Environmental determinants of malaria risk in Europe : past, present and future

Abstract

Malaria is today the most important vector-borne disease in the world. Like all vector- borne diseases, its distribution depends on a complex array of environmental factors, including climate. The historical global distribution of malaria was considerably greater than today’s, and there has been much recent speculation addressing whether the predicted climate changes could lead to the re-emergence of malaria in previously endemic areas, such as Europe. This thesis quantifies the role of climate and other environmental factors in the temporal and spatial pattern of malaria risk within England and across Europe using historical and current data and uses the findings to simulate the likely changes in malaria risk in Europe as the result of global warming. The inter-annual variability and spatial variation in ague (malaria) mortality rates in English and Welsh counties during the 19lh century were significantly related to changes in marsh wetlands, cattle density, average temperature and rainfall. Model simulations indicated that increasing cattle densities and decreasing marshlands accounted for at least 19% of the drop-off in English malaria. Further simulations showed that the changes in climate predicted for 2050 may increase malaria risk by 8-14%, clearly insufficient to lead to the re-emergence of the disease in England. Temporal and spatial patterns of malaria incidence throughout Europe during the 20th century were significantly related to changes in woodlands, rice cultivation, cattle densities and average temperature. According to model simulations, at least 12-15% of the decrease in European malaria was due to increased woodland coverage and cattle densities. Simulations of global warming predicted for the 2050s showed that malaria transmission is likely to increase by 10-24% with more pronounced effects in northern areas. In comparison to current levels of indigenous transmission, these increases are insignificant and will not lead to the re-emergence of local transmission in Europe. Current malaria risk across Europe was quantified using environmentally-based risk maps of the present distribution of five mosquito vectors. For each anopheline, spatially defined measurements of relative vectorial capacity (RVC) were calculated from published temperature-dependent and independent measurements of relevant parameters and then converted into an absolute measure of malaria risk, Ro, using country-specific ratios predicted from GDP and life expectancy. The relationship between these national indicators and the ratios of RVC: Ro was empirically derived from the ratio of recent indigenous: imported malaria cases in 5 European countries with available data. Simulations indicated that projected climate changes may expand the geographical distribution and/or increase the abundance of both southern and northern vectors. Calculations of RVC showed that there are only few locations in Europe where the five mosquitoes can currently act as efficient vectors of P. vivax malaria. With projected climate changes, these areas expand so that by the 2080s, three of the five mosquitoes may have more important roles in malaria transmission across Europe. Ro was currently predicted to be above 1 (i.e. conditions suitable for malaria transmission) in areas of Moldova and Romania and projected climate changes could considerably increase the risk of transmission in these areas by the 2080s while the risk of disease in the rest of Europe remains very low.