The human ecology of climate change - Climate Change: Biological and Human Aspects - page 403

The seasonality of Lyme disease is also likely to be affected by climate change. It
was first identified in the USA in 1975 and then noticed in much of Europe. Its name
comes from a district in Connecticut, which is one of the many areas in which it is
endemic. The disease is caused by the spirochaete bacterium Borrelia burgdorferi ,
with mammals acting as reservoirs, and is transmitted by ixodid ticks such as Ixodes
ricinus . Humans are not the preferred host, but nonetheless Lyme disease is a public-
health concern in a number of rural areas in Europe and especially the USA, where
there are many thousands of cases a year. In the UK and Ireland the incidence of
the disease has been sporadic with typically just a few scores of cases a year. It
results in arthritis of large joints and may also inflame the brain and nervous system.
Transmission occurs during the spring, summer and early autumn months, which is
also when humans are likely to be outdoors in rural areas and exposed to the vectors.
Because of climate change both the seasonality of the disease and also its geographic
distribution may be extended.
Climate change not only affects the propensity of a disease to affect a human
population but can also change the species range of disease vectors. Of the major
vector-borne diseases, malaria is the most significant. In the 1990s it had the potential
to have an impact on more than 2.1 billion people and successfully infected some
300-500 million, of whom some 90% are in tropical Africa. Malaria currently kills
more people than any other infection except tuberculosis, and has an annual death toll
of 1-2 million people, mainly young African children. It possibly accounts for 24% of
child deaths. Annual deaths from malaria rose steadily from around a million in 1980
to about 1 900 000 in 2005. Following substantial investment in control (development
assistance for tackling malaria increased from US$149 million in 2000 to almost $1.2
billion in 2008) deaths from malaria declined to around 1 300 000 in 2010 (Murray
et al., 2012). This peak-and-decline pattern of mortality is reflected in WHO data,
although the WHO's figures (WHO, 2011) are broadly around 25% lower than this.
Having said that, the methodology of Murray et al. has caused some debate, relying
as it does on interviews with next of kin and friends of the deceased (Anon, 2012).
The range of malaria, among other diseases, has also changed with climate, and will
undoubtedly continue to do so. The disease itself is caused by protoctistan parasite spe-
cies of Plasmodium , namely Plasmodium ovale , Plasmodium vivax , Plasmodium mal-
ariae and Plasmodium falciparum , which are transmitted to (and between) humans
by mosquitoes belonging to the genus Anopheles. P. ovale results in a less-severe
form of malaria compared with the other three species, and Hypocrites ( c . 2500 years
ago) divided malaria into three different types that, with the benefit of subsequent
knowledge, largely appear to relate to three of the different Plasmodium species. As
for the vector, there are some 380 species of Anopheles , of which about 60 transmit
malaria.
Genetic studies suggest that the virulent form of malaria evolved some 10 000
years ago just after the last glacial. This broadly coincides with the development
of agriculture and the supposition has been posited that water held for irrigation
provided an ideal habitat for the Anopheles mosquito to evolve into its current form,
but this has not been proven.
Malaria evolved in the tropics and subtropics of the Old World (Africa) and it did
not arrive in the New World (the Americas) until around 1500 with European settlers