Geoscience Reference
In-Depth Information
Currently milder dengue has all-year transmission, but mostly during and shortly
after the rainy season around the tropics, between about 25 N and 25 S, where the
principal vector is Aedes aegypti (which is also the vector for yellow fever arbovirus,
now largely eliminated from the USA, Central America and most of South America
and still present in much of tropical Africa). The WHO estimates that there are
50 million cases of dengue infection each year in more than 100 countries. This
includes 100-200 cases reported annually to the US Centers for Disease Control and
Prevention (CDC), mostly in people who have recently travelled abroad. From 1977
to 1994, US health-care workers reported to the CDC 2248 cases of dengue that
had been imported into the USA. Many more cases probably go unreported because
some doctors do not recognise the disease. Cases of dengue began to increase in
many tropical regions in the late 20th century. Epidemics also began to occur more
frequently, and to be more severe. The concern for Europe is that with climate change
another dengue vector, Aedes albopictus , is currently increasing its range and can
already be found in Italy and parts of the Balkans.
In North America West Nile fever has become a concern in recent years. It is
caused by a member of a group of viruses called flaviviruses, which are transmitted
mainly by birds to humans via mosquitoes. It results in a mild, influenza-like illness
that is often called West Nile fever. More severe forms of the disease, which can
be life-threatening, may be called West Nile encephalitis, West Nile meningitis or
neuroinvasive disease. Although many people are bitten by mosquitoes that carry
West Nile virus, most do not know that they have been exposed. Few people develop
severe disease or even notice any symptoms at all. It is thought that only one in five
infected people develop mild illness, and that only 1 in 100 of fever-infected people
go on to develop brain inflammation (meningitis or encephalitis). By 2000 the disease
was considered firmly established in the USA and it is of concern in Canada. In 2002
and 2003 in the USA it caused the death of 284 and 264 people, respectively. By 2002
in Canada several populations of birds were found to have the disease. It was originally
found throughout much of Africa, from the Cape of Good Hope to Cairo, and is also
widespread in the Middle East and India. The disease occasionally appears in parts of
southern Europe, such as Italy, and in Belarus, Ukraine and southern Russia. In 1997
some 500 in the Bucharest region of Romania caught the disease, with 50 people
dying.
In 2004 there was sufficient concern over West Nile fever that the UK Department
of Health issued a short bulletin alerting clinicians to be vigilant. The document
described a plan to control key mosquito populations and a contingency plan for
when cases were diagnosed (Department of Health, 2004).
In North America, West Nile virus (which emerged in New York city in 1999 before
spreading across much of the continent) has been implicated in large-scale declines
of bird populations. In 2007 Shannon LaDeau and colleagues from the Smithsonian
Migratory Bird Center used a 26-year bird breeding survey to demonstrate that the
populations of seven species from four families of birds had declined, coincident
with the predicted areas of the pathogen. These were American crow ( Corvus bra-
chyrhnchos ), blue jay ( Cyanocitta cristata ), tufted titmouse ( Baeolophus bicolour ),
American robin ( Turdus migratorius ), house wren ( Troglodytes aedon ), chickadee
( Poecile spp.) and the eastern bluebird ( Sialia sialis ). The American crow population
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