Adding screens to the windows and doors of houses can help reduce the number of malaria-carrying mosquitoes and illness in children, researchers have found.

In Thursday's online issue of The Lancet, British researchers say the number of mosquitoes in houses fell by 49 per cent for houses with screened ceilings and 59 per cent for those with full screening of windows, doors, eaves and other holes, compared with unscreened properties in Farafenni, in the Republic of The Gambia.

"House screening substantially reduced the number of mosquitoes inside houses and could contribute to prevention of anemia in children," Prof. Steve Lindsay, now at the London School of Hygiene and Tropical Medicine, and his colleagues concluded.

In the study, mosquitoes that entered the homes were caught in traps.

The frequency of parasites found in the blood did not differ between the groups. The researchers said that was not surprising, since it would only decline if the infection level was substantially suppressed.

Not a replacement for bed nets

It could be that house screening reduced malaria "superinfection" of children, a condition that leads to anemia, the team said.

Children were clinically assessed by researchers who did not know what type of house where each child resided, which could potentially bias the results.

The study's authors said house screening could easily be incorporated into programs to reduce mosquitoes. Because it does not rely on insecticides, it may be particularly beneficial in areas where insecticide resistance develops.

The team advocated house screening to augment rather than replace insecticide-treated bed nets.

A higher proportion of participants in the full screening group said the additions improved privacy, appearance of the house and stopped entry of mosquitoes and other pests.

Full screening was largely made using local materials and installed by local carpenters, which cost about $10 US per person if the netting is donated free of charge.

"We also hope that the results of our trial will stimulate the development of additional sustainable methods that, in combination with improved health care and access to treatment, can help to strengthen efforts to eliminate malaria," the researchers said in calling for a larger trial to assess if screening reduces clinical episodes of malaria elsewhere.

In a commentary accompanying the study, Dr. Laurence Slutsker and Dr. John Gimnig from Atlanta's Centers for Disease Control and Prevention, said that house screening to prevent malaria historically in the U.S., Europe and Tanzania may have contributed more to malaria control and elimination than previously thought.

They also urged further research to determine when and how screening should be integrated with insecticide-treated bed nets for extra benefits, and weigh the costs involved.