Visceral leishmaniasis on the Indian Subcontinent: Efficacy of fipronil-based cattle treatment in controlling sand fly populations is dependent on specific aspects of sand fly ecology.

Background: Visceral leishmaniasis (VL) is a deadly disease transmitted by the sand fly Phlebotomus argentipes on the Indian subcontinent, with a promising means of vector control being orally treating cattle with fipronil-based drugs. While prior research investigating the dynamic relationship between timing of fipronil-based control schemes and the seasonality of sand flies provides insights into potential of treatment on a large scale, ecological uncertainties remain. We investigated how uncertainties associated with sand fly ecology might affect our ability to assess efficacy of fipronil-based control schemes. To do this, we used a previously-described, individual-based, stochastic sand fly model to quantify how uncertainties associated with 1) the percentage of female sand flies taking blood meals from cattle, and 2) the percentage of female sand flies ovipositing in organic matter containing feces from treated cattle might impact the efficacy of fipronil-based sand fly control schemes. Principal findings: Assuming no prior knowledge of sand fly blood meal and oviposition sites, the probabilities of achieving effective sand fly population reduction with treatments performed 3, 6 and 12 times per year were ≈5–22%, ≈27–36%, and ≈46–54%, respectively. Assuming ≥50% of sand flies feed on cattle, probabilities of achieving efficacious control increased to ≈8–31%, ≈15–42%, and ≈52–65%. Assuming also that ≥50% of sand flies oviposit in cattle feces, the above probabilities increased further to ≈14–53%, ≈31–81%, and ≈89–97%. Conclusions: Our assessments of the efficacy of fipronil-based cattle treatments in controlling sand fly populations depend on our assumptions regarding key aspects of sand fly ecology. Assessments are most sensitive to assumptions concerning the percentage of sand flies ovipositing in feces of treated cattle, thus emphasizing the importance of identifying sand fly oviposition sites. Our results place the evaluation of fipronil-based cattle treatment within a broader ecological context, which could aid in the planning and execution of a largescale field trial. Author summary: Visceral leishmaniasis is a virulent disease transmitted to man by phlebotomine sand flies. Prior research has suggested the potential for fipronil-based drugs administered orally to cattle to successfully control adult sand flies feeding on cattle blood and larvae developing in cattle feces. However, field studies have yet to be conducted and uncertainties surrounding sand fly ecology may impact the success of treatment. We use an individual-based, stochastic model representing the sand fly life cycle, previously used to evaluate a number of potential fipronil-based sand fly control schemes, to investigate two of these uncertainties: 1) the percentage of sand flies feeding on cattle blood, 2) the percentage of sand flies laying eggs in cattle feces. Results indicate that changes in our assumptions regarding these two uncertainties have a significant impact on our estimates of the probable success of fipronil-based control schemes. Additionally, simulations suggest that the results are most sensitive to changes in the percentage of sand flies ovipositing in cattle feces. Hence, our results place the evaluation of fipronil-based cattle treatment within a broader ecological context and emphasize the need for novel approaches for determining the oviposition sites of sand flies. [ABSTRACT FROM AUTHOR]