The links between climate change and viral infection

Climate change is one of the most complex challenges of this century. Globalisation and climate change have caused an unprecedented impact on emerging and re-emerging diseases including zoonoses (diseases that can be passed from animals to humans) in recent years. Emerging infectious diseases refer to diseases caused by newly identified and previously unknown infectious agents; they have the potential to cause immense burden on public health both locally and internationally. On the contrary, re-emerging infectious diseases, caused by agents that have been known for some time and have fallen to very low levels, are now showing an upward trend in incidence worldwide. It is worth noting that viruses and biological vectors (e.g. mosquitoes) swim in the evolutionary stream—they swim so fast that even any thoughtful intervention usually fails to stop them from infiltrating the system.

Global warming and climate change have taken diseases like dengue and other vector-borne viral diseases to new dimensions. Climatic factors, particularly temperature and rainfall, affect the ability of viral disease propagation and potential mosquito vectors to coexist long enough to maintain and increase the rate of transmission. The decreased prevalence of infectious diseases in western countries in the 20th century was due to urban sanitation, improved housing, personal hygiene, antisepsis and immunisation. Since the last quarter of the 20th century, there has been a resurgence of infectious diseases: certain viral diseases (Avian influenza, Ebola, Marburg, Rift Valley fever, chikungunya, dengue, Japanese encephalitis) have emerged or re-emerged while others (smallpox, poliomyelitis, measles) have declined significantly.

Zoonotic transmission of infectious agents from animals (wild and domestic) to humans constituted more than two-thirds of emerging infections. Contact among animals and people is another driving force behind the emergence of new infections. Deforestation forces wild animals into closer contact with humans. Increased possibility for agents to breach species (host) barrier between animals and humans is responsible for the spread of diseases like Lassa fever, yellow fever and swine flu while global warming facilitated the spread of vector-borne diseases such as dengue, chikungunya and Japanese encephalitis.

Rapid urbanisation and population displacement have given rise to the growth of densely populated cities with substandard housing, unsafe water, poor sanitation, overcrowding, indoor air pollution (triggering incidence of viral diarrhea), acute respiratory tract infection, and many other microbial infections. Recognising the complexity of the diverse sociocultural processes involved in the emergence/re-emergence of infectious diseases, many researchers in the fields of biology, medicine, and public health are calling for inputs from experts in the social, economic and behavioural sciences. With its integrative approach to complex bio-cultural issues, anthropology is well-positioned to make significant theoretical and practical contributions. Climate change has been responsible for at least one emerging or re-emerging disease in many countries and the number of such countries is gradually increasing.

Diseases such as severe acute respiratory syndrome (SARS), one of the first emerging viral diseases of the 21st century, in one country are an alarming threat to all travellers with a tremendous negative economic impact on trade, travel and tourism. Nipah virus infection is becoming endemic in Bangladesh as cases have been continuously detected since 2001. Avian influenza (H5N1) has been detected since November 2003 in birds and affected 60 countries across Asia, Europe, Middle East and Africa, and more than 220 million birds were killed by the virus or culled to prevent further propagation. Swine-origin influenza A (H1N1) which leads to swine flu causes respiratory diseases in pigs; pigs can get infected by human, avian and swine influenza viruses. In late 2009 and early 2010, the global pandemic of swine flu caused great panic. Although a few cases of swine flu were detected, there were only two recorded deaths in Bangladesh.  

The outbreak of dengue has taken place over the past 40 years with a 20-fold increase to nearly 0.5 million cases from 1990 to 1998. The medical community of Bangladesh was fairly unfamiliar about the presence of dengue in the country before 2000. Since its outbreak beginning in the summer of 2000, cases have been reported every year. Chikungunya fever is also a re-emerging condition in previously unaffected areas with possibly changing epidemiology and severity of the disease. This tends to be clustered geographically and overlap with dengue because they share some common clinical features.

The role of climate as well as environmental changes on the growing burden of emerging and re-emerging infections calls for a new approach so as to prevent these threats. The response options need to be appropriate keeping in mind the nature of vulnerabilities that might affect demographic transitions due to climate change. Health, nutrition and population experts must address these areas of public-health issues related to climate change with the required responses. Member countries have given the World Organisation for Animal Health a mandate to address the issue by using its scientific capabilities and networks at the global, regional and sub-regional levels. The aim is to prevent or reduce the effects of climate change on animal diseases which are transmissible to humans. In order to offer a multidisciplinary perspective to mitigate the problem, infectious disease specialists, epidemiologists, geneticists, microbiologists, and population biologists need to join hands to address questions about the definition, identification, factors responsible for and multidisciplinary approaches to viral infections. There is also a need for monitoring at the national, regional and global levels which can be done by taking an epidemiological, laboratory-based, ecological and anthropological approach and adopting early control measures.

The role of public-health professionals is to establish monitoring and surveillance for unusual diseases and drug-resistant agents as well as ensure laboratory capacity to identify new agents and develop plans to handle outbreaks of unknown diseases. Finally, socio-political commitment at both the national and international levels is crucial for effective containment of these dangerous diseases.

Prof Dr Iftikhar Ahmed, MBBS, M Phil (Microbiology), is a WHO Fellow and Head, Department of Microbiology, Enam Medical College, Savar, Dhaka. Email: ia65831@gmail.com