Biology Reference
In-Depth Information
mice, birds, and bats). Environmental conditions have a definitive influence
on the propagation of vectors and, to a lesser extent, on the development of
pathogens. Although not a topic of this chapter, environmental conditions
may also influence disease transmission between humans by modulating
the contact rates among humans.
In this section, we will discuss the environmental determinants for some of
the more well-known infectious diseases, as well as those for the more recent
H5N1 avian influenza, which has affected a significant part of the world and
caused human deaths in at least 12 countries.
3.2.1 Malaria
Malaria is a parasitic disease caused by a number of
Plasmodium
species. It
is transmitted by infected female
Anopheles
mosquitoes. Malaria is endemic
in a significant part of the world's tropical regions, including sub-Saharan
Africa, South and Southeast Asia, and Latin America. The WHO estimates
more than 500 million cases (and 1 million deaths) of malaria occur every
year worldwide. Climate and environmental conditions play an important
role in the spread and transmission of malaria (Kiang et al. 2006). For exam-
ple, rainfall provides mosquitoes breeding sites. It also increases humidity,
which prolongs their life span. However, that the lack of rainfall has also
been shown to create new breeding sites, such as pools and puddles in
some regions; as such, little rainfall can increase vector population as well.
Alternatively, intense and prolonged rainfall may flush away larval habitats
and reduce malaria transmission (Kovats 2003). Natural bodies of water such
as ponds and streams serve as breeding sites as well. Temperature is another
factor that can influence the transmission of malaria. Warmer temperature
accelerates larval and vector development (Craig 1999) and shortens the
sporogonic cycle; this prolongs the time a mosquito can transmit malaria.
Warmer air holds more moisture. Suitable or relative humidity is needed
for the survival of
Anopheles
mosquitoes. Studies have shown that optimal
relative humidity is between 55% and 80% for active malaria transmission
to occur in India (Bhattacharya 2006). Vegetation, which is associated with
mosquito breeding, feeding, and resting sites, is another important factor for
malaria transmission.
3.2.2 Dengue Fever
Dengue fever or dengue is a viral disease transmitted by infected
Aedes aegypti
and
A.
albopictus
m o is q u i t o e is . S i m i l a r t o m a l a r i a , d e n g u e f e v e r i is m o is t p r e v a l e n t
in the Tropics. Four related serotypes are involved in the diseases. However,
there is no cross-protection among serotypes. A severe form of the disease is
dengue hemorrhagic fever, which can be fatal. According to the WHO, about
2.5 billion people, or 40% of the world's population, are currently at risk for
this disease. Dengue is considered an urban disease as it is predominately
