For the past week, we have been talking about the severe dengue situation in Singapore. In part 1, we have specifically identified which areas in Singapore are in the dengue danger zone and also what do the colours of the zone exactly represent. In part 2, we talked about the transmission of dengue, from human-to-mosquito and from mosquito-to-human.
Today, in part 3 we will be looking at the Aedes Mosquito which is the main culprit behind the dengue disease.
The Aedes mosquito
The Aedes aegypti mosquito is the main vector that transmits the viruses that cause dengue. The viruses are passed on to humans through the bites of an infective female Aedes mosquito, which mainly acquires the virus while feeding on the blood of an infected person.
Virus transmission in the mosquito
Within the mosquito, the virus infects the mosquito mid-gut and subsequently spreads to the salivary glands over a period of 8-12 days. After this incubation period, the virus can be transmitted to humans during subsequent probing or feeding. The immature stages are found in water-filled habitats, mostly in artificial containers closely associated with human dwellings and often indoors.
When are Aedes Mosquitoes the most active?
Aedes aegypti is a day biting mosquito. That means that the mosquito is most active during daylight, for approximately two hours after sunrise and several hours before sunset. The mosquito rests indoors, in closets and other dark places. Outside, they rest where it is cool and shaded. The males of all species of mosquitoes do not bite humans or animals of any species, they live on fruit.
However, Aedes aegypti breed indoors and are capable of biting anyone throughout the day. The indoor habitat is less susceptible to climatic variations and increases the mosquitoes’ longevity.
Dengue Mosquito Life Cycle
Figure 3: Aedes aegypti life cycle
Mosquitoes have a complicated life cycle (Figure 3). As they develop, mosquitoes change their shapes and habitats. Female mosquitoes generally lay their eggs above the water line inside containers that hold water.
These containers include tires, buckets, birdbaths, water storage jars, and flowerpots. Mosquito larvae hatch from the eggs when the containers fill with water, in many cases after a rainfall. The larvae are aquatic, meaning that they live in the water and feed on microorganisms found in the water.
Larvae go through developmental stages in which they molt, or shed their skin, three times. These larval stages are called the first to fourth instars. When a larva is a fully grown fourth instar, it undergoes metamorphosis into a new form called a pupa, the “cocoon” stage for the mosquito. This stage of the mosquito’s life is also aquatic. After two days, the fully developed adult mosquito forms and breaks through the skin of the pupa. The adult mosquito is able to fly and is no longer aquatic. It has a terrestrial habitat.
Flight range studies suggest that most female Aedes. aegypti may spend their lifetime in or around the houses where they emerge as adults and they usually fly an average of 400 metres. This means that people, rather than mosquitoes, rapidly move the virus within and between communities and places.
Hence it is especially important to remove stagnant water from places such as tires, buckets, birdbaths, water storage jars, and flowerpots to prevent the growth of Aedes mosquitoes.
What happens if there is no rain?
Aedes aegypti mosquitoes have adapted so that their eggs can survive dry conditions for several months.
If eggs are laid in a dry container, new mosquitoes only develop when the container is filled with water. This adaptation has made it very difficult to eliminate mosquito populations completely. In many areas of the world, dengue outbreaks occur every year during the rainy season, when conditions are perfect for mosquito breeding.
Dengue can pose a particular threat in highly populated regions because epidemics are more likely where there are large numbers of people in contact with large numbers of mosquito vectors than in more isolated areas. In countries in the equatorial zone that experience tropical monsoon seasons — such as Indonesia, India, Brazil, Thailand, Sri Lanka, and Myanmar — dengue epidemics are a serious public health problem.
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