Practicalities of vector control methods: Bed-nets 

Here we look at the use of bed nets as a vector control tool in more detail. This step will highlight what should be considered when deploying bed nets as part of a vector control programme.

We know that the use of insecticide-treated bed nets has been successful in reducing deaths from malaria but their effectiveness against sand flies is not so straightforward^[1]. Studies have found that long-lasting insecticide treated nets (LLINs) can reduce sand fly numbers but had little impact on VL transmission^{[2, 3]}. LLINs are not used in India’s national VL control programme.

Sand flies are smaller than mosquitoes so if bed nets are to be deployed as part of a vector control programme, the “mesh” or hole size is important. However when impregnated with insecticide, the mesh size is less relevant and sand flies will come into contact with insecticide when passing through the holes and die. For example, the PermaNet 2.0 is impregnated with deltamethrin) has no less than 24 holes/cm2. It is important to remember there is conflicting data on whether LLINs have a significant impact on VL transmission.

To date, published field studies looking at the impact of LLINs on sand fly behaviour are extremely limited. One study, in Nepal, looked at different mesh sizes and found that nets with a smaller mesh size provided a more effective barrier to sand flies and the standard impregnated nets of 24 holes/cm2^[4]. It now generally acknowledged that mesh size of at least 100 holes/cm2 is more effective at preventing sand fly biting unless nets are impregnated with insecticide.

Placement of bed nets

Perhaps a more important factor to consider is that sand flies will feed indoors or outdoors depending on where the “food” is. Humans sleeping outdoors to avoid the heat of summer months will be bitten by sand flies if there is no barrier, such as an LLIN, to sleep under. Any intervention using bed nets must be adapted to human behaviour, in other words, deploy them indoors and/or outdoors depending on where people sleep. However, when impregnated nets are being used outdoors, it is crucial to protect them from UV light as they will rapidly lose their efficacy due to degeneration of the insecticide.

The availability of suitable nets and the means/method to maintain them correctly is a key factor in implementing a successful vector control programme. For insecticide-impregnated bednets to have an impact on controlling VL and its sand fly vector, it is important that the majority of households in an area use the intervention