Research in Senegal has found that growing resistance to a common class of insecticide by Anopheles gambiae (the species of mosquito responsible for transmitting malaria to humans in Africa), coupled with increasing susceptibility of older children and adults (probably due to decreasing protective immunity because of a lack of exposure to malaria) has resulted in a resurgence of malaria. The findings, published Online First in The Lancet Infectious Diseases, suggest that new strategies are urgently needed to mitigate the potentially devastating effects of insecticide resistance on malaria control in Africa.
The introduction of artemisinin-combination therapies (ACTs) and widespread distribution of insecticide-treated bednets have resulted in major reductions in malaria transmission in Africa. But little is known about the extent to which these interventions might reduce malaria morbidity in the long-term. Indeed, recent reports from Africa suggest that resistance to common pythethroid insecticides is on the rise with 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, Dakar, Senegal led a team to assess the impact of the introduction of ACTs as the first-line treatment for malaria (June 2006) and the roll-out of long-lasting insecticide (deltamethrin*)-treated bednets (LLINs; July 2008) in a rural west African population. In the village of Dielmo in Senegal, data on malaria morbidity, mosquito populations, and asymptomatic infections were collected 1.5 years before the scale-up of LLINs and 2.5 years after.
During the two years (August 2008 to August 2010) after the distribution of LLINs there was a marked reduction in malaria attacks. But, between September and December 2010 (27 months after the roll out of LLINs) malaria attacks increased to even higher levels than before the introduction of LLINs in adults and older children.
The researchers also found that 37% of A gambiae mosquitoes were resistant to deltamethrin in 2010, and that the kdr mutation (conferring resistance to pythethroid insecticides) increased from 8% in 2007 to 48% in 2010.
The authors suggest 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 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."
In a Comment, Joseph Keating and Thomas Eisele from Tulane University, New Orleans, USA urge that caution is needed before generalising these results to other regions of Africa: "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 (ie, before the scale-up of malaria control) loses their acquired immunity, especially in adults with decades of exposure to repeated malaria parasite infections."
Dr Jean-François Trape, Institut de Recherche pour le Développement, Dakar, Senegal. T) +33 4 6773 6422 or +221 77 819 2400 E) jean-francois.trape@ird.fr
Professor Joseph Keating, Tulane University, New Orleans, USA. T) +1 504 988 1348 E) jkeating@tulane.edu
Notes to Editors: *Deltamethrin is one of the main insecticides used for malaria control in Africa and is recommended by WHO for bednet treatment.
** The persistence of immunity acquired during early childhood depends on sustained exposure to malaria and that immunity decreases when exposure is reduced, resulting in an increase in cases of malaria in older children and adults.
Journal
The Lancet Infectious Diseases