Mosquitoes are rapidly developing resistance to insecticide-treated bed nets (ITNs), according to a group of 18 research scientists led by Jean-Francois Trape from the Institut de Researche pour le Developpment and the Pasteur Institute of Dakar.
The results of the study is an important development in the fight against the deadly disease malaria as insecticide-treated nets were previously thought to be the most effective method of reducing malaria morbidity, mortality and prevalence around the world. The researchers surmise in their study reported by the British Journal, The Lancet Infectious Diseases, that the “findings are of great concern” not least because “they support the idea that insecticide resistance might not permit a substantial decrease in malaria morbidity in many parts of Africa where Anopheles gambiae (the most common species of malaria spreading mosquito in Africa) is a vector.”
The results of the study conducted in Dielmo, Senegal between January 2007 and December 2010 aimed to assess the effect of the distribution of insecticide-treated bed nets and combination therapies on malaria morbidity, mosquito populations and infections. The scientists monitored inhabitants for the disease in the first year and a half of the programme. In July 2008, the scientists then offered long-lasting insecticide (Deltamethrin) treated nets to all the villagers.
The results of the study showed that the incidence of malaria was 13 times lower than before the nets were introduced. Further, the proportion of the Anopheles gambiae mosquitoes resistant to Deltamethrin, the chemical advocated by the WHO for use on the bed nets, was 37%. The study also found that the incidence of the disease rose to significant levels during the last few months of the study. In essence, the reports of malaria attacks were higher than recorded rates prior to the introduction of the ITNs. The researchers attributed this marked spike in disease incidence rates, especially in older children and adults, to the increasing resistance of A. gambiae mosquitoes to insecticides allied with loss of immunity or reduced immunity when the mosquito population declined.
Inevitably, the results of the study bears serious implications on the global fight against the spread of the disease especially if the Millennium Development Goals target of reversing the incidence of malaria by 2015 is to be achieved. ITNs were thought to be the most effective method of preventing the disease which is prevalent in sub-Saharan Africa, Asia, Latin America and to a lesser extent the Middle East and parts of Europe. The disease is also said to infect between 400 and 500 million people a year and also kills nearly two million people yearly; 90% of these deaths occur in sub-Saharan Africa.
The use of ITNs has resulted in a reduction of malaria incidence, morbidity and prevalence rates. UNICEF states that between 2004 and 2010, more than 400 million nets were delivered to malaria-endemic countries, with 290 million delivered since 2008 alone. It also states that this has led to the reduction in global malaria deaths by 20 per cent between 2000 and 2009.
However, John Keating and Thomas P. Eisele; both of the Department of Global Health Systems & Development at Tulane University, New Orleans aver that the results of the survey should be assimilated with caution. The researchers point to the relatively short duration of the research (3 – 4 years) which inhibits the assessment of long-term shifts and transmission of malaria epidemiology. An additional unanswered question they state regards how quickly a person previously exposed to malaria (before use of ITNs) loses their acquired immunity. The researchers add that this becomes an even more important issue especially in the case of adults who have experienced decades of exposure in endemic areas. Also, the fact that the study was conducted in a single village and further over a short period of time is said to be a limitation of the study, not least because it may thus be impossible to generalise its findings across Africa.
Despite these criticisms, the resurgence in malaria mortality in Rwanda, Sao Tome and Zambia since 2009 perhaps lends credence to the researchers’ fears. Worryingly, the prevalence of malaria in Kenya has increased in the last 3 years, according to a government survey. The survey indicates an overall malaria prevalence rate of 11% at present, with the disease affecting about 8% of all children under 5 years, compared with 3% in 2007. Interestingly, these countries have all implemented programmes aimed at distributing the WHO endorsed ITNs to their population.
Against this background it is perhaps now, and more than ever before necessary for scientists and researchers to develop effective malaria prevention and control strategies which protect individuals in endemic areas and which also reduces the speed of transmission of the disease. Therefore recent news that the PATH Malaria Vaccine Initiative is to collaborate with Glaxosmithkline (GSK) & Crucell in development of second-generation malaria vaccine is heart warming. At the time of writing, the RTS,S vaccine candidate is in the midst of a large-scale phase 3 clinical efficacy trial. The WHO anticipates that a policy recommendation for RTS,S may be possible by early as 2015. Dr Christian Loucq, director of PATH Malaria Vaccine Initiative, states that: “for the first time, we have a malaria vaccine in late phase development.” He adds that the new approach has the potential to “move us closer to the internationally agreed upon goal of an 80% effective second-generation vaccine by 2025.” It is no doubt hoped that development of a vaccine can be accelerated and made available and accessible to the developing world in the near future. Without this, the air of victory over malaria which currently pervades is just as likely to revert back into the sense of helplessness which previously abounded.
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