Predicting Scenarios for Successful Autodissemination of Pyriproxyfen by Malaria Vectors from Their Resting Sites to Aquatic Habitats; Description and Simulation Analysis of a Field-Parameterizable Model

Abstract

Background: Large-cage experiments indicate pyriproxifen (PPF) can be transferred from resting sites to aquatic habitats by Anopheles arabiensis--malaria vector mosquitoes to inhibit emergence of their own offspring. PPF coverage is amplified twice: (1) partial coverage of resting sites with PPF contamination results in far higher contamination coverage of adult mosquitoes because they are mobile and use numerous resting sites per gonotrophic cycle, and (2) even greater contamination coverage of aquatic habitats results from accumulation of PPF from multiple oviposition events.

Methods and findings: Deterministic mathematical models are described that use only field-measurable input parameters and capture the biological processes that mediate PPF autodissemination. Recent successes in large cages can be rationalized, and the plausibility of success under full field conditions can be evaluated a priori. The model also defines measurable properties of PPF delivery prototypes that may be optimized under controlled experimental conditions to maximize chances of success in full field trials. The most obvious flaw in this model is the endogenous relationship that inevitably occurs between the larval habitat coverage and the measured rate of oviposition into those habitats if the target mosquito species is used to mediate PPF transfer. However, this inconsistency also illustrates the potential advantages of using a different, non-target mosquito species for contamination at selected resting sites that shares the same aquatic habitats as the primary target. For autodissemination interventions to eliminate malaria transmission or vector populations during the dry season window of opportunity will require comprehensive contamination of the most challenging subset of aquatic habitats [Formula: see text] that persist or retain PPF activity (Ux) for only one week [Formula: see text], where Ux = 7 days). To achieve >99% contamination coverage of these habitats will necessitate values for the product of the proportional coverage of the ovipositing mosquito population with PPF contamination (CM) by the ovitrap-detectable rates of oviposition by wild mosquitoes into this subset of habitats [Formula: see text], divided by the titre of contaminated mosquitoes required to render them unproductive [Formula: see text], that approximately approach unity [Formula: see text].

Conclusions: The simple multiplicative relationship between CM and [Formula: see text], and the simple exponential decay effect they have upon uncontaminated aquatic habitats, allows application of this model by theoreticians and field biologists alike.

Fig 1. A schematic illustration of how partial coverage all resting sites is amplified in two steps as PPF contamination is transferred to the adult mosquito population and then onwards to the larval habitats.

The proportional coverage of the resting sites (C _r), ovipositing adult mosquito population (C _M) and larval habitats (C _l) is depicted as a proportion of all resting sites (r), adult mosquitoes (M) and larval habitats (l) covered with PPF contamination (c).

Fig 2. Evaluation of the proportion of all larval aquatic habitats which are effectively contaminated with PPF C _l at different values for the mean time that habitats persist but remain unproductive (U).

This figure presents combinations of values for proportional coverage of the ovipositing adult mosquito population with PPF contamination C _M, ovitrap-detectable rates of oviposition by wild mosquito (m _l,z,d), and the titre of contaminated mosquitoes required to render habitats unproductive (T _l,z,d) that may lead to specific values of C _l at different values of U.

Fig 3. An illustration of the main three input parameters in predicting the proportion of all aquatic habitats contaminated with PPF.

This figure presents combined influence of C _M, $m_{l_{z,d}}$, and $\mathrm{1}/T_{l_{z,d}}$ upon the availability of uncontaminated aquatic habitats (1-C _l) to the vector population as a simple exponential decay, so that the increasing threshold values required to achieve specific larval habitat coverage targets can be visualized as a log-linear function of their product.