Topic 2 – Biology, Ecology And Behaviour Of Dengue And Zika Vectors
By Micah Moen
Assalamualaikum and good day everyone. In previous video, you’ve learn about the biology of dengue and zika viruses and how these viruses are transmitted to susceptible hosts. Now, in this video, we are going to explore another interesting topic in Dengue and Zika, which is the Biology,Ecology and behaviour of Dengue and Zika Vectors. In this video, you will learn about the vectors that are responsible for the disease transmission. From that you will understand why the biology, ecology and behaviour of the vectors contribute to the expansion of Dengue and Zika diseases around the world. Before we move on to the key topics that will be covered in this video, let’s have a quick look at the definition of some important terms that you will hear in this video. Anautogenous – is a condition when blood meal is needed by female mosquito to develop eggs. Species that do not need a blood meal to lay eggs undergo autogenous development. Gravid- is a condition when eggs are fully developed in mosquito abdomen or literally pregnant. Arthropophilic means species that has an affinity for human blood. Zoophilic is species that prefers to feed on animal’s blood. Mosquitoes that prefer to rest indoor are called endophilic and those that prefer to rest outdoor are called exophilic. After you have finished watching this video, we expect you to be able to: Identified the vectors for Dengue and Zika You should be able to describe the life cycle and breeding requirements of the vectors And…You should be able to describe the behaviour of the vector which is related to their efficiency in transmission of dengue and zika viruses. In trying to achieve those learning outcomes, we are going to cover a few main topics. They are: Morphological characteristics of vectors Host Preferences by the vectors Blood Feeding and Egg development process in vectors Biting and resting habits Breeding sites characteristics Life cycle, and Environmental thresholds that limits vector distribution and expansion. Dengue and Zika viruses are transmitted from one to another host (human or animal) through Aedes mosquito bites. There are two species of Aedes mosquitoes that have been recognized as vectors for the diseases. The primary vector is Aedes aegypti While the secondary vector is Aedes albopictus. In areas where Ae. aegypti population is not well-established, Ae. Albopictus will be the primary vector for dengue and Zika Historically, Aedes aegypti is originated from Africa. While Aedes albopictus is originated from the South-East Asia Both species are first discovered from forested areas, in tree holes as habitats. Adults of Ae. aegypti and Ae. albopictus can be easily differentiated based on their morphological characteristics. Adults of Ae. aegypti are relatively small and has a black and white pattern due to the presence of silver scale patches on a black background on its leg and other parts of the body. The prevailing diagnostic character is the presence of silver scales in a shape of a lyre on a black background on the scutum which is the dorsal part of thorax. Similar to Aedes aegypti, Adult Aedes albopictus are easily recognized by the bold black shiny scales and distinct silver white scales on the legs. However, the scutum has a single longitudinal silvery dorsal stripe down the center beginning at the dorsal surface of the head and continuing along the thorax. Adult mosquitoes of both sexes obtain nourishment for basic metabolism and flight by feeding on nectar. In addition, Aedes female mosquitoes are anautogenous where they need a blood meal to develop eggs. They suck blood via specialized piercing-sucking mouthparts. Aedes aegypti prefer mammalian hosts and will preferentially feed on humans, even in the presence of alternative hosts. They also feed multiple times during one gonotrophic cycle (feeding, egg-producing cycle) which has implications for disease transmission. Aedes albopictus is an opportunistic feeder. Blood hosts include humans, domestic and wild animals, reptiles, birds and amphibians. However they have shown a preference for human blood meals. After taking its blood meal The abdomen of a female mosquito will be extended and become red in colour Later the colour of the abdomen becomes dark red. Eggs which are white in colour in the abdomen will develop when the blood has been digested. At this stage, the posterior part of the abdomen is whitish and the anterior is reddish in colour. This stage is called half-gravid. When the blood is finally digested, the abdomen is enlarged and becomes whitish because all the eggs are develop. The female mosquito is now in gravid stage and is ready to lay its eggs. The process of taking blood meal, digesting the blood and laying the eggs is called the gonotrophic cycle. Gonotrophic cycle length range between 2 to 4 days. Female can produce an average 100 to 200 eggs per batch. After lay eggs, the female mosquito now is ready to take another blood meal and the gonotrophic cycle will be repeated. For Aedes aegypti, Breeding sites often not found further than 100 meters from human habitations although some studies have shown that breeding habitats can also be found away from human dwellings. Aedes aegypti prefer human habitations as they provide resting and host-seeking possibilities and as a result will readily enter buildings. Aedes albopictus is currently considered a serious biting nuisance for humans. Adult females bite aggressively, usually during the day and preferably outdoors with many vegetation. However, there are reports that Ae. albopictus is becoming partially endophilic, and is found to be biting indoors. Blood-fed females were reported to be found indoors, indicating that local mosquito populations could spend time resting indoors after a blood meal. Aedes aegypti and Aedes albopictus are short distance fliers. The flight distance is up to 500 meters or shorter.
Both are diurnal or active during daytime Their biting peaks at change of light intensity which is after sunrise and before sunset. This information is crucial for planning Aedes control program. Aedes aegypti and Aedes albopictus are sympatric species, where they are overlapping in distribution and shared or have similar requirement for breeding. Both species are Container/tree hole breeders. These mosquitoes lay eggs on the walls of a container just above the water line. The eggs are flooded when rainfall raises the water level. Aedes aegypti and Aedes albopictus larvae can be found in stagnant water. The water is usually clear but not necessarily clean. The breeding sites maybe anything from water in discarded automobile tires and axils of plants, to tanks for water storage and plastic sheet used for coverage. Studies have shown most breeding were found in containers made of plastic over other types of containers. Both species prefer man-made or artificial containers to oviposit eggs. Since plastic containers are abundantly made, used and discarded, this provides opportunity for both Aedes vectors to proliferate. Breeding can be found in shaded or partially shaded container to lay eggs over containers located in the open. Aedes albopictus is exophilic and usually found to breeding in outdoor containers such as discarded containers. The leaves debris in the water can be the food source for the larvae. While Aedes aegypti are endophilic and prefer to breed in indoor containers such as indoor water tank in bathroom. Aedes aegypti has adapted to urban and and suburban domestic habitats exploiting a wide range of artificial containers such as vases, water tanks and tyres. It is also found utilising underground aquatic habitats, such as septic tanks, and adapting to use both indoor and outdoor aquatic container habitats in the same area. Adaptation to breeding outdoors may allow for increased population numbers and difficulty in implementation of control methods. Aedes albopictus has spread globally and endemic in tropical and subtropical Asia, Central and South America and Tropical Island. The breeding sites can be found in natural and artificial containers in urban, suburban and rural areas. Aedes albopictus geographical spread has mostly occurred via passive transport of eggs in used tires and lucky bamboo. Public and private transport from heavily-infested areas has also resulted in the passive transportation of Ae. albopictus into new areas. Both Aedes aegypti and Aedes albopictus prefer darker colours of black and red. This is very important information to develop effective egg and adult trapping devices. Let’s take a look at some containers that usually found in and around houses which contribute to the breeding sites of Aedes aegypti and Aedes albopictus. As we have known about the host preferences, egg development process and breeding sites of Aedes mosquitoes, Let us take a look at the Aedes life cycle. All mosquitoes including Aedes mosquitoes undergo complete metamorphosis, having egg, larval and adult stages. All mosquitoes gone through four separate and distinct stages of its life cycle:
Egg, Larva, Pupa, and Adult. Each of these stages can be easily recognized by its special appearance. Water is a necessary part of their habitat. For Aedes aegypti and Aedes albopictus mosquitoes
Eggs are laid singly. Aedes mosquitoes lay their eggs on wet surfaces such as at the edge of the water surface in tree holes, leaves, rock pool or on the inside walls of artificial containers just above the water line. The eggs are black in colour and shape of a rugby ball. Aedes eggs will desiccate and perish easily when first laid. However, after embryo development with each egg, the eggs can withstand dry conditions for long periods of time. This trait has allowed Aedes mosquitoes to use temporary water bodies for breeding such as artificial containers. Eggs must be submerged in water in order to hatch. Hatching occurs in batches. Most eggs hatch into larvae within 48 hours. Also, Aedes mosquito have been inadvertently been carried to many parts of the world as dry eggs in tires, water jars, or other containers. Larva: The larva (plural – larvae) lives in the water and comes to the surface to breathe. Larvae have short and blunt siphon tubes for breathing and hang upside down from the water surface. This is different from Anopheles larvae as they do not have a siphon and lie parallel to the water surface to get a supply of oxygen through a breathing opening. The Aedes larvae feed on microorganisms and organic matter in the water. Larvae shed (molt) their skins four times, growing larger after each molt. During the fourth molt the larva changes into a pupa. Pupa: The pupal stage is a resting, non-feeding stage of development, but pupae are mobile, responding to light changes and moving (tumble) with a flip of their tails towards the bottom or protective areas. This is the time the mosquito changes into an adult. Pupae give rise to adult mosquitoes in 1 to 2 days. The emergence process begins with splitting of the pupal skin along the back. Adult: The newly emerged adult rests on the surface of the water for a short time to allow itself to dry and all its body parts to harden. The wings have to spread out and dry properly before it can fly. Accordingly, this is critical stage in the survival of mosquitoes. If there is too much wind or wave action, the emerging adult will fall over, becoming trap on the water surface to die. This is the reason that little if any mosquito breeding occurs in open water. Blood feeding and mating does not occur for a couple of days after the adults emerge. How long each stage lasts depends on both temperature and food source. With optimal food and temperature, Aedes mosquito can complete their entire life cycle as short as 7 days. Differences in morphology between male and female of both species include the antennae of the male are plumous and mouthparts are modified for nectar feeding. Field identification is very easy because of these distinct features. The adult of Aedes mosquitoes have an average life span of 30 days depending on environmental conditions. In tropical areas, Aedes aegypti and Ae. albopictus breeding may continue year-round. However, this wont occur in temperate region. Aedes aegypti, unlike Ae. albopictus is not able to undergo winter diapause as eggs, and this therefore limits their ability to exploit more northerly temperate region. However this species readily adapts to breeding in subterranean structures that provide a suitable and protective environment from adverse climatic conditions. Aedes aegypti also has limited dispersal capability as adults. Rainfall may affect abundance and productivity of breeding sites but this species’ preference for artificial water containers means it does not have to rely on rainfall for larval development sites. Coupled with its preference for feeding and resting indoors, these aspects make this species less susceptible to the effects of climatic factors which could influence its distribution. In temperate climates, Aedes albopictus produce dormant eggs that can survive the harsh effects of winter. A mean winter temperature of>0degree celcius permit egg overwintering and a mean annual temperature of>11degree celcius allow adult survival and activity. At least 500mm of annual rainfall is a pre-requisite for the maintenance of aquatic habitats. Now, having watch this video, Can you recognized both Aedes aegypti and Aedes albopictus adults morphological characteristics? Can you describe the host seeking, resting habits and breeding sites of the mosquitoes? Can you explain the Blood feeding, egg development process and the life cycle of Aedes mosquitoes? Can you describe why these species are efficient vectors for dengue and zika? I hope you have enjoyed this video and slide shows, and have gain knowledge on dengue and zika vectors. In the next video, you’re going to explore another interesting topic, which is vector Control Strategies. Thank you