Mosquitoes developing resistance to the insecticide used in long-lasting insecticidal nets (LLINs) and mosquitoes changing their biting (feeding) behaviour and biting not between 10pm and 2am but towards the early morning ie 5am – or rather, in both cases, natural selection occurring favouring those mosquitoes that develop some level of resistance or having a later biting time – is starting to be seen. The extent to which either or both of these factors become a major problem for the effectiveness of LLINs is currently not clear.
Currently both issues – resistance to pyrethroids and changed time of biting - are not widespread. Currently LLINs remain highly effective in reducing the incidence of malaria.
Professor Steve Lindsay of Durham University comments: 'It is more likely we will see an increase of mosquitoes biting outdoors in the early evening, before people go to bed, although we do not have evidence for this occurring in Africa at present. Nonetheless, what is a common finding is that large-scale use of LLINs has resulted in the near extermination of Anopheles gambiae sensu stricto in houses in East Africa, with An. arabiensis remaining as the major vector, biting outdoors. In this case, the view is that there is no increase in An. arabiensis populations.'
On resistance to pyrethroids, Steve Lindsay comments: 'Resistance to pyrethroids is a rapidly growing phenomenon. We do not know whether this is impacting the control of malaria yet, but it is likely to be the case in the future. For the present they are probably effective in most places. New nets with different combinations of insecticides may provide better protection where pyrethroid resistance occurs, but these products are in the early stages of testing. And we shouldn't forget that an intact net is protective against malaria. For the moment, don't stop rolling out the nets.'
It is perfectly possible either or both – resistance to pyrethroids and changed time of biting - will remain a relatively minor problem without widespread impact.
It is also perfectly possible, over time, either or both - resistance to pyrethroids and changed time of biting - will become a material issue. This is the more likely scenario.
Resistance has been seen in other areas so it would not be a great surprise to see resistance developing to an insecticide. For example, the malaria parasite (rather than the mosquito in this case) first showed resistance to chloroquine
in the 1950s and more recently, in 2004, evidence of resistance to Artemisinin Combination Therapy (ACT)
, the primary drug currently used to treat malaria.
This highlights the importance of meta-research studies that look at both these issues - resistance to pyrethroids and changed time of biting - in many more locations to both establish how prevalent the problems are and to monitor developments.
This is a very important time for research so we gain clearer information that can influence decisions.
A solution to mosquitoes developing pyrethroid resistance is the use of non-pyrethroid insecticides. Currently there aren’t any that can be used on nets. Significant research is being carried out by a variety of groups and organisations to develop such insecticides. No-one can give a clear timeframe or the chances of success although some in this research area suggest a three to ten year timeframe is likely before products are able to come to market given the strict testing of such insecticide-including products.
In 2010, a net incorporating a second chemical, referred to as a synergist, was introduced to the market. There is some, but not conclusive, evidence that the combination of the pyrethroid and the synergist is more effective than pyrethroid-only nets at achieving knock-down of mosquitoes developing pyrethroid resistance. Further research studies, including field studies, are currently underway. More are required. The conclusion currently therefore, is there is no clear indication that the synergist treated nets are the nets of choice for areas where resistance is indicated, or indeed for other areas to prevent resistance developing. However, they do show promise and they might prove to be better than non-synergist-including nets. The synergist treated nets are currently 30% more expensive than the pyrethroid-only treated nets.
A solution to mosquitoes changed time of biting is less clear, as is the extent of the problem. There is an interesting article
that discusses some of the current factors that may influence how serious, or not, a problem this becomes.
The bottom line:
- Long-lasting insecticide treated nets (LLINs) remain the most effective means of malaria prevention.
- More research needs to be funded to understand the extent of problems such as mosquito resistance to the insecticide used on nets (pyrethroids) and change in time of day of biting of some mosquitoes.
- Research into both non-pyrethroid insecticides and chemical combinations (with pyrethroids) is very important.