1.3.7.2 Patterns in vector-borne diseases

Vector-borne diseases are known to be sensitive to temperature and rainfall (as shown by the ENSO effects discussed above). Consideration of these relationships suggests that warmer temperature is likely to have two major kinds of closely related, potentially detectable, outcomes: changes in vectors per se, and changes in vector-borne disease outcomes (Kovats et al., 2001). Insect and tick vectors would be expected to respond to changes in climate like other cold-blooded terrestrial species (Table 1.9). There is some evidence that this is occurring in relation to disease vectors, but the evidence for changes in human disease is less clear.

Tick vectors

Changes in the latitudinal distribution and abundance of Lyme disease vectors in relation to milder winters have been well documented in high-latitude regions at the northern limit of the distribution in Sweden (Lindgren et al., 2000; Lindgren and Gustafson, 2001), although the results may have been influenced by changes due to reporting and changes in human behaviour. An increase in TBE in Sweden since the mid-1980s is consistent with a milder climate in this period (Lindgren and Gustafson, 2001), but other explanations cannot be ruled out (Randolph, 2001).

Malaria

Since the TAR, there has been further research on the role of observed climate change on the geographical distribution of malaria and its transmission intensity in African highland areas but the evidence remains unclear. Malaria incidence has increased since the 1970s at some sites in East Africa. Chen et al. (2006) have demonstrated the recent spread of falciparum malaria and its vector Anopheles arabiensis in highland areas of Kenya that were malaria-free as recently as 20 years ago. It has yet to be proved whether this is due solely to warming of the environment. A range of studies have demonstrated the importance of temperature variability in malaria transmission in these highland sites (Abeku et al., 2003; Kovats et al., 2001; Zhou et al., 2004) (see Chapter 8, Section 8.2.8.2 for a detailed discussion). While a few studies have shown the effect of long-term upward trends in temperature on malaria at some highland sites (e.g., Tulu, 1996), other studies indicate that an increase in resistance of the malaria parasite to drugs, a decrease in vector-control activities and ecological changes may have been the most likely driving forces behind the resurgence of malaria in recent years. Thus, while climate is a major limiting factor in the spatial and temporal distribution of malaria, many non-climatic factors (drug resistance and HIV prevalence, and secondarily, cross-border movement of people, agricultural activities, emergence of insecticide resistance, and the use of DDT for indoor residual spraying) may alter or override the effects of climate (Craig et al., 2004; Barnes et al., 2005).

There is a shortage of concurrent detailed and long-term historical observations of climate and malaria. Good-quality time-series of malaria records in the East African and the Horn of Africa highlands are too short to address the early effects of climate change. Very few sites have longer data series, and the evidence on the role of climate change is unresolved (Hay et al., 2002a, 2002b; Patz et al., 2002; Shanks et al., 2002), although a recent study has confirmed warming trends at these sites (Pascual et al., 2006).