Review of Issues on Residual Malaria Transmission

Abstract

Residual malaria transmission is the actual maintained inoculation of Plasmodium, in spite of a well-designed and implemented vector control programs, and is of great concern for malaria elimination. Residual malaria transmission occurs under several possible circumstances, among which the presence of exophilic vector species, such as Anopheles dirus, or indoor- and outdoor-biting vectors, such as Anopheles nili, or specific behavior, such as feeding on humans indoors, then resting or leaving the house the same night (such as Anopheles moucheti) or also changes in behavior induced by insecticides applied inside houses, such as the well-known deterrent effect of permethrin-treated nets or the irritant effect of DDT. The use of insecticides may change the composition of local Anopheles populations, such as A. arabiensis taking up the place of A. gambiae in Senegal, A. aquasalis replacing A. darlingi in Guyana, or A. harrisoni superseding A. minimus in Vietnam. The change in behavior, such as biting activity earlier than usually reported-for example, Anopheles funestus after a large-scale distribution of long-lasting insecticidal nets-or insecticide resistance, in particular the current spread of pyrethroid resistance, could hamper the efficacy of classic pyrethroid-treated long-lasting insecticidal nets and maintained transmission. These issues must be well documented in every situation to elaborate, implement, monitor, and evaluate tailored vector control programs, keeping in mind that they must be conceived as integrated programs with several well and appropriately coordinated approaches, combining entomological but also parasitological, clinical, and social methods and analyses. A successful integrated vector control program must then be designed to reduce transmission and incidence rates of malaria morbidity and overall mortality.

Keywords: Residual malaria transmission; outdoor biting behavior; vector behavior changes; vector control methods.

Figure 1.

Biting activity and parity rate of Anopheles nili for indoor (blue) and outdoor (orange) human habitations. A, Biting activity as hourly number of bites in humans. B, Parity rate per hour.

Figure 2.

Breeding sites of Anopheles vectors in Vietnam. A, Anopheles dirus from Khan Hoa Province (photograph provided by P. C.). B, Anopheles minimus from Hoa Binh Province (photograph provided by S. M.).

Figure 3.

Main behaviors induced by chemicals implemented in houses for vector control (source: Guillaume Carnevale).

Figure 4.

Monthly human biting rate of Anopheles gambiae in control and DDT-treated houses of the pilot zone of Bobo-Dioulasso, Burkina Faso [28].

Figure 5.

Number of specimens of Anopheles vectors caught in controls and permethrin-treated nets (PTNs) in Soumousso experimental huts. A, Anopheles gambiae. B, Anopheles funestus.

Figure 6.

Experimental huts in the World Health Organization (WHO) collaborative field station in Soumousso, Burkina Faso. A, Bobo hut design. B, Mossi hut design. (Photographs provided by Frédéric Darriet) [29].

Figure 7.

Number of Anopheles collected in 3 locations inside the experimental huts, inside the hut (blue), under nets (orange), or in veranda traps (green) in Bobo and Mossi huts with either untreated nets (Controls) or permethrin-treated nets (PTNs). A, Anopheles gambiae. B, Anopheles funestus.

Figure 8.

Monthly evolution of the biting density (bites per person per hour) of Anopheles vectors indoors and outdoors in controls and Olyset net villages. A, Anopheles dirus; B, Anopheles minimus.

Figure 9.

Fish used in biological control against mosquito larvae. A, Aphanius díspar. B, Poecilia reticulata (guppy). C, Gambusia affinis.

Figure 10.

Releasing larvivorous fish, Chanthaburi, Thailand (photograph by Tawat Kantasri, courtesy of Frédérick Gay) [78].

Figure 11.

Mosquito larval source reduction in Indonesia. A, Elimination of manmade breeding places carried out by the Department of Public Works. B, Larviciding by health department personnel (photographs by Bangkit Hutajulu, courtesy of Frédérick Gay) [80].

Figure 12.

Outdoor residual spraying at a malaria epidemic focus in Dak Lak Province, Vietnam (photograph by Huu Thuy, courtesy of Frédérick Gay); [81].

Figure 13.

Field conditions and protection against outdoor-biting mosquitoes. A, Woodcutters in their temporary shelter in Vietnam (photograph by Vo Hinh, courtesy of Frédérick Gay) [82]. B, Long-lasting insecticide-treated hammock (source: Guillaume Carnevale).

Figure 14.

Use of ZeroFly insecticide-treated plastic sheeting (ITPS) in villages in Angola. A, Hanging ITPS on the wall. B, Sleeping on ITPS spread on the floor. (Photographs provided by P. C.)

Figure 15.

Example of a wall lining withdrawn from houses and laid on the fence in Angola (photograph provided by P. C.).