Download Climate Change and Infectious diseases

Climate Change and Infectious diseases
Dr. Andrew K. Githeko PhD
Kenya Medical Research Institute
Climate and Human Health Research Unit
Conference communications
Introduction
The transmission of many human infectious diseases such as malaria, cholera, dengue
fever, meningitis, and Lyme disease is a function of interaction with the environment
which results in varying rates of transmission. Climate, which is a major component of
the environment, has a direct impact on the population sizes of the disease vectors and the
development rate of the infectious agents in the vectors and in the environment. Climate
is one of the key determinants of diseases distribution in time and space. The interactions
of the atmosphere, oceans, terrestrial and marine biospheres, the cryo-sphere and land
surface determine the Earth’s surface climate. Human activities such as use of fossil
fuels, land use change and agriculture has increased the concentration of greenhouse
gases namely carbon dioxide (CO2 ), methane (CH4 ) and nitrous oxide (N 2O). These
gases trap heat in the atmosphere thus increasing the earth’s surface temperatures and
the precipitation regimes. Its is estimated that by the end of this century the mean
earth’s temperature will increases by 1.4 to 5.6 oC and accompanied by a more
frequent and intense extreme events associated with outbreaks of infectious diseases.
Climate change involves a change in the mean state and in the departure from the mean.
While the change in the mean state is a slow process, climate variability is increasing at
a higher rate resulting in frequent diseases outbreaks.
A reversal of climate change may take several decades even after reducing the
greenhouse gases and therefore human beings must adapt to the health consequences of
climate change to reduce its potential negative effects. Adaptation requires skills,
considerable level of economic development, and political will, factors that are not
found in sufficient quantities in developing countries. Thus developing countries, many
of them located in endemic areas, are likely to be most adversely affected by climate
change.
Infectious disease
Vector-borne diseases
Recent studies of disease variations associated with inter-annual climate variability (such
as those related to the El Niño cycle) have provided much useful evidence of the
sensitivity to climate of many disease processes. This is particularly so for mosquitoborne diseases. The combination of knowledge from such empirical research; the
resultant theoretical understanding of biological and ecological processes; and the output
of scenario-based modelling; leads to several conclusions about the future effects of
climate change on human populations.
Higher temperatures, changes in precipitation and climate variability would alter the
geographic range and seasonality of transmission of many vector-borne diseases. Mostly,
range and seasonality would be extended; in some cases reduced. Currently 40% of the
world population lives in areas in which endemic malaria occurs. In areas with limited or
deteriorating public health infrastructure, increased temperatures will tend to expand the
geographic range of malaria transmission to higher altitudes and latitudes. Higher
temperatures in combination with conducive patterns of rainfall and surface water will
extend the transmission season in some locations. Changes in climate mean conditions
and variability would affect many other vector-borne infections (such as dengue,
leishmaniasis, Lyme disease, and tick-borne encephalitis) at the margins of their current
distributions. For some vector-borne diseases in some locations, climate change will
decrease the likelihood of transmission via a reduction in rainfall, or temperatures that are
too high for transmission.
A range of mathematical models, based on observed climatic effects on the population
biology of pathogens and vectors, indicate that climate change scenarios over the coming
century would cause a small net increase in the proportion of the world population living
in regions of potential transmission of malaria and dengue. An alternative modelling
approach, based on a direct correlation of the observed distribution of disease distribution
against a range of climate variables, suggests that there will be little change in malaria
distributions, as areas that become permissible for transmission are balanced by others
that become unsuitable for at least one climatic factor. Neither approach attempts to
incorporate the effects of socioeconomic factors or control programmes on the
distribution of current or future disease.
Table 1 Main vector-borne diseases: populations at risk and burden of diseases
Disease
Vector
Population Number
Disability
Present
at risk
currently
adjusted
distribution
infected or life
years
new cases lost a
per year
Malaria
Mosquito
2
400 272 925 000 39 300 000
Tropics/
million
subtropics
(40% world
population)
Schistosomiasis Water snail 500-600
120 million
1 700 000
Tropics/
million
subtropics
Lymphatic
Mosquito
1
000 120 million
4 700 000
Tropics/
filariasis
million
subtropics
African
Tsetse fly
55 million
300 00 –
1 200 000
Tropical
trypanosomiasis
500 ,000
Africa
(Sleeping
sickness)
Leishmaniasis
Sand Fly
350 million 1.5
2 1 700 000
Asia,
million
Onchocerciasis
River blindness
Black fly
American
Triatomine
trypanosomiasis bug
(Chagas’
disease)
Dengue
Mosquito
Yellow fever
Mosquito
Japanese
encephalitis
Mosquito
120 million
18 million
100 million
16-18
million
3
000
million
468 million
in Africa
300 million
Tens
millions
200,000
50,000
1 100 000
600 000
of
1 800 000b
Not
available
500 000
Africa,
Southern
Europe,
Americas
Africa,
Latin
America,
Yemen
Central and
South
America
All tropical
countries
Asia
a
The Disability –Adjusted Life Year (DALY) is a measure of population health deficit
that combines chronic illness or disability and premature death. Numbers are rounded up
to nearest 100 000.
b
Data from Gubler and Metzer.
Water-borne infectious diseases
There are complex relationships between human health and water quality, water quantity,
sanitation and hygiene. Increases in water stress are projected under climate change but
it is difficult to translate these changes into risk of water-related diseases.
Heavy rainfall events can transport terrestrial microbiological agents into drinking-water
sources resulting in outbreaks of crytosporidiosis, giardiasis, amoebiasis, typhoid and
other infections. Recent evidence indicates that copepod zooplankton provide a marine
reservoir for the cholera pathogen and thereby facilitate its long-term persistence and
disseminated spread to human consumers via the marine food-web. Epidemiological
evidence has pointed to a widespread environmental cause for recent outbreaks of
cholera, rather than a point source contamination as seen in Peru in 1991 and East Africa
in 1997/98. Strong links are found between cholera infections, bathing and drinking
water from east African lakes. Cholera epidemics also are associated with positive
surface temperature anomalies in coastal and inland lake waters.
Global warming is expected to lead to changes in the marine environment that alter risks
of bio-toxin poisoning from human consumption of fish and shellfish. For example, biotoxins associated with warm waters, such as ciguatera in tropical waters, could extend
their range to higher latitudes. Higher sea surface temperatures would also increase the
occurrence of algal blooms that may affect human health directly, and which are also
ecologically and economically damaging.
Changes in surface water quality and quantity are likely to affect the incidence of
diarrhoeal diseases. This group of diseases includes conditions caused by bacteria such as
cholera and typhoid as well as parasitic diseases such as amebiasis, giardiasis and
cryptosporidium. Infections with cholera and typhoid bacteria are dependent on the
concentration of the pathogens in water or food. Currently the World Health
Organization (WHO) estimates more than 1 billion people worldwide to be without
access to safe drinking water, and that every year approximately 4 million die
prematurely because they do not have access to safe drinking water and sanitation.
Climate can increase directly the amount of pathogen in the water through increasing the
biotic reservoir of the infectious agent (cholera) or by decreasing the amount of water in a
river or a pond and thus raising concentration of the bacteria (typhoid). Floods can cause
contamination of public water supplies with both bacteria and parasites as surface
discharge flows into rivers and reservoirs, while drought can increase the concentration
of pathogens in the limited water supplies. A reduction in the availability of clean water
increases the risk of drinking contaminated supplies and also reduces the amount of water
available for personal hygiene thus leading to skin infections.
Africa
It is estimated that by 2050 the Sahara and the semi-arid parts of southern Africa may
warm by 1.6OC, while equatorial countries such as Cameroon, Uganda and Kenya could
experience warmer temperatures of 1.4 OC . Recent analysis of global mean surface
precipitation from 1901-1995 indicates that some regions in Africa have an increasing,
while others have a decreasing trend. While East Africa has had an increasing trend of
wetness, West and North Africa, are likely to have a decreasing trend . These are broad
overviews and may have low certainty when the regions are downscaled.
Climate change will have short and long-term impacts on disease transmission. For
example, a short-term increase in temperature and rainfall as was seen in the 1997/98 El
Niño caused Plasmodium falciparum malaria epidemics and Rift Valley fever in Kenya.
This may have been due to accelerated parasite development and an explosion of vector
populations. This phenomenon can be attributed to inter-annual climate variability.
However, these changes can also reduce malaria transmission as was observed in
Tanzania. There is emerging evidence that besides the seasonal extreme climatic events,
there is a general elevation of mean temperatures and in some cases, precipitation. For
example, the mean rate of temperature change in Africa from 1901-1995 has been 0.39
o
C/century. While there has been a reduction in precipitation in many part of the
continent there has been a mean increase of 300 mm/century in the East African region.
Such changes are likely to support rapid development of malaria vectors and parasites in
regions where there has been low-temperature transmission restriction. On the other
hand, increased warming will have a negative effect at the extreme high-temperature
range of malaria vectors. In Senegal Anopheles funestus has virtually disappeared and
malaria prevalence dropped by more than 60% over the last 30 years due to reduced
precipitation and drought.
Currently the seventh cholera pandemic is active across Asia, Africa and South America.
During the 1997/98 El Niño, the rise in sea-surface temperature and excessive flooding
provided two conducive factors for cholera epidemics which were observed in Djibouti,
Somalia, Kenya, Mozambique and the United Republic of Tanzania, all of which border
the Indian Ocean. Cholera epidemics have also been observed in areas surrounding the
Great Lakes in the Great Rift Valley region. A significant association between bathing,
drinking water from Lake Tanganyika and the risk of infection with cholera has been
found. It is likely that warming in these African lakes may cause conditions that increase
the risk of cholera transmission.
Major epidemics of bacterial meningococcal infection usually occur every 5-10 years
within the African meningitis belt, and typically start in the middle of the dry season and
end a few months later with the onset of the rains. Between February and April 1996, the
disease affected thousands of people in parts of northern Nigeria, many of whom died31 .
This epidemic spread from the traditional meningitis belt to Kenya, Uganda, Rwanda,
Zambia and the United Republic of Tanzania. One of the environmental factors that
predispose to infection and epidemics is low humidity. To date this disease has been
limited to the semi-arid areas of Africa, suggesting that future distribution could expand
due to increased warming and reduced precipitation.
Plague is a flea-borne disease and the major reservoirs of infection are rodents such as the
common rat. Rodent populations fluctuate widely with the availability of food, which in
turn depends on rainfall. Exceptionally heavy rainfall can increase food abundance; as a
consequence the population of rodents and fleas may multiply rapidly. During severe
droughts, rodents may leave their wild habitats in search of food in human houses and
this can also increase the risk of plague transmission. Plague outbreaks in Africa have in
the last few years been reported in Mozambique, Namibia, Malawi, Zambia and Uganda
Europe
The most important vector-borne diseases in Europe and some of the former Soviet
Union Republics are malaria, which is transmitted by mosquitoes and Lyme disease by
ticks. The evidence that climate change has increased the risk of these diseases is weak,
because of the relatively subtle changes in climate to date and the over-riding impact of
major environmental changes created by expanding populations, alterations in
agricultural practice and changing socio-economic conditions. However, there should be
no room for complacency, as the capacity exists for an increase and expansion of many
vector-borne diseases in many parts of the continent.
Some countries in Eastern Europe with restricted access to water at home could be
affected by any climate-related decrease in supplies. For instance, an increase in the
frequency and intensity of extreme precipitation could increase the risk of transmission of
cryptosporidiosis.
The distribution of carriers of food-borne diseases such as flies, cockroaches and rodents
could change due to climate change. In the United Kingdom of Great Britain and
Northern Ireland, a study of food-borne illness found a strong relationship between
incidence and temperature in the month preceding the illness.
Leptospirosis, a disease associated with flooding, is a major concern in some parts of
Europe. Outbreaks of the disease have been reported following floods in Ukraine and the
Czech Republic in 1997 and Portugal in 1967. As well as the direct injuries and
infections resulting from flooding, psychological distress including cases of suicide has
been associated with the event.
South America
The most important climate-sensitive vector borne-diseases in South America, as far as
the numbers of people affected are concerned, are malaria, leishmaniasis, dengue fever,
Chagas disease and schistosomiasis. Numbers of cases of these diseases reported to the
Pan American Health Organization in 1996 are shown below:
Table: Vector– borne diseases in South America, 1996 – Source: PAHO, 1998
Disease
Malaria
Dengue
*Chagas disease
Schistosomiasis
Number of cases
877,851
276,758
5,235,000
181,650
*Cases of Chagas disease have been estimated from the number of people exposed
New cases of cutaneous leishmaniasis varied from 250 per year in Bolivia (1975-1991) to
24,600 in Brazil (1992) and those of onchocerciasis were around 9,200 in 1992 .
Other vector borne diseases which have a relatively low number of cases occurring each
year, and which may be sensitive to climate shifts are yellow fever (522 cases in 1995),
plague (55 cases in 1996), Venezuelan equine encephalitis (25,546 cases in 1995), and
other arbovirus infections. Up to 1991, in the Brazilian Amazonia alone, 183 different
types of arbovirus were isolated and among these 34 are known to cause human disease,
sometimes in explosive epidemics. One of these, the Oropouche Fever Virus, is known to
occur in cycles associated with the beginning of the rainy season.
Recent estimates based on the Hadley Centre's coupled atmosphere-ocean general
circulation model, HadCM3, projected that additional people at risk of infection due to
year round transmission of malaria in South America will range from 25 million by year
2020 to 50 million by 2080
In semi-arid zones in Mexico, rainfall has been observed to cause outbreaks of bubonic
plague probably as a result of an increase in the rodent reservoirs. Rodents escaping
floods in Colombia are suspected to have been the primary cause of leptospirosis
outbreaks. The effects of water-borne diseases are well documented in this region.
Between 1991 and 1996 cholera affected 21 counties in Peru resulting in almost 200 000
cases and 11 700 deaths. Climate variability was linked to later outbreaks in Peru and
Ecuador during the 1997/98 El Niño event. Besides cholera in Peru, other diarrhoeal
diseases such as Salmonella typhi have been linked to environmental change, climate and
sanitary conditions.
North America
The recent importation of West Nile viral encephalitis into the New York area in 1999
marked the first time this virus had been found in North America. Whether or not the
extreme record-breaking summer drought along the East Coast affected Culex mosquito
populations that can carry West Nile virus currently remains unanalyzed.
Birds are the natural hosts for the West Nile virus, a zoonotic disease. Such zoonotic
diseases, including tick-borne diseases, some mosquito-borne encephalitides, and rodentand flea-borne diseases are more difficult to predict and control since these involve
intermediate reservoir hosts in the environment. Also, to the extent that climate change
may have indirect impacts on vegetation and ecosystems that can affect determinants of
these diseases, projected changes are not straightforward.
The hard tick, Ixodes scapularis, transmits Borrelia burgdorferis, a spirochete, and the
causative agent for Lyme disease, the most common vector-borne disease in the U.S.,
with 15,934 cases in 1998. Other tick-borne diseases are the Rocky Mountain Spotted
Fever (RMSF), and Ehrlichiosis, the later having first being recognized in the mid-1980s.
Tick and host mammal populations involved are influenced by land use/land cover, soil
type, elevation, and the timing, duration, and rate of change of temperature and moisture
regimes. The relationships between vector life stage parameters and climatic conditions
have been verified experimentally in both field and laboratory studies. Climate change,
therefore, could be expected to alter the distribution of these diseases. According to one
modelling study, in the southern U.S., RMSF may decline due to ticks' intolerance of
high temperatures and diminished humidity .
Of the water-borne diseases, giardia cysts are fairly common in treated water in Canada
(18.2%) and very frequent in raw sewage samples (73%). The pathogen cryptosporidium
is also widely distributed and capable of causing large-scale outbreaks. For example, in
1993, more that 400 thousand cases (including 54 deaths) from a cryptosporidium
outbreak were reported in Milwaukee, Wisconsin. A positive correlation between rainfall,
concentration in river water and human diseases has been noted for both
cryptosporidiosis and giardiasis.
Asia, Australia and the West Pacific Islands
The Asian continent spans through the tropical to the temperate region. Plasmodium
falciparum and P. vivax malaria, dengue and dengue hemorrhagic fever and
schistosomiasis are endemic in parts of tropical Asia. In the past 100 years mean surface
temperatures have increased by 0.3-0.8 OC across the region and is projected to rise by
0.4- 4.5 OC by the year 2070.
An increase in temperature, rainfall and humidity in some months in the Northwest
Frontier Province Pakistan has been associated with an increase in the incidence P.
falciparum malaria while in the North east region of the Punjab, malaria epidemics
increased five- fold in the year following the El Niño and in Sri Lanka the risk of malaria
epidemic increases four- fold during an El Niño year. In the Punjab, epidemic are
associated with above normal precipitation while in Sri Lanka, below normal
precipitation.
According to WHO, many countries in Asia experienced unusually high levels of
dengue/dengue hemorrhagic fever in 1998 the activity being higher that in any other year.
It has been suggested that a major contributing factor to this activity may be changes in
.
the weather patterns, such as the El Niño
Laboratory experiments have demonstrated that the incubation period of dengue (DEN-2)
could be reduced from 12 days at 30OC to 7 days at temperatures equal to or greater that
5
32 OC (32-35 OC) in Ae. egypti thus climate change can have an effect on the
transmission of the virus. Dengue has been reported in several Small Island States in the
Pacific region. It has been shown in some of these islands that rainfall and local
temperatures correlate with the southern oscillation index (SOI) a component of ENSO
(El Niño Southern Oscillation). Furthermore a positive correlation was found between
SOI and dengue in 10 out of 14 of the island states.
Water-borne diseases such as cholera, and various diarrhoeal diseases such as giardiasis,
salmonellosis and cryptosporidiosis, occur commonly with contamination of drinking
water in many south Asian countries. These diseases could become more frequent in
many parts of South Asia in a warmer climate.
Conclusions
Besides the existing divers of infectious diseases such as seasonal weather variation,
social-economic status, vector control programmes, environmental changes and drug
resistance, climate change and variability are highly likely to impact upon the current
disease epidemiology. The effects are likely to be expressed in short-term epidemics to
long-term gradual changes in disease epidemiology. Recent results in Kenya suggest that
climate accounts for up to 50% of the anomalies in hospital based highland malaria cases.
Adaptation to climate change will depend to a certain extent of the level of health
infrastructure in the affected regions. Some regions such as Africa and South America
have a great diversity of disease vectors that are sensitive to climate change and more
efforts will be required to contain the expected disease epidemiology. Furthermore
climate variability, unlike any other epidemiological factor, has the potential to
precipitate simultaneous and multiple diseases epidemics. Climate change has far
reaching consequences beyond health and touches on all life support systems. It is
therefore a factor that should be rated high among those that affect human health and
survival
References
1
Githeko A. K. Lindsay S. W. Confalonieri U, and Partz J. (2000) Climate changes and
Vector borne diseases: A regional analysis Bulletin of the World Health Organization 78
:1136-1147
2
Climate Change and Human Health 2 nd Edition. Risks and Responses
World Health Organization: 2004