Dr Mark Cresswell Impacts: Disease 69EG6517 – Impacts & Models of Climate Change

Topics Scale of the problem Some terminology Malaria Respiratory diseases Impacts Future impacts Summary

Scale of the problem Many countries are vulnerable to diseases directly influenced by the weather Vector-borne diseases (like malaria) Respiratory illnesses (like meningitis) Water-borne diseases (like cholera) Stress illnesses (heat-stroke or hypothermia) Illnesses caused by “mechanical” effects of extreme weather events

Some Terminology An epidemic occurs when the number of cases exceeds the background average by a significant number (>2SD) An outbreak is a localised number of cases En epidemic wave describes an epidemic that follows a geographical spread as the disease is transferred from one person to the next Many epidemic follow a “saw tooth” pattern over a number of years

Some Terminology Mortality refers to deaths Morbidity refers to cases of disease who subsequently recover – but nevertheless present the symptoms of a disease A DALY is a Disability Adjusted Life Year and can be used to measure the economic impact of disease on the human population and for countries as a whole

Vector-Borne Diseases Malaria is an example of a well known tropical vector-borne disease Any agent of transmission of a bacterium, virus or pathogen is a vector Diseases transmitted by insects are known as vector-borne Malaria is caused by a parasite of the genus Plasmodium The parasite enters the human host after biting from a female Anopheline mosquito

Stable transmission Unstable transmission

Respiratory Diseases Just as vector-borne diseases such as malaria are highly dependent on the weather, so too are respiratory illnesses such pneumonia and meningitis Dry air may desiccate the upper respiratory tract (nasopharyngeal tract or NPT) Since air entering lungs must have 43 gm -3 of absolute humidity and shortfall must be supplied by the NPT

Respiratory Diseases The drier the ambient air, the more moisture the NPT must supply to tidal air entering the lungs As a general rule, air drier than 10 gm -3 absolute humidity may desiccate the NPT If the NPT dries, bacteria are able to penetrate deep lung tissue – triggering the disease The spread of a disease is enhanced by dry and dusty conditions (Harmattan)

Respiratory Diseases Many respiratory illnesses such as meningitis and pneumonia are highly dependent on the weather Changes in climate may act as a trigger or provide more favourable conditions for the disease to flourish ABOVE: CSM bacteria (stained) LEFT: From Cheesbrough et al. 1995

Using GIS Most layers of biologically relevant environmental information are combined within a Geographical Information System (GIS) Humidity is currently a “missing” layer

Malaria Model simplified schematic of Liverpool model Mosquito Human UninfectedInfectedInfectious UninfectedInfectedInfectious death Maturing larvae Infection Underlying model is similar to that described by Aron and May (1982) Model assumes no immunity, no superinfection (Sporogonic cycle)

Malaria Model prevalence and ERA rainfall

Model Output: West Africa interannual variability PrevalenceRainfall

Impacts The following changes to our climate will make the prevalence of diseases such as malaria, meningitis and cholera more acute: Enhanced precipitation in wet season Warmer temperatures in upland areas as temperatures rise Drier air – with very low absolute humidities in dry season Changes in vegetation patterns Floods in lowland areas Migration of refugees as a result of extreme weather Based on IPCC projections

Impacts

ENSO ENSO events are steadily increasing in frequency Cresswell et al, 1999

ENSO ENSO provides us with a means of identifying the impact of climate change on insect populations A study was undertaken in the Wajir district of Kenya in early Under normal weather conditions this region is too dry for the vectors and very little transmission occurs. There had not been a malaria epidemic since 1952 and the local health sector was unprepared for the major outbreak that followed the heavy rains as a result of ENSO Cresswell and Kovats, 1999

Role of Climate?

Future Impacts In the 2080s it is estimated that some 290 million additional people worldwide will be exposed to malaria – due to climate change (McMichael et al, 2003) In the absence of climate change, in the 2080s about 9 billion people (~80% of world pop) would live in areas potentially able to support malaria – climate change represents only a 1 to 3% increase in population at risk

Future Impacts Increases in summer time mortality due to enhanced heat stress…and decreases in winter time mortality due to milder winters in urban population have been modelled in accordance with projections of global warming Ingress of brackish water to coastal zones as sea-level rises will enhance population numbers of mosquito vectors (IPCC)