Mosquitoes

Mosquitoes

Mosquitoes undergo a complete metamorphosis which consists of 4 distinctive life stages: egg, larva, pupa and adult. The eggs can hatch into larvae in a few days to months, larvae then take 4-5 days to become pupae, and the adults emerge from pupae after 2 days. Only Aedes eggs are able to hatch into larvae after a few months (up to a year).

Mosquito larvae can be found in low volumes of stagnant water. They remain on or come to the water surface at frequent intervals to take in oxygen through breathing tubes (siphon) or spiracles. They are sensitive to light and movement, and will quickly wriggle to the bottom of water when disturbed. The larvae molt 4 times during growth, growing larger after each molt. When the 4th instar larva molts, it becomes a pupa. The pupa is the resting or non-feeding stage in which they will stay on the water surface when not moving. It is a coma-like structure which will emerge into an adult in 1 to 2 days. The newly emerged adult then rests on the water surface for a short time to allow itself to dry and for the body parts to harden.

Both male and female mosquitoes require nectar or plant sap as food. The sugar is burnt as fuel for flight and is replenished on a daily basis. However, female mosquitoes suck blood in order to obtain the protein in the blood for egg maturation. This turns them into a vector that spreads diseases.

The three most common species found in Singapore include Aedes mosquitoes, Culex mosquitoes and Anopheles mosquitoes.

Aedes mosquito

Adults have black and white patches
Most active during daytime (dawn/dusk)
Breed in clear water
Disease: Dengue, Dengue hemorrhagic fever, Chikungunya, Zika, Yellow fever

Culex mosquito

Adults are brownish
Most active during nighttime
Prefer polluted water
Disease: Filariasis, Japanese encephalitis

Anopheles mosquito

Adults are dark brown to black
Most active during nighttime
Prefer clean, unpolluted water with vegetation
Disease: Malaria

Vector-borne diseases

Vector-borne diseases are a dominant cause of morbidity and mortality worldwide, especially in the tropics and subtropics. They account for more than 17% of the global burden of infectious diseases, with over 80% of the world’s population at risk from at least one vector-borne disease. According to World Health Organization (WHO) estimation, mosquito-borne diseases, including dengue, chikungunya, malaria and yellow fever, cause more than 700,000 deaths per year. Mosquito-borne diseases are rising at an alarming rate driven by several reasons such as country urbanization, global population movement and climate change. As a result, the Global Vector Control Response (GVCR) 2017–2030 approved by the World Health Assembly in 2017 calls for strengthening vector control programs by providing strategic guidance to countries and development partners to prevent diseases and respond to the outbreaks.

The Singapore context

Singapore is a heavily urbanized tropical city located at the southern tip of the Malay Peninsula and its climate is characterized by uniformly high temperatures, abundant rainfall with high humidity all year round. Being a tropical city with a warm and humid climate, Singapore serves as a prime breeding ground and provides suitable habitat for the mosquitoes to breed. Therefore, Singapore is endemic to dengue, with a cyclical pattern of outbreaks every 5 to 7 years. Exposure to other mosquito-borne diseases, such as chikungunya, Zika and yellow fever is relatively low in Singapore, while malaria was eliminated in Singapore in 1981.

Dengue cases in Singapore

According to the National Environment Agency (NEA), there are a total of 6,423 dengue cases reported during the first four months of 2022, exceeding the total of 5,258 cases reported for the whole of 2021. This unexpected rise has continued even before Singapore reaches the usual peak dengue season from June to October.

Dengue is a viral infection that is transmitted to humans through the bite of an infected Aedes mosquito. In Singapore, Aedes aegypti mosquitoes act as the primary dengue vector, while Aedes albopictus mosquitoes are considered a secondary vector. Dengue is caused by any one of four serotypes of dengue virus (DENV-1, DENV-2, DENV-3 and DENV-4) and one can be infected as many as four times in their lifetime. The symptoms of dengue include a fever that is accompanied by severe headache, muscle and joint pains, nausea, vomiting, swollen glands or rash. The symptoms may last for 2 to 7 days, usually after an incubation period of 4 to 10 days after being bitten by an infected Aedes mosquito. Severe dengue can lead to death due to plasma leaking, fluid accumulation, respiratory distress, severe bleeding or organ impairment.

Mosquito control in Singapore

In order to reduce dengue cases and keep the mosquito population low, there have been many mosquito control initiatives happening in Singapore. However, the control programme in Singapore is challenging due to the changes in vector behaviour and rapid urbanization and industrialization process that provide many additional potential mosquito breeding habitats in the city. With a rising trend of dengue haemorrhagic fever in Singapore in the 1960s, the public health response to dengue began in 1966 by setting up a Vector Control Unit within the Quarantine and Epidemiology Branch.

The main focus of mosquito control operations in Singapore includes source reduction, surveillance, public education and law enforcement. Close collaboration between government agencies, private sectors, communities and research institutions plays an important role in order to make the programme succeed — this includes the local pest control and mosquito control services in Singapore.

Community engagement programme such as the “Do the Mozzie Wipeout” campaign was launched in 2013 and since then, the annual campaign encourages the communities to actively check for and to remove stagnant water in and around their homes by practising the 5-Step Mozzie Wipeout. (B: Break up hardened soil, L: Lift and empty flowerpot plates, O: Overturn pails and wipe their rims, C: Change water in vases, K: Keep roof gutters clear and place BTI insecticide). House-to-house inspections and ground surveillance accompanied by a strong legislative framework with monetary penalties help to support compliance.
In 2016, Singapore embarked on a multi-phased field project to fight dengue and other mosquito-borne diseases, namely Project Wolbachia – Singapore, to suppress the Aedes mosquito populations. This project involves the release of non–biting male Wolbachia – carrying Aedes mosquitoes to mate with the field female mosquitoes. These mating produce eggs that do not hatch due to the biological incompatibility, and hence result in reduced mosquito populations. According to the National Environment Agency (NEA) in July 2021, the impact of Project Wolbachia – Singapore is encouraging and had achieved up to 98% suppression of the urban Aedes aegypti mosquito population and 88% fewer dengue cases at existing project study sites.

Do you know…

Not all mosquitoes feed on blood.

Only female mosquitoes bite and suck blood from mammals and other animals, as they require the protein in the blood for egg development and maturation. The mouthpart with a straw-like structure, also known as proboscis, is used to pierce through the skin. This feeding behaviour makes them a vector that spreads diseases.

Male mosquitoes do not feed on blood but take flower nectar or plant sap as food. The sugar is burnt as fuel for flight and replenished on a daily basis.