The Millennium Development Goals (MDGs) include a target of 80 percent coverage of insecticide-treated bed nets (ITNs) for children under five and pregnant women.  Although young children in malarial affected areas are most at risk, increasing the use of ITNs throughout the general population may bring community-wide benefits that are just as important.

This was the conclusion of a study carried out by a team attached to the Ifakara Health Research and Development Centre in Tanzania. The team used models of malaria transmission to assess the effectiveness of population–wide bed net coverage, as opposed to specifically targeting pregnant women and children. The study sought to calculate the impact of ITNs on mosquitoes’ access to human hosts and their ability to survive. The malaria parasite needs eight days to mature within the mosquito before transmission to a human host. The completion of its life cycle therefore depends on the mosquito’s ability to live for a minimum of 10 days and to feed on human blood.

Two models were used: one in which the mosquitoes had access to human blood only; the other including the presence of large numbers of cattle, which offer an alternative food source. The models were used to calculate the likelihood over time of a mosquito being able to find, attack and feed on a human host, without being killed in the attempt. The models used quantitative data from the village of Namwawala in southern Tanzania, chosen because a great deal is known about malaria transmission and mosquito prevalence in the village and surrounding areas. ITN coverage was expressed in terms of nightly use, rather than simple ITN ownership.

Findings include:

• Targeting young children and pregnant women required a coverage level of 80 percent to achieve limited protection for those groups.
• ITNs provide a much greater safeguard against human exposure to malaria if half or more of the whole population is covered.
• Where there are no cattle to provide alternative hosts for mosquitoes, 35 percent of the population must sleep under bed nets to achieve community-wide protection equal to targeted individual protection.
• Where cattle offer an alternative food source, the same target is achieved at 55 percent coverage.

The findings have important implications for MDG and Roll Back Malaria Partnership targets for coverage of pregnant adults and the under-fives:

• If the majority of people regularly used bed nets, all non-users including children would receive an equivalent level of protection as that achieved through programmes of ITN coverage targeted at children alone.
• Limiting policy to programmes targeted at achieving the MDG of 80 percent coverage among pregnant women and children under five leaves out the substantial benefits of communal protection for the remaining 20 percent.
• Where cost-sharing is needed to complement public subsidies for ITNs, engagement and awareness raising among target populations is essential.
• More studies are needed to quantify and verify the levels of protection possible, but the results indicate that expanding ITN use could help protect all pregnant women and vulnerable children.

Source(s):
‘Preventing Childhood Malaria in Africa by Protecting Adults from Mosquitoes with Insecticide-treated Nets’, PLoS Medicine 4(7), by Gerry F Killeen, Tom A Smith, Heather M Fergusin, Hassan Mshinda, Salim Abdulla et al, 2007 Full document.

Funded by: Swiss National Science Foundation; Bill and Melinda Gates Foundation; the Biotechnology and Biological Research Council of the United Kingdom; the Wellcome Trust

id21 Research Highlight: 18 February 2008

Further Information:
Gerry F Killeen
Ifakara Health Research and Development Center
Ifakara
Kilombero District
Tanzania

Tel: 255-748-477-118
Fax: 255-23-262-5312
Contact the contributor: gkilleen@ihrdc.or.tz

Ifakara Health Research and Development Center, Tanzania

Other related links:
‘Low transmission but high risk: planning for malaria epidemics in Uganda’

‘Gender and treatment-seeking for child malaria in Ghana’

‘Anti-malarial treatment in Tanzania: differences in willingness to pay’

‘Widening the net: supplying insecticide-treated bed nets in Kenya’