Dengue (pronounced 'Dhen gey') is a mosquito-borne infection, which in recent years has become a major international public health concern. Dengue is found in tropical and sub-tropical regions around the world, predominantly in urban and semi-urban areas.

Dengue fever is found mostly during and shortly after the rainy season in tropical and subtropical areas of

• Africa
• Southeast Asia and China
• India
• Middle East
• Caribbean and Central and South America
• Australia and the South and Central Pacific

Dengue fever is an infectious disease caused by a virus, called the dengue fever virus.

The virus

Viruses are extremely small infectious particles consisting of an outer envelope, composed of proteins. Within this envelope of proteins is a string of molecules, called nucleic acid, which helps the virus replicate (form numerous copies of itself), in the animal it infects.

There are four types of dengue fever virus. These are classified based on the presence of certain proteins found in the virus, called antigens. These antigens are recognized by the human immune system, which produces substances against them, called antibodies.

Based on its antigens, the four types of dengue fever virus are called DEN-1, DEN-2, DEN-3, and DEN-4.

A person can get infected by any one of these subtypes, and infection with one does not prevent infection with another, from occurring. However, once a person has been infected with a particular type, he is protected from infection from the same type for the rest of his life.

Unfortunately, infection by the several types in sequence predisposes an individual to a more severe form of the illness, called dengue hamorrahgic fever.

The virus is transmitted from an individual infected with the virus to another via a vector (which transmits the infecting agent, but does not show signs of the disease itself). In the case of dengue fever, the vector is a day-biting female of the mosquito called Aedes aegyptii and Aedes albopictus. The mosquito gets infected when it bites a person infected with the virus and then transmits the infection to a healthy individual. Once the mosquito sucks the blood of an infected person, it takes approximately 8-10 days for it to become infective. This is called the vector incubation period. Once infective, the mosquito remains infective for the rest of its life span. Once the virus enters the human blood stream, it circulates in the blood for 2-7 days, during the phase of fever. It is during this time, that a person can transmit the infection to a mosquito.

It is believed that the interaction between the antibodies and antigens leads to the formation of complexes, which are responsible for the bleeding seen during the disease.

The disease

Dengue fever often occurs in epidemics every three to five years. The world's largest known epidemic occurred in Cuba in 1981, when more than 116,000 people were hospitalized, some 11,000 in a single day. We are presently in the midst of a dengue fever pandemic (an epidemic across several countires), primarily affecting India, Vietnam and the Caribbean.

Dengue commonly occurs in children, because adults, having been exposed to the virus at sometime in their lives, develop immunity.

The illness can vary in severity from a mild flu-like illness to a life threatening condition.

The clinical features of dengue fever vary according to the age of the patient. Infants and young children may have a non-specific fever with rash.

Older children and adults may have either a mild fever or the classical incapacitating disease with abrupt onset and high fever, severe headache, pain behind the eyes, muscle and joint pains, and rash. It is often called 'breakbone fever' because of the severe muscle and joint pains. The rash usually comes in two waves- the first occurs as a generalized rash lasting 1-2 days followed 1-2 days later by a rash, which spares the palms and soles, lasting for around 5 days.

Dengue hemorrhagic fever is a potentially deadly complication that is characterized by high fever, bleeding (haemorrhagic phenomena)-often with enlargement of the liver-and in severe cases, shock (circulatory failure). The illness commonly begins with a sudden rise in temperature accompanied by facial flush and other non-specific symptoms of dengue fever. The fever usually continues for two to seven days and can be as high as 40-41°C, possibly with febrile seizures and bleeding.

In moderate cases, all signs and symptoms abate after the fever subsides. In severe cases, the patient's condition may suddenly worsen after a few days of fever; the temperature drops, followed by signs of shock (inadequate blood supply), and the patient may rapidly go into a critical state of shock and die within 12-24 hours, or quickly recover following appropriate therapy.

Several complications may arise as a result of the infection such as chest complications (like pneumonia or collection of fluid within the chest), brain haemorrhage due to the bleeding tendency, bleeding from other sites, kidney failure and shock.

Treatment

Unfortunately, there is no known treatment for dengue fever. Most treatment is supportive- that is, treatment of the symptoms and treatment of anticipated complications.

Mild forms of the disease are virtually indistinguishable from any minor viral illness and do not require hospitalization.

Severe forms including the dengue haemorrhagic fever and the dengue shock syndrome need hospitalization, investigations such as blood tests to look for any bleeding tendency and imaging such as an X-ray of the chest and CT scan of the brain. These tests may need to be repeated often during the hospital stay to monitor the progress of the illness. Once appropriately treated the mortality for the severe forms of the illness is around 3%.

Prevention

Prevention is the best form of treatment. Unfortunately, at present there are no effective vaccines to prevent the disease, mainly because there are four different types of virus.

At present, the only method of controlling or preventing dengue is to combat the vector mosquitoes.

In Asia and the Americas, Aedes aegypti breeds primarily in man-made containers like earthenware jars, metal drums and concrete cisterns used for domestic water storage, as well as discarded plastic food containers, used automobile tyres and other items that collect rainwater. In Africa it also breeds extensively in natural habitats such as tree holes and leaf axils.

Vector control is implemented using environmental management and chemical methods. Proper solid waste disposal and improved water storage practices, including covering containers to prevent access by egg laying female mosquitoes are among methods that are encouraged through community-based programs.

The application of appropriate insecticides to larval habitats, particularly those which are considered useful by the householders, e.g. water storage vessels, prevent mosquito breeding for several weeks but must be re-applied periodically. Small, mosquito-eating fish and copepods (tiny crustaceans) have also been used with some success. During outbreaks, emergency control measures may also include the application of insecticides as space sprays to kill adult mosquitoes using portable or truck-mounted machines or even aircraft. However, the killing effect is only transient, variable in its effectiveness because the aerosol droplets may not penetrate indoors to microhabitats where adult mosquitoes are sequestered, and the procedure is costly and operationally very demanding. Regular monitoring of the vectors' susceptibility to the most widely used insecticides is necessary to ensure the appropriate choice of chemicals. Active monitoring and surveillance of the natural mosquito population should accompany control efforts in order to determine the impact of the program.

Personal protection should be used in endemic areas such as the use of protective clothing, use of mosquito nets and use of mosquito repellents such as DEET and pyrethrin extracts. Keep clean surroundings, eliminate potential areas for mosquito breeding, and
just plain good old common sense!!

Expert: Dr. Thimappa Hegde