Climate change as cause and catalyst of infectious diseases

Climate change as cause and catalyst of infectious diseases

Climate change is a global phenomenon characterised by changes in temperature, precipitation, wind patterns, etc, and is especially caused by human activities. As a result, the sustainability of the planet’s ecosystems is under threat, as well as the future of humankind.
Humans have known that climatic conditions affect epidemic diseases from long before the role of infectious agents was discovered late in the nineteenth century. Roman aristocrats retreated to hill resorts each summer to avoid malaria. South Asians learnt early that in high summer, strongly curried foods were less likely to cause diarrhoea. Infectious agents vary greatly in size, type and mode of transmission. There are viruses, bacteria, protozoa and multicellular parasites. Those microbes that cause “anthroponoses” have adapted, via evolution, to the human species as their primary, usually exclusive, host. In contrast, non-human species are the natural reservoir for those infectious agents that cause “zoonoses”. There are directly transmitted anthroponoses (such as TB, HIV/AIDS, and measles) and zoonoses (e.g., rabies). There are also indirectly-transmitted vector-borne anthroponoses (e.g., malaria, dengue fever, yellow fever) and zoonoses (e.g., bubonic plague and Lyme disease).
Human exposure to waterborne infections occurs by contact with contaminated drinking water, recreational water, or food. This may result from human actions, such as improper disposal of sewage wastes, or be due to weather events. Rainfall can influence the transport and dissemination of infectious agents, while temperature affects their growth and survival.
There are three categories of research into the linkages between climatic conditions and infectious disease transmission. The first examines evidence from the recent past of associations between climate variability and infectious disease occurrence. The second looks at early indicators of already-emerging infectious disease impacts of long-term climate change. The third uses the above evidence to create predictive models to estimate the future burden of infectious disease under projected climate change scenarios.
There is much evidence of associations between climatic conditions and infectious diseases. Malaria is of great public health concern, and seems likely to be the vector-borne disease most sensitive to long-term climate change. Malaria varies seasonally in highly endemic areas. The link between malaria and extreme climatic events has long been studied in India. Early last century, the river-irrigated Punjab region experienced periodic malaria epidemics. Excessive monsoon rainfall and high humidity was identified early on as a major influence, enhancing mosquito breeding and survival. Globally, temperature increases of 2-3ºC would increase the number of people who, in climatic terms, are at risk of malaria by around 3 to 5%, i.e., several hundred million. Further, the seasonal duration of malaria would increase in many currently endemic areas.
There are many pathways that lead to certain environmental changes and consequently, the incidence of acquiring infection. Decrease in sanitation practices, hygiene and increase in water contamination leads to certain environmental changes and thereby give rise to diseases like cholera, dengue, cutaneous leishmaniensis. Likewise, many of the diseases like malaria, river blindness, Lyme disease, red tide, hantavirus pulmonary syndrome, etc, are due to occurrence of diverse environmental changes. Today, worldwide, there is an apparent increase in many infectious diseases, including some newly-circulating ones (HIV/AIDS, hantavirus, hepatitis C, SARS, etc.). This reflects the combined impacts of rapid demographic, environmental, social, technological and other changes in our ways of living.
The main types of models used to forecast future climatic influences on infectious diseases include statistical, process-based, and landscape-based models. Statistical models require, first, the derivation of a statistical (empirical) relationship between the current geographic distribution of the disease and the current location specific climatic conditions. This describes the climatic influence on the actual distribution of the disease, given prevailing levels of human intervention (disease control, environmental management, etc.).Process-based (mathematical) models use equations that express the scientifically documented relationship between climatic variables and biological parameters – e.g., vector breeding, survival, and biting rates, and parasite incubation rates. Since climate also acts by influencing habitats, landscape-based modelling is also useful. This entails combining the climate-based models described above with the rapidly-developing use of spatial analytical methods, to study the effects of both climatic and other environmental factors (e.g., different vegetation types – often measured, in the model development stage, by ground-based or remote sensors).
It is thus evident that infectious disease transmission patterns are a likely major consequence of climate change. Therefore it is important to learn more about the underlying complex causal relationships, and apply this information to the prediction of future impacts, using more complete, better validated, integrated models.

—The writer is a master’s student of Zoology at Central University of Kashmir. Saadattansur.st@gmail.com

 

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