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Disease outbreaks, another effect of climate change?

We know that many infectious diseases depend on climatic factors such as temperature. So, can climate change cause an increase of the outbreaks? Let’s find out!

HEALTH AND CLIMATE CHANGE

 According to some surveys conducted by the Pew Research center, 54% of respondents believe that climate change is a serious problem and their major concerns include drought, intense rainfall and heat. If you are interested in to learn more about this survey, you can find them in the following article.

These changes have a negative effect on human health. The World Health Organization (WHO) expected that between 2030 and 2050 climate change will cause some 250,000 additional deaths a year. The effects can be very varied: deaths by  heat, floods, increase in respiratory diseases, stress etc. One of the important health effects is an increase in the transmission of infectious diseases.

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Graphic of impacts of climate change on human health (Photo: CDC)

Infectious diseases are closely related to  environment’s characteristics (such as temperature and humidity). In some cases, these diseases are transmitted by vectors (bats, arthropods, snails, rodents, ticks…). A  temperaturerising  will modified its geographical distribution, seasonality and population size. An example is  the presence of the mosquito Aedes albopictus, known as mosquito tigre, in Spain.

On the other hand, changes in the use of the soil, overcrowding of cities, poor hygienic habits and other socio-economic factors also have an effect in the transmission of certain diseases. For example, deforestation and poor hygiene of the population increases the breeding sites of the mosquitoes, causing an increase in the probability of malaria transmission.

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Human activities may effect diseases transmission rate. (Photo: OMS)

VECTOR DISEASES

Vector diseases are those that are transmitted through a vector animal (whether a mosquito, rodent, tick, snail, bat…). These diseases may be zoonotic (animal to human, as rabies) or antroponotic (among humans, such as malaria or dengue). If you want to know more about the effects of climate change on vector, feel free to access this article.

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Different types of vector diseases. (Photo: OMS)

There are many vector diseases which should be monitored in the coming years, as for example the malaria, dengue fever chikungunya, Boutonneuse etc. Let’s look at the two best known infectious diseases.

MALARIA

This disease is caused by parasites of the genus Plasmodium, which is transmitted by the bite of mosquitoes of the genus Anopheles. There are four different types of malaria, but the most deadly is that caused by the species Plasmodium falciparum.

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Plasmodium falciparum gametocyte. (Photo: CDC)

The WHO estimates that in the year 2013, 198 million people were infected,  584,000 of which died. It is expected that these numbers will increase due to climate change. Temperature rise leads to an increase in the infective period of the mosquito and the modification of vector’s geographical distribution. Possibly in the next few years, if the trend does not change, there will be an increase in the spread of the disease in endemic areas  and will probably resurface in other areas (red areas on the map).

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Estimation of the spread of malaria in 2050 (Photo: Randolph Rogers)

In Spain, the autochthonous malaria was eradicated in 1964. Currently, the spanish cases of malaria are imported from countries with indigenous malaria. Even so, note the geographic situation of our country, the rising temperatures, the presence of a competent vector and the presence of imported parasit, significantly increase the likelihood of disease’s transmission.

DENGUE FEVER

This is a viral disease (caused by viruses of the genus Flavivirus) that is transmitted by the bite of mosquitoes of the Aedes genus (including the Tiger mosquito). Dengue fever is a widespread disease in tropical countries, although its suffering geographical changes due to changes in temperature, precipitation and a demographic overcrowding of the cities.

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Structure of dengue fever virus (Photo: César Cabezas)

Before 1970, only nine countries had experienced serious dengue epidemic episodes. In recent decades, the cases have increased sharply. According to WHO estimates, each yerar are produced about 390 million infections,  23%  of which are clinically manifested.

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forecast of the spread of dengue fever in Europe during the twenty-first century. Expressed in nº of cases /100.000 habitants. (Photo: Moha Bouzid)

As in the case of malaria, current climatic variations alter the geographical distribution of the vector. As we can see in the previous map, the predictions for this century, if conditions do not change, are a significant dengue fever cases increase in Northern Europe (lighter areas are potential sites of infection). As we see in the case of Spain, the Mediterranean would be the region that would have more cases of dengue fever.

WATERBORNE DISEASES

Climate change also affects the water cycle. The news about weather disasters (floods, strong drought, torrential rains, hurricanes…) never cease to appear in the media. These climatic variations affect those diseases that are spread by water, either by contamination of the flows, by human migration and low hygiene that exist in certain places of overcrowded cities.

The most known diseases associated with floods and droughts are infections of Cryptosporidium or cholera. Let’s look at this last example.

CHOLERA

Vibrio cholerae is a bacilar bacteria that causes this disease. It is a diarrheal infection that suffer every year between 1.4 and 4.3 million people, 142,000 which end up dying. The transmission of this Bacillus is closely linked to environmental mismanagement. Heavy rains or flooding can cause water pollution, and extreme drought increases the bacterial charge of the existing flows.

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Microphoto of Vibrio cholerae (Photo: Louisa Howard).

During the 19th century, cholera spread across the world from Ganges (India). The last cholera epidemic began, as we can see in the map, in the South of Asia in 1961. Now cholera has been distributed worldwide due above all to human migrations (bacillus carriers), the agglomeration of people in suburban areas without hygiene habits and climate disasters. The WHO estimates that by 2030 there will be 10% more cases due to climate change.

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evolution of last epidemic of cholera (1961-2004). (Photo: IPCC)

It may not be possible to quantify in that measure climate change can affect the transmission of these diseases, since these depend on many other factors (demographic dynamics, immunization, etc.). Is worth mentioning, that the provisions set out in this article are assumptions obtained from current data. That means, that if the mechanisms for the reduction of global climate change works and environmental conditions improve, these data would no longer have any statistical value

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Remember that it is better to be safe than sorry!

Cares for the environment: the Earth is your home. 

REFERENCES

Maribel-anglès

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