But, rates of malarial morbidity and mortality have been declining rapidly in parts of sub-Saharan Africa, including Eritrea, Rwanda, Zanzibar, Pemba, Tanzania mainland, Kenya and Zambia (as reported here), and in some cases it's not at all clear why. Some of the decline is due to widespread use of bed netting to prevent mosquito bites, some to improved medical treatment, and some to use of pesticides, but the decline is also being seen in areas where none of this is happening. A paper in the current issue of Malaria Journal by Meyrowitsch et al. suggests:
...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 e.g. improved housing construction.Or, the decline might also be attributable to a decrease in the mosquito population due to changing rainfall patterns caused by climate change, an hypothesis tested by Meyrowitsch et al. They collected mosquitoes weekly in light traps in 50 households in northeast Tanzania in in two separate study periods (1998 - 2001 and 2003 - 2009), an area with no organized mosquito control. It's a rural area; the study communities have around 1000 inhabitants, and people live in "mud-walled houses thatched with dried coconut leaves." There are generally 2 rainy periods per year here, a long one in March-June and a shorter one in October-November.
Insect counts showed a marked decreased in the mosquito population over the 11 year study period (the primary mosquito vectors for malaria in sub-Saharan Africa are Anopheles gambiae and Anopheles funestus).
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.Below are the tables of results for the two sampling periods. (If you click on a table, it will actually be readable.) If you look at rows 5 and 6 of, the total mosquito counts by year, you'll notice that the decline is not linear. Instead, it seems something dramatic happened between 1998/9 and 1999/2000, the year with the most significant decline in mosquito numbers, and then again between 2004 and 2005. The number of traps used in the first first period was less than the second, so the two periods aren't totally comparable so let's just stick with the second. After 2006, the number of An, gambiae rose again, and then something happened between 2008 and 9 to drastically reduce insect numbers by 2009. And it doesn't seem to be differences in total rainfall. The authors don't discuss this apparent flux, but instead treat the decline as a general trend. But, it might be that something distinct happened in 1999 and 2004 that explains the sharp decline, which could be overlooked by treating the decline as linear.
In any case, clearly the mosquito population has dropped precipitously. The authors don't have morbidity and mortality statistics for the study period, but they assume they both fell. And these same kinds of results have been reported for various other parts of sub-Saharan Africa. Indeed, they point out that a study on the island of Pemba, Tanzania, found that malaria transmission began to fall before the start of the malaria control program there.
They conclude that the unpredictability of the rainfall resulting, presumably, from climate change could be the cause of these declining mosquito counts, rather than absolute differences in monthly rainfall. And/or the decline may be due to:
...changes in socio-ecological conditions in the study area (e.g. changes in temperature, ability for water to pool, deforestation or land-use, change in the use of agricultural pesticides or insecticide-like compounds not directly applied for targeting malaria vectors, improved house constructions or changes related to agricultural activities). An increase in predatorily pressure on the mosquito population (e.g. birds or invertebrates) or an insect pathogen that specifically targeted mosquitoes, e.g. a bacterial, viral or fungi infection, could also potentially have induced the observed declines.This decline in mosquito numbers, and thus in malarial infection, is very interesting in its own right, at least to us, not to mention very important if it signals the beginning of the decline in malarial infection. But it's also interesting that the reason for the decline is so elusive -- another instance of the difficulty of determining causation. Indeed, the explanation for the elimination of malaria from the United States early in the 20th century is still debated. If the decline is in fact a trend, leading to elimination of the mosquito in areas where malaria has been endemic, it doesn't matter so much why it happened -- again, as in the US. If, however, the population numbers are going to continue to jump around, potentially rising again, it's very important to figure out why. And, as the authors state, if malaria is going to stick around, even if at lower levels, and children aren't going to be exposed as frequently, they won't develop immunity so that the few infections they do get will make them sicker.
One can say that the rate of malaria is not just about physical ecology or human biological susceptibility, but also about human culture. If that's the explanation, it's very curious and interesting. That's because in the first place, endemic malaria may have been due to the spread of settled agriculture, exposing land to water pooling where nearby mosquitoes could breed. So culture enabled malaria to rise. And now, if the suggestions are true, culture is leading to its decline. In both cases much, at least, of this was unintended (such as global warming). And if mass scale agriculture or global human population eventually decline, fewer people sharing the world's resources may ironically mean higher risk of malaria, as in the bad old days.