Pilot study to assess the impact of feed-through insecticide on the sand fly density in an endemic area of zoonotic cutaneous leishmaniasis in Morocco

BACKGROUND: Cutaneous leishmaniasis (CL) remains a significant public health concern in Morocco. Leishmania major, the principal cause of zoonotic CL (ZCL) in Morocco, is transmitted by the female sand fly Phlebotomus papatasi, with the great gerbil (Meriones shawi) serving as the primary reservoir host. Current control strategies predominantly rely on strychnine-treated bait targeting gerbils. Recently, insecticide-treated rodent bait has emerged as a promising alternative for ZCL control. This study evaluated the village-level impact of feed-through insecticide, involving two intervention villages and two control villages, with outcomes assessed through female sand fly density and Leishmania infection rates.

METHODOLOGY/PRINCIPAL FINDINGS: The study was conducted in the Province of Zagora, southeastern Morocco. Three applications of fipronil-treated bait were performed during July and August 2020. Sand fly populations were monitored through nine surveys per village, with each survey spanning three consecutive nights. Two control and two intervention villages were included. In each village sand flies were collected in three epidemiological relevant locations, i.e., in the field where the application was implemented, and indoors and outdoors in the villages. Blood-fed sand flies were prioritized for analysis to increase the likelihood of detecting Leishmania circulation and because blood feeding sand flies are the primary target of feed-through insecticides. A negative binomial generalized linear model was used, with the number of female sand flies as the response variable. Explanatory variables included village status (intervention or control), application number (0, 1, 2, 3), and their interaction. The initial two applications of fipronil did not result in a significant reduction in sand fly density in the sampling location 'field' of the intervention villages (Interaction term: application 1*village type: IRR 0.65, CI95: 0.39-1.09; Interaction: application 2*village type: 0.81, CI95: 0.52-1.27). Only after application 3, a significant reduction in sand fly density was observed in the 'field' of the intervention villages. This reduction was observed in blood-fed sand flies only. No effect of the intervention was observed on the sand fly Leishmania infection rates. Furthermore, despite processing more than 3,600 sand flies, we did not detect L. major, the presumed cause of ZCL in this region. Yet, we detected the circulation of L. infantum, L. tropica, and L. tarentolae in the study area.

CONCLUSIONS/SIGNIFICANCE: This study assessing the impact of feed-through insecticide on sand fly density and Leishmania infection rates in an endemic ZCL area in Morocco found insufficient evidence of impact on sand fly densities to achieve epidemiological relevance. Additionally, the study revealed gaps in the understanding of the transmission cycle, as two Leishmania species other than L. major were identified as potential causes of CL in the area. These findings underscore the need for improved knowledge of the transmission dynamics to enhance control measures.