Household modification

VANESSA CHEN-HUSSEY: Welcome to this presentation on House Modification and its use in the control vector borne diseases. By the end of this presentation, you will have been introduced to the concept of house modification as a vector control tool and seeing examples of its use in malaria and Chagas control. We will look at the potential impact of house modification on disease transmission and its place in control programmes. The specific vector-proofing strategies required will depend on the local vector species. For example, plastering walls will remove harborage for triatomine bugs but won’t have much effect on anopheline mosquitoes. We will examine house modifications separately for different vectors.

But it is important to bear in mind that some areas will have multiple vectors and may require a combination strategy. Many factors are found biting, breeding, and resting in and around the house. Therefore, the design of human living spaces allows numerous opportunities for interventions to reduce vector contact. These interventions start with physically preventing entry through screening of windows or other entry points. The peridomestic environment should be considered next to ensure potential breeding sites are removed. And finally, internally the house should be made inhospitable to vectors that might find refuge there. House screening as an intervention against malaria was first tested by Angelo Celli in 1899.

He was able to demonstrate that screening windows and doors effectively protected railway workers and their families against malaria in Italy. This low-tech intervention spread and house screening was used throughout the tropics in the early part of the 20th century. A number of other house-based interventions have been attempted since then, including closing gaps in walls and around doors, elevating houses, closing eaves, doors, and windows screens, and insecticidal screening. A Cochrane review is planned that will look at various house interventions for preventing malaria. They’ve so far identified a number of interventions ranging from those that are incorporated into the initial building of a house, such as increased house elevation, to those that can be retrofitted later, such as window screens.

Anopheles gambiae, the most important factor of malaria in Sub-Saharan Africa, feeds indoors at night. Therefore, house screening– whether windows, doors, and eaves– are secured against mosquito entry can work very well. Jetta et al looked at the effect of different house designs on mosquito entry in the Gambia and found that closing eaves reduced Anopheles gambiae entry to thatched houses by 94%, even if doors were poorly fitted. However, in houses with metal roofs, closed eaves and poorly-fitted doors resulted in a greater number of Anopheles gambiae entering. This could be due to either higher carbon dioxide levels or higher temperatures in the metal-roofed houses.

A systematic review and meta analysis looked at 90 studies of the effect of house materials and screening on malaria. The quality of evidence was unfortunately low. But taken together, there was a consistent trend in the study outcomes that showed an association of improved housing with lower malaria. Aedes aegypti and Aedes albopictus are the principal vectors of the viruses that cause dengue, chikungunya, Zika, and yellow fever. They are aggressive biters and feed both indoors and outdoors during the evening before people are in bed. However, they also tend not to fly very far. So control can focus on source reduction. Potential 80s larval sites, such as artificial containers, should be targeted for removal.

Water containers that cannot be removed, such as storage tanks, should be treated with a larvicide or biological control method. Triatomine bugs are responsible for the transmission of Trypanosoma cruzi, the protozoan parasite that causes Chagas disease. There are more than 140 triatomine species. And while most are probably capable of transmitting T cruzi, only a few are considered important vectors. Triatomine species that are highly adapted to living in and around houses are targeted by removing harborages and indoor insecticide spraying. Harborages are found in cracks and crevices, so interventions include plastering walls, cementing floors, and replacing thatch or sod roofs with tiles or corrugated iron. These improvements should equally apply to other nearby structures, such as animal houses.

A community-based study in four villages in Guatemala used both spraying and low-tech wall plastering to interrupt persistent triatomine re-infestations. The proportion of triatomines found inside houses as opposed to outside houses reduced from 52% to 17%.

There are a proportion of triatomines even of domesticated species that are found in the forest and are able to reinvest properties after an intervention. In these situations, as with mosquitoes, insecticide-treated window screens and curtains can be an effective control tool. A trial in Mexico found that triatomine abundance inside houses was reduced by over 96% after screens and curtains were installed. Unlike insecticide spraying which only lasted one season, the effect of screens and curtains persisted into the following year, still reducing indoor triatomines by 87%. House improvement is a very old idea in the fight against vector-borne disease. It is not easy or cheap. And the arrival of cost effective insecticides meant that this strategy was almost forgotten.

Today, there is renewed urgency to learn from these lessons. Linking housing and vector-borne disease is vital to achieving the UN sustainable development goals. As the urban environment expands rapidly, knowledge and regulation of this area is necessary to prevent poor housing contributing to disease outbreaks once more.