Australia’s Dengue Risk: Human Adaptation to Climate Change - Recent Advances and Issues in Environmental Science - page 142

today's climate throughout the southern Australian mainland. This potential distribu-
tion includes the metropolitan areas of Brisbane (pop 1.8 million), Sydney (pop 4.2
million), Adelaide (pop 1.1 million), and Perth (pop 1.5 million). Additionally the
climate change temperature limit projections for the mid scenario 2050 see this range
expand to include Melbourne (pop 3.6 million). The addition of a theoretical dengue
virus transmission limit parameter (we used a 14.2°C wet bulb isotherm) suggests an
overlapping dengue risk in many of Australia's metropolitan regions during the sum-
mer months (December-February).
The potential for dengue virus introduction to these regions through travelers from
endemic regions (including north Queensland) during summer presents a transmission
risk that can be inferred by the current incidence of imported and endemic cases of
dengue in Australia--many of which enter Australia through national and interna-
tional transport nodes. For example, for the year to June, 2008 there were 250 den-
gue notifi cations for Australia, of which 113 came from Queensland (most via local
transmission), 72 from NSW, 15 from NT, 12 from SA, 8 from VIC, and 28 from WA.
Notifi cations from NSW, South Australia, Victoria and Western Australia exceeded the
5-year mean in each jurisdiction suggesting that the frequency of dengue is increasing
[32].
Understanding the relationship between climate and dengue transmission is dif-
fi cult because non-linear relationships exist between the daily survival of Ae. aegypti ,
the extrinsic incubation period (EIP) of the virus, temperature and humidity [33-35].
Forecasted regional warming in Australia may lengthen and intensify the dengue
transmission season by shortening the mosquitoes' EIP, although it is important to note
that dengue epidemics appear to be more strongly infl uenced by intrinsic population
dynamic (epidemiological) processes than by climate [36]. Even so, any temporal ex-
tension effect in the transmission season will follow the expansion of potential larval
sites that is now underway in Australia. Thus, while the issue of regional warming is
important, the expansion of large rainwater tanks throughout urban regions of Australia is
at present a prevailing human adaptation with more immediate possibilities for chang-
ing vector distributions in Australia than the direct warming effects projected by an-
thropogenic climate change scenarios. Whether southern Australia's current drought is
due to the region's natural climate variability or part of a changing climate pattern, will
continue to be debated by some. Nonetheless, it is important to avoid the cycle where
human changes in water storage result in an Ae. aegypti range expansion followed by
dengue epidemics seeded by viremic travelers [4, 37]. Additionally, domestic water
storage can sustain Ae. aegypti populations (and dengue transmission) in regions not
normally suitable for its survival [38], while active government and community con-
tributions can remove established Ae. aegypti populations (and dengue) from areas
where it has been endemic [39]--and both of these are human modifi cations.
In Australia, ineffectively screened domestic rainwater tanks have been identifi ed
as key containers with respect to Ae. aegypti productivity [40, 41]. The introduction of
reticulated water systems in towns and cities throughout Australia is believed respon-
sible for a major range contraction of Ae. aegypti over the last 50 years. This trend may
now be reversed as humans adapt to climate-change-induced drought conditions--the