Mosquitoes carrying the malaria parasite have virtually disappeared in several parts of sub-Saharan Africa, Danish researchers reported in the journal Malaria Journal; they add that the incidence of Malaria in those regions is dropping dramatically, and nobody appears to know what the reason is. The scientists say they have some hypotheses, but cannot tell whether the disease is being eradicated, or whether it is just a case of malaria resting up before coming back with a vengeance.

Dan Meyrowitsch, Associate Professor at the Dept of Health Services Research, the University of Copenhagen, said:

“Many of our fellow malaria researchers think that the fall in countries such as Tanzania, Eritrea, Rwanda, Kenya and Zambia shows that all the control programmes are working, particularly the use of mosquito nets.

“That just isn’t the whole story. For more than ten years we have been collecting and counting the number of mosquitoes in Tanzanian villages. The number in our traps fell from 5300 in 2004 to just 14 in 2009, and these were from villages without mosquito nets.”

There was a 99% drop in the mosquito population that carries malaria towards the end of the 1990s, which experts put down to a serious fall in rainfall. This present phenomenon could be caused by global climate changes, the authors wrote.

Meyrowitsch said:

“From 2003 to 2009 the volume of precipitation was more stable, but the rain was more chaotic and fell outside the rainy season. And this may have disturbed the natural cycle of mosquito development.

Of course it is great that the number of malaria-related fatalities among children has fallen drastically in the last five or six years, but we need to know why!” “

As the investigators can rule out mosquito nets, they are wondering whether the mosquitoes have become ill, perhaps people have been applying pesticides, or whether it has something to do with the chaotic precipitation patterns that have developed recently.

Meyrowitsch explained:

“Unless we find the answer we will not be able to predict when the malaria mosquitoes will come back, and that could rapidly prove critical.”

A large number of people, including children, have not been exposed to malaria over the last six years. They have either lost or never developed immunity to Plasmodium falciparum, the malaria parasite that historically has caused many deaths in sub-Saharan Africa.

If infected mosquitoes suddenly return in large numbers, there is a risk of serious malaria epidemics and many deaths, unless the people and their health authorities are prepared.

Historically, malaria with the parasite Plasmodium falciparum has been the cause of many deaths and illnesses in sub-Saharan Africa. Huge areas of SSA (sub-Saharan Africa) over the last few years have become virtually disease free, even in areas with no or very little intervention.

The Danish research team set out to determine what temporal changes had occurred in Anopheline populations in two communities of north east Tanzania from 1998 to 2009 – these two communities have historically been highly malaria-endemic ones.

Their research covered two periods, the 1st Period (1998-2001) and the 2nd Period (2003-2009). CDC light traps were used to collect mosquitoes on a weekly bases from 50 family homes. Nearby climate stations were used to gather rainfall data. This data was analyzed to determine whether climate could be linked to mosquito populations.

They gathered data on Anopheles gambiae and Anopheles funestus, two types of mosquitoes that can carry the malaria parasite.

Below are some highlighted results of their findings:

  • 1st Period (1998-2001)
    Anopheles gambiae population dropped by 76.8%
    Anopheles funestus population dropped by 55.3%
  • 2nd Period (2003-2009)
    Anopheles gambiae population dropped by 99.7%
    Anopheles funestus population dropped by 99.8%
    During a 12-month period in 2009 only 14 mosquitoes (of both types) were found, out of 2,368 traps.

The researchers identified a link between a drop in the An. Gambiae mosquito population and a decline in rainfall only during the 1st Period, not the 2nd Period.

Written by Christian Nordqvist