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It has been widely documented that the global climate is changing; the globe as a whole is warming up with such effects as the melting of the polar ice caps, and severe and erratic weather such as hurricanes, floods, and drought [Nabi & Qader, 2008]. The effect is also seen in form of changing vegetation with the ice caps on mountains giving way to warmer forests.
Insects are very sensitive to ambient temperatures; this is mainly due to their small size and cold-blooded nature; this affects their size, rate of maturation, reproduction, and activity [Biswas, et al, 1993]; taking this into account, there has been a growing concern regarding the effects of an increasingly warmer globe on the occurrence of insect-borne diseases in the world [Nabi & Qader, 2008]. Among the potential changes that may occur is the spread of disease to new areas stemming from the new ability of a vector to survive in that area. Alternatively, a change in climate may alter the magnitude and severity of outbreaks of disease [Biswas, et al, 1993].
This paper seeks to explore the nature of two vector-borne diseases, malaria, and dengue fever, in regards to the characteristics that would make them prone to effects of climate change, and to highlight some of the documented cases of such change.
The paper also seeks to give possible areas of research to provide a clearer picture of the effects of climate change on the occurrence of vector-borne diseases.
Malaria
Malaria is a vector-borne disease that has widespread occurrence in the tropical regions of the world; including parts of Asia, Africa, and America. The disease is potentially fatal and causes many deaths, especially in sub-Saharan Africa.
Etiology
The disease is caused by protozoan blood parasites of the genus plasmodium; the members of this genus can infect a wide range of animals ranging from mammals to birds; humans however can only be infected by four species; Plasmodium falciparum, Plasmodium over, Plasmodium vivax, and Plasmodium malariae.
Epidemiology
The parasites are spread through a mosquito bite from a female mosquito of the anopheles genus. The disease has a worldwide distribution although it is found mostly in the tropical regions where the climate can support the vector; consequently, the distribution of malaria is similar to the distribution of the vector; additionally, the prevalence of the disease in a population is pegged to the mosquito activity in the area; thus, disease outbreaks are usually seen in hot and wet weather [Rodgers, 2000]; as the multiplication of the vectors is highest during this weather.
About 250 million cases of malaria are reported worldwide every year; from these, about one million cases are fatal [Rodgers, 2000].
Clinical disease
The disease presents with a characteristic fever accompanied by joint pains, chills, emesis, and hemoglobinuria. The disease severity is usually, associated with the speed and effectiveness of the administration of therapy, the age of the patient (with children showing a more severe f,orm), and the natural immunity of the person against malaria parasites.
Dengue fever
Synonyms; Break-bone fever, Dengue hemorrhagic fever
This is an acute viral disease of man that is also spread by blood-sucking mosquitoes; it presents in form of a potentially fatal hemorrhagic fever [Kour, 2001].
Etiology
The disease is caused by viruses of the genus Flavivirus with four serotypes being attributed to the causation of disease; the viruses are spherical, enveloped, and possess a single-stranded RNA genome [Kour, 200]. The serotypes are sufficiently different to show no cross-protection between them.
Epidemiology
The disease is spread to man via a bite from an infected female mosquito of the species Aedes aegypti which is the main vector of the disease; however, Aedes albopictus has also been shown to have the capacity to spread the disease [Kour, 2001]. Members of the Aedes genus feed during the day.
The disease also has a worldwide prevalence with the tropical regions having the bulk of the burden; areas of Southeast Asia and parts of Australia are particularly endemic; the disease is however endemic in over 100 countries worldwide. It is estimated that there are 50 million cases worldwide with a larger 2.5 billion people being at risk of contracting the disease [Kour, 2001]. The misdiagnosis of mild febrile disease as influenza allows people to travel and spread the disease to new areas when they arrive; the disease can however only be spread via a mosquito bite or through exchthe range of blood products during the febrile period.
Clinical disease
The clinical presentation of the disease may range from an in-apparent condition to a fatal hemorrhagic fever. The common sincludeslude muscle and joint pains, rashes, fever; may progress into an abdominal involvement such as gastritis with accompanying emesis nausea and diarrhea [Kour, 2001]. Mild cases of undifferentiated fever are usually misdiagnosed as influenza.
Vector borne diseases and changing climate
As mentioned before, the environmental temperature and humidity are major factors determining the presence and the activity of insect vectors; the rate of maturation from an egg to an adult insect determine the time when the new insect will also produce eggs; therefore determining the rate in increase of the insect population in the environment [Rasgon et al, 2003]. This has been the mechanism that has been fuelling the seasonal variation of the population of mosquitoes (and by extension the prevalence of vector-borne disease) in tropical areas during the rainy season; apart from the temperature and humidity, the season also determines the availability of stagnant water breeding sites.
However, with the change in the global temperatures, the seasonal dynamics of the population may be altered. A net rise in temperature would increase the metabolic rate of the insect (as they are poikilotherms) thus allowing them to mature faster than before and also more activity. The change may also alter the way the pathogen is growing inside the insect vector. In addition to this, the ambient environment may dictate how long the disease vector survives; thus generally setting the period in which the vector has the opportunity to spread the disease [Thomas et al, 2004].
The rate of development of the pathogen inside the vector and the lifetime of the infected vector in the environment are both very important factors in determining whether the pathogen is passed to the next human host before the vector dies.
The average incubation period of malaria and dengue pathogens before a mosquito can become effective is fourteen days; on average, Aedes aegypti can survive for 50 days under natural conditions, thus having 29 days to spread the virus to a new host [Rasgon et al, 2003]. Any change that would increase this lifespan would definitely have an impact on the prevalence of the disease in the area; the opposite is also true. Experiment to reduce the lifespan of the mosquito by more than half (to 21 days) by infecting them with a bacterium of the genus Wolbachia, a common pathogen of the fruit fly led to less infective mosquitoes [Rasgon et al, 2003].
On the other hand, a change in precipitation, for example as a result of floods, may lengthen the breeding period of the insects thus allowing them to populate the environment over a longer period than before.
A change in climate could also alter the vegetation of an area; for example, global warming is resulting in the melting of ice caps on highland regions with the replacement of the vegetation with forests due to the warmer temperature and the moisture; this would allow an altitudinal and a longitudinal enlargement of the vector (and disease) zones.
Dengue fever: Climate change effects
In this case, elevated ambient temperatures have been shown to affect the Aedes mosquito and thus the prevalence of the disease in the vector zone. Laboratory experiments show that by elevating the temperature during the development of this vector in the larval stage, the resultant adult was smaller in size. Additionally, the range of flight and the rate of biting was also increased [Pant, 1973; Reiter, 1996].
Elevation of developmental temperature was also shown to increase the duration of survival of the adult mosquito [Ruenda et al, 1990; Tun-Lin et al, 2000]. In addition to this, the high temperatures were also shown to enhance the replication of the Dengue virus inside an infected mosquito thus making bites more infective [Vithanomsat et al, 1983; Watts et al, 1987; Koopman et al, 1991]. These findings were documented in Thailand where Dengue fever is endemic and occurs in seasonal variation linked to the rainy season [Thammapalo et al, 2005]. Similar findings linking the elevation of temperature to the rise in the prevalence of Dengue fever have been documented in Mexico, Australia, and America [Patz et al, 1996; Herrera et al, 1992].
Malaria: Climate change effects
There has been a steady increase in the prevalence of malaria in the world, especially in developing countries. There is, however, no consensus regarding the cause of this increase [Hay, 2000].
One school of thought blames the change in weather. This has a potential two-fold effect. Cases of elevated rainfall and flooding stemming from severe weather precipitating from global warming cause a lengthened period of breeding for the mosquito vectors; the result is a more severe outbreak of malaria that is above the expected prevalence of the rainy season [Hales, 2003].
Alternatively, the changing climate causes the changing of (particularly) highland climate and vegetation allowing for breeding of mosquitoes in areas where they previously couldn’t; thus leading to an altitudinal and longitudinal expansion of the malaria-endemic zones. The increase in the prevalence of malaria in the highlands of East Africa has been attributed to this phenomenon [Shanks et al, 2002].
The increase in the prevalence of malaria in Africa and other developing countries has alternatively been blamed on the failure of the public health authorities in the country to control mosquitoes and thus prevent malaria. It has also been blamed on poverty, where the people cannot buy facilities that would enable them to fight the disease; poverty also limits their access to healthcare [Hay, 200]. Poverty also dictates the living conditions of these people; relegating them to unsuitable habitats such as marshland or where public works such as faulty systems allow breeding of mosquitoes. The increase has also been linked to the acquisition of resistance to drugs by the Plasmodium parasite.
The effect of global warming has never therefore been conclusively linked to the increasing prevalence of malaria in the world. However, events that have been linked to warming such as the El-Niño phenomenon have caused a significant spike in malaria prevalence; thus increasing the suspicion of climate change. Therefore, there is no denying that the prevalence of malaria is linked to the climatic condition of the area.
Discussion
It has been proven by scientists above all doubt that the climate of the globe is changing. Of particular concern is the increase in the global temperature. Since 1861, the global temperature was shown to have increased by 0.60 centigrade [Nabi, Qader, 2008]. By the year 2100, it has been estimated that if the current trend continues, the temperature will rise by 1.4 to 5.8 degrees centigrade.
This rise in temperature not only has an effect on the metabolism of insects, but it also causes another climatic factor, such as floods, that alter the dynamics of the vector population. It would therefore be unwise to ignore the potential health factors regarding insect-borne diseases that might arise from this change [Epstein, 2000]. The steps that must be taken include the following fields.
Research vector control
Insects have in the past shown the ability to develop resistance to some of the chemicals formulated to control them. Consequently, reliance on these chemicals as the sole means of controlling a possible vector population explosion may be unwise. Research on vector control should concentrate on formulating new, more innovative methods of controlling disease vectors and stay prepared to fight such a war.
An example of these innovations is the use of bacteria that are harmful to the vectors to shorten their life-spans [Rasgon, 2003].
Research epidemiology
As we have seen in the case of malaria, scientist cannot agree whether the increase in the incidence can be attributed to climate change or to other factors. My recommendation is for the thorough study and characterization of all the disease that may have an implication of the future health of the world stemming from climate change. This would allow for proper mechanisms to be put in place either to avoid this situation or to deal with it when it arises.
Mathematical models
These have been used for a long time to predict the behavior of diseases as pertains their epidemiology. However, they have not, for a long time, been factoring in the issue of global warming as a possible factor in the outcome of a disease. This may have stemmed from the fact that the phenomenon has only been recently accepted as a scientific fact.
The epidemiologists should now endeavor to come up with models that can accurately predict the possible future effects of climate change on the incidence and prevalence of insect borne diseases.
Conclusion
The worst outcome that would arise from an outbreak of a disease is to be caught completely unawares. With the world getting smaller due to much improved transportation, effect of a potential such outbreak would not just affect the area, but also the whole world.
Some disease have been unanimously linked to the prevailing climate, more so the vector borne disease, since the climate is changing, it would be expected for the dynamics of the disease to change with it.
It is therefore prudent to stay on guard for such future occurrence.
References
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