized An. arabiensis population were simulated with a simple but process-explicit deterministic model and relevance to other vectors examined through sensitivity analysis.

Results: Where most humans use bed nets, two thirds of An. arabiensis blood feeds and half of malaria transmission events were estimated to occur outdoors. However, it was also estimated that most successful feeds and almost all (>98 %) transmission events are preceded by unsuccessful attempts to attack humans indoors. The estimated proportion of vector blood meals ultimately obtained from humans indoors is dramatically attenuated by availability of alternative hosts, or partial ability to attack humans outdoors. However, the estimated proportion of mosquitoes old enough to transmit malaria, and which have previously entered a house at least once, is far less sensitive to both variables. For vectors with similarly modest preference for cattle over humans and similar ability to evade fatal indoor insecticide exposure once indoors, >80 % of predicted feeding events by mosquitoes old enough to transmit malaria are preceded by at least one house entry event, so long as ≥40 % of attempts to attack humans occur indoors and humans outnumber cattle ≥4-fold.

Conclusions: While the exact numerical results predicted by such a simple deterministic model should be considered only approximate and illustrative, the derived conclusions are remarkably insensitive to substantive deviations from the input parameter values measured for this particular An. arabiensis population. This life-history analysis, therefore, identifies a clear, broadly-important opportunity for more effective suppression of residual malaria transmission by An. arabiensis in Africa and other important vectors of residual transmission across the tropics. Improved control of predominantly outdoor residual transmission by An. arabiensis, and other modestly zoophagic vectors like Anopheles darlingi, which frequently enter but then rapidly exit from houses, may be readily achieved by improving existing technology for killing mosquitoes indoors.

No MeSH data available.


Related in: MedlinePlus

Dependence of the population-wide mean proportion of human exposure occurring outdoors with (Ω = N) and without (Ω = 0) 80 % bed net usage upon the proportion occurring indoors in the absence of any nets, for a vector population with the same ability as Anopheles arabiensis to avoid fatal contact with LLINs or IRS after entering houses [16–19], so that the insecticide treatment status of the net is irrelevant
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Fig2: Dependence of the population-wide mean proportion of human exposure occurring outdoors with (Ω = N) and without (Ω = 0) 80 % bed net usage upon the proportion occurring indoors in the absence of any nets, for a vector population with the same ability as Anopheles arabiensis to avoid fatal contact with LLINs or IRS after entering houses [16–19], so that the insecticide treatment status of the net is irrelevant

Mentions: Of course An. arabiensis populations, with these particular behavioural parameters in this well-studied part Tanzania, are merely a motivating example of an understudied phenomenon that has also been described among several important malaria vectors of Latin America, and may be widespread amongst a variety of Anopheles populations across the tropics [14]. Applying sensitivity analysis to the same model illustrates how remarkably high proportions of residual human exposure to biting vectors can occur outdoors when nets are widely used, even in settings where the vector has a strong preference for feeding indoors (Fig. 2). Obviously, where a vector preferentially attempts to attack humans almost exclusively indoors (πh,i,N = ≥90 %), most transmission will still occur indoors even if high LLINs coverage is achieved. However, this relationship is very sensitive to even slight shifts towards preference for outdoor biting, so outdoor biting represents a considerable proportion of residual human exposure following LLIN scale up where vectors have even a partial ability to feed outdoors (Fig. 2). If even as little as a quarter of exposure of residents lacking a net occurs outdoors (πh,o,0 = 25 %), slightly more than half of all exposure across the entire human population is expected to occur outdoors (πh,o,N = 54 %). Where human exposure in the absence of nets is equally likely to occur indoors or outdoors (πh,o,0 = πh,i,0 = 50 %), more than three quarters of residual transmission is predicted to occur outdoors when nets are used at high coverage (πh,o,N = 78 %).Fig. 2


Most outdoor malaria transmission by behaviourally-resistant Anopheles arabiensis is mediated by mosquitoes that have previously been inside houses.

Killeen GF, Govella NJ, Lwetoijera DW, Okumu FO - Malar. J. (2016)

Dependence of the population-wide mean proportion of human exposure occurring outdoors with (Ω = N) and without (Ω = 0) 80 % bed net usage upon the proportion occurring indoors in the absence of any nets, for a vector population with the same ability as Anopheles arabiensis to avoid fatal contact with LLINs or IRS after entering houses [16–19], so that the insecticide treatment status of the net is irrelevant
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4837512&req=5

Fig2: Dependence of the population-wide mean proportion of human exposure occurring outdoors with (Ω = N) and without (Ω = 0) 80 % bed net usage upon the proportion occurring indoors in the absence of any nets, for a vector population with the same ability as Anopheles arabiensis to avoid fatal contact with LLINs or IRS after entering houses [16–19], so that the insecticide treatment status of the net is irrelevant
Mentions: Of course An. arabiensis populations, with these particular behavioural parameters in this well-studied part Tanzania, are merely a motivating example of an understudied phenomenon that has also been described among several important malaria vectors of Latin America, and may be widespread amongst a variety of Anopheles populations across the tropics [14]. Applying sensitivity analysis to the same model illustrates how remarkably high proportions of residual human exposure to biting vectors can occur outdoors when nets are widely used, even in settings where the vector has a strong preference for feeding indoors (Fig. 2). Obviously, where a vector preferentially attempts to attack humans almost exclusively indoors (πh,i,N = ≥90 %), most transmission will still occur indoors even if high LLINs coverage is achieved. However, this relationship is very sensitive to even slight shifts towards preference for outdoor biting, so outdoor biting represents a considerable proportion of residual human exposure following LLIN scale up where vectors have even a partial ability to feed outdoors (Fig. 2). If even as little as a quarter of exposure of residents lacking a net occurs outdoors (πh,o,0 = 25 %), slightly more than half of all exposure across the entire human population is expected to occur outdoors (πh,o,N = 54 %). Where human exposure in the absence of nets is equally likely to occur indoors or outdoors (πh,o,0 = πh,i,0 = 50 %), more than three quarters of residual transmission is predicted to occur outdoors when nets are used at high coverage (πh,o,N = 78 %).Fig. 2

Bottom Line: Life histories of a well-characterized An. arabiensis population were simulated with a simple but process-explicit deterministic model and relevance to other vectors examined through sensitivity analysis.The estimated proportion of vector blood meals ultimately obtained from humans indoors is dramatically attenuated by availability of alternative hosts, or partial ability to attack humans outdoors.While the exact numerical results predicted by such a simple deterministic model should be considered only approximate and illustrative, the derived conclusions are remarkably insensitive to substantive deviations from the input parameter values measured for this particular An. arabiensis population.

View Article: PubMed Central - PubMed

Affiliation: Environmental Health and Ecological Sciences Thematic Group, Ifakara Health Institute, Ifakara, Kilombero, Morogoro, United Republic of Tanzania. gkilleen@ihi.or.tz.

ABSTRACT

Background: Anopheles arabiensis is stereotypical of diverse vectors that mediate residual malaria transmission globally, because it can feed outdoors upon humans or cattle, or enter but then rapidly exit houses without fatal exposure to insecticidal nets or sprays.

Methods: Life histories of a well-characterized An. arabiensis population were simulated with a simple but process-explicit deterministic model and relevance to other vectors examined through sensitivity analysis.

Results: Where most humans use bed nets, two thirds of An. arabiensis blood feeds and half of malaria transmission events were estimated to occur outdoors. However, it was also estimated that most successful feeds and almost all (>98 %) transmission events are preceded by unsuccessful attempts to attack humans indoors. The estimated proportion of vector blood meals ultimately obtained from humans indoors is dramatically attenuated by availability of alternative hosts, or partial ability to attack humans outdoors. However, the estimated proportion of mosquitoes old enough to transmit malaria, and which have previously entered a house at least once, is far less sensitive to both variables. For vectors with similarly modest preference for cattle over humans and similar ability to evade fatal indoor insecticide exposure once indoors, >80 % of predicted feeding events by mosquitoes old enough to transmit malaria are preceded by at least one house entry event, so long as ≥40 % of attempts to attack humans occur indoors and humans outnumber cattle ≥4-fold.

Conclusions: While the exact numerical results predicted by such a simple deterministic model should be considered only approximate and illustrative, the derived conclusions are remarkably insensitive to substantive deviations from the input parameter values measured for this particular An. arabiensis population. This life-history analysis, therefore, identifies a clear, broadly-important opportunity for more effective suppression of residual malaria transmission by An. arabiensis in Africa and other important vectors of residual transmission across the tropics. Improved control of predominantly outdoor residual transmission by An. arabiensis, and other modestly zoophagic vectors like Anopheles darlingi, which frequently enter but then rapidly exit from houses, may be readily achieved by improving existing technology for killing mosquitoes indoors.

No MeSH data available.


Related in: MedlinePlus