|Malaria in Kenya
Cool It, British
Cool It, American
AL GORE´S PRESENTATION
In his film, Al Gore shows a diagrammatic picture of mosquitoes swarming around at the slopes of a mountain range, first at relatively low altitudes, and later also at higher altitudes. The text is:
"There are cities that were founded because they were just above the mosquito line. Nairobi is one. Harare is another. There are plenty of others. Now the mosquitoes with warming are climbing to higher altitudes."
There seems to exist no official definition of what is meant by a "mosquito line". You might compare with the term "tree line", which is a line above which no trees can grow. By analogy, you would assume that a "mosquito line" is a line above which no mosquitoes can live. But this interpretation would not fit in relation to malaria in Kenya.
Instead, you would have to understand the term as a line above which malaria is not present every year, but only in certain years, when epidemics occur.
LIMITING THE OCCURRENCE OF MALARIA
There are several malaria parasites, the most widespread of which are Plasmodium vivax and Plasmodium falciparum. The former occurs or has occurred also at rather high latitudes on the northern hemisphere, and is not the most dangerous one. The latter is restricted to the tropics, and is the most deadly species.
The number of
days required for P. falciparum parasites to complete that part of its
development which takes place inside the mosquito depends on
temperature. The lower the temperature, the longer the duration of
parasite development, and the lower the probability that the mosquito
is still alive when the parasites are ready to infect humans. The
relationship is as follows (source:this link):
survival in mosquito
Below 18° C parasite survival is so low
that a population of parasites cannot survive. At 22° C and above,
survival is so high that a successful life cycle from mosquitoes to
humans and back to mosquitoes will permanently function. In regions
with such temperatures, there will be "stable transmission" of malaria,
i.e. there is a permanent risk of infection. When temperatures range
between 18° and 22° C, the risk of infection is variable.
In addition, rainfall is also important,
determining the abundance of mosquitoes.
a country like Kenya may be divided into four different zones with
respect to malaria: 1) zones with stable malaria. 2) zones with
seasonal malaria, i.e. only at those times of the year when
conditions are favourable. 3) zones prone to malaria epidemics, i.e.
areas where malaria is not present every year, but with outburst of
epidemics in certain years when favourable weather conditions prevail
for at least c. three months. 4) zones with no malaria (mainly zones
where malaria never has entered, or has been eradicated).
depend on altitude. Every 100 m increase in altitude corresponds to a
0.5° C decline in temperature. In Kenya, regular occurrence of
malaria is roughly restricted to altitudes below 1600 m, but epidemics
occur in some years at 1700-2300 m, and malaria has occasionally been
recorded as high as 2400 m.
situated at 1660 m and thus may be said to be just above the line
delimitating the zones of regular occurrence. The coldest month, July,
has average maximum temperatures of 20.6° C, i.e. temperatures less
than optimal for the P. falciparum parasite. Thus, it is true that
Nairobi is just inside the zone where malaria occurs only as epidemics,
not as a permanent or seasonal penomenon. If this is what is understood
by "the mosquito line", Al Gore is approximately right.
MALARIA IN RELATION TO ALTITUDE ETC.
The Nandi and Kericho districts in the western Kenyan highlands were originally free of malaria. For this reason, they were the preferred districts for European settlers in the years around 1900. A sanatorium for colonial officials was built in Nandi because malaria did not exist there. The construction of a road connecting Uganda with the sea led to increases in transport, trade and settlement. The first malaria epidemic occurred in 1918, when troops were being resettled after the First World War. Apparently, infected people brought the disease to the district. Over the following twenty years, malaria spread up onto the high plateaus.
infected persons seems to have broken the infectious cycles - i.e. too
few parasites survived in humans to efficiently infect the next
generations of mosquitoes. In any case, epidemics became rarer, and
without any special efforts against malaria, the Nandi and Kericho
districts had very little malaria during the years 1950 to 1970. After
1970, malaria became gradually more frequent, and a series of severe
epidemics occurred during the 1990s. One of the causes may have been
the increasing human population - the more people, the greater the
chance that an infected mosquito actually finds a person to bite. In
addition, parasites that had become resistant to the anti-malarial drug
chloroquine began to spread in the 1980s (Björkman &
Bhattarai (2005): Acta tropica 94: 163-169).
is if changing climate may also have contributed to this pattern. For a
country in the tropics, we would expect that average temperatures have
increased by less than c. 0.5° C, and such a slight increase would
be barely noticeable. It would correspond to temperature zones climbing
100 m upwards on the mountain slopes. There exists a series of weather
data for a tea estate in the Kericho district, see this
figure 4.12. The text accompanying the figure states that there are no
obvious effects of "warming" in this area since 1967. This is not quite
true, however. The maximum
temperatures show a slightly rising trend (non-significant); the trend
appears much more clearly if you count how many consecutive months each
year have temperatures above 22° C. Only from 1987 onwards do we
have years where the maximum temperatures are 22° or more the whole
year round, that is, only from this year onwards could malaria occur
permanently throughout the year. Thus, the slight rise in coherent
periods with relatively high temperatures, especially during the 1990s,
coincides rather precisely with the increasing permenanence of malaria
in the district.
analysed this temperature series. A study group led by S. I. Hay has
analysed weather data from the region; they have found that although
there is a significant upward trend in temperatures from 1970 onwards,
this trend is what mathematicians call "a random walk" and thus does
not represent a true upward trend (see this link). In another paper (see this link), the authors write: "In essence, climate
and climate variability had not changed, so could not have caused the
malaria resurgences". However, others dispute their conclusions (link), and a repeated statistical study of
temperature changes in the region, by M. Pascual and others (link), finds that temperature trends in the
region parallel global trends: a temperature decline from 1950 to 1970,
and a rise of about 0.5° C from 1970 onwards. This trend is
significant beyond what is explained by "red noise", approximately the
same as "random walk". Thus, they directly reject the conclusion of Hay
et al. Furthermore, they find that the temperature increase is within
an interval where the effect on the malaria parasite could be
factors that might favour the malaria parasite have acted
simultaneously: increasing human populations, increasing resistance to
drugs, and increasing temperatures. It is not easy to tell what factor
has been most important. It is quite possible that the factors have
interacted: if climate change and population increase create better
conditions for the malaria parasite, then the chance that the parasite
will develop drug resistance is increased. Another study in a district
likewise shows a temperature increase and a concomitant increase in
malaria in 1987 (Loevinsohn (1994): Lancet 343: 714-718). In this study, variations in malaria
incidence could be explained very precisely by variations in
temperature. And, as a contrast to the situation in the Kericho
district in Kenya, there were no confounding factors: no road building,
no increase in population, no local increase in drug resistance etc.
This points to temperature as a decisive factor.
There is also
a study of malaria on Mount Kenya (Chen et al 2006)
which indicates that malaria and its mosquito host have spread there
recently at rather high altitudes.
Furthermore, a relationship between temperature changes and malaria is demonstrated by a study performed in Madagascar, see this link. Here, the author found that when analysing temperatures of just those months in the year when the infectious cycle would normally start, these temperatures are found to explain a very large proportion of the variation in malaria incidence. In the author´s words: "This review suggests that the rejection of climate-disease associations in studies so far published may not have used biologically relevant climate parameters."
relationship between altitude and the rate of infection with the
falciparum malaria parasite does exist, is well demonstrated in this
paper: Drakeley et al. (2005): Journal of infectious diseases
191: 1589-1598. Here, the incidence is
found to decrease by 20 % for every 100 m increase in altitude.
In conclusion, we see just what we would
expect to see when trends are just emerging: an increasing prevalence
of malaria, especially at higher altitudes, and a relationship between
this and the variations in temperatures. But the climate changes are
still so small that they are barely significant, and only very precise
analyses, analysing exactly those months that are most important for
the life cycle of the malaria parasite, give significant results. There
are several other factors which very likely contribute to the increase
in malaria incidence, and this further obfuscates the relationship
with temperature. So, if you believe that climate change is underway,
then you can say that the "signal" (the malaria-temperature
relationship) is only just beginning to appear above the "noice" (other
factors affecting malaria). If temperatures indeed continue to
increase, the malaria "signal" should become stronger in future.