Small Bite, Big Threat

Deadly Infections Transmitted by Aedes Mosquitoes

Jagriti Narang, Manika Khanuja

• Publisher: Jenny Stanford Publishing
• Year: 2020
• ISBN:
  9781000754483
• Edition: 1
• Topic: Medicine
• Subtopic: Biotechnology in Medicine

Mosquitoes are significant vectors that transmit various pathogens to humans and other mammals. Mosquitoes seem to be omnipresent and easily breed in climates favourable to them. Life cycle of the Aedes species of mosquitoes is similar to others of its genera. This book focuses on Aedes mosquitoes that are responsible for many dreadful diseases and discusses every stage in the life cycle of the species.

The contributing authors of this book have extensive teaching and research experience in the field of detection of viruses of Dengue, Chikungunya, yellow fever and West Nile. One of the contributing authors, Prof. Vinod Joshi, has researched on Dengue viruses for 17 years. The book provides a detailed account of the distribution of Aedes mosquitoes, their role as a vector and their control through various methods. Currently, there has been increased interest among researchers to mitigate the threat caused by Aedes mosquitoes and substantial investigation is being done on the mosquito's history, in characterizing present circumstances and to collaborate future efforts.

Chapter 1

Aedes Mosquitoes: The Universal Vector

Annette Angel,^a Bennet Angel,^b Neelam Yadav,^c,d Jagriti Narang,^e Surender Singh Yadav,^f and Vinod Joshi^b

a Division of Zoonosis, National Center for Disease Control, 22 Sham Nath Marg, Civil Lines, Delhi, India

b Amity Institute of Virology and Immunology, Amity University, Sector 125, Noida, India

c Centre for Biotechnology, Maharshi Dayanand University, Rohtak, India

d Department of Biotechnology, Deenbandhu Chhotu Ram University of Science and Technology, Murthal, Sonipat, India

e Department of Biotechnology, Jamia Hamdard University, New Delhi, India

f Department of Botany, Maharshi Dayanand University, Rohtak, India [email protected], [email protected], [email protected]

Mosquitoes play a vital role in transmitting various pathogens to humans and other mammals. They are considered to be one of the successful creatures of this planet. Their different forms occupy every corner of the earth, be it land or water. In this chapter, a general account of the mosquito, its morphology and anatomy as well as different stages of its development are discussed. Though the book is specifically on Aedes mosquitoes, yet it is very important to have a general understanding about mosquitoes before moving forward.

1.1 Background

Mosquitoes act as an important vectoral component for disease transmission of many of the classified pathogens and parasites, including viruses, bacteria, fungi, protozoa, and nematodes. This is because when they lay eggs, they require blood, for which they feed on vertebrate hosts. During this event, the pathogens residing in them find a route to travel from the infected host to the mosquito and vice versa, though in some cases humans may serve as a dead end. Hence the term vector has been functionally assigned to them (Arbovirus Summary Archives, 2008). Of the vast population of mosquitoes inhabiting the planet, the ones belonging to the family Culicidae (order: Diptera) are known to play a crucial role as vectors of arbovirus transmission (Arbovirus Summary Archives, 2008). They are found throughout the world except places that are permanently frozen, and they occupy the tropics and sub-tropics where the climate seems favorable and efficient for their development (Clements, 2000). Within this family Cuclicidae, the genus Aedes is involved in the vicious cycle of transmission of many diseases of infectious and communicable nature, such as dengue fever, Chikungunya fever, West Nile fever, Zika, yellow fever, etc. (CDC, 2016). There are many species of Aedes depending on ecological distribution and spatiation and spread across the globe (Arbovirus Summary Archives, 2008). Although there is not much dissimilarity or variation among them, the differences may have been incorporated due to the genetic and environmental changes that might have occurred due to adaptation or continental drift since time immemorial. The genus Aedes originated mainly from Africa and might have found ways to spread throughout the world via seaports, which were the main places for export of goods during the 18th century. With increasing globalization, the species might have evolved from dwelling in forests to colonizing urban areas (CDC, 2016).

1.2 Life Cycle

The life cycle of Aedes species is similar to that of any other mosquitoes of its genera. These mosquitoes undergo an indirect development with well-defined stages: eggs æ larvae æ pupae æ adult (Figs. 1.1 and 1.2). The larval stage can be further divided into four stages or instars reflected by its size and development (Gubler, 1989).

Figure 1.1 Stages of Aedes mosquito. Figure taken from www.cdc.gov/dengue/entomologyecology/m_lifecycle.html#stages.

Figure 1.2 Time duration of development of Aedes mosquito under favorable environmental conditions. Figure taken from https://www.cdc.gov/dengue/resources/factSheets/MosquitoLifecycleFINAL.pdf

1.2.1 The Egg

Aedes are known as indoor breeding mosquitoes, that is, they breed in freshwater containers usually found inside or around human dwellings (domestic and peri-domestic containers, respectively). For laying eggs in foci favorable within these dwellings, the females search for damp or moist places that form little water bodies (Gubler, 1989). The preferable sites where eggs are seen are the water collected in tree holes, on the edges of artificial containers such as cement, clay, metallic, and plastic tanks. (Fig. 1.3). They lay around 50–100 eggs at one time. The eggs when laid appear white but change to black as they meet the atmosphere after 2 h (Nelson, 1986). They are oval shaped, covered with shell, and 1 mm in length (Fig. 1.4). The shell is partly secreted by the mother and partly by the embryo. They are soft and flexible when laid but later become hard and waterproof. The embryo develops inside the eggs within 2–3 days depending on atmospheric conditions. These eggs can remain viable up to a year (Foster and Walker, 2002). If an infected female has laid the eggs, then there are chances of the progeny getting infected; in other words, the female is capable of vertical transmission (egg to progeny). The process of laying eggs at a suitable place is termed oviposition, and this is done with the help of fine sensory hairs present on the lower parts of the abdomen (Clements, 2000).

Figure 1.3 Some breeding containers preferred by Aedes mosquitoes for laying eggs.

Figure 1.4 Eggs of Aedes mosquitoes as observed under a light microscope (10× magnification).

1.2.2 The Larvae

White minute larvae, approximately 2.5 mm in length, emerge from the eggs and start wriggling around in the water. This is known as the first instar (Fig. 1.4, right). The larvae soon start feeding and use atmospheric air for breathing (Gubler, 1989). Thus, it ensures its survival by two important features: use of water-borne particles for food and use of atmospheric oxygen for respiration. But they move away from light and prefer dark places. They feed on particulate matter and aquatic microorganisms such as bacteria, diatoms, algae, and their detritus (Nelson, 1986). The mouth brushes provided in front of their mouth help them to locate food particles. Continuous beating of these brushes creates a water current, which helps them to sense food particles ahead. The atmospheric oxygen is taken with the help of spiracles situated in the rear end of the abdomen (Fig. 1.5). The larvae of Aedes species can be distinguished from the other members of the family through their respiratory pattern. Every time they need air, the larvae travel to the water surface and align themselves 90° such that the spiracles are held at the upper end (Figs. 1.6 and 1.7) (Nelson, 1986). Inside the larval body, the spiracles reach t...