1. The Potential Impact of Global Climate Change on the
Facilitated Emergence of Malaria and Leishmaniasis in Non-Endemic
Areas and the Possible Shortcomings of Climate Change Research
Modeling Dave Porter, 2011 ABSTRACT In recent decades, the impacts
of global warming has seen increased discussion, and opposite sides
are currently locking horns about whether it is anthropologically
accelerated or a further progression of a natural cyclical process.
However, there are some solid conclusions that can be made, given
the levels of current data. It has been observed that eleven of the
last twelve years were among the warmest since the beginning of
systematized temperature recording, in terms of average global
temperature (Holy et al, 2010). Rising sea levels, increased
degradation of our atmospheric ozone, and the increase in average
temperature in both terrestrial and aquatic environments, the
latter being incredibly sensitive to change, may create
irreversible changes to our world and way of life. There is another
potential effect of global warming that is backed up by strong
observational evidence there is a link to the increased emergence
of parasitic infections and the increase in global temperatures.
Traditionally, the historical patterns of vector range ecology and
disease transmission rates have been used as a basis for the design
of control and prevention strategies to protect the health of the
public as well as the agriculture that sustained them. That had
been possible only because the global climate and most regional
environmental conditions seemed to remain relatively constant year
in and year out (Suntherst, 2004). This is no longer the case, and
scientists and researchers must adapt to these changing conditions.
As the distribution range of parasitic infections increases beyond
the usual tropical and sub-tropical limitation seen, populations of
humans and animals will be put at risk for infections not normally
observed in the region. This report will detail the emergence of
leishmaniasis and malaria and trematode infections in non-endemic
areas, facilitated by the onset of global warming as well as
potential shortcomings of the computer models usedto predictthese
events. THE POTENTIAL INCREASES IN MALARIA EMERGENCE RISK
FACILITATED BY CLIMATECHANGE Malaria is a parasitic disease caused
by members of the Plasmodium genus, which are transmitted to human
hosts by mosquitoes of the genus Anopheles. Four major species of
Plasmodium cause disease in humans P. falciparum (having the most
significant fatality risk), P. vivax, P. ovale, and P. malariae.
Malaria is endemic to Africa (especially sub-Saharan Africa, where
its economic and social effects are devastating), Southeast Asia
(where drug- resistant strains are emerging at alarming rates) and
parts of Central and South America. The Anopheles vectors require
ample rainfall and humid climates with consistently high
temperatures for breeding and this coincides with the incidence of
malarial transmission as well (Paaijmans et al, 2009). The
sporogonic phase of the parasite, occurring in the mosquito, is
most critically impacted by temperature. Paaijmans et al reference
the basic reproductive number (R0), which defines the number of
cases of a disease that arise from the index case introduced into a
population of susceptible hosts. R0 can be linked exponentially
with the success of the sporogonic phase of the Plasmodium life
cycle, because it influences the number of infected vectors that
survive long enough no transmit the parasites to humans during
blood
2. meals (Paaijmans et al, 2009). The possible interaction with
global warming rates is obvious as the latitudinal and altitudinal
range of vectors increases, coupled with a productive sporogonic
phase, the population of infected mosquito vectors is increased in
these areas (Holy et al, 2010). Interestingly, there is a strong
statistical correlation between the diurnal temperature range and
the sporogonic development of Plasmodium (Alonso et al, 2011,
Paaijmans et al, 2009). Diurnal temperature fluctuations straddling
a mean temperature of more than 21o C (70o F) decrease the rate of
parasite development (thereby decreasing R0) compared to constant
diurnal temperatures at respective level (Paaijmans et al, 2009).
Conversely, fluctuations around mean diurnal temperatures less than
21o C increase parasite development (increasing R0) compared to
constant equivalent temperatures. It should be noted that adult
mosquitoes themselves are not adversely affected by temperature
until it surpasses 35- 36o C (95-97o F), when the mosquitos
mortality rate begins a significant increase (Paaijmans et
al,2009). There is a meteorological principle that states that
large bodies of water tend to reduce daily temperature ranges
relative to those of waterless areas, and this may in turn allow
global warming facilitated malaria emergence to become more
prominent in coastal regions and maritime harbors. The humidity in
air near bodies of water is higher than that of dry, inland air,
and because of waters high specific heat value, the air temperature
of this moist air will be less (assuming equal sunlight). The
observation that P. falciparum sporogonic rates are increased when
mean diurnal temperature fluctuate slightly around 21o C (Paaijmans
et al, 2009) lends credence to this hypothesis. Areas near a
significant water source tend to have larger human populations than
dry landlocked regions as well, because of ease of commerce and
transport. In addition, it should be noted that these bodies of
water might promote breeding of mosquitoes, if the salinity levels
of the water doesnotimpede larval growth. Some opponents to these
theories argue that increases in epidemic malaria in East African
highlands are not due to climate change, but due to other factors
(Alonso et al, 2010). Findings found that although temperatures
tended to fluctuate around a mean high temperature of 19- 21o C,
malaria outbreaks in 1994 and 1997 were not strongly correlated
with temperature, at least to the extent that Paaijmans et al
describe. A consolidated figure from the 2010 work of Alonso et al
is featured below. From this figure, which shows the number of
clinically diagnosed malaria cases recorded from 1966-2002 at an
inpatient hospital in highlands of the Kericho district of western
Kenya, along with the respective temperature readings for each
respective year, it can be seen at first glance that spikes in
temperature fluctuating near 21o C do not always coincide with
increased malaria diagnoses in this region (Alonso et al, 2010).
There are several other factors that may contribute to this
finding; not limited to an increase in clinical knowledge and
malarial diagnostic prowess from 1970 to 2002, the emergence of
HIV/AIDS in the population, an increase in travel to endemic
lowland areas by the observed population. Land use was not likely
to be significantly affected, because Alonso et al
3. stated that this data was taken on a permanent tea farm with
a relatively constant population of workers and their dependents. A
constant increase in temperature,leading to a net increase of 1o C
in 30 years, was observed and documented (Alonso et al, 2010). This
observation was further investigated through climatological
modeling, which led to interesting results. A condensed figure
depicting the results of Alonso et al is shownbelow. This figure
shows the observed cases of malaria taken from the aforementioned
Kenyan hospital records (a red line in both graphs) with the median
number of expected cases of malaria (the dark grey line in both
graphs; calculated from simulated models) and the expected cases in
the 5th -95th percentile of expected cases (the shaded light grey
areas; also calculated from simulated models) to serve as a margin
of error (Alonso et al, 2010). The top graphs dark and light grey
components represent the estimated number of malaria diagnoses
modeled with the trend of increasing temperature and standard
environmental variables. The dark and light grey components of the
bottom graph depict the estimated number of cases without
temperature increases of 1o C per 30 years being considered in the
model. The significant discrepancyin malarial epidemics in the late
1990s in the bottom graph is indicative that temperature may play a
factor in determining malarial epidemics. Comparing the two graphs
in this figure can allow for the visualization of this principle in
the top graph, the median number of expected malaria cases appears
to be on a steady increase, which may coincide with the average
temperature increase in the region. The analogous line on the
bottom graph does not increase at all, further illustrating the
possibility that temperature is a factor in malarial epidemics.
Although a correlation may be made, more analytical research and
experimentation is needed to demonstrate the strengthof
thiscorrelation. In another modeling experiment, Holy et al
describe a potential emergence of malaria in Germany, where the
disease was once endemic. Historically, through various
anthropological interventions such as wetland drainage, DDT
application and improved medical care and diagnosis, P. vivax was
eradicated from Germany, although it should be noted that the
indigenous Anopheles vectors have persisted up to the present day
(Holy et al, 2010). Because the vector is still present and
breeding successfully, a return of Plasmodium to the region through
global warming facilitated migration of infected mosquitoes could
have grave consequences. In August 1997, two autochthonous cases of
malaria were reported from a hospital in Germany which could be
traced back to infections by Anopheles plumbeus, which had bitten
an immigrant from Angola, a country in south-central Africa, with a
chronic case of P. falciparum malaria (Holy et al, 2010). Two maps
of Germany, quantifying the potential malarial transmission months
during two differing timeframes are shown. These maps were created
by modeling the basic reproductive rate (R0) of the Anopheles
vector native to Germany Anopheles atroparvus infected with
Plasmodium vivax, the malarial agent most likely to returnto
Germany(Holyetal, 2010).
4. From these figures, which colorfully depict the seasonal
malaria transmission months from 1961-1990 and 1991-2007 in
Germany, it can be deduced that the length of the malaria
transmission season has increased in a majority of Germany (Holy et
al, 2010). This information can be extrapolated and used to predict
the risk of malaria infection if the causative organism presents
itself in Germany once more. Of significant interest are the
regions in southwest Germany, where the transmission season lasts
for five months out of the year (the regions of dark red). These
areas, near the Frankfurt/Rhine-Main Metropolitan Region
(population 5.6 million), are of elevated risk of potential
malarial epidemics, should Plasmodium return to the region for
multiple reasons, not limited to the length of the malarial
transmission season and the relatively dense populations. It is of
epidemiological significance that this metropolis borders the Rhine
and Main Rivers, which could facilitate the spread of malaria to
other major cities in Germany through the transport of Plasmodium
along established commercial shipping routes. The Rhine and Main
could serve as breeding grounds for the local Anopheles as well,
perpetuating the effects of a potential Plasmodium resurgence.
Additional modeling should be done to predict the effects of these
possible events. THE POTENTIAL INCREASES IN LEISHMANIASIS EMERGENCE
RISK FACILITATED BY CLIMATECHANGE Another parasitic disease that
may potentially become more prevalent worldwide through global
warming facilitated ecological range expansion is leishmaniasis.
Leishmaniasis is caused by three different complexes of Leishmania
parasites, and is distributed worldwide. In North America, the
female sand flies of the genus Lutzomyia are responsible for
transmitting the New World form of the disease through their blood
meals while in parts of Europe, Africa, Asia and the Middle East,
the responsible vectors are sand flies of the genus Phlebotomus.
Phlebotomus are the major vectors involved in the transmission of
Old World cutaneous leishmaniasis, which is caused by the
Leishmania tropica complex. In Central and South America, New World
cutaneous leishmaniasis is caused by the L. mexicana complex. The
L. donovani complex the causative agents of visceral leishmaniasis
(the most deadly form of leishmaniasis) are distributed in Central
America (L. chagasi), India (L. donovani) and the Middle East (L.
infantum)
5. and are of significant public health importance in these
respective regions. Mucocutaneous leishmaniasis is caused by the L.
braziliensis complex, and is endemic to the countries of the Amazon
River Basin, most notably Brazil. Although the increase in
leishmaniasis in recent decades is due to economic development,
human migration and civil wars, rapid global urbanization and the
subsequent invasion of the sand fly habitat by humans, the spread
of leishmaniasis may also be affected by climate change and the
currenttrendof global warming. New World cutaneous leishmaniasis,
caused by the L. mexicana complex, is endemic along the border
between the United States and Mexico, where it is transmitted
mainly by Lutzomyia diabolica and Lutzomyia anthophora with a
highest incidence in the autumn months (Gonzlez et al, 2010).
Several reservoir hosts are suspected, including the wood rats of
the genus Neotoma, which occupy niches along the USA- Mexico
border, as well as in the eastern United States. Gonzlez et al
constructed a computer model that would predict the climate change
induced ecological range expansions of Leishmania, Lutzomyia as
well as Neotoma in an effort to predict the spread of cutaneous
leishmaniasis in the United States and northern Mexico. Current
warming rates, regional economics, environmental use trends, and
the population density of vectors, reservoir hosts and potential
human hosts were taken into account while programming the model
(Gonzlez et al, 2010). A consolidated figure from the work of
Gonzlez et al is shown showing the estimates of the ecological
range of Lutzomyia diabolica and Neotoma floridana isshowntothe
right. The top graph depicts the modeled predictions of the range
of the Leishmania vector, Lutzomyia diabolica, in 2080 while the
bottom graph depicts the modeled range of the common Leishmania
reservoir host, Neotoma floridana in 2080. Experts do not currently
classify leishmaniasis as an endemic disease to the eastern United
States, although isolated cases have been documented (Gonzlez et
al, 2010). However, given the nature of the modeled predictions in
the aforementioned figure, current climate change trends may
increase the range of Lutzomyia and Neotoma, posing public health
risks to the populations of these regions. Though the predicted
ecological range expansion of Lutzomyia diabolica is less extensive
than that of fellow vector Lutzomyia anthophora, the northward
shift is more pronounce, as indicated in figures not shown in this
report. Because of this result, its a strong possibility that the
potential spread of New World cutaneous leishmaniasis in eastern
North America will be limited by the ecological expansion of the
sand fly Lutzomyia diabolica (Gonzlezetal,2010). Also worth
elaborating upon is the potential that the current southern
boundary range of Neotoma floridana may potentially become
unsuitable for the reservoir host to flourish, and would likely
migrate northward (Gonzlez et al, 2010). Judging by additional
modeling figures of other species of Neotoma as well as that of the
aforementioned N. floridana in the work of Gonzlez et al, which are
not shown in this report, this is a very possible outcome.
6. However, it is to be noted that there are multiple reservoir
hosts for New World cutaneous leishmaniasis not in the Neotoma
genus, including other rodents native to northern Mexico(Wynsberghe
etal,2000). REVIEW OF SEVERAL CONFOUNDING VARIABLES THATMAY EFFECT
CURRENTCLIMATECHANGEMODELS In addition, it is very important to
remember that global warming facilitated parasitic disease
emergence in non-endemic regions should not be considered an event
solely caused by holistic climate change, itself. Climate change is
a complex,multifactorial event. The multiple anthropological and
natural environmental alterations that contribute to climate change
may have profound effects on parasites, their vectors, and their
reservoir hosts as shown in the chart below (Suntherst, 2004),
which proposes multiple ways that certain contributors to
accelerated climate change can also affect parasitic disease
emergence. All these factors contribute in different ways to
parasitic disease emergence. It is important to consider each both
individually and holistically when proposing climate change models
as well as subsequent parasitic emergence prevention and control
measures. For example, if global warming (an increase in
temperature) is being caused by the destruction of large forested
regions (a decrease in habitat); it would be inappropriate to only
program the climate change computer
7. model to take only temperature into account. Deforestation
removes heat sequestering and shade-providing vegetation, which
leads to an increase in sunlight and UV rays reflecting off the
now-barren ground, which increases temperature. In addition,
deforestation lowers humidity levels (less evapotranspiration from
the forest), increases the speed of winds (due to less resistance
from foliage) and prevents the sequestration of atmospheric toxins,
such as ground level ozone. It is also important to take these
factors into account when proposing intervention strategies and
prevention plans, because they may allow for more pronounced
outcomes(Bushetal,2011). There exists an opposition to the methods
used to predict global warming, saying that cannot possibly
accurately predict the effects of global warming on a grand scale,
because of the multifactorial causes. For example, some experts
argue that it is not plausible to use models to predict the future
distribution of parasitic vectors because it is not possible to
know how land-use will change (Alonso et al, 2010). Alonso et al
also criticize models that do not take the effect of temperature
and humidity into account, which may confound the results and
invalidate the conclusions made from said results. Another
informative figure from the elegant work of Robert Suntherst can
further illustrate this principle,andcanbe seenabove. The
differences between these maps of Australia can be easily seen. Map
A shows the risk of P. falciparum malaria in a generalized way,
taking into account only the temperature needed for sporogonic
development within the mosquito. Map A does not include the
important variables of ideal developmental humidity and the
geographical range of A. farauti; the only Anopheles vector
indigenous to Australia, which only inhabits the northern
peninsulas of the Australian continent (Suntherst, 2004). These
maps can serve as forewarnings for the scientific community and
those involved in climate change research and its effects on
multiple factors, not just parasitic disease emergence. More
accurate modeling methods are needed before the risks and potential
consequences of climate change can be concretelydetermined.
CONCLUSIONS Whether or not it is due to actions of humankind or a
natural cycle, the Earth is warming at a rate that may have
potential consequences. Although the future effects of this
relatively slow process (slow by our own reckoning, not that of
geologic time) may not be realized in our lifetimes, our
descendants may experience detrimental effects to the biosphere not
limited to coral reef destruction due to increasing ocean
temperatures, ozone depletion due to increased greenhouse gas
production, and parasitic disease emergence through expansion of
natural vector and parasite biotic ranges.
8. Some climatologists have predicted that if current rates of
global warming continue unchecked, average global temperatures will
rise by 6.4o C by 2099 (Holy et al, 2010). The effects of range
expansion of malaria and leishmaniasis may have far-reaching
consequences as the diseases emerge in latitudinal and altitudinal
areas where they are not currently found due to climate
restrictions. It is important to remember that climate science is a
relatively new discipline and modeling methods need additional
improvements before solid predictions may be made about the future
effects of climate change. However, it would be unreasonable to
dismiss the very real possibilities of future vector and parasitic
ecological range expansions due to the effects of climate change.
More research must be done on this topic to discover possible
preventativeactions. REFERENCES 1.) AlonsoD, et al.(2010).
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