How Dengue Virus Is Transmitted By Aedes Aegypti Biology Essay


Mauritius is a small tropical island located at latitude 20 18 0 S and longitude 57 34 60 E. It has a tropical climatic condition. Mauritius has an area of about 2,040 sq km and is located to about 2000 kilometers from east west of Africa and some 800 kilometers from Madagascar. The positioning of Mauritius makes the latter a tropical country with moderately lofty temperature throughout the year. Winter and summer are the seasons that manifest onto the island.


Fig 1: World map showing location of Mauritius Island in the Indian Ocean

The island of Mauritius finds itself as one of the most accessible islands in the Indian Ocean. Situated amid Réunion island and Rodrigues island, the island of Mauritius has gained the reputation, through the course of time of that of the 'key and star' of the Indian Ocean. The Mauritian population estimates for the year 2008 was about 1, 260, 781 with an annual growth rate of 0.7 %.

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Since the country is undergoing major developmental changes many industries have implanted here and thus the number of expatriates in the country is on the rise. These people may be a carrier of the disease and of course those Mauritians visiting the dengue endemic areas can also become infected and bring the disease in the country.

It is an indisputable fact that during the lapsed decades, Mauritius has witnessed a multitude of diseases. The most prominent and recent one being Chikungunya which has infested merely about 12000 Mauritians. Furthermore, the history of diseases in Mauritius is marked with Malaria epidemics since colonial regimes and through the intensive effort of the Public Health sector, the latter has been proclaimed eradicated by the World Health Organization in 1973.

Some years ago many of the realm's citizens were not aware of what was dengue fever even though it had already occurred in the country but there was not mass infection by the virus. Providentially, the number of cases reported beforehand was only one or two and through the close collaboration between the Ministry Of Health and the infected person the situation was under control and hence no further positive case of dengue were recorded.

The Mauritian government is putting forward all steps to prevent an epidemic rather than to rush for controlling it when it has already hit the population. The Ministry of Health is working on a list which highlights all water retaining sites and is identifying the hotspots of such sites that are liable to cause proliferation of mosquitoes; this process is carried out each year. Furthermore, an action plan is being prepared by the ministry which gives a layout of which and what job is to be done by which section of the ministry or other stakeholders (anonymous, 2009).

Dengue viruses are transmitted by the Aedes species. Two known species the Aedes aegypti and Aedes albopictus are vectors of the disease. The Aedes albopictus can be found in large quantity all around the island whereas Aedes aegypti is said to be eradicated from the country. Surveillance on the abundance of mosquitoes is carried out by the entomological section throughout the year. All sites where mosquitoes that can be vectors of disease are seen, they are referred to the nearby health office for a larviciding to be carried out at that place and in the vicinity.

Aedes albopictus (Skuse) is known as the Asian Tiger mosquito (Robertson and Hu, 1988). Aedes albopictus is native to Southeast Asia, but now occurs throughout the world. The worldwide spread of Aedes albopictus during the precedent 20 years has caused apprehension in the midst of public health officers and scientists over the possibility that the introduction of this species will amplify the risk of epidemic dengue fever and other arboviruses in countries where it has become established (Gubler, 2003).


The aim of this study is mainly to evaluate the effectiveness of the control measures taken to prevent dengue fever in Mauritius. Emphasis will be laid on the steps taken before, during and after the disease occurrence. This might highlight the shortcomings that Mauritius face in order to manage outbreaks of diseases.

Objectives of study

The objectives of this dissertation are to evaluate the management, procedures and legislation that are implemented in Mauritius during outbreaks of dengue fever. Furthermore, most interest is geared towards the application of chemicals, preventive measures, and health education of the public carried out by the Ministry of Health & Quality of Life to prevent the occurrence of the disease and also to annihilate if ever found in the island. To elucidate the effectiveness of fogging, larviciding carried out in the country and health education of the public.



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2.0 Dengue

2.0.1 General considerations

Dengue fever and dengue hemorrhagic fever were first identified in the 1950's, during the dengue epidemics in Philippines and Thailand and by 1975 it had become a leading cause of hospitalization and death among children in many countries found in that region (Lloyd, 2003). In the year 1779 Egypt and Java had dengue-like epidemics, but it is thought that they were caused by the chikungunya virus (Carey, 1971).

Dengue virus belongs to the genus Flavivirus, Family Flavivaridae and there are four serotypes of the virus (DEN-1, DEN-2, DEN-3 and DEN - 4). All the four serotypes can cause dengue fever, dengue hemorrhagic fever and even dengue shock syndrome (Ramchurn et al, 2009). The four viruses are closely related but are distinct. Millions of people residing in tropical areas of the world are affected by epidemics of dengue fever. Dengue fever is associated with the severe form dengue hemorrhagic fever/ dengue shock syndrome (DHF/DSS) that is seen mostly in children and nevertheless adults also are attained by the disease.

In the 19th and early 20th centuries dengue or dengue-like epidemics were reported in the Middle East, Southern Europe, Asia, North Africa, the Americas, and Australia and on various islands found in the Indian Ocean, South and Central Pacific and the Caribbean (Ehrenkranz et al, 1971). Generally these epidemics consisted of nonfatal feverish illnesses, often coupled with rash and either muscle or joint pains (Carey, 1971). In 1897 during dengue epidemics in Australia and in Greece in 1928, over 1000 deaths were reported (Halstead, 1980). Hemorrhagic demonstrations, including gastrointestinal bleeding, were seen during epidemics of dengue in Texas and Louisiana in the year 1922 (Scott, 1923). Nonetheless through the earliest half of the 20th century, dengue was generally thought to be a self-limited, nonfatal febrile illness, with occasional hemorrhagic manifestations such as red spots, acute hemorrhage from the nostril, nasal cavity, or nasopharynx, gingival bleeding and menorrhagia that only once in a blue moon resulted in more stern or fatal outcomes (Gubler et al. 1992).

During the last decade, dengue infection along with its complications has been on the rise all over the world. Their geographical spread is increasing: only 5 countries documented dengue in the 1950's but to date there are more than 100 countries reporting the incidence of dengue fever and dengue hemorrhagic fever (Guha -Sapi & Schimmer, 2005). Dengue is found in tropical and sub-tropical climates worldwide, mostly in urban and semi-urban areas (WHO, 2009).

Dengue fever is a very infectious mosquito borne viral disease characterized by either a mild febrile syndrome or the classic incapacitating disease with abrupt onset of high fever, severe headache, pain behind the eyes, muscle and bone or joint pains, nausea and vomiting and rash. Skin hemorrhages are not uncommon. Leukopenia is usually seen and thrombocytopenia may be observed (WHO 1997).

Dengue is a flu-like mosquito-borne disease and has a soaring capacity for epidemic outbreaks, which according to the World Health Organization (2009) affects 50-100 million people each year in the tropical and sub-tropical areas of the world. Dengue is cited as being one of the most significant mosquito-borne disease affecting humans and as a major international public health concern (WHO 2009). Dengue fever is predominantly transmitted by Aedes species which have adapted themselves to living near human habitation (Hales et al., 2002).

The dengue virus is a member of the family Flaviviridae virus, transmitted through the biting of infected Aedes aegypti and Aedes albopictus mosquito. The Aedes aegypti mosquito normally bites indoor and late in the afternoon whereas the proficient mosquito Aedes albopictus is an aggressive daytime biter, which is also known to bite early in the morning, late afternoon (Knight and Hull, 1952) and at night (Murray and Marks, 1984). This biter is usually an outdoor biting mosquito, but it also bites indoors (Hawley, 1988). Generally the mosquitoes bite at ground level (MacDonald and Traub, 1960, cited in Hawley, 1988). Females will bite any area of exposed skin, but prefer the ankles and knees (McClelland et al., 1973; Robertson and Hu, 1935). The time amid the bite of a mosquito carrying dengue virus and the apparition of symptoms ranges from 4 to 6 days, with a range of 3 to 14 days.

2.0.2 Pathogenicity of Dengue fever:

1. Asymptomatic and mild infection

It is very common.

2. Dengue Fever (primary infection)

Dengue fever is characterized by increase in body temperature; severe aching of the forehead; retro-ocular pain; muscle and joint pain; and widespread maculopapular inflammation. Conjunctiva may become red. Other common problems that may arise are diarrhea, vomiting, nausea and abdominal pain. Fear of light, sore throat, increase in the size of the lymph node and bleeding tendencies may also happen. The illness lasts 5 to 7 days.

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Immunity is lifelong. On the other side the incidence of Dengue Hemorrhagic Fever or Dengue Shock syndrome increases if the person has immunity or has already been infected before with a different serotype. Even after several months of recovery some patients may experience depression and fatigue.

3. Dengue Hemorrhagic fever

The well-known feature is bleeding. It happens when a person is infected twice but with a different dengue virus serotypes or infrequently by primary infection is common in kids Under 15 years of Age (Rigall-Pewrez et al.1998). There is sudden rise in temperature and other manifestations of Dengue fever. Petechiae, effortless bruising, gingival bleeding and epistaxis are common. In severe cases bleeding of the gastrointestinal tract can be observed. In children, we can have an increase in the size of the spleen and the liver.

4. Dengue Shock Syndrome

The prominent feature is hypotension. It normally occurs in people below 15 years of age. The clinical features include weak pulse with narrow blood pressure, cold and clammy skin (Rigall-Pewrez et al.1998).

2.0.3 Mode of transmission of dengue virus:

Dengue and chikungunya viruses are transmitted to humans by the bites of infected mosquitoes. In contrast, Aedes albopictus can be easily found and may be the only key vector of these viruses on the islands. Both species bite mostly during the daytime, predominantly in the early hours following dawn and for 2-3 hours prior to darkness. Aedes albopictus is more active outdoors whereas Ae. aegypti typically feeds and rests more indoors (WHO 2008).

In the cycle of dengue, the vertebrate host is man and the Aedes species the vectors. The disease is acquired only when bitten by female mosquitoes, as the female feed on blood in order for the development of their eggs whereas the male mosquitoes are not infectious due to the fact that they feed only on nectars rather than blood. In 8-10 days the infected mosquito is able to transmit the virus to other people. Thus the cycle of transmission takes only 14 days. One dengue-infected female mosquito is capable of biting and infecting several people during one feeding session.

The dengue mosquito frequents backyards in search of containers holding water inside and outside the home, such as: cans, buckets, jars, and vases, pot plant dishes, birdbaths, boats, tyres - discarded with no rims, roof gutters blocked by leaves striking containers, tarpaulins and black plastic.

Figure 2.1: showing blocked state of rain water drain and a good site for breeding of mosquitoes.

It can also breed in natural containers like:


fallen palm fronds.

In drier conditions it also breeds in water in subterranean sites such as: wells, telecommunication pits, sump pits, gully traps.

Transmission cycle of dengue results from a complex system based on several main constituents like: the density of susceptible hosts, environmental conditions and the presence of one or more serotypes of the dengue virus. The number of confirmed dengue cases has been increasing owing to the fact that the world is undergoing rapid urbanization and its population is also on the rise, disposal of non-biodegradable containers, rapid transportation and poor living conditions such as poor water supply and very rare scavenging services at squatter areas (Satwant, 2001).

Various studies have shown that the Aedes albopictus is able to transmit all the 4 serotypes of dengue. Aedes albopictus mosquito can serve as an important maintenance vector of dengue viruses in endemic areas, and new endemic areas may be initiated by importation of vertically infected eggs (Gubler, 2002). That is the infected Aedes mosquito can pass the dengue virus to its progeny and when the eggs will develop into mature mosquitoes they will be already infected, hence capable of causing infection of human beings or even pass the virus to their progeny.

Figure 2.2: how dengue virus is transmitted by Aedes aegypti (source: CDC Dengue Slideset).

Transmission cycle of dengue virus by the Aedes aegypti mosquito starts with a person infected with the dengue virus. The blood of the person will contain the virus thus circulating in his body and this is called a viremia which will last for about 5 days. During this period, an uninfected female Aedes aegypti mosquito bites the infected person and acquires the dengue virus.

Within the mosquito, replication of the dengue virus occurs and this process usually takes between 8-12 days, after which the female mosquito can transmit the virus upon a blood meal. Once infected the virus takes 4-7 days to replicate within the new host (the person whom the infected mosquito bite) before inception of symptoms.

Symptoms may last from three to 10 days, with an average of five days, after the onset of symptoms. Hence, the disease persists several days after apparition of symptoms (CDC Dengue Slideset).

2.0.4 Lifecycle of Aedes mosquito:

The mosquito goes through four separate and distinct stages during its life cycle; and are as follows: Egg, Larva, pupa, and adult. Each of these stages can be easily identified by their particular appearance (West Umatilla).

Egg: Eggs are laid one at a time and they float on the surface of the water. Aedes lay their eggs on damp soil that will be flooded by water. Most eggs hatch into larvae within 48 hours (Alexandria Health Dept. 2010)

Larva: The larva lives in the water where they eventually undergo a molting process to become a pupa.

Pupa: It is the non-feeding, only and is the time the mosquito turns into an adult. It takes about two days before the adult is fully developed and upon complete development, the pupal skin splits and the mosquito emerges as an adult.

Adult: On the surface of the water the just turned into adult mosquito rests for a dumpy time before flying away (West Umatilla).

In the Aedes mosquito family only the female bites because it requires protein to develop eggs, therefore if it bites a person infected with the dengue virus the mosquito becomes infectious after approximately 7 days. The mosquitoes are known to be biting at a highest frequency at dawn and dusk. Some more facts:

The average lifespan of a mosquito of the genus Aedes in Nature is 2 weeks

Mosquitoes may lay eggs about 3 times in his life, and about 100 eggs are produced each time.

The eggs can live in dry conditions until approximately 9 months, after which they can hatch if it is subject to conditions, i.e food and water

Source: [accessed on 05.12.09]

Figure 2.4: Picture of lifecycle of Aedes mosquito in water

2.0.5 Investigation for dengue infections:

Laboratory results

Decrease in the number of white blood cell and peripheral neutrophils in the blood, abnormal increase in the number of lymphocytes in bloodstream and very low amount of platelets in the blood (Rigall-Pewrez et al. 1998).


X-ray of the chest normally shows pleural effusion and seldom pericardial effusion (Rigall-Pewrez et al.1998).


Used to detect pericardial effusion and 2) presence of excess fluids in the gap amid the tissues lining the abdomen and abdominal organ (Rigall-Pewrez et al.1998).


Laboratory diagnosis is done by detection of virus in specimen-serum at the virology laboratory. Culture is done in cell line derived from A. albopictus cell. Immunoflurescent techniques are used to detect viral replications. The virus can be isolated in patients with fever (Rigall-Pewrez et al.1998) .


By the 6th days of infection IgM is detectable in 90 % of patients. False negative results may be seen if the serum had been collected early. Even after 2-3 months of infection IgM can be detected. In case where the IgM investigation is Positive it may imply recent infection with Dengue fever. However more promising diagnosis can be made if the virus is detected r by using PCR the genome of the virus is detected (Rigall-Pewrez et al.1998). Seroconversion or boost in titer may indicate fresh infection.

Plasma and serum are the best samples for PCR tests. Molecular test is highly sensitive unfortunately it can be used in patients only with viraemia (Rigall-Pewrez et al.1998).

2.0.6 Treatment:

The managing of dengue fever can be enhanced with bed rest, passable fluid intake, plus control of fever and pain with antipyretics in addition to analgesics (e.g. paracetamol). For the supplementary ruthless manifestations of dengue virus infection, correct management requires early identification and swift intravenous fluid substitution. Blood transfusion may be necessary in cases.

There is currently no vaccine is available to shield against dengue infection. The current lack of a booming vaccine against the dengue virus causes prevention methods to be approached by plummeting disease vector population, with Integrated Pest Management programs for mosquito control.

These employ a mishmash of control strategies, including mosquito surveillance, source diminution, eradicating larvae and eradicating adult mosquitoes (Ooi et al. 2007). Eradicating adult mosquitoes alone is fruitless in controlling mosquito populations because it is complex to treat the unattainable habitat of the adults. Mosquito larvae are left to carry on their development, and they quickly swap the adults. Nevertheless, mosquitoes can become resistant if pesticides are overused.

2.0.7 Dengue fever in Mauritius:

Dengue virus infections are emerging as the major ones in Southeast Asia. Global warming may worsen the occurrence of dengue fever. Since very last few years mixed outbreak of chikungunya and periodic cases of dengue fever have been reported on Réunion Island and other South West Indian Ocean countries. From March 2005 till March 2006 it is estimated that about 204000 people in Réunion Island may have been infected by the chikungunya virus, which furthermore shows that there is presence of the transmitting vectors of the disease on the island which are also the vectors of dengue fever as well. Hereafter, the other South West Indian ocean countries were not spared from infection from the chikungunya virus.

An outbreak of dengue fever was reported in Madagascar more specifically in the city of Toamasina that started mid-January 2006 and rare cases of chikungunya were also reported mid-February. Maldives also have suffered from a dengue outbreak in year 2006 where 602 people were suspected to be infected among which there were some severe form of dengue fever that is 64 dengue hemorrhagic fever cases and 9 cases of dengue shock syndrome (WHO 2006).

In Mauritius the first case of dengue fever dates to the 1976's and it was contained thus limiting the disease from spreading. Then we had a case of imported dengue from a person who visited an endemic dengue area in January 2008 (CDCU). The main vectors of the disease remain the Aedes mosquitoes, among which the Aedes aegypti mosquito is the primary vector and Aedes albopictus the secondary one. The mosquito found to be spreading dengue fever and Chikungunya in Mauritius is the Aedes albopictus (CDCU 2009). It is to be noted that in Mauritius we had both the Aedes aegypti and Aedes albopictus mosquitoes, due to the intense anti-malaria campaign during the year 1952 the primary carrier of the dengue fever, the Aedes aegypti have been successfully eradicated. Still very minute amounts of this mosquito can be seen whereas the Aedes albopictus is abundant. Dengue is transmitted from person to person through the biting of infected mosquitoes.

Most recently we had a short-lived epidemic of re-emerged dengue fever in Mauritius that started in the month of June 2009 which was imported. The mild fever was first localized in the city of Port Louis, where there were 192 cases and then we did have some sporadic cases in other regions of the island. Mosquito fogging and larviciding in whole Port Louis started on 3rd June 2009, and were repeated every seven days. Fogging was carried out outdoors early in the morning, early evenings and sometimes till late in the evenings (Dengue Unit 2009).

The Ministry of Health and Quality of Life of Mauritius took the situation as being severe and all medium possible to contain the disease were put into action. Like the Special Mobile Force and manpower from other Ministries which joined the Ministry of Health to fight the dengue fever. Public alertness campaigns on the requisite to hunt and eliminate mosquito breeding sites at home and in the neighbourhood and to protect oneself against mosquito bites were carried out through radio, television and the press through a public private partnership. Detailed information leaflets were also distributed, door to door distribution of pamphlets showing pictures of possible breeding sites for mosquitoes and products to be used to prevent mosquito bite were carried out by the primary health care personnels. Target groups included the public, community groups and school children (Ramchurn et al, 2009).

By the end of the month August no new or suspected cases of dengue were recorded in any of the country's hospital. But still the control and prevention program were continued throughout the island as the summer season was coming near hence reappearance of the dengue fever was possible due to the ambient temperature, favorable for larvae development. The fear of having the virus again was due to the possibility of the infected mosquitoes to pass the virus to their progeny. Fortunately, till February 2010 no suspected case of dengue fever was reported from any in the country (Dengue Unit 2010).

2.1 Vector surveillance and control program

Ever since mosquitoes are capable of transmitting diseases like dengue and chikungunya, till now it has not been possible to eradicate the mosquitoes completely from their originating site. The best way to monitor or control vector-borne diseases is to control or limit the population of the vector to such an extent that disease transmission is very low or even stopped. In order to achieve this goal, it is imperative to know all about the mosquito involved in the transmission of the disease.

Detailed knowledge of all aspects such as the breeding sites, different features of the mosquito at different stages, feeding habits, mating, resting and structure and most importantly without forgetting the lifecycle of the mosquito, are the main required things in order to be able to break the chain of transmission. Furthermore, the only way to prevent infection of people who have not suffered from dengue is to control the population of dengue vector (Ooi et al.2001) and of course personal precaution has also proved to be effective in reducing the risk of being infected by a mosquito.

Since no vaccine is yet available for dengue the only mode to control dengue fever is the control the amount of the disease vector that is of the Aedes mosquitoes. The control strategies of these mosquitoes are 1) carrying out larviciding -spraying a chemical called abate in any water retaining place which kill the larvae of the mosquitoes hence interrupting the cycle to be completed, 2) fogging operation- a thermal fogger is used to propel fumes of Aqua K-Othrine® which when is in contact with a mosquito kills it, thus the amount of developed or simply mature mosquitoes are reduced and 3) health education- talks are organized for the members of the public, for children in schools, colleges, etc…

Entomological survey is an important and integral part of dengue prevention and control. The effect of the intervention by the community can directly affect the ecology of the vectors that is the Aedes mosquitoes.

The Communicable Disease Control Unit (CDCU) is the unit which is mostly concerned for the control of communicable diseases such as Malaria, Dengue fever, Chikungunya, and other infectious diseases. In Mauritius, surveillance, disease prevention and education of infectious diseases are mainly carried out by the Health Inspectorate Cadre. In Mauritius, we have the Public Health Act (Section 32A) which is used in case where there is presence of a mosquito borne disease in the island.

The potential for predation to prevent pathogen invasion or reduce disease prevalence in a host population also has implications for the biological control of vector populations. Predators have been introduced, or proposed, as biological control agents of vectors for various diseases such as malaria, dengue fever and Lyme disease (Jenkins 1964; Legner 1995; Stauffer et al. 1997; Samish & Rehacek 1999; Scholte et al. 2005; Kumar & Hwang 2006; Ostfeld et al. 2006; Walker & Lynch 2007). Several recent studies suggest that predator introductions led to a decline in local cases of dengue fever in Vietnam and Thailand (Kay & Nam 2005; Kittayapong et al. 2008), and malaria in India (Ghosh et al. 2005; Ghosh & Dash 2007).

2.2 Biology of Aedes albopictus (Skuse)

Aedes albopictus are two winged insects from the family Culicidae of the order Diptera. They are among the best known groups due to their importance as pests and as vectors of diseases. They are easily identified due to a combination of the following characters: long trunk projecting head; charisma of scales on the wing veins, a tassel of scales along the posterior boundary of the wing, and the typical wing venation, the second, fourth and fifth longitudinal veins being branched (Miyagi and Toma 2000).

Female mosquitoes feed on blood and they have highly specialized mothparts for piercing host skin and blood sucking (Wahid et al. 2002). Aedes species are normally day-time bitters and active during the day. During this time, they have peaks of landing and biting activity. The peak time for Aedes albopictus occurred about one hour after sunrise and then before sunset (Abu Hassan et al. 1996). Nevertheless, the rate of biting varies depending on the mosquito age and time of the day (Xue and Barnard 1996).

Source: CDC, Public Health Image Library (PHIL)

Figure 2.5: Aedes Albopictus mosquito