Although introducing artemisinin-combination therapies (ACTs) and widespread distribution of insecticide-treated mosquito-nets achieved major reductions in malaria transmission in Africa, little is known about the extent to which these interventions might reduce malaria morbidity in the long-term.
Recent reports from Africa indicate that resistance to common pythethroid insecticides is on the rise. Seeing that there are few effective and inexpensive alternative insecticides safe for humans, this rise causes serious implications for malaria control strategies, particularly as there are few alternative insecticides that are effective, inexpensive, and safe for humans.
Jean-François Trape from the Institut de Recherche pour le Développement in Dakar, Senegal and his team assessed the consequences of introducing ACTs as the first-line treatment for malaria (June 2006) and the roll-out of long-lasting insecticide (deltamethrin*)-treated mosquito-nets (LLINs; July 2008) in a rural west African population.
Researchers collected data on malaria morbidity, mosquito populations and asymptomatic infections in the Senegalian village of Dielmo for 1.5 years before the scale-up of LLINs and 2.5 years after. Between August 2008 and August 2010, they discovered a marked reduction in malaria attacks after the distribution of LLINs, however, 27-30 months after the roll out (Sept. and Dec. 2010), the attacks increased to even higher levels than before the introduction of LLINs in adults and older children.
In 2010, the researchers also discovered a 37% resistance of A gambiae mosquitoes to deltamethrin and that the kdr mutation (conferring resistance to pythethroid insecticides) increased by 40% in 3 years (from 8% in 2007 to 48% in 2010).
Researchers believe that the "rebound" in malaria attacks in older children and adults was the result of a combination of lowered immunity because of a lack of exposure to malaria** due to intervention-suppressed transmission and insecticide resistance that increased exposure to A gambiae mosquitoes.
They authors conclude,
"These findings are of great concern, since they support the idea that insecticide resistance might not permit a substantial decrease in malaria morbidity in many parts of Africa where A gambiae is the major vector and acquired clinical immunity is a key epidemiological factor."
Joseph Keating and Thomas Eisele from Tulane University in New Orleans, USA, stress the need for caution before making the assumption that these results apply to other African regions. In a comment they said,
"The study was done over a short period of time (4 years) which restricts any attempt to differentiate between short-term heterogeneity in malaria transmission and long-term shifts in malaria epidemiology...[and] also raises questions about how fast an individual previously exposed to stable transmission (i.e., before the scale-up of malaria control) loses their acquired immunity, especially in adults with decades of exposure to repeated malaria parasite infections."