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High latitudinal spread of ticks - vectors for Lyme disease - with milder winters in Sweden and the Czech Republic

Lindgren et al., 2000; Danielova et al., 2006

Food-and water-borne diseases

Salmonellosis in Australia associated with higher temperatures

E. coli and Cryptosporium outbreaks could not be attributed to climate change

D'Souza et al., 2004 Charron et al., 2004

Pollen- and dust-related diseases

Increasing pollen abundance and allergenicity have been associated with warming climate

Pollen abundance also influenced by land-use changes

Levetin, 2001 ; Beggs, 2004

1.3.7.1 Effects of patterns in heat and cold stress

Episodes of extreme heat or cold have been associated with increased mortality (Huynen et al., 2001; Curriero et al., 2002). There is evidence of recent increases in mean surface temperatures and in the number of days with higher temperatures, with the extent of change varying by region (Karl and Trenberth, 2003; Luterbacher et al., 2004; Schär et al., 2004; IPCC, 2007). This increase in heatwave exposures, where heatwaves are defined as temperature extremes of short duration, has been observed in mid-latitudes in Europe and the USA. Individual events have been associated with excess mortality, particularly in the frail elderly, as was dramatically illustrated in the 2003 heatwave in western and central Europe, which was the hottest summer since 1500 (Luterbacher et al., 2004; Chapter 8, Box 8.1).

In general, high-income populations have become less vulnerable to both heat and cold (see Chapter 8, Section 8.2). Studies in Europe and in the USA of mortality over the past 30 to 40 years found evidence of declining death rates due to summer and winter temperatures (Davis et al., 2003a, b; Donaldson et al., 2003). Declines in winter mortality are apparent in many temperate countries primarily due to increased adaptation to cold (Chapter 8, Section 8.2.1.3) (Kunst et al., 1991; Carson et al., 2006). However, the mortality associated with extreme heatwaves has not declined. The 25,000 to 30,000 deaths attributed to the European heatwave is greater than that observed in the last century in Europe (Kosatsky, 2005). Analyses of long-term trends in heatwave-attributable (versus heat-attributable) mortality have not been undertaken.

1.3.72 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).

1.3.73 Emerging food- and water-borne diseases

Food- and water-borne diseases (WBD) are major adverse conditions associated with warming and extreme precipitation events. Bacterial infectious diseases are sometimes sensitive to temperature, e.g., salmonellosis (D'Souza et al., 2004), and WBD outbreaks are sometimes caused by extreme rainfall (Casman et al., 2001; Curriero et al., 2001; Rose et al., 2002; Charron et al., 2004; Diergaardt et al., 2004) but, again, no attribution to longer-term trends in climate has been attempted.

1.3.7.4 Pollen- and dust-related diseases

There is good evidence that observed climate change is affecting the timing of the onset of allergenic pollen production. Studies, mostly from Europe, indicate that the pollen season has started earlier (but later at high latitudes) in recent decades, and that such shifts are consistent with observed changes in climate. The results concerning pollen abundance are more variable, as pollen abundance can be more strongly influenced by land-use changes and farming practices (Teranishi et al., 2000; Rasmussen, 2002; Van Vliet et al., 2002; Emberlin et al., 2003; WHO, 2003; Beggs, 2004; Beggs and Bambrick, 2005) (see Section 1.3.5). There is some evidence that temperature changes have increased pollen abundance or allergenicity (Beggs, 2004) (see Chapter 8, Section 8.2.7). Changing agricultural practices, such as the replacement of haymaking in favour of silage production, have also affected the grass-pollen season in Europe.

The impact on health of dust and dust storms has not been well described in the literature. Dust related to African droughts has been transported across the Atlantic to the Caribbean (Prospero and Lamb, 2003), while a dramatic increase in respiratory disease in the Caribbean has been attributed to increases in Sahara dust, which has in turn, been linked to climate change (Gyan et al., 2003).

1.3.75 Summary of human health

There is now good evidence of changes in the northward range of some disease vectors, as well as changes in the seasonal pattern of allergenic pollen. There is not yet any clear evidence that climate change is affecting the incidence of human vector-borne diseases, in part due to the complexity of these disease systems. High temperature has been associated with excess mortality during the 2003 heatwave in Europe. Declines in winter mortality are apparent in many temperate countries, primarily due to increased adaptation to cold.

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