The world is currently plagued by an upsurge in outbreaks of diseases like Ebola, Zika and MERS. Is global warming responsible?
New York: Global temperature is rising at an alarmingly high rate, and will keep on rising, probably at a faster rate, if left unchecked. For decades, global warming has been implicated in a multitude of public health woes. For the last two decades, a sudden surge in outbreaks of new or lesser known diseases like Ebola, SARS, MERS and Zika have many directly pointing fingers at global warming as the trigger behind these devastating scourges. However, finding a direct link between global warming and infectious diseases is notoriously difficult.
Over the last century, the temperature of our world has consistently increased as a direct result of many human activities. Fueled by economic growth, we have achieved a dramatically high population size and rapid urbanisation to accommodate more people than ever that are moving into towns and cities. The result of both of these achievements is an unsustainably high consumption rate. To meet the demand of this insatiable consumption, we have, in turn, converted large swaths of primary forests into agricultural land. All these activities either contribute large amount of greenhouse gases (like carbon dioxide and methane that facilitate increase in atmospheric temperature) to the atmosphere or create urban heat islands, which eventually cause global warming. This man-made warming of the world is unlikely to be checked any time soon as developed countries are reluctant to reduce their consumption while developing powerhouses like India and China try to emulate the developed countries by achieving higher economic growth.
Global warming and disease risk
With the increase in global temperature, it was expected that many newer, nastier pathogens will emerge from places such as melting arctic ice that may have ancient pathogens trapped in them. Global warming can also facilitate invasion of vectors, blood-sucking tropical insects, to temperate regions that are becoming warmer. This invasion by vectors, in turn, can spread vector-borne infectious diseases beyond their present distribution. For instance, by 2020, it was predicted that globally 60% more people could be at risk of getting malaria with an increase in global temperature. The effect of climate change is also likely to be prominent in case of pathogens that are transmitted environmentally. This makes sense because these pathogens, such as fungi, parasitic worms and water-borne bacteria are dependent on environmental conditions for their development, replication and transmission.
A direct link between climate warming and infectious diseases is often difficult to prove. To begin with, we are only starting to understand how a host or a parasitic organism may respond to changes in ambient temperature. Likely, the response will manifest in their physiology, immunology or even behaviuor. Additionally, global warming brings with it changes in precipitation, humidity and myriad other environmental factors. How organisms will respond to the sum total of these environmental factors is expected to be extremely complex. Furthermore, the effects of climate change in the form of diseases are often visible only in the form of higher-level (population or community) disruptions such as mass mortality events [Box 1]. But, to understand the connection between the two, it is essential to observe the initiation of diseases in individuals, which often goes unnoticed. For instance, two decades ago biologists first observed frog deaths worldwide. Only much later, we came to realise that the reason behind the amphibian extinctions was a group of parasitic fungi called chytrids. These fungi became particularly lethal after changes in environmental temperature that caused depressed immunity and higher susceptibility to diseases in amphibians. However, such seminal and painstakingly done studies connecting evidence between different levels, from individuals to ecosystem, are few and far between.
Finally, interventions by humans often obscure the signature of climate change. This is particularly true for human diseases, since humans regularly interfere with the spread of disease with medical treatments, vector-control and infrastructural changes. However, this is not often the case for diseases of the wildlife, and hence the link between climate warming and infectious diseases is more readily seen in the wild. In spite of these challenges, scientists have recently recorded significant expansion in the geographic range of multiple human viruses including bird flu, coronaviruses (that caused severe acute respiratory syndrome; SARS), Ebola and arboviruses(dengue, chikungunya and Zika to name a few) due to factors such as urbanisation, globalisation and habitat destruction.
Indeed, global data on infectious diseases points to a recent, multifold increase in Emerging Infectious Diseases (EIDs) that are newly spreading, such as Ebola and Zika, or re-surging of diseases that were once considered eradicated or under-control, such as malaria and Lyme disease. There is also evidence that warmer temperature can increase human exposure to diarrhoeal diseases such as cholera. With the rise of sea level—a result of global warming—the risk is only likely to increase in near future.
The Saiga antelope die-off
In May of 2015, conservation biologists heard about die-offs in Saiga antelopes – an odd looking ungulate (“hoofed animal”) – that once used to roam a vast area spread between Mongolia in the east to Romania in the west. During the nineteenth and early twentieth century, rampant hunting, however, decimated 95% of the historic population size restricting the animals to only a few small populations mainly in Kazakhstan. Die-offs are not uncommon for ungulates, so biologists were not particularly alarmed. However, nothing could prepare them when they visited the Kazakhstan populations. They witnessed the death of whole population – 60,000 animals in total – within four days of their arrival. This scale of decimation – 100% mortality within days – of a wild ungulate population is unprecedented.
Later conservationists found out that the die-offs were occurring across the species’ distribution and in total more than 120,000 animals were dead by late May. At that point, due to the lightning speed of the population crash, it was clear that some virulent infectious agents were involved – perhaps Pasteurella or Clostridia – the usual suspects. What was mysterious, however, was the devastating speed at which the infections decimated populations that were far away from each other. What may have triggered such widespread outbreaks? In January, 2018, the bacterium, Pasteurella multocida type B, was finally identified as the pathogen that killed the antelope. Interestingly, this bacterium is common even in healthy animals but was known to turn virulent with a change in environmental conditions. Indeed, scientists found that the climate in Kazakhstan did gradually turn unusually warmer and humid before precipitating the widespread epidemic of pasteurellosis in Saiga antelopes.
Studies may initially focus on how changing temperature may influence different types of diseases such as vector-borne, environmentally or directly transmitted ones, taking into consideration thermal physiology, ecology and behaviour of the organisms involved. Later, other important climatic factors such as humidity can also be incorporated into these models. Furthermore, such investigations are needed to be done at different levels of organisations, from individuals to communities and finally, different ecosystems, such as terrestrial and marine. To trace the mechanisms in the background, the investigations will need to be finally complimented with experiments. It is imperative that we strive to first understand these background mechanisms before attributing all new maladies to global warming.