What is Dengue ? Transmission by Aedes Aegypti

Overview

Dengue fever is a mosquito-borne viral disease that has recently spread to all WHO regions. Female mosquitoes, primarily Aedes aegypti and, to a lesser extent, Aedes albopictus, transmit dengue virus. These mosquitos also transmit the chikungunya, yellow fever, and Zika viruses. Dengue fever is common throughout the tropics, with local variations in risk influenced by climate as well as social and environmental factors.

Dengue causes a wide range of diseases. This can range from subclinical disease (where people are unaware they are infected) to severe flu-like symptoms in those who are infected. Although it is less common, some people develop severe dengue, which can result in a variety of complications such as severe bleeding, organ impairment, and/or plasma leakage. Severe dengue has a higher risk of death if not treated properly. Severe dengue was first identified during dengue epidemics in the Philippines and Thailand in the 1950s. Severe dengue fever now affects the majority of Asian and Latin American countries, and it is a leading cause of hospitalization and death among children and adults in these regions.

Dengue is caused by a virus of the Flaviviridae family, and the virus has four distinct but closely related serotypes (DENV-1, DENV-2, DENV-3 and DENV-4). It is believed that recovering from infection provides lifelong immunity against that serotype. However, after recovery, cross-immunity to the other serotypes is only partial and only temporary. Subsequent infections (secondary infection) with other serotypes increase the risk of severe dengue.

Dengue has distinct epidemiological patterns associated with the virus's four serotypes. Within a region, these can co-circulate, and many countries are hyper-endemic for all four serotypes. Dengue fever has a serious impact on human health as well as the global and national economies.

Transmission

Through mosquito bite

Humans are infected with the virus through the bites of infected female mosquitos, primarily the Aedes aegypti mosquito. Other Aedes species can also act as vectors, but their contribution is secondary to that of Aedes aegypti.

The virus replicates in the mosquito midgut after feeding on a DENV-infected person, before spreading to secondary tissues such as the salivary glands. The time between ingesting the virus and transmitting it to a new host is referred to as the extrinsic incubation period (EIP). When the ambient temperature is between 25 and 28°C, the EIP takes about 8-12 days.

Variations in the extrinsic incubation period are influenced not only by ambient temperature, but also by a number of factors such as the magnitude of daily temperature fluctuations, virus genotype, and initial viral concentration. Once infected, the mosquito can transmit virus for the rest of its life.

Human to mosquito

Mosquitoes can become infected with DENV from people who are infected. This can include people who have a symptomatic dengue infection, people who have not yet developed a symptomatic infection (they are pre-symptomatic), and people who show no signs of illness (they are asymptomatic).

Human-to-mosquito transmission can occur up to two days before symptoms appear and up to two days after the fever has subsided.

High viremia and fever in the patient are associated with a higher risk of mosquito infection; on the other hand, high levels of DENV-specific antibodies are associated with a lower risk of mosquito infection (Nguyen et al. 2013 PNAS).

The majority of people are viremic for about 4-5 days, but viremia can last up to 12 days.

Maternal transmission

Mosquito vectors are the primary mode of transmission of DENV between humans. However, there is evidence of the possibility of maternal transmission (from a pregnant mother to her baby). While vertical transmission rates appear to be low, the risk of vertical transmission appears to be associated with the timing of the dengue infection during pregnancy. When a mother has a DENV infection while pregnant, her babies may have pre-term birth, low birthweight, and fetal distress.

Ecology of Vector

The mosquito Aedes aegypti is thought to be the principal DENV vector. It could breed in natural containers like tree holes and bromeliads, but it has now adapted to urban habitats and now breeds primarily in man-made containers like buckets, mud pots, discarded containers and used tyres, storm water drains, and so on, making dengue an insidious disease in densely populated urban areas. Female Aedes aegypti feeds during the day; its peak biting periods are early in the morning and before dusk. Aedes aegypti feeds several times between egg-laying periods, resulting in clusters of infected individuals.When a female lays her eggs, the eggs can survive in dry conditions for several months until hatching when they come into contact with water.

The secondary dengue vector Aedes albopictus has expanded to more than 32 states in the United States and more than 25 nations in Europe, owing to international traffic in old tyres (a breeding habitat) and other items (e.g. lucky bamboo). It prefers breeding locations in dense vegetation, such as plantations, which has been associated to an increased risk of exposure for rural workers in rubber and palm oil plantations, but it has also been reported to be abundant in metropolitan areas. Aedes albopictus is a very adaptable species. Its geographical distribution is partly owing to its ability to tolerate cooler temperatures as an egg and adult.

Aedes albopictus is a day biter like Aedes aegypti, and it has been implicated as the principal vector of DENV in a small number of outbreaks where Aedes aegypti is absent or present in low numbers.

Sign and Symptoms

Acute Dengue

When a high fever (40°C/104°F) is accompanied by two of the following symptoms during the febrile phase (2-7 days), dengue should be suspected:

Muscular and joint discomfort, severe headache, pain behind the eyes,rash, nausea, vomiting, swollen glands are common symptoms.

Severe Dengue

Doctors should look for the following warning signs:

Severe abdominal discomfort, constant vomiting, rapid breathing, bleeding gums or nose, fatigue, restlessness, liver enlargement, and blood in the vomit or stool.