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climatechange 27 07 03 - WHO

6 climatechange AndInfectiousDiseases Today, worldwide, there is anapparent increase in manyinfectious diseases, includingsome newly-circulating ones(HIV/AIDS, hantavirus,hepatitis C, SARS, etc.). This reflects the combinedimpacts of rapiddemographic, environmental,social, technological andother changes in our ways-of-living. Climate change willalso affect infectious CHANGE AND HUMAN HEALTH - RISK AND RESPONSES 16 Humans have known that climaticconditions affect epidemic diseasesfrom long before the role ofinfectious agents was discovered,late in the nineteenth aristocrats retreated to hillresorts each summer to avoidmalaria. South Asians learnt earlythat, in high summer, stronglycurried foods were less likely tocause diarrhoea. Infectious agents vary greatly insize, type and mode oftransmission. There are viruses,bacteria, protozoa and multicellularparasites.

6 Climate Change And Infectious Diseases Today, worldwide, there is an apparent increase in many infectious diseases, including some newly-circulating ones

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Transcription of climatechange 27 07 03 - WHO

1 6 climatechange AndInfectiousDiseases Today, worldwide, there is anapparent increase in manyinfectious diseases, includingsome newly-circulating ones(HIV/AIDS, hantavirus,hepatitis C, SARS, etc.). This reflects the combinedimpacts of rapiddemographic, environmental,social, technological andother changes in our ways-of-living. Climate change willalso affect infectious CHANGE AND HUMAN HEALTH - RISK AND RESPONSES 16 Humans have known that climaticconditions affect epidemic diseasesfrom long before the role ofinfectious agents was discovered,late in the nineteenth aristocrats retreated to hillresorts each summer to avoidmalaria. South Asians learnt earlythat, in high summer, stronglycurried foods were less likely tocause diarrhoea. Infectious agents vary greatly insize, type and mode oftransmission. There are viruses,bacteria, protozoa and multicellularparasites.

2 Those microbes thatcause anthroponoses haveadapted, via evolution, to thehuman species as their primary,usually exclusive, host. In contrast,non-human species are the naturalreservoir for those infectious agentsthat cause zoonoses (Fig ).There are directly transmittedanthroponoses (such as TB,HIV/AIDS, and measles) andzoonoses ( , rabies). There arealso indirectly-transmitted, vector-borne, anthroponoses ( , malaria,dengue fever, yellow fever) andzoonoses ( bubonic plague andLyme disease ).Vector-borne and water-borne diseases Important determinants of vector-borne disease transmission include:(i) vector survival and reproduction,(ii) the vector s biting rate, and (iii)the pathogen s incubation ratewithin the vector organism. Vectors,pathogens and hosts each surviveand reproduce within a range ofoptimal climatic conditions:temperature and precipitation arethe most important, while sea levelelevation, wind, and daylightduration are also important.

3 Human exposure to waterborneinfections occurs by contact withcontaminated drinking water,recreational water, or food. Thismay result from human actions,such as improper disposal ofsewage wastes, or be due to weatherevents. Rainfall can influence thetransport and dissemination ofinfectious agents, while temperatureaffects their growth and survival. Observed and predictedclimate/infectious disease linksThere are three categories ofresearch into the linkages betweenclimatic conditions and infectiousdisease transmission. The firstexamines evidence from the recentpast of associations between climatevariability and infectious diseaseoccurrence. The second looks atearly indicators of already-emerginginfectious disease impacts of long-term climate change. The third usesthe above evidence to createpredictive models to estimate thefuture burden of infectious diseaseunder projected climate EvidenceThere is much evidence ofassociations between climaticconditions and infectious is of great public healthconcern, and seems likely to be thevector-borne disease most sensitiveto long-term climate varies seasonally in highlyendemic areas.

4 The link betweenmalaria and extreme climatic eventshas long been studied in India, forexample. Early last century, theriver-irrigated Punjab regionexperienced periodic malariaepidemics. Excessive monsoonrainfall and high humidity wasFigure : Four main types of transmission cycle for infectious diseases (reference 5)AnthrAnthroponosesoponosesHUMANSHUMANS ANIMALSANIMALSHUMANSZ oonosesDirect transmissionHUMANSHUMANSANIMALSANIMALSI ndirect transmissionVECTOR/VEHICLEVECTOR/VEHICLE VECTOR/VEHICLEVECTOR/VEHICLEHUMANSSUMMAR Y17identified early on as a majorinfluence, enhancing mosquitobreeding and survival. Recentanalyses have shown that themalaria epidemic risk increasesaround five-fold in the year after anEl Ni o impacts of climate changeThese include several infectiousdiseases, health impacts oftemperature extremes and impactsof extreme climatic and weather events (described in section 5 above).

5 Predictive Modeling The main types of models used toforecast future climatic influenceson infectious diseases includestatistical, process-based, andlandscape-based types of model addresssomewhat different questions. Statistical models require, first, thederivation of a statistical (empirical)relationship between the currentgeographic distribution of thedisease and the current location-specific climatic conditions. Thisdescribes the climatic influence onthe actual distribution of thedisease, given prevailing levels ofhuman intervention (diseasecontrol, environmentalmanagement, etc.). By thenapplying this statistical equation tofuture climate scenarios, the actualdistribution of the disease in futureis estimated, assuming unchangedlevels of human intervention withinany particular climatic have been applied toclimate change impacts on malaria,dengue fever and, within the USA,encephalitis.

6 For malaria somemodels have shown net increases inmalaria over the coming half-century, and others little (mathematical)models use equations that expressthe scientifically documentedrelationship between climaticvariables and biological parameters , vector breeding, survival, andbiting rates, and parasite incubationrates. In their simplest form, suchmodels express, via a set ofequations, how a givenconfiguration of climate variableswould affect vector and parasitebiology and, therefore, diseasetransmission. Such models addressthe question: If climatic conditionsalone change, how would thischange the potential transmissionof the disease ? Using morecomplex horizontal integration ,the conditioning effects of humaninterventions and social contextscan also be modelling method has beenused particularly for malaria anddengue malariamodelling shows that smalltemperature increases can greatlyaffect transmission , temperature increases of2-3 C would increase the numberof people who, in climatic terms,are at risk of malaria by around 3-5%, several hundred , the seasonal duration ofmalaria would increase in manycurrently endemic climate also acts byinfluencing habitats, landscape-based modeling is also useful.

7 Thisentails combining the climate-basedmodels described above with therapidly-developing use of spatialanalytical methods, to study theeffects of both climatic and otherenvironmental factors ( differentvegetation types often measured,in the model development stage, byground-based or remote sensors).This type of modelling has beenapplied to estimate how futureclimate-induced changes in groundcover and surface water in Africawould affect mosquitoes and tsetseflies and, hence, malaria andAfrican sleeping in infectious diseasetransmission patterns are a likelymajor consequence of climatechange . We need to learn moreabout the underlying complexcausal relationships, and apply thisinformation to the prediction offuture impacts, using morecomplete, better validated,integrated, models. Environmental changesExample diseasesPathway of effectDams, canals, irrigationSchistosomiasisSnail host habitat, human contactMalariaBreeding sites for mosquitoesHelminthiasiesLarval contact due to moist soilRiver blindnessBlackfly breeding, diseaseAgricultural intensificationMalariaCrop insecticides and vectorresistanceVenezuelan rodent abundance, contacthaemorraghic feverUrbanization, Cholerasanitation, hygiene.

8 Water urban crowdingcontaminationDengueWater-collect ing trash, Aedesaegyptimosquito breeding sitesCutaneous leishmaniasisproximity, sandfly vectorsDeforestation and new MalariaBreeding sites and vectors, habitationimmigration of susceptible peopleOropouchecontact, breeding of vectorsVisceral leishmaniasiscontact with sandfly vectorsReforestationLyme diseasetick hosts, outdoor exposureOcean warmingRed tideToxic algal bloomsElevated precipitationRift valley feverPools for mosquito breedingHantavirus Rodent food, habitat, pulmonary syndromeabundance Table : Examples of how diverse environmental changes affect the occurrenceof various infectious diseases in humans (Refernce 5)increasereduction


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