A Good Example of Medical and Environmental Mystery — Disappearing Malaria Mosquitoes and the Paradoxical Concern for Children’s Future Health

© 2011 Peter Free

 

25 August 2011

 

 

Be careful what you wish for?

 

What follows is a potential case of what seems to be good news may actually have a “gotcha” lining.

 

Today’s reduced malarial mosquito populations may bounce back to cause even more disease than before in children who now lack malarial immunity.

 

The science underlying this concern illustrates the complexity of most kinds of environmental causation.

 

 

Malaria is declining in some countries — but why?

 

The incidence of Plasmodium falciparum malaria (the worst kind) declined 50 percent during the last decade in Eritrea, Kenya, Pemba, Tanzania (mainland), Zambia, and Zanzibar.

 

Most people assume that bed-net programs, indoor spraying, intermittent presumptive medical treatment, better diagnosis, and more effective combinations of drugs caused the reduced burden of the disease.

 

But recent research indicates that these interventions may not be the sole (or even the largest) reason for the reduced incidence.  Malaria also declined in places that lacked organized public health interventions.

 

Malaria vector mosquito populations have been declining in this region.  No one knows why.  So, we cannot be sure that the public health interventions that we thought were successful are actually the main cause of the reduction in malaria’s burden.

 

A recent paper focused attention on the causation question:

 

 

The present study assessed temporal changes in Anopheline populations in two highly malaria-endemic communities of NE Tanzania during the period 1998-2009.

 

 

Methods

 

 

Between 1998 and 2001 (1st period) and between 2003 and 2009 (2nd period), mosquitoes were collected weekly in 50 households using CDC light traps. Data on rainfall were obtained from the nearby climate station and were used to analyze the association between monthly rainfall and malaria mosquito populations.

 

 

Results

 

 

The average number of Anopheles gambiae and Anopheles funestus per trap decreased by 76.8% and 55.3%, respectively over the 1st period, and by 99.7% and 99.8% over the 2nd period.

 

 

During the last year of sampling (2009), the use of 2368 traps produced a total of only 14 Anopheline mosquitoes.

 

 

With the exception of the decline in An. gambiae during the 1st period, the results did not reveal any statistical association between mean trend in monthly rainfall and declining malaria vector populations.

 

 

Conclusion

 

 

A longitudinal decline in the density of malaria mosquito vectors was seen during both study periods despite the absence of organized vector control.

Part of the decline could be associated with changes in the pattern of monthly rainfall, but other factors may also contribute to the dramatic downward trend.

 

 

© 2011 Dan W Meyrowitsch, Erling M Pedersen, Michael Alifrangis, Thomas H Scheike, Mwelecele N Malecela, Stephen M Magesa, Yahya A Derua, Rwehumbiza T Rwegoshora, Edwin Michael, and Paul E Simonsen, Is the current decline in malaria burden in sub-Saharan Africa due to a decrease in vector population?, Malaria Journal 10:188, doi: 10.1186/1475-2875-10-188 (13 July 2011) (paragraph split)

 

 

Which other factors might account for reduced mosquito populations?

 

Changes in agriculture, land use, housing, and climate.

 

 

“Why should we care, Pete?”

 

Reduced populations of malaria-vectors (mosquitoes) means that human immunity weakens.  This most impacts children under five years old.

 

If we guessed wrong on why the incidence of malaria is declining, resurgent mosquito populations may send the malaria burden soaring beyond the peaks we used to have.

 

 

What to do now? — Prove causation

 

The research authors think that it is important to pin down what actually contributed to the decline in malarial incidence.

 

If public health interventions are not the primary cause for reduced disease incidence, then the risk for a resurgence of malaria is significant.  And we’ll essentially be back to step one:

 

In order to monitor and predict the risk of sudden epidemics it is, therefore, of outmost importance to identify the underlying causative factor or factors.

 

The findings suggest that the pronounced decline in malaria mosquito vectors over the study periods is not a consequence of bed net use or indoor residual spraying.

 

Although there may be an intriguing link to recent changes in rainfall patterns that requires further investigation, other potential explanations for the observed decline in vector populations in the two study communities should be explored, including an assessment of the potential role of changes in temperature, the ability for water to pool, agricultural activities, land use, vegetation, sibling species composition and impact of toxic substances and insect pathogens.

 

© 2011 Dan W Meyrowitsch, Erling M Pedersen, Michael Alifrangis, Thomas H Scheike, Mwelecele N Malecela, Stephen M Magesa, Yahya A Derua, Rwehumbiza T Rwegoshora, Edwin Michael, and Paul E Simonsen, Is the current decline in malaria burden in sub-Saharan Africa due to a decrease in vector population?, Malaria Journal 10:188, doi: 10.1186/1475-2875-10-188 (13 July 2011) (paragraph split)

 

 

The moral? — When evidence is limited, sound conclusions in science and medicine are hard to come by

 

Biology and public health are usually more complicated than they initially appear to be.