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The dengue virus is transmitted to humans via the bite of an infected mosquito. Only a few mosquito species are vectors for the dengue virus. What is a vector? A vector is a vehicle that carries and transmits a disease to its host organism. Vectors include animals and microorganisms that transmit different diseases. The most common vectors are arthropods, which are invertebrate animals with an external skeleton called an exoskeleton. Arthropods include mosquitoes, ticks, lice, flies, and fleas. For instance, ticks can carry Lyme disease, and some mosquitoes can carry yellow fever, malaria, and dengue fever.
When a mosquito bites a person who has dengue virus in his or her blood, the mosquito becomes infected with the dengue virus. An infected mosquito can later transmit that virus to healthy people by biting them. Dengue cannot be spread directly from one person to another, and mosquitoes are necessary for transmission of the dengue virus.
Can any type of mosquito carry dengue? The dengue virus is carried and spread by mosquitoes in the genus Aedes, which includes a number of mosquito species. Of these species, the primary vector of the dengue virus is the species Aedes aegypti. It is the principal dengue vector responsible for dengue transmission and dengue epidemics. Other mosquito species in the genus Aedes — including Aedes albopictus, Aedes polynesiensis, and Aedes scutellaris — have a limited ability to serve as dengue vectors.
Aedes aegypti is a small, dark mosquito that can be identified by the white bands on its legs and a silver-white pattern of scales on its body that looks like an ancient Greek musical instrument called a lyre (Figure 1). Where are these mosquitoes found? Aedes aegypti dwell in tropical and subtropical regions all over the world, mainly between the latitudes of 35°N and 35°S where the winter temperature is no colder than 10°C. Although some mosquitoes may travel farther north or south of these latitudes, they are unable to survive cold winters. Because Aedes aegypti require a warm climate, they typically do not live at altitudes above 1000 m, where the temperature is colder. These mosquitoes are associated with the living spaces of humans. They generally spend their entire lives in and around the houses where their eggs hatched.
How does an Aedes aegypti mosquito become a dengue vector? After a mosquito feeds on the blood of someone infected with the dengue virus, that mosquito becomes a dengue vector. The mosquito must take its blood meal during the period of viremia, when the infected person has high levels of the dengue virus in the blood. Once the virus enters the mosquito's system in the blood meal, the virus spreads through the mosquito's body over a period of eight to twelve days. After this period, the infected mosquito can transmit the dengue virus to another person while feeding. Does a mosquito infected with the dengue virus only transmit the virus to the next person it feeds on? No, once infected with dengue, the mosquito will remain infected with the virus for its entire life. Infected mosquitoes can continue transmitting the dengue virus to healthy people for the rest of their life spans, generally a three- to four-week period.
Both male and female mosquitoes feed on plant nectars, fruit juices, and other plants sugars as their main energy source. Why, then, do mosquitoes bite humans? Female mosquitoes require blood to produce eggs, so they bite humans. Each female mosquito can lay multiple batches of eggs during its lifetime, and often Aedes aegypti take several blood meals before laying a batch of eggs. When a female mosquito is infected with the dengue virus, the virus is present in its salivary glands. How does the virus travel from the mosquito's salivary glands into a human? When taking a blood meal, an infected female mosquito injects its saliva into the human host to prevent the host's blood from clotting and to ease feeding. This injection of saliva infects the host with the dengue virus.
Are mosquito bites the only way the dengue virus can be transmitted to humans? In rare events, dengue can be transmitted during organ transplantations or blood transfusions from infected donors. There is also evidence that an infected pregnant mother can transmit the dengue virus to her fetus. Despite these rare events, the majority of dengue infections are transmitted by mosquito bites.
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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