Introduction
The genus Aedes has close to one thousand species that bite and are vectors of various diseases. Among them is the Aedes aegypt i species, which has proven a risk to health due to its high breeding and spread rate, given that transfer of this species from one place to another is quite easy. This could be because they can be carried in containers, tires, shoes, clothes, etc. They also undergo behavioral changes in the new area, so adapt and reproduce very fast. This could also be attributed to the encroachment of natural habitat by man due to activities that interfere with their natural habitat. The mosquitoes, therefore, as any other living creature would do, have been forced to undergo natural selection. Those that are not able to adapt become extinct. The natural selection process is what has led to domestication. This means that their source of food, which is primarily blood, is from humans ( Brown et al.,2014) . Anthropophagy causes transfer of diseases that were originally found in non-human animals only to humans. This is because the insects now breed close to human habitats due to encroachment. When the mode of feeding was exclusively zoophagy, the pathogens transmitted by these mosquitoes remained confined to non-human hosts. Domestication has further caused the Aedes aegypti species to become invasive. They have developed adaptation mechanisms that allow them to co-exist with humans, such that they become a nuisance in the human habitat.
Origin of the Aedes aegypti species
The common name for Aedes aegypti is yellow fever mosquito or scientifically as Stegomyia aegypti. This is because it was initially associated with the spread of yellow fever, but the cases have significantly dropped after the invention of its vaccine. It is now associated with dengue fever and chingukunya viruses ( Watts et al.,2014) . These two viruses are becoming an epidemic, given that they spread so easily yet these are not stable therapy for them.
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It is hypothesized that the origin of the Aedes aegypti is in Sub-Saharan Africa. Some forms of the ancestral species still exist in Sub-Saharan Africa, majorly the species formosus. This species has the non-mutated characteristics of the mosquito, which include dark patterns, laying eggs in tree holes, and feeding exclusively on non-human blood meals. This species has experienced little disturbance in its normal habitat. The mutated form of Aedes aegypti , however, has undergone a major transformation as a mechanism of adaptation (Powell et al., 2013). The mutated form has a lighter pattern, lays eggs on any undisturbed place, and feeds on both human and non-human blood. Since the movement was by ship, the mosquitoes adapted to living in human-made conditions. However, a debate on whether the adaptation came during the travel time or after arrival is still ongoing. Its arrival was however proved by a yellow fever outbreak in Yucatan in 1648( Spiegel et al., 2007) . The introduction of the mosquito-spread along the trade routes of that time and was evident in Australia and parts of Asia. The species has now spread to West Africa and East Africa, areas in which yellow fever was unheard of. Dengue fever and chingukunya have also been reported in East Africa, proving the mutated/domesticated species of Aedes had reached the region. Although the species was long reported in the southeastern parts of the United States of America, It was not until 2013 summer that it was first reported in California.
Ecology
The Aedes aegypti mosquito undergoes the ideal complete lifecycle, with an egg, larvae, pupae, and adult stage.
Factors that would favor breeding of the Aedes aegypti mosquito include:
Size of the area of target- a larger container, a bigger area covered by grass, or a large abandoned car will be more likely to be a breeding ground compared to a tiny container.
Activity- a place where there is little to no activity is more likely to attract the mosquitoes since chances of them maturing to adults are higher. The accumulated nutrients are also more.
Temperature- This species prefers warm conditions to cold ones. A place/container that receives direct sunlight is, therefore, more attractive to them ( Cavalcanti et al. 2014) .
The egg stage: The females Aedes aegypti mosquito lays 100-200 eggs, depending on the amount of blood meal taken in. The eggs are laid in damp places, mainly in areas where rainwater has collected. This makes it very easy to access their human hosts, given they lay the eggs right in their backyard. The female does not lay its eggs all in one cluster. They are spread, such that they exist at different water levels. This could be to increase the chances of survival, given that the species is rapidly destroyed by man. The eggs are shiny and blackish, ovoid, smooth and long, about a millimeter long. They hatch when inundated by water, and can take a few days to a week to develop, depending on the temperature. In a warmer climate, they hatch faster than in cold regions. Due to the harsh conditions, they have adapted by hatching immediately when laid in water. When laid in dry places, they can survive for up to one year, until they are submerged in water (Meerauset al.,2008). This poses a major challenge in containing diseases spread by this mosquito.
The larvae stage: After the eggs hatch, they develop into larvae, which feed on organic matter in the water such as algae. Larvae spend most of their time on the surface of the water and occasionally swim to the bottom to feed or to hide. The larval stage has four sub-stages, taking the longest time in the fourth instar. Warm conditions favor the development of larvae, with males developing faster than females. If the temperature is not conducive, they stay dominant in this stage for months (Verna et al., 2013).
The pupae stage: The pupae are mobile, and take in oxygen to develop into adults, then split into two and emerge out of the water. The adult is ready to fly and bite
The bite pattern: Mosquitoes bite individuals who are still. For this reason, people who stay at home are bitten by mosquitoes more, depending on the availability of breeding grounds nearby. Although other Aedes species feed on blood from non-human sources, the Aedes aegypti species feeds almost exclusively on human blood (99%). The remaining 1% is mainly from domestic animals such as pigs, cats, and dogs. The mosquitoes are attracted to carbon(iv) oxide from humans, which is a sign of food for them. They detect the smell, including sweat, from humans as good smell, so are attracted to them. However, males do not feed on blood, as much as they can detect the good smell. They come to the human body in search of mates who might be having meals at that time. This species feeds during the day, unlike other mosquitoes.
The Aedes aegypti mosquito has become difficult to contain since its eggs can survive in harsh conditions for so long. For example, they egg, if laid in a dry place, will stay viable until it comes across the water so that it can hatch. This means that even if all surfaces and water puddles are spayed and destroyed, adult mosquitoes killed or trapped, there are high chances of viable eggs being left out. Once it rains, the eggs will hatch, and the cycle starts once again(Wallis et al., 2015).
Dispersal and Distribution of Aedes aegypti
According to Kamgang et al. (2018), Aedes aegypti is considered one of the world’s most widely distributed mosquitoes. It is commonly acknowledged that the species have a short range of flight – they fly approximately 50 -100m in their entire lifetime. However, the rate at which the mosquitoes colonize and inhabit new geographical regions is alarming. This is manifested by rapid breakout and prevalence of epidemics the mosquitoes transmit such as yellow and dengue fever. This rapid distribution of the mosquitoes is attributed to the mechanisms with which the female mosquitoes lay their eggs. The females lay a small number of eggs in numerous locations within the geographical area they fly. This way, the batches of the mosquito eggs may be distributed over a considerable distance. Given the short life cycle of the mosquitoes, the next generations of females repeat the sequence of oviposition. Eventually, the mosquitoes will have inhabited a large geographical area within a short period (Kamgang et al., 2018). Accurate methods of determining the distribution of these species remain elusive. Public health practitioners and organizations have relied on epidemiological studies to ascertain the distribution of the organisms in different, by the geographical staging of disease outbreaks. Other scientific methods that have been applied in studying the distribution of the mosquitoes based on their oviposition tendencies is by experimentally labeling the deposited egg using rubidium – a non-radioactive element.
In an experiment to investigate the vertical and horizontal dispersal of the mosquito species, females of the mosquito species were fed in blood containing rubidium. The element is detectable in the eggs of the mosquito using Graphite Furnace Atomic Absorption Spectrophotometry (GFAAS). The mosquitoes were then released in urbanized and semi-rural areas and allowed to lay eggs in their natural ecosystem. Ovitraps spanning a radius of 320m from the release point were used to detect the eggs. Rb-marked eggs were detected unilaterally across the marked array (Kamgang et al., 2018). This study confirms that the females of the mosquitoes can easily and rapidly disperse to different parts within an ecosystem in search of an oviposition site. This is the primary mechanism of dispersal of Aedes aegypti.
The dengue-bearing mosquitoes were first known to originate from South East Asia and Sub-Saharan Africa, before dispersing to other continents following the increased domestication and encroachment of humans into the wild ecosystems. The colonization of the mosquitoes in the United States originated from South America. The mosquitoes have since moved to southern Californian and Las Vegas. According to the Center for Disease Control and Prevention, the mosquitoes are very likely to live and reproduce in the southern part of the U.S. This includes in states such as Texas, Florida, Tennessee, Arizona, Missouri, Georgia, Kentucky, North and South Carolina, etc. Human encounters with the mosquitoes both in the domestic and wild setting have reposted being attacked by large, voracious mosquitoes.
California has also joined in the war against mosquitoes. There have been a door to door campaigns to educate communities on eradication of the mosquitoes. Twenty million mosquitoes have also been bacteria treated and released into the environment to curb reproduction of Aedes aegypti species.
Diseases spread by Aedes aegypti
This species is known to be a vector of various viruses, but mainly Zika, Dengue, Chingukunya, and yellow fever viruses.
Yellow fever- Mosquitoes transmit the virus when they transfer blood containing the virus from an infected person to a healthy person. The transmission cycles are grouped into three: jungle/sylvatic, intermediate and urban cycles. The jungle cycle is whereby the virus is transferred from non-human hosts to humans e.g.from monkeys. The intermediate, also called savannah cycle involves the virus being transferred from infected humans and non-humans to humans. The urban cycle happens in the human set up, where the virus is transferred from one person to another. Symptoms include: chills, nausea, vomiting, weakness, fatigue and sudden onset of fever
Zika virus infection- this virus, transferred by Aedes aegypti mosquito has fatal effects on the brain, such as causing microcephaly. The infection can be passed to the fetus during pregnancy, during sexual intercourse, through blood transfusion or by accidental piercing in a clinical setup. If any, symptoms are mild. They include; red eyes, fever, headache, joint pain, muscle pain, and skin rash.
Dengue fever-mainly transmitted by Aedes aegypti but may be transmitted from mother to child in rare cases. Symptoms include a severe headache, loss of appetite, skin rash, diarrhea and bleeding of gums and nose ( Asilakis et al.,2011) .
Chingukunya virus infection- transmitted by female Aedes aegypti mosquitoes. Symptoms of infection include joint pain, fever, muscle pain, and headache.These fevers share signs and symptoms and may easily be misdiagnosed.
Since they are all transmitted by the same vector, the means of containing their spread is similar. Most of them target the breeding sites so that the situation is contained at an earlier stage. They include larvicides, which target the larvae stage of the mosquito. They are directly applied by spraying the breeding area( Ballenger et al., 2009). Organophosphates and mineral oils can also be applied to stagnant water to kill the larvae and pupa stages. Biological control using the Bacillus species of bacteria is also applied to curb their spread (Barrera et al., 2009). The same can also be achieved by using Gambusiaaffinis(mosquitofish). The mosquitofish feed on the mosquitoes in the larval stage. Commonly used in California since they adapt and multiply very fast when introduced into a pool. Cuba has successfully reduced to almost zero percent, all achieved by household personal responsibility. Each household has a “mosquito gun.”The gun is a fumigant that makes breeding harder by creating harsh conditions.
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