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On last week’s blog, in part 1 we have mentioned about the different clusters of dengue in Singapore such as the Red, Yellow, Green as well as what do these 3 colours represent.

For part 2, we will be looking at the transmission of dengue, mainly the method in which dengue is being infected from one person to another.

Transmission

There are namely 2 types of transmission, from mosquito to human and from human to mosquito transmission. We have adapted the following details of transmissions from the official WHO website: who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue

Mosquito-to-human transmission

The virus is transmitted to humans through the bites of infected female mosquitoes, primarily the Aedes aegypti mosquito.

After feeding on an DENV (Dengue virus) -infected person, the virus replicates in the mosquito midgut, before it disseminates to secondary tissues, including the salivary glands. In dengue virus infected mosquito's, the virus is present in the salivary glands of the mosquito.

When a female Aedes aegypti bites a human for food, she injects saliva into the wound where the anti-coagulants contained in her saliva facilitate feeding. Without knowing, the mosquito also injects the dengue virus into the host. 

The time it takes from ingesting the virus to actual transmission to a new host is termed the extrinsic incubation period (EIP). The EIP takes about 8-12 days when the ambient temperature is between 25-28°C.

Variations in the extrinsic incubation period are not only influenced by ambient temperature; a number of factors such as the magnitude of daily temperature fluctuations, virus genotypeand initial viral concentrationcan also alter the time it takes for a mosquito to transmit virus. Once infectious, the mosquito is capable of transmitting virus for the rest of its life.

Human-to-mosquito transmission

Mosquitoes can become infected from people who are viraemic with DENV. This can be someone who has a symptomatic dengue infection, someone who is yet to have a symptomatic infection (they are pre-symptomatic), but also people who show no signs of illness as well (they are asymptomatic).

Human-to-mosquito transmission can occur up to 2 days before someone shows symptoms of the illness, and up to 2 days after the fever has resolved.

Risk of mosquito infection is positively associated with high viremia and high fever in the patient; conversely, high levels of DENV-specific antibodies are associated with a decreased risk of mosquito infection Most people are viraemic for about 4-5 days, but viremia can last as long as 12 days.

Other modes of transmission

The primary mode of transmission of DENV between humans involves mosquito vectors[1]. There is evidence however, of the possibility of maternal transmission (from a pregnant mother to her baby).

While vertical transmission rates appear low, with the risk of vertical transmission seemingly linked to the timing of the dengue infection during the pregnancy. When a mother does have a DENV infection when she is pregnant, babies may suffer from pre-term birth, low birthweight, and fetal distress.

What type of mosquito transmit dengue?

Dengue viruses are spread to people through the bites of infected Aedes species mosquitoes (Aedes aegypti

or Aedes albopictus).  These are the same types of mosquitoes that spread Zika and chikungunya viruses.

Fig 1: Aedes aegypti

Fig 2: Aedes albopictus

• These mosquitoes typically lay eggs near standing water in containers that hold water, like buckets, bowls, animal dishes, flowerpots, and vases.
• These mosquitoes prefer to bite people, and live both indoors and outdoors near people.
• Mosquitoes that spread dengue, chikungunya, and Zika bite during the day and night.
• Mosquitoes become infected when they bite a person infected with the virus. Infected mosquitoes can then spread the virus to other people through bites.

Find out more about the behaviours of these mosquitoes at: https://www.futurelearn.com/courses/preventing-zika/0/steps/14641#:~:text=aegypti%20mosquitoes%3B%20they%20are%20both,the%20top%20of%20the%20thorax

Do mosquitoes die immediately after spraying it with insect repellent?

A common misconception that people have with insect repellent is that it kills mosquitoes instantaneously. Well, the answer is no. Repellents are substances that help people avoid mosquito bites. Repellents do not kill mosquitoes and other insects, but they will help deter them from biting people.

The active ingredient that we use in our insect repellent products, neem does not instantaneously kill mosquitoes.

Firstly, neem has a strong garlic smell which deters mosquito away from you and your skin.

When you spray the repellent on surfaces or on you skin, should a mosquito come into contact with the surface, it will be coated by the neem substance.

However, the mosquitoes do not die on the spot. What the neem does is to disrupt the mosquitoes inhabiting behaviour such as:

• Disrupting or inhibiting the development of eggs, larvae, or pupae.
• Blocking the moulting of larvae or nymphs.
• Disrupting mating and sexual communication.
• Repelling larvae and adults.
• Deterring females from laying eggs.
• Sterilizing adults.
• Poisoning larvae and adults.
• Deterring feeding.
• Blocking the ability to "swallow" (that is, reducing the motility of the gut);
• Sending metamorphosis awry at various stages

Hence after a period of time, due to its disrupted behaviour which affects its feeding habits, it will then eventually die.

Adapted from:

https://www.cdc.gov/dengue/transmission/index.html#:~:text=Aedes%20aegypti%20mosquito.,albopictus

https://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue

https://www.nature.com/scitable/topicpage/dengue-transmission-22399758/

http://www.denguevirusnet.com/aedes-aegypti.html#:~:text=Aedes%20aegypti%20is%20a%20day,closets%20and%20other%20dark%20places.

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[1] 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.