A new study carried out in Tanzania has indicated a drastic decline in malaria infection and disease rates following changes in Anopheline populations in highly malaria endemic communities.
The study titled “is the current decline in malaria burden in sub-saharan Africa due to a decrease in vector population?” was published in the July edition of the Malaria Journal.
The study conducted by scientists from Denmark, Tanzania and the USA found that despite the absence of organised vector control a longitudinal decline in the density of malaria mosquito vectors was seen during the study periods.
The scientists collected mosquitoes weekly from 50 households between 1998 and 2001 (1st period) and between 2003 and 2009 (2nd period) using CDC light traps.
The scientists led by Prof. Dan Meyrowitsch from the University of Copenhagen noted that scale-up of malaria control interventions in high-endemic countries has without doubt contributed to the observed decline in malaria cases and deaths.
“However, other factors not related to intervention could potentially have an impact on mosquito vectors, and thereby reduce transmission, which subsequently will result in reductions in number of infected cases. Among these factors are urbanization, changes in agricultural practices and land use, and economic development resulting in improved housing construction,” they said.
They said global climatic changes resulting in changes in rainfall, humidity and temperature may also impact malaria vectors.
Meyrowitsch said the decline cannot be linked entirely to the changing climate.
“It could be partly due to this chaotic rainfall, but personally I don’t think it can explain such a dramatic decline in mosquitoes, to the extent we can say that the malaria mosquitoes are almost eradicated in these communities.
“What we should consider is that there may be a disease among the mosquitoes, a fungi or a virus, or they’re may have been some environmental changes in the communities that have resulted in a drop in the number of mosquitoes,” Meyrowitsch said.
Despite the relevance of these multiple determinants and their potential roles in the declining malaria burdens, the majority of field studies reporting changing malaria epidemiology focus exclusively on the change in the occurrence of human infections and disease.
In this respect, there have also been only limited research efforts invested in systematic data collection and mapping of long-term trends in vector densities and other vector related aspects of transmission in the areas where declining malaria infection and disease burdens have been reported.
Approximately half of the world’s population is at risk of malaria, and an estimated 243 million infected cases resulted in nearly 863,000 deaths in 2008.
In sub-Saharan Africa (SSA), where 91 per cent of all malaria-related deaths take place, malaria is estimated to result in an annual loss of 35.4 million Disability Adjusted Life Years with 85 per cent of the deaths amongst children below five years of age and around 40 per cent of all public health spending is related to malaria.
Despite these distressing records, reductions in the numbers of malaria cases and malaria-related deaths by up to 50 per over the past decade have been reported from several high burden African countries, including Eritrea, Rwanda, Zanzibar, Pemba, Tanzania mainland, Kenya and Zambia.
The declining infection rates and overall disease burdens as well as reduction in asymptomatic carriers are considered to be a consequence of improved quality of health systems, including improved case management, such as enhanced diagnostics and implementation of highly effective anti-malarial drugs.
Large scale investments in intervention programmes specifically aimed at achieving high coverage of bed nets, campaigns of indoor residual spraying (IRS) and implementation of intermittent presumptive treatment (IPT) in vulnerable groups, have further reduced the malaria burden significantly.