Climate change, along with biodiversity loss and intensification of farming, are combining to cause unpredictable and unstable conditions; ones that are perfect for new diseases to emerge.
Some emerging diseases are fairly rare; mercifully, most people have never heard of Nipah virus, but the horrific Ebola virus is infamous, though so far it has not gone global. However, COVID-19 has spread worldwide, and after more than two months of lockdown, and hundreds of thousands of deaths across the globe, we know all too well about that one.
In this blog, I will explore, from two angles, whether COVID-19 is ultimately caused by climate change. Firstly, by looking at the behaviour of animal hosts – animals that become infected with, and amplify, viruses – and then by looking at the behaviour of the viruses.
Since the study of COVID-19 is a new field, information about it is often missing or contradictory – for instance, it is difficult to talk about seasonality for a virus that we’ve only known about for six months. So I will discuss studies of other human coronaviruses, to fill the gaps and provide a broader context.
What role do animals play in COVID-19?
Zoonotic pathogens – animal pathogens that are transmitted to humans – make up 60% of human infectious diseases. Wild animal host species sustain parasite (e.g. virus) cycles when they are not infecting humans, and serve as a source of infection and potential reinfection of humans. These three lethal viruses, Ebola, Nipah and COVID-191, are all zoonotic, with bats as presumed reservoir hosts. Coronaviruses (CVs), in particular, are associated with bats; 17 of 46 CV species were first found in bats and the closest genetically related CV to COVID-19 was found in a horseshoe bat. Scientists believe that MERS (Middle East Respiratory Syndrome), SARS (Severe Acute Respiratory Syndrome) and most mammalian CVs evolved from bat CVs.
Currently, many bat species are threatened with extinction, from habitat loss, disturbance at their roosting sites, and hunting. Bat response to climate change is largely unknown. In all likelihood, each species will respond differently. To generalise, insect-eating bats seem to increase with high humidity, which increases insect prey populations, and decline in dry conditions. So, where climate change leads to increased rainfall, it may increase the abundance, range of bats and bat viruses.
In dry areas, the bat population may decrease. One South East Asian model2 predicted conditions would become unsuitable for 25% of the region’s bats by 2080. Potentially, areas outside South East Asia will become suitable for those bat species under future climate conditions – bat populations might shrink in size, relocate, or even become extinct in the region. Let’s not forget biodiversity loss, and the intensification of farming, which are both forms of environmental change. With less natural habitats and food, bats have to look elsewhere for resources – for example, fruit eating bat numbers may increase near fruit plantations. This could lead to an increased likelihood of interactions between different species, and increased chance of disease spill over.
Bat behaviour in flux
Recently, 300,000 fruit bats invaded Ingham, Australia, in response to rising temperatures, habitat loss, and the attraction of human farming. This brought a large bat population into contact with people, pets and livestock. Elsewhere bat migration patterns have altered. The combination of warmer winters and dependable human crops allow bats to forego migration. This increases contact time with humans, increasing the likelihood that contact coincides with peak virus shedding. Potentially, alternative food resources found in the human-dominated landscape are poorer quality for the bats than their natural diet, and a poor diet could contribute to increased infection. At present, bat behaviour is in flux.
What about the viruses?
CVs were first detected in the 1930s in chickens – the Infectious Bronchitis Virus (IBV). IBV infected newly hatched chicks, leaving them listless and gulping for air, with mortality rates ranging between 40-90%. The first two in humans, discovered in the 1960s, were fairly benign, usually causing “colds”. Then SARS emerged in 2003, and changed the way virologists viewed CVs. It was really nasty, killing nearly 800 people, and infecting ten times that number.
Since then, the other four CVs have emerged, with COVID-19 being the most recent and serious to date. Five human CVs, two which cause serious disease, and one (COVID-19) that is a global horror show, have emerged in the last 20 years. In stark contrast, only two were described in the previous 40 years. This is, in part, because we are better at detecting CVs than we were 20 years ago: new tools like genetic sequencing are cheaper and vastly more efficient, and many scientists are interested in CVs.
Still, such a dramatic increase in diseases, including three that have become household names, seems like big news. Outside Influenza, which has always been a major health problem (e.g. Spanish flu in 1918), no other virus group has produced so many new dangerous pathogens in the past 20 years.
Recent emergence of CVs may be explained by changes in host behaviour. Can we see evidence that they are affected by climate change? There has been no investigation yet of how coronaviruses respond to climate change. However, several published articles 3-7 investigate seasonality in coronavirus transmission. Seasons are marked by changes in temperature and precipitation. This is not the same thing as climate change, but a disease that thrives in hot conditions might be expected to become more prevalent as global temperatures rise. Seasonal variation in disease prevalence is sometimes a direct response to fluctuation in temperature or humidity. However, seasonal variation in disease prevalence may be due to confounding, non-meteorological factors, such as seasonal immune depression, or perhaps due to a cultural timing of when an immune naïve cohort of children gather together for their first school year, for example.
If evidence of seasonality in CV transmission were clear, it might point to trends that require further investigation. However, evidence that CVs or COVID-19 are affected by season is currently contentious. One study of four human CVs found that they displayed marked winter seasonality (circulating between December and April) and were not detected in the summer3. In contrast, a study of MERS found worldwide infections to be highest in June, particularly in Saudi Arabia4. While a study of Human Coronavirus in China yielded the less reassuring results that peak transmission occurred in both summer and winter months, with transmission persisting throughout the year5. Two contributions on COVID-19 contradict each other, with a Spanish study finding that transmission will be reduced by summer heat6, and a Canadian study finding that transmission will likely continue through the summer7. Perhaps, six months into COVID, is too early to talk about seasonality though. Sadly, at this point there is no strong evidence that summer heat will bring a respite from the COVID-19 pandemic. Currently (June 5 2020), as transmission is highest in Brazil, USA, India, Russia and Pakistan, a mix of areas entering winter and summer, I doubt it will.
The emergence of coronaviruses has been surprisingly high in the last 20 years. This coincides with climate change, but also with increasing human population, biodiversity loss and intensification of farming. There is no clear evidence that climate change is directly affecting virus frequency, however, there is pretty strong evidence that a combination of climate change and other human activities destabilises the ecology of bats, and other disease hosts. It is difficult to predict whether this will lead to increased or decreased interactions, or shifts in foci of transmission. We should, however, be prepared for unforeseen future coronavirus trends.
Dr Richard Williams recently presented a webinar on this topic.
The University of Derby’s Environmental Sustainability Research Centre is running a series of climate change-focused events bringing together researchers and practitioners to present the science and action of climate change. The seminars are currently running online. The next event is on June 10. Upcoming events from the Research Centre
1 Zhou P, Yang XL, Wang XG, Hu B, Zhang L, Zhang W, Si HR, Zhu Y, Li B, Huang CL, Chen HD. A pneumonia outbreak associated with a new coronavirus of probable bat origin. nature. 2020 Mar;579(7798):270-3.
2 Hughes, A.C., Satasook, C., Bates, P.J., Bumrungsri, S. and Jones, G., 2012. The projected effects of climatic and vegetation changes on the distribution and diversity of Southeast Asian bats. Global Change Biology, 18(6), pp.1854-1865.
3 Gaunt, E.R., Hardie, A., Claas, E.C., Simmonds, P. and Templeton, K.E., 2010. Epidemiology and clinical presentations of the four human coronaviruses 229E, HKU1, NL63, and OC43 detected over 3 years using a novel multiplex real-time PCR method. Journal of clinical microbiology, 48(8), pp.2940-2947.
4 Nassar, M.S., Bakhrebah, M.A., Meo, S.A., Alsuabeyl, M.S. and Zaher, W.A., 2018. Global seasonal occurrence of middle east respiratory syndrome coronavirus (MERS-CoV) infection. Eur Rev Med Pharmacol Sci, 22(12), pp.3913-3918.
5 Feng, L., Li, Z., Zhao, S., Nair, H., Lai, S., Xu, W., Li, M., Wu, J., Ren, L., Liu, W. and Yuan, Z., 2014. Viral etiologies of hospitalized acute lower respiratory infection patients in China, 2009-2013. PloS one, 9(6)
6 Araujo, M.B. and Naimi, B., 2020. Spread of SARS-CoV-2 Coronavirus likely to be constrained by climate. medRxiv.
7 Jüni, P., Rothenbühler, M., Bobos, P., Thorpe, K.E., da Costa, B.R., Fisman, D.N., Slutsky, A.S. and Gesink, D., 2020. Impact of climate and public health interventions on the COVID-19 pandemic: a prospective cohort study. Cmaj, 192(21), pp.E566-E573.