Abstract
BackgroundAn important advantage of pyrethroid-treated nets over untreated nets is that once nets become worn or holed a pyrethroid treatment will normally restore protection. The capacity of pyrethroids to kill or irritate any mosquito that comes into contact with the net and prevent penetration of holes or feeding through the sides are the main reasons why treated nets continue to provide protection despite their condition deteriorating over time. Pyrethroid resistance is a growing problem among Anopheline and Culicine mosquitoes in many parts of Africa. When mosquitoes become resistant the capacity of treated nets to provide protection might be diminished, particularly when holed. An experimental hut trial against pyrethroid-resistant Culex quinquefasciatus was therefore undertaken in southern Benin using a series of intact and holed nets, both untreated and treated, to assess any loss of protection as nets deteriorate with use and time.ResultsThere was loss of protection when untreated nets became holed; the proportion of mosquitoes blood feeding increased from 36.2% when nets were intact to between 59.7% and 68.5% when nets were holed to differing extents. The proportion of mosquitoes blood feeding when treated nets were intact was 29.4% which increased to 43.6–57.4% when nets were holed. The greater the number of holes the greater the loss of protection regardless of whether nets were untreated or treated. Mosquito mortality in huts with untreated nets was 12.9–13.6%; treatment induced mortality was less than 12%. The exiting rate of mosquitoes into the verandas was higher in huts with intact nets.ConclusionAs nets deteriorate with use and become increasingly holed the capacity of pyrethroid treatments to restore protection is greatly diminished against resistant Culex quinquefasciatus mosquitoes.
Highlights
An important advantage of pyrethroid-treated nets over untreated nets is that once nets become worn or holed a pyrethroid treatment will normally restore protection
This paper describes an experimental hut trial conducted in southern Benin using untreated and insecticide-treated nets that were deliberately holed in order to explore the relationship between insecticide resistance, insecticide treatment and deterioration in the physical integrity of the material on the capacity of ITNs to protect against Culex quinquefasciatus mosquitoes
Over the course of the trial, 1265 Culex quinquefasciatus and 39 Anopheles gambiae s.l. females were caught in the huts
Summary
An important advantage of pyrethroid-treated nets over untreated nets is that once nets become worn or holed a pyrethroid treatment will normally restore protection. The malaria vector Anopheles gambiae sensu stricto has developed resistance to pyrethroids in West and East Africa [7,8] and, as in Culex, the resistance can be caused by kdr, oxidases, esterases or a combination of mechanisms [7,9,10]. A form of pyrethroid resistance has been found in An. gambiae of the M molecular form ( associated with the kdr mechanism) that seems to be highly protective [12,13] Resistant mosquitoes of this M biotype readily penetrate holed ITNs and feed more successfully than mosquitoes of the S molecular form [11,12]. The failure of ITNs once holed to protect against certain types of pyrethroid-resistant Culex and Anopheles needs further study since it is the relationship between pyrethroid dosage on nets, the level of resistance in the mosquito, and number or area of holes that determines whether an ITN is protective to users
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