Medicosnext
2020 will always be remembered as the year that COVID-19 pandemic spread across the world. The SARS-CoV-2 virus was transmitted from bats to humans and started a pandemic that is still ongoing almost two years later, and is sure to be remembered in the history books. Transmissible diseases have long been historically relevant, whether it’s the bubonic plague that wiped out almost one-third of Europe; or the introduction of communicable disease like measles, smallpox, whooping cough, etc., from the Old World to the newly discovered Americas during the Columbian Exchange that led to the decimation of more than 80 percent of the Indigineous Native American population from 15th century onwards.
The past hundred years have been crucial for the eradication of many infectious diseases. Till date only two diseases have been completely eradicated: smallpox and rinderpest. But through vaccination, new innovations in the medical field, and many global campaigns to eradicate diseases, we have successfully reduced the prevalence of many diseases such as polio, guinea worm, measles, rubella, mumps, etc., and even completely eliminated these diseases from certain countries or regions of the world.
However, while human led efforts have driven many diseases to the ground, other human activities have also created a new threat that looms on the horizon. Since the industrial revolution, human influences have led to the changes in the climatic conditions on earth. In the past several decades, these changes have become even more pronounced. Climate change is bringing in new threats that may lead to new communicable diseases or bring back old ones that could be devastating to people all over the world.
Climate change has become a much discussed issue in the national and international forums for good reason. Eight of the ten hottest years on record have occurred after 2010, and all ten of them have occurred after the year 2000. This has led to many global threats, including the melting of the polar ice caps and glaciers, rising sea levels, desertification, increasing frequency and intensity of natural disasters like hurricanes, floods, etc., and extinction of certain species and many more. These changes in the environment have also brought about many changes in the living environment of many disease causing germs and vectors, thereby increasing the threat of diseases, as a result of climate change.
When it comes to diseases, climate change impacts communicable disease in three different ways. The first way is by expanding the geographical area over which vectors and pathogens can survive. There are three determinants that affect vector-borne disease transmission – the vector survival and reproduction, the vector’s biting rate, and the pathogen’s incubation rate. Increasing temperatures and erratic rainfall patterns have led to serious changes in the environmental conditions all over the world and have increased the area over which pathogens and their vectors, such as insects and rodents can survive and reproduce. Pathogens, their vectors and hosts, are each able to survive and reproduce within a specific range of optimal climatic conditions. While the temperature and precipitation are the most important factors for their survival, other factors like altitude, wind, and even daylight duration play an important part in their life cycle and reproduction.
According to a 2008 study in the journal Nature, nearly one-third of emerging infectious diseases over the past 10 years were vector-borne. This finding correlated with the unusual changes in the climate. With infection-bearing vectors like mosquitoes and ticks chasing warmer temperatures, the diseases they carry are becoming more frequent and moving to newer areas than before.
A major vector borne illness is malaria which is transmitted by mosquitoes and occurs seasonally in South Asia. This disease and its link to climatic conditions has been studied for a long time. In the early twentieth century, the river-irrigated Punjab region experienced periodic malaria epidemics. It was then determined that excessive monsoon rain and high humidity was a major factor that increases mosquito breeding and survival, leading to higher occurrence of malaria. Recent studies have also shown that malaria risk increases every five years after an El-Nino event that disrupts the annual monsoon season.
Modelling of malaria shows that increase in temperature increases transmission of the disease. An increase of 2-3 degree Celsius in global temperature would increase the number of patients at risk of malaria by 3-5 percent, i.e. several hundred million. Further, the temperature increase would increase the seasonal duration during which malaria causing mosquitoes would be active, thereby increasing the seasonal duration of malaria.
Dengue is another climate sensitive disease that has substantially expanded its global range in the past 50 years. Dengue is now responsible for an estimated 10 000 deaths and 100 million infections a year in over 125 countries.
Similarly, other diseases like chikungunya and Zika are also expanding their reach as the geographical range of their vector, female mosquitoes of Aedes aegypti and Aedes albopictus species, increases due to climate change and their adaptation to human environment. These diseases have extended their reach beyond their endemic areas in Africa and Asia. Chikungunya reached the Americas in 2013 and led to more than a million cases within a year.
In the Americas, as the temperature increases, ticks and mosquitoes now thrive in places they had never been seen before. These tropical species are moving north, and they are bringing dangerous pathogens with them. The Zika virus, which is endemic to Africa, has spread to southern states of the United States like Texas and Florida, where they are being transmitted locally by mosquitoes.
The second reason why climate change increases the risk of communicable disease is because it increases incidences of interaction between humans and animals, leading to more instances of viruses jumping from animals to humans. Climate change and ensuing biodiversity loss has created imbalances in the natural ecosystems all around the world. Loss of predator species is creating an environment where their natural prey like rodents and insects can thrive. These species are also more resilient and are more capable of surviving in human made environments and more importantly, are more likely to carry diseases. Furthermore, due to the encroachment of humans into the natural habitats of animals through forest clearings and resource extraction, many species are now more vulnerable to human influences and more likely to pass along powerful pathogens to humans, thus increasing the chances of zoonotic diseases.
The current COVID-19 virus is an example of a disease with zoonotic origin where the virus jumped from bats to humans. Similarly, previous types of coronaviruses, including the SARS epidemic in the early 2000s, was transmitted from civet cats to humans, and MERS outbreak in 2012 travelled from camels to humans. The 2014 Ebola outbreak in West Africa is believed to have been transmitted from animals to humans when a boy dug into a tree stump that happened to be the roost of bats carrying the virus. Even HIV/AIDS has a zoonotic origin wherein the SIV virus switched from primates to humans. According to the U.S. Centers for Disease Control and Prevention, three-quarters of all new viruses have emerged from animals.
Scientists studying the different types of coronaviruses in southern China for years have long warned that swift environmental changes due to climate change, loss of biodiversity and human encroachment, was going to help viruses jump from animals to people.
Loss of biodiversity has also led to an increase in diseases in several parts of the world. In America, as the migratory birds have become less diverse, the birds that do survive also happen to be the ones that host the West Nile virus that causes West Nile encephalitis. This has resulted in an increased risk of the disease spreading to people more than before.
A study published in the journal Proceedings of the Royal Society B found that as the population of larger mammals is declining due to hunting and shifting climate patterns, smaller species like rats, bats, and other rodents are thriving as they are better able to adapt to the degraded environment. These rodents forage in the garbage bins, build nests in buildings, and live among the humans. They are able to transmit diseases to humans. In Panama, in the year 1999, when the annual rainfall increased by three times, the rat population exploded. In the same year, a fatal lung disease called hantavirus pulmonary syndrome was transmitted from the rats to humans through their saliva, feces, and urine.
Another example of a disease outbreak caused by zoonotic causes is the Nipah Outbreak in Malaysia in the year 1999 which inspired the movie Contagion. The clearing of the forests for palm plantation drove the fruit bats living in the forests to the villages, where they roosted along a hog farm. Their droppings fell into the pigsty and were eaten by the pigs. When the pigs were slaughtered and consumed by the humans it started an outbreak that killed more than a hundred people.
The final manner in which climate change could cause disease outbreak is by bringing back ancient “zombie” pathogens that have been frozen in the ice, and reviving them. Climate change is causing the melting of the polar icecaps and glaciers and thawing of permafrost. Permafrost is any type of ground that has been frozen for a long period of time (at least 2 years). Lying frozen and dormant in these permafrost are microbes that no longer exist elsewhere in the world. As the permafrost thaws, scientists speculate that they could release pathogens causing old diseases to emerge or reemerge.
In 2016, a remote part of Yamal Peninsula in Russia saw an outbreak of anthrax. The source of this disease was a frozen carcass of an infected reindeer that had thawed and released spores to nearby water and soil. The reindeer died more than 75 years ago and the anthrax causing bacteria remained in the ice in the carcass until the ice thawed and then went on to create a disease outbreak.
In 2015, researchers collected ice cores from the Guliya glacier in Tibet. In the ice cores they found 33 different viruses that had been frozen for nearly 15000 years. Twenty-eight of those viruses were new to science.
The bacteria and viruses found in the permafrost can be revived. Scientists have successfully revived bacteria retrieved from permafrost around the Kolyma River in northeastern Siberia and used it to infect amoeba. As the climate change leads to more thawing of the permafrost we might yet see diseases that have long been lost in time come back to haunt us.
As climate change causes more and more unforeseen effects in the environment, it will bring about more and more complications, many of which will affect human health. In the long term, the impact of climate change on human health will depend on whether or not we take effective steps to mitigate climate change and its effects.
