Humans have known that climatic Figure 6.1: Four main types of transmission cycle for infectious diseases (reference 5) conditions affect epidemic diseases Anthroponoses from long before the role of Direct transmission Indirect transmission 6 infectious agents was discovered, late in the nineteenth century. HUMANS HUMANS VECTOR/VEHICLE Roman aristocrats retreated to hill VECTOR/VEHICLE Climate resorts each summer to avoid HUMANS HUMANS malaria. South Asians learnt early Zoonoses Change And that, in high summer, strongly curried foods were less likely to ANIMALS ANIMALS VECTOR/VEHICLE cause diarrhoea. VECTOR/VEHICLE Infectious ANIMALS ANIMALS Infectious agents vary greatly in HUMANS HUMANS Diseases size, type and mode of transmission. There are viruses, bacteria, protozoa and multicellular optimal climatic conditions: variability and infectious disease Today, worldwide, there is an parasites. Those microbes that temperature and precipitation are occurrence. The second looks at apparent increase in many cause “anthroponoses” have the most important, while sea level early indicators of already-emerging infectious diseases, including adapted, via evolution, to the elevation, wind, and daylight infectious disease impacts of long- human species as their primary, duration are also important. term climate change. The third uses some newly-circulating ones usually exclusive, host. In contrast, the above evidence to create (HIV/AIDS, hantavirus, non-human species are the natural Human exposure to waterborne predictive models to estimate the hepatitis C, SARS, etc.). reservoir for those infectious agents infections occurs by contact with future burden of infectious disease that cause “zoonoses” (Fig 6.1). contaminated drinking water, under projected climate change This reflects the combined There are directly transmitted recreational water, or food. This scenarios. impacts of rapid anthroponoses (such as TB, may result from human actions, demographic, environmental, HIV/AIDS, and measles) and such as improper disposal of Historical Evidence zoonoses (e.g., rabies). There are sewage wastes, or be due to weather There is much evidence of social, technological and also indirectly-transmitted, vector- events. Rainfall can influence the associations between climatic other changes in our ways- borne, anthroponoses (e.g., malaria, transport and dissemination of conditions and infectious diseases. of-living. Climate change will dengue fever, yellow fever) and infectious agents, while temperature Malaria is of great public health zoonoses (e.g. bubonic plague and affects their growth and survival. concern, and seems likely to be the also affect infectious disease Lyme disease). vector-borne disease most sensitive occurrence.1 to long-term climate change. Vector-borne and water-borne diseases Observed and predicted Malaria varies seasonally in highly Important determinants of vector- climate/infectious disease links endemic areas. The link between borne disease transmission include: malaria and extreme climatic events (i) vector survival and reproduction, There are three categories of has long been studied in India, for (ii) the vector’s biting rate, and (iii) research into the linkages between example. Early last century, the the pathogen’s incubation rate climatic conditions and infectious river-irrigated Punjab region within the vector organism. Vectors, disease transmission. The first experienced periodic malaria pathogens and hosts each survive examines evidence from the recent epidemics. Excessive monsoon and reproduce within a range of past of associations between climate rainfall and high humidity was 16 CLIMATE CHANGE AND HUMAN HEALTH - RISK AND RESPONSES identified early on as a major forecast future climatic influences management, etc.). By then affect transmission potential. influence, enhancing mosquito on infectious diseases include applying this statistical equation to Globally, temperature increases of breeding and survival. Recent statistical, process-based, and future climate scenarios, the actual 2-3ºC would increase the number analyses have shown that the landscape-based models.3 These distribution of the disease in future of people who, in climatic terms, malaria epidemic risk increases three types of model address is estimated, assuming unchanged are at risk of malaria by around 3- around five-fold in the year after an somewhat different questions. levels of human intervention within 5%, i.e. several hundred million. El Niño event.2 any particular climatic zone.These Further, the seasonal duration of Statistical models require, first, the models have been applied to malaria would increase in many Early impacts of climate change derivation of a statistical (empirical) climate change impacts on malaria, currently endemic areas. These include several infectious relationship between the current dengue fever and, within the USA, diseases, health impacts of geographic distribution of the encephalitis. For malaria some Since climate also acts by temperature extremes and impacts disease and the current location- models have shown net increases in influencing habitats, landscape- of extreme climatic and weather specific climatic conditions. This malaria over the coming half- based modeling is also useful. This events (described in section 5 above). describes the climatic influence on century, and others little change. entails combining the climate-based the actual distribution of the models described above with the Predictive Modeling disease, given prevailing levels of Process-based (mathematical) rapidly-developing use of spatial The main types of models used to human intervention (disease models use equations that express analytical methods, to study the control, environmental the scientifically documented effects of both climatic and other relationship between climatic environmental factors (e.g. different Table 6.1: Examples of how diverse environmental changes affect the occurrence variables and biological parameters vegetation types – often measured, of various infectious diseases in humans (Refernce 5) – e.g., vector breeding, survival, and in the model development stage, by biting rates, and parasite incubation ground-based or remote sensors). Environmental changes Example diseases Pathway of effect rates. In their simplest form, such This type of modelling has been Dams, canals, irrigation Schistosomiasis Snail host habitat, human contact models express, via a set of applied to estimate how future Malaria Breeding sites for mosquitoes equations, how a given climate-induced changes in ground Helminthiasies Larval contact due to moist soil configuration of climate variables cover and surface water in Africa River blindness Blackfly breeding, disease would affect vector and parasite would affect mosquitoes and tsetse Agricultural intensification Malaria Crop insecticides and vector biology and, therefore, disease flies and, hence, malaria and resistance Venezuelan rodent abundance, contact transmission. Such models address African sleeping sickness. haemorraghic fever the question: “If climatic conditions Urbanization, Cholera sanitation, hygiene; water alone change, how would this Conclusion urban crowding contamination change the potential transmission Dengue Water-collecting trash, Aedes of the disease?” Using more Changes in infectious disease aegypti mosquito breeding sites complex “horizontal integration”, transmission patterns are a likely Cutaneous leishmaniasis proximity, sandfly vectors the conditioning effects of human major consequence of climate Deforestation and new Malaria Breeding sites and vectors, habitation immigration of susceptible people interventions and social contexts change. We need to learn more Oropouche contact, breeding of vectors can also be incorporated. about the underlying complex Visceral leishmaniasis contact with sandfly vectors causal relationships, and apply this Reforestation Lyme disease tick hosts, outdoor exposure This modelling method has been information to the prediction of Ocean warming Red tide Toxic algal blooms used particularly for malaria and future impacts, using more 4 Elevated precipitation Rift valley fever Pools for mosquito breeding dengue fever. The malaria complete, better validated, Hantavirus Rodent food, habitat, modelling shows that small integrated, models. pulmonary syndrome abundance temperature increases can greatly increase reduction SUMMARY 17.