Dengue Transmission
Introduction
The risk of contracting dengue infection has increased dramatically since the 1940s. This upward trend is due to increases in long-distance travel, population growth and urbanization, lack of sanitation, ineffective mosquito control, and increases in the surveillance and official reporting of dengue cases. Dengue has spread through Southeast Asia, the Pacific Island countries, and the Middle East. Today, approximately 40% of people live in regions of the world where there is a risk of contracting dengue. Dengue is an endemic disease, which means that it occurs regularly, in tropical regions of the world. The disease is endemic in more than one hundred countries in Africa, the Americas, Asia, the Caribbean, and the Pacific. How does dengue spread, and how is this disease transmitted to humans?
How Does Dengue Spread?
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.
Aedes Mosquitoes
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.
Figure 1: Aedes aegyptiAedes aegypti is the principal vector responsible for dengue transmission.Courtesy of James Gathany/PHIL/CDC. Some rights reserved.
How Is Dengue Transmitted to Humans?
Figure 2: Dengue transmissionThe dengue virus is spread through a human-to-mosquito-to-human cycle of transmission.© 2007 Nature Publishing Group Adapted from Whitehead, S. S. et al. Prospects for a dengue virus vaccine. Nature Reviews Microbiology 5, 518–528 (2007). All rights reserved.
The dengue virus is spread through a human-to-mosquito-to-human cycle of transmission (Figure 2). Typically, four days after being bit by an infected Aedes aegypti mosquito, a person will develop viremia, a condition in which there is a high level of the dengue virus in the blood. Viremia lasts for approximately five days, but can last as long as twelve days. On the first day of viremia, the person generally shows no symptoms of dengue. Five days after being bit by the infected mosquito, the person develops symptoms of dengue fever, which can last for a week or longer.
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.
Dengue Mosquito Life Cycle
Figure 3: Aedes aegypti life cycleFemale Aedes aegypti commonly lay eggs on the inner walls of artificial containers. When the containers fill with water, mosquito larvae hatch from the eggs. After developing through four larval stages, the larvae metamorphose into pupas. Like the larval stage, the pupal stage is also aquatic. After two days, a fully developed adult mosquito forms and breaks through the skin of the pupa. The adult mosquito can fly and has a terrestrial habitat.© 2011 Nature Education All rights reserved.
What are the life stages of mosquitoes? 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 flower pots. 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.
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.
Summary
The dengue virus is spread through a human-to-mosquito-to-human cycle of transmission, with the mosquito Aedes aegypti as the primary vector. These mosquitoes live near humans in tropical and subtropical regions of the world. Female Aedes aegypti become dengue vectors after feeding on the blood of a person infected with the dengue virus. Infected mosquitoes continue to transmit dengue with each blood meal for the rest of their lives. Aedes aegypti have a complex life cycle that includes aquatic and terrestrial stages. These mosquitoes lay their eggs inside containers, and new Aedes aegypti hatch when the containers are filled with water. Dengue poses the greatest risk in highly populated regions with rainy seasons where there are large populations of Aedes aegypti with a high degree of contact between the mosquitoes and humans.