Anthropogenic climate change, human health and the emergence of new diseases

Throughout history, the Earth has experienced several periods in which the climate changes radically. These changes occurred due to a mixture of different factors that eventually caused extreme changes in the organisms that inhabited the planet. Climate change is not a new phenomenon on the planet. However, current climate change is anthropogenic. “Anthropogenic” refers to the fact that changes in a certain environment are derived from human intervention or activities.

Anthropogenic climate change is today an undeniable fact. This phenomenon has generated alterations in the global climate system that currently affect millions of people in the world. One of the most relevant points when addressing this issue is that "climate change is an emerging phenomenon with an inequitable distribution, since the greatest risks are suffered by the poorest populations, who contribute the least to the growth of said phenomenon" [1].

According to the Pan American Health Organization (PAHO) climate change is the greatest threat to human health in the 21st century [2]. This organization points out that changes in climate will have direct and indirect impacts on human health. The direct impacts are related to the effects that heat waves, droughts, storms, hurricanes, the rise in sea level, etc. may have on health. While the indirect impacts are related to the increase in respiratory diseases, vector-borne diseases, food insecurity, malnutrition, etc. It is important to note that the impacts of climate change on human health will mainly affect vulnerable populations as well as people living in countries with weak health institutions.

The rise of zoonotic diseases

The World Health Organization (WHO) defines zoonosis as “a disease or infection that is naturally transmitted from vertebrate animals to humans” [3]. Anthropogenic climate change has generated countless alterations in many ecosystems on the planet. Many of the human activities involve humans being in contact with animals that may be carriers of disease.

According to the WHO, zoonotic pathogens can spread to humans through any point of contact with domestic, agricultural or wild animals. It is important to note that for decades human beings have been in contact with domestic animals. However, it is increasingly common for human activities to bring us into contact with wild species that could carry unknown diseases. The destruction of habitats has also caused many animal species to migrate to spaces closer to human settlements, increasing the possibility of wild animal-human contact.

The WHO currently has a record of more than 200 kinds of zoonoses: "rabies, leptospirosis, anthrax, SARS, MERS, yellow fever, dengue, HIV, Ebola, chikungunya and coronaviruses are phenomena zoonotic, as well as common influenza” [4]. The World Wildlife Fund (WWF) has indicated in several of its reports that the probability of a virus being transmitted between a wild animal and a human being has been increasing due to a series of factors such as: the destruction of natural ecosystems, the increase in human populations in contact with wild animals, the trade and consumption of wild species, among others.

Alterations in the global climate derived from climate change are also a factor that favors the increase in zoonoses. Roberto Mendoza Alfaro, professor at the Faculty of Biological Sciences of the Autonomous University of Nuevo León, explained how "the increase in temperature in various regions of the planet as a consequence of climate change has led to diseases such as malaria, African trypanosomiasis, the disease Lyme disease, tick-borne encephalitis, yellow fever, and dengue fever have the necessary conditions to spread over a broader geographic range” [5].

Case study: Nipah virus

According to the WHO, the Nipah virus was first recognized in 1998 in Malaysia during an outbreak among pig farmers. The natural host of the virus is the fruit bat of the family Pteropodidae, genus Pteropus [6]. This virus was transmitted to humans through a pig that had been in contact with a carrier bat. During subsequent outbreaks in Bangladesh and India in 2001, the Nipah virus spread directly from person to person. The WHO has estimated that the estimated case fatality rate of the virus is 40% to 75%. The foregoing has varied in each of the outbreaks that have occurred since the appearance of the virus to the present.

When the virus began to be studied, experts found that the Nipah virus was new to science. Medical studies later revealed that fruit bats in the region have carried the Nipah virus for centuries. The transmission of this virus from bats to humans was possible due to a combination of factors such as: destruction of bat habitat, changes in weather patterns in the region, expansion of industrial agriculture, and human-animal contact. infected households.

As of 2018, records indicate that there have been sporadic Nipah virus outbreaks in Malaysia, Singapore, India, Bangladesh, Cambodia, Thailand, and Madagascar [7]. The transmission of this virus spread through several villages and it was not until it reached India that health experts began to worry about the possibility of the Nipah virus spreading rapidly through the large population of this country.

Since 2015, the WHO has considered the Nipah virus as a "priority disease" due to its potential to become a pandemic virus. However, to this day there is still no vaccine or treatment for those infected by this virus. Nipah outbreaks have occurred mainly in rural and marginalized areas of the aforementioned countries.

The communities in which people infected with this virus have been detected are usually dedicated to livestock or agriculture. In most cases, these communities do not have basic health services, which has made it difficult to quickly detect people who could be carriers of the virus. It should be noted that countries where Nipah outbreaks have been detected are particularly susceptible to the effects of climate change.

Final thoughts

According to WHO data, between 2030 and 2050 climate change will cause approximately 250,000 additional deaths each year, due to malnutrition, malaria, diarrhea and heat stress. It is possible that over time the effects of climate change generate suitable factors for the appearance of new diseases such as the Nipah virus. A study published in the journal Science of the Total Environment pointed to the possibility that the effects of climate change in China have led to the outbreak of Covid-19 [8] .

Studies on Covid-19 are extremely new. It is clear that with the passage of time the experts will have more information about the virus. However, it is a fact that Covid-19 is a zoonotic virus, which implies that an infected animal transmitted the virus to humans. As of April 2021, around 3 million people have died from this virus worldwide. It is possible that in the future the numbers of people who have died as a result of climate change will have to include the deaths of people from new viruses.

Many countries implement actions to combat the effects of climate change. However, the reality is that much work remains to be done on the matter. Currently many zoonotic diseases could be linked to the effects of climate change. The Covid-19 pandemic works as a current example of what a pandemic implies in the 21st century. The pandemic scenario invites us to reflect on whether we can somehow avoid an outbreak of equal or greater magnitude in the future.

It is important to highlight that climate change especially affects marginalized populations, which means that these people are at risk of being the focus of the emergence of new diseases. The Nipah virus case illustrates how a disease can easily spread among rural communities in various countries without people having access to adequate health resources. Today more than ever, the fight against climate change is of vital importance to the entire world. With special emphasis on managing to safeguard the lives of the most vulnerable populations, which are generally the ones that contribute the least to the increase in climate change.

The effects of climate change will eventually modify many of the planet's spaces. It is important to advance in the fight to protect natural spaces and habitats for wild animals to try to keep the ecological balance intact. One must never lose sight of the fact that any change in the ecological balance of the Earth can have great consequences for all the inhabitants of the planet, including consequences for our life as we know it.