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MOSQUITOES-BORNE VIRAL DISEASESPart 1: Dengue

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PART 1: DENGUE

Dengue is the most rapidly spreading mosquito-borne viral disease in the world.

Approximately 2.5 billion people live under the threat of dengue transmission

Like malaria, dengue fever exists throughout the tropics, and seems particularly prevalent in Asia, the Middle East and Latin America,

Dengue in Africa appears to be mainly concentrated in the eastern half of the continent.

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DENGUE REGIONS OF THE WORLD

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DENGUE

Dengue virus in many ways seems to be the obverse of malaria.

While malaria transmission occurs most frequently in rural areas, dengue is a city disease.

While the Anopheles vectors of malaria bite mainly at night, the Aedes vectors of dengue bite mainly in the daytime.

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TRANSMISSION

Dengue is caused by several closely related viruses, called dengue virus

Dengue virus (DEN) is a small single-stranded RNA virus comprising four distinct serotypes

1. DEN-1 2. DEN-23. DEN-3 4. DEN-4 These closely related serotypes of the dengue

virus belong to the genus Flavivirus, family Flaviviridae.

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The disease is transmitted from person to person mainly by Aedes aegypti, but Aedes albopictus can also act as a vector.

In rare cases dengue can be transmitted in organ transplants or blood transfusions from infected donors, and there is evidence of transmission from an infected pregnant mother to her fetus.  

But in the vast majority of infections, a mosquito bite is responsible.

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Two forms of the disease occur- dengue fever - dengue haemorrhagic fever.

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DENGUE FEVER

Dengue fever occurs in explosive epidemics that mainly affect adults, sometimes with tens of thousands of cases, especially in urban areas.

It is an acute febrile disease that starts suddenly and lasts for a week or more, causing intense headache, pain in joints and muscles, and rash.

Infection rarely results in death. It occurs in most tropical countries and in some

subtropical areas. It can occur in rural and urban areas, if suitable

vector mosquitos are present.

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DENGUE HAEMORRHAGIC FEVER

Dengue haemorrhagic fever is a severe illness that occurs in south-east Asia and has appeared relatively recently in the Americas and the South Pacific, mainly affecting children.

Infection starts with high fever, vomiting, headache, difficulty in breathing and pain in the abdomen.

Signs of internal bleeding are common. Dengue shock syndrome may develop as a result of

loss of blood and lowered blood pressure. If treatment is not available, as many as 50% of

patients with shock may die, but overall mortality from dengue haemorrhagic fever is usually in the range of 5–10%.

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Rash, which shows up 3-4 days after the start of the symptoms, starting from the trunk region and spreading to the face, arms and legs.

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DENGUE: TREATMENT

There is no specific medication for treatment of a dengue infection, but patients with dengue shock syndrome can be treated by rapid administration of fluid and plasma and the monitoring of vital signs.

Bed rest Paracetamol Oral fluids and electrolyte therapy

Dengue vaccines in development are of four types: live attenuated viruses, chimeric live attenuated viruses, inactivated or sub-unit vaccines, and nucleic acid-based vaccine

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GRADING THE SEVERITY OF DENGUE INFECTION

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CONTROL OF DENGUE/ DHF

No drug/vaccine available

Control of Aedes aegypti only method of choice

Personal protection against daytime-biting mosquitoes, including the use of protective clothing, repellents and house screening.

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The most effective preventive measures aim at reducing the population density of the vector, Aedes aegypti.- large-scale prevention of breeding, through removal or filling of breeding habitats in man-made and natural containers, the burning of organic waste, screening or fitting mosquito-proof lids to drinking-water storage containers, installing piped drinking-water supply, and, if other methods are not feasible or practical, applying safe and effective larvicides to breeding sites.

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In epidemic situations the same measures should be taken, but attempts should also be made to reduce populations of adult mosquitoes rapidly by outdoor space spraying with insecticides.

Insecticidal sprays are usually applied to the parts of towns where abundant breeding sites are available, supporting large populations of Aedes.

Space sprays can be applied with knapsack or hand-carried fogging machines and by truck- or aircraft-mounted machines.

Residual wall spraying against Aedes aegypti is generally ineffective as this species normally rests indoors on surfaces that are not suitable for spraying, such as curtains and other fabrics.

Stocks of insecticides should be kept for emergencies.

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MOSQUITOES-BORNE VIRAL DISEASESPart 2: Japanese Encephalitis

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JE OUTBREAKS IN MALAYSIA (1995)

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WHAT IS JAPANESE ENCEPHALITIS? A viral disease- Flavivirus

Transmitted by infective bites of female mosquitoes mainly belonging to:

1. Culex tritaeniorhynchus2. Culex vishnui3. Culex pseudovishnui

JE virus is primarily zoonotic in its natural cycle and man is an accidental host

JE virus is neurotropic and arbovirus and primarily affects central nervous system

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TRANSMISSION CYCLE

Culex- Vector Pigs- Amplifier host - a vertebrate that has high level of pathogen that

a feeding vector will likely become infectious. Ardeid birds (Cattle egret, Pond heron)- Natural

hosts Man- Dead end host - a vertebrate that harbors the pathogen and is

severely affected by it, yet the level of pathogen in its body is too low for blood sucking vector to become infective after feeding on the host.

- Mosquitoes do not get infection from JE patient

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The virus also can be transmitted when an infected pig's body fluids such as saliva, nasal discharge, blood, uterine discharge, urine and faeces, had a direct contact with human mucus.

For example, a worker handling pigs sometimes feeds the pigs by hand without any protection gears such as gloves or mask.

This could lead to pig's mucus secreted from its nostril to have contact with worker's hand or palm.

Then, the worker does not take care of his own hygiene by eating without washing hands properly.

This careless mistake mentioned above can lead to the infection of JE virus.

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The transmission all started when the JE virus multiplies in the pig's body and if the female Culex mosquito sucks the blood of the pig, she will picks up the JE virus.

After an incubation period of 14 days, the Culex mosquito is able to transmit the virus to a new host, especially pigs.

JE can infect humans and animals. Besides pigs as the host for the virus, other

animals like cows, buffaloes, goats, sheep, dogs and cats also have shown positive reaction rates ranging from 6-66%.

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The virus cannot be spread from human to human.

Eating pork too cannot transmit the disease.

Leading cause of viral encephalitis in Asia with 30-50,000 cases reported annually

Fewer than 1 case/year is reported in U.S. civilians and military personnel traveling to and living in Asia

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Countries which have had major epidemics in the past, but which have controlled the disease primarily by vaccination

China KoreaJapan Taiwan Thailand

Countries that still have periodic epidemicsVietnam CambodiaMyanmar IndiaNepal Malaysia

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JE endemic areas

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WHY JE IS CONCENTRATED IN ASIA?

JE occurred in most of the Asian countries because these countries such as China, Malaysia, Taiwan and many more are the main exporter of pork in Asia that is still practising the traditional ways of rearing pigs.

The farmers that are still using the old ways, did not vaccinated their pigs eventhough the farms are built in areas of known JE activity.

The staff that works in these farms are lacking of vaccination too.

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As a result of using the traditional ways, the rivers are clogged with pig waste because those farmers just "dump everything" into the river.

The clogged river then becomes the perfect place for Culex mosquito to breed.

To make the situation worse, those farmers and workers does not wear any protection gear such as mask, apron and gloves during their daily work in those pig farm, making them an easy target for the Culex mosquito.

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Majority of those farmers also stay near their pig farms.

This also contribute to the spread of the disease.

To summarize the whole scenario, the spread of this disease is because of poor management and condition of those pig farms.

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SYMPTOMS

JE begins clinically as a flu-like illness with headache, fever and often gastrointestinal symptoms.

At the early stage, confusion and disturbances in behaviour may occur.

After 3-14 days of fever and headache, drowsiness and disorientation would take place.

Later, the illness may progress to a serious infection of the brain that can lead to coma within 24-48 hours.

A quarter of patients with symptoms die and the remaining one third of survivor will suffer brain damage.

Seizures are common, especially among children

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PREVENTION, TREATMENT AND CONTROL

A vaccine against JE is used to immunize children in some of the Asian countries in which the disease is endemic.

The drug Ribavirin is now being given to all JE patients or those suspected to have been infected with the disease to help improve their chance of recovery

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Infections can be prevented by using protective clothing, repellents, house screening, mosquito nets, coils and mats, and by the avoidance of outdoor activities in the evening.

The spraying of houses and animal shelters in rural areas to control the Culex vectors of JE is generally ineffective because of the outdoor biting and resting habits of the vector species.

In some areas control is possible by measures that prevent breeding in rice fields and irrigation systems.

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Outdoor space spraying with insecticides can be carried out where epidemics occur.

In endemic areas it is recommended that domestic animals be housed away from human habitations.

This applies especially to pigs in areas where JE is endemic.

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MOSQUITOES-BORNE VIRAL DISEASESPart 3: Yellow Fever

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INTRODUCTION

Yellow fever is an acute disease of short duration which often causes death.

The disease starts with a high fever, headache, body aches, vomiting and sometimes jaundice (which gives the patient a yellow colour).

This is followed by internal haemorrhages (bleeding) and vomiting.

Death may occur within three days after the onset of the disease.

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TRANSMISSION AND DISTRIBUTION

The yellow fever virus mainly occurs in populations of monkeys in dense forests and gallery forests in Africa and South and Central America.

It is transmitted from monkey to monkey by forest-dwelling mosquitos (Aedes species in Africa, Haemagogus and Sabethes in South and Central America;

These mosquitos occasionally bite humans when they enter forests and may thus transmit the virus from the monkey reservoir to the human population.

There is evidence in some areas of endemicity that the virus is maintained in mosquito populations through transovarial transmission in the absence of a vertebrate reservoir.

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In Africa, monkeys sometimes leave the forest in search of bananas in plantations and may then infect the local mosquito species, which in turn infect humans living or working on the plantations.

People infected in or near forests can carry the virus to rural or urban areas where Aedes aegypti or related mosquitos can pick it up and transmit it among the human population.

Such situations can result in serious epidemics and many deaths.

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In the Americas, urban outbreaks used to be extremely severe but have not occurred since 1954.

However, the risk remains and cases are reported each year among people working in forests.

In Africa, urban or rural outbreaks are occasionally reported from areas near forests and may cause thousands of deaths.

People working in forests also become infected regularly.

Yellow fever has never been reported in Asia.

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PREVENTION AND CONTROL

Yellow fever is best prevented by immunization, which is recommended for all persons working in or visiting forests where yellow fever occurs.

Immunization is also indicated for people in urban or rural areas at risk.

Vaccination normally provides protection for at least 10 years and revaccination every 10 years is required by the port or frontier health authorities in a number of tropical countries.

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Epidemics can be controlled by vaccinating all persons living in affected areas; by space-spraying with insecticides against adult mosquitos; and by appropriate larval control measures.

Non-immunized people can reduce the risk of infection by protecting themselves from mosquito bites with protective clothing, repellents and screens to prevent daytime biting.

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MOSQUITOES-BORNE VIRAL DISEASESPart 4: Chikungunya Fever

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Chikungunya fever is a viral disease transmitted to humans by the bite of infected mosquitoes.

Chikungunya virus is a member of the genus Alphavirus, in the family Togaviridae.

Chikungunya virus was first isolated from the blood of a febrile patient in Tanzania in 1953, and has since been cited as the cause of numerous human epidemics in many areas of Africa and Asia and most recently in limited areas of Europe.

Chikungunya virus is not currently found in the United States

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Chikungunya virus is spread by the bite of an infected mosquito.

The Aedes aegypti mosquito is the principle vector responsible for transmitting the chikungunya virus to humans.

Mosquitoes become infected when they feed on a person infected with chikungunya virus.

Infected mosquitoes can then spread the virus to other humans when they bite.

Monkeys, and possibly other wild animals, may also serve as reservoirs of the virus.

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SYMPTOMS

Chikungunya virus infection can cause a debilitating illness, most often characterized by fever, headache, fatigue, nausea, vomiting, muscle pain, rash, and joint pain.

The term ‘chikungunya’ means ‘that which bends up’ in the Kimakonde language of Mozambique.

Acute chikungunya fever typically lasts a few days to a few weeks, but as with dengue, West Nile fever and other arboviral fevers, some patients have prolonged fatigue lasting several weeks.

Additionally, some patients have reported incapacitating joint pain, or arthritis which may last for weeks or months.

Fatalities related to chikungunya virus are rare.

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TREATMENT, CONTROL AND PREVENTION

There is no vaccine or specific antiviral treatment currently available for chikungunya fever.

Treatment is symptomatic and can include rest, fluids, and medicines to relieve symptoms of fever and aching such as ibuprofen, naproxen, acetaminophen, or paracetamol.

Aspirin should be avoided. Infected persons should be protected from further

mosquito exposure (staying indoors in areas with screens and/or under a mosquito net) during the first few days of the illness so they can not contribute to the transmission cycle.

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Pregnant women can become infected with chikungunya virus during all stages of pregnancy and have symptoms similar to other individuals.

Most infections occurring during pregnancy will not result in the virus being transmitted to the fetus.

The highest risk for infection of the fetus/child occurs when a woman has virus in her blood (viremic) at the time of delivery.

There are also rare reports of first trimester abortions occurring after chikungunya infection.

Pregnant women should take precautions to avoid mosquito bites.

Products containing DEET can be used in pregnancy without adverse effects.

Currently, there is no evidence that the virus is transmitted through breast milk.

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The best way to prevent chikungunya virus infection is to avoid mosquito bites.

Prevention tips are similar to those for other viral diseases transmitted by mosquitoes, such as dengue 

Additionally, a person with chikungunya fever should limit their exposure to mosquito bites to avoid further spreading the infection.

The person should use repellents when outdoors exposed to mosquito bites or stay indoors in areas with screens or under a mosquito net.