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Malaria morbidity and mortality (1) is typically a function of:

  • Vector control as in control of malaria-infected mosquitoes.
  • Ease of access to medical service, accurate and timely diagnosis, and Artemisinin-based combination therapies.
  • Rural-to-Urban human population shift.
  • Clean housing, ample nutrition, hygienic water management.

In other words, more resources spent, better malaria control. After all, malaria and poverty go hand in hand. Obviously w.r.t. climate change, vector control or capacity of mosquitoes to breed and spread is the weakest link in the chain.

Increased emission of greenhouse gases into the atmosphere triggers climate change. In turn, this affects temperature, rainfall and relative humidity, the same variables that influence the habitats of vectors like mosquitoes (2, 3, 4).

  • More humans now live in cities, a change that in turn parallels the range of anthropophilic mosquitoes such as Aedes aegypti and Aedes albopictus (5).
  • For example, each degree increase in temperature per century has increased the Northern Italy and Alps habitat of the tiger mosquito, Aedes albopictus (6).
  • Tiger mosquito habitat is also expanding along the East and West coasts of the US (7).
  • The tiger mosquito in particular has relevance for human diseases because of key features that make it an optimal disease-spreading agent
    • Thrives in both urban and rural human habitats.
    • Flexible breeding sites.
    • Vast geographic range that includes both US coasts, Southern Europe, South America, Africa and Southern Asia.
  • During drought, again an increasing feature of climate change, we store water for longer periods, with fewer changes and cleaning, in turn leading to greater mosquito breeding and disease outbreaks (8, 9, 10).

Climate change and Malaria
Do temperature, rainfall and relative humidity affect malaria transmission? Epidemiological and lab studies suggest yes.

  • When annual average relative humidity is <60%, malaria transmission is strongly inhibited (11).
  • As temperatures increase, mosquitoes proliferate faster and bite more (12).
  • At 19oC, immature Plasmodium vivax takes 30 days to develop fully inside the mosquito At 28oC? Only 13 days (13).

Could climate change-related environment changes impact mosquito-borne diseases? Studies suggest yes.

  • One study (14) showed that coastal brackish water increases can influence mosquito-borne diseases in two ways
    • Increase local density of salt-tolerant mosquitoes like Anopheles sundaicus and Culex sitiens.
    • Make freshwater mosquitoes such as Anopheles culiefaciens, Anopheles stephensi, Aedes aegypti, Aedes albopictus salt-tolerant.
  • The compelling human relevance of this study?
    • For one, >50% of the global human population lives on land <60km from the seashore (14).
    • For another, coastal area population density is projected to increase from 87 persons per sq. km in 2000 to 134 persons per sq. km in 2050 (15).
    • Human population density is likely to increase particularly in tropical areas, where mosquito-borne diseases like malaria are already endemic.
    • Thus, as a result of climate change-related changes in relative water salinity, both mosquito breeding grounds and salt-adapted mosquito species could expand. In turn, they increase likelihood of spread of mosquito-borne diseases.
  • In the final analysis, climate change-related changes have the capacity to both augment and reduce spread of malaria as this table suggests (16; see figure below).


Climate change and Chikungunya
Let’s consider another mosquito-borne disease likely to spread due to climate change, Chikungunya.

  • With clinical symptoms similar to Dengue fever, this novel arbovirus was first isolated in 1952 among eastern Tanzania’s Makonde people suffering from a febrile illness similar to Dengue.
  • Named Chinkungunya which in the native Makonde language means ‘that which bends up‘, obviously a reference to the body contortions associated with its characteristic severe arthralgia (17, 18, 19).
    • ~2 to 6 days incubation following bite of an infected mosquito.
    • Starts with abrupt onset high fever for 1 to 7 days, crippling myalgias and lower back pain.
    • Severe arthralgias of ankles and wrists, conjunctivitis, maculopapular rash.
  • Infectious bites have a nearly 100% transmission rate (20).
  • Rarely fatal, complete recovery expected but with likely chronic arthritis for several months to years (21, 22).
  • For decades Chikungunya was known to cause episodic outbreaks in Africa, India and southeast Asia without much notice from global health monitors until 2005 when an outbreak on the French island of Reunion spread to India affecting at least 1.4 million people (23, 24, 25).
  • Many features were different about the 2005 outbreak
    • Mortality of 0.3 to 1/1000 symptomatic cases, much higher than in previous Chikungunya epidemics (26).
    • Hepatic failure, spontaneous hemorrhage, vertical mother-infant transmission (27, 28, 29).
    • Chronic, persistent arthritis with symptoms lasting even two years later (30, 31), leading to considerable disability, and public health and economic burden (32).
    • In the 2005 Reunion outbreak, Chikungunya switched vectors to the tiger mosquito, Aedes albopictus (33, 34). What are the implications of this switch?
    • Where previously, Chikungunya was primarily associated with woodland (sylvatic) mosquitoes such as Aedes africanus, A. furcifer, A. luteocephalus, A. neoafricanus, A. taylori (35, 36) that have limited habitat distribution, now it can spread faster and further, piggybacking on the vast range of the tiger mosquito (37, 38).
  • Is climate change the cause of Chikungunya’s increased severity? Maybe not. What’s apparent though is that its vector switch enables it to capitalize on the climate change-induced increased spread of Aedes albopictus. As a result, Chikungunya is likely to become endemic worldwide (5, 39, 40).

Climate change and Cholera

  • Vibrio cholerae, a bacterium, causes cholera. It associates with planktons and tiny crustaceans (copepods) for its survival, reproduction and transmission (41).
  • As sea surface temperatures rise, phytoplanktons increase, in turn causing the increase of cholera’s reservoir, i.e.  zooplanktons such as copepods (42).
  • For example, cholera outbreaks in coastal Bangladesh have been linked to plankton increases associated with local sea surface temperature increase (43).

Bibliography

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https://www.quora.com/Will-climate-changes-have-an-impact-on-health-and-affect-the-spread-of-diseases-such-as-malaria/answer/Tirumalai-Kamala

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