Scientists at the Ifakara Health Institute (IHI) say the efforts to eliminate malaria would finally yield good results if more investment is put in curbing the remaining burden of malaria in rural Tanzania which is now attributed to one unique mosquito species, called anopheles funestus.

The researchers have called for complementary control approaches to eliminate malaria, after learning that this species of mosquitoes remains notorious for transmitting about 90 per cent of the malaria burden in rural Tanzania.

This mosquito occurs in very small numbers compared to other mosquito species, but is extremely effective at transmitting malaria to humans, they say.

Results of a new study conducted in south eastern Tanzania by Emmanuel Kaindoa of IHI and others, and published in the health research journal, PLoS ONE, today suggests that anopheles funestus mosquito is now the major malaria vector.

Nearly nine in every ten malaria infections in rural Tanzania are now carried by this mosquito species.

The scientists also showed that this last remaining vector is resistant to chemicals used in long-lasting insecticide treated nets (LLINs) – currently the key intervention against the killer disease.

The mosquitoes can survive even after contacting insecticide-treated mosquito nets commonly used in households.

“We have clearly shown that anopheles funestus mosquitoes are highly resistant to pyrethroids used on LLINs, and that they also survives unexpectedly longer than the other malaria mosquitoes, thus requiring new control approaches to tackle the residual malaria,” Kaindoa said.

“Nearly nine in every ten malaria infections in rural Tanzania are now transmitted by just one mosquito species called anopheles funestus,’’ he says.

Tanzania has experienced a decline in malaria transmission following the introduction of insecticide treated nets (ITNs) and the scaling up of LLINs.

A health worker examines a specimen for presence of malaria. © 2007 Bonnie Gillespie, Courtesy of Photoshare

By 2010, the country had made significant progress, and most areas that were experiencing prevalence above 50 per cent in 2000, now had below 10 per cent prevalence.

However, results of the new study by Kaindoa and his colleagues, suggest that the focus should now be changed or rather shifted from relying solely on LLINs as a major control method to looking for alternative interventions, particularly those that can effectively address the problem of anopheles funestus.

The suggestion is timely, with the recent malaria indicator survey 2016 returning an average prevalence of 14.8 per cent prevalence in children under 5 years.

In his summary report provided to Ifakara Health Institute’s blog, Sayansi na Ubuntu, Kaindoa emphasized their conclusion that interventions which effectively target Anopheles funestus mosquitoes could significantly improve control of the ongoing persistent malaria transmission in south-eastern Tanzania.

Malaria is transmitted by many Anopheles species. The transmission occurs when sporozoites found in the mosquito salivary gland (infective stage of the malaria parasite, Plasmodium) are injected in human blood by an Anopheles mosquito during its blood meal.

The contribution of Anopheles mosquitoes to malaria transmission vary across different settings. In this study, Kaindoa and colleagues assessed contributions of major malaria vectors, An. arabiensis and An. funestus, in endemic villages in rural south-eastern Tanzania.

Their findings yielded evidence that anopheles funestus contributed to 86.2 per cent and An. arabiensis contributed to 17.8 per cent of the ongoing malaria transmission in the study villages in South-eastern Tanzania.

Furthermore, they found that An. funestus mosquito is resistant to pyrethroids used on LLINs and survives unexpectedly longer than An. arabiensis, thus requiring new control approaches. Due to the above challenges, the current interventions cannot fully suppress malaria transmission in rural Tanzania.

Moreover, the study found that An. funestus exclusively feed on human, unlike An. arabiensis which were found to also feed on cattle.

The high human blood index observed in the An. funestus clearly suggests that the species could still be best targeted by household-based interventions that protect humans.

“Though An. arabiensis is still the most abundant vector species here, ongoing residual transmission is predominantly mediated by An. funestus, possibly due to high resistance and high survival probabilities,”he said.

Sustained use of insecticide treated bed nets in rural Tanzania have effectively eliminated Africa’s main malaria mosquito, anopheles gambiae, from many Tanzanian villages, but malaria transmission has persisted, though at much lower levels than before.

The World Health Organization (WHO) report indicates that malaria continues to be one of the most significant mosquito-borne parasitic diseases, affecting about 212 million people, causing 429,000 deaths annually and adversely affect socioeconomic development in sub-Saharan African countries.

The report further highlights the success toward malaria control, showing that that there has been a decline in malaria burden, and that morbidity worldwide has been reduced by 21 per cent and mortality by 29 per cent between 2010 and 2015. However, sub-Saharan Africa still accounts for approximately 92 per cent of all malaria deaths and cases.

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