- Education and Science
What Is Dengue Fever?
Dengue fever (DF) is not a recent discovery. It has been afflicting mankind since ages. DF-like illnesses were described in Chinese medical writings dating back to 265AD. The outbreaks of febrile illnesses compatible with DF have been recorded throughout history. In 1789, Benjamin Rush, MD, published an account of a probable DF epidemic that had occurred in Philadelphia in 1780. Rush coined the term break-bone fever, based on the intense description of symptoms reported by one of his patients. The probable outbreaks of DF occurred sporadically every 10-30 years until after the Second World War (WWII). The socio-economic disruptions caused by WWII resulted in an increased spread of dengue viruses globally.
Analysis Of Dengue Fever
DF is one of the many arthropod (insect) borne illnesses that affect humans. It is classified as a major global health threat by the World Health Organisation (Who). A yearround transmission of dengue fever 25ºN and 25ºS has been established. The mosquito responsible for the spread of the disease is Aedis aegypti which thrives in places where water is stored in containers.
The mosquito typically breeds near human habitation, using relatively fresh water from sources such as water jars, vases, discarded containers, coconut husks and old tires. It usually inhabits dwellings and bites during the day. The mosquito not only carries the dengue virus but can also transmit other viruses producing diseases that can be similar to the dengue fever. So it is not the mosquito which causes this illness - it is the virus (called the dengue virus) that it carries, thus transmitting the virus into the human body through its bite. After two to seven days of being bitten, the patient suddenly develops a fever, headache, generalised body aches, pain behind eyes, back pain, along with severe muscle pain, hence the famous colloquial term break-bone fever.
There is often a rash on the skin on the first day along with redness in the eyes. The rash is gradually spreads on the arms and legs but not on the palms and soles. There may also be signs of anorexia, nausea or vomiting with marked skin hyper sensitivity. The illness lasts around a week and is treated, using some supportive measures. Patients usually recover completely without needing any specific antiviral drug. The dreadful complication of dengue fever which is called the viral hemorrhagic fever (VHF) or dengue hemorrhagic fever (DHF) occurs only rarely and under some specific circumstances.
VHF is actually a group of viral diseases that result in bleeding from different parts of body and is caused by many other viruses (besides the dengue virus). Examples of such viruses include the Congo virus, Lassa virus and yellow fever virus. This is a life-threatening situation but the mechanisms involved in dengue fever are different.
As a general rule, the human body, whenassaulted by micro organisms (like bacteria and viruses) by virtue of its sophisticated immune system, produces many cells and other compounds (called antibodies) to combat the unwanted invaders. These cells and antibodies are destructive and are usually able to take care of the problem.
However, sometimes, these antibodies start attacking structures of the body that, in some way, resemble the original pathogen, misinterpreting them as enemies. This unusual phenomenon is termed autoimmunity. The same mechanism is involved in the pathogenesis of VHF in patients with dengue infection.
The main culprit, other than the notorious mosquito, is the dengue virus. It has four distinct types, namely dengue 1-4. These are termed in the medical literature as serotypes. If an individual is infected for the first time s/he may develop immunity from further infections. However, this immunity does not last long.
VHF occurs if there is a second infection in a patient infected previously and recovered by a serotype different from that causing the previous illness. The antibodies produced from previous infection, instead of protecting from further infections, facilitate the entry of the virus into the cells of the patient, thus helping the virus to replicate and produce more viruses in increased numbers.
Sometimes, this enhancing antibody is acquired in newborn children via direct transmission from the mother to the foetus. However, this is uncommon and does not happen all the time. Only rarely does the primary or the first infection results in hemorrhagic fever - the pathogenesis in this situation is not known.
In VHF, along with fever, there is increased tendency to bleed. The cells and antibodies, through various interactions, destroy the blood vessels (capillaries) and they start losing the blood which they contain. Other cells, which protect us from bleeding by plugging gaps and holes in the vessel walls (the platelets), are also reduced in number.
DHF is identified when a patient with dengue fever develops increasing lethargy, restlessness and either an increased tendency to bleed (bruises and petechiae which are small, purplish red pin-point spots on the skin) or overt bleeding. Along with this, the platelets decrease strikingly. In more severe cases, the patient can even become unconscious, causing a decrease in blood pressure and severely bleeds through the gastrointestinal tract. Such a situation is described by doctors as dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS).
This macabre situation can last for one or two days after the patients' usual response to oxygen administration, intravenous fluids and close monitoring. Many patients may require platelet transfusions if the platelet count is below 20,000. Doctors usually try to keep platelet count above 50,000. The diagnosis is confirmed by blood tests detecting antibodies. The case fatality rate varies greatly with case ascertainment and quality of treatment. The overall mortality rate in an experienced centre in the tropics is as low as one per cent.
The first epidemic of DHF was described in Manila in 1953. After that, the outbreaks of DF became more common. A pattern developed in which DF epidemics occurred with increasing frequency and were associated with occasional DHF cases. Later, DHF epidemics occurred every few years. In due course, they occurred yearly with major outbreaks occurring approximately every three years. This pattern has repeated itself as DF has spread to new regions. It is interesting to know that children of a few countries, like USA, are unlikely to have the severe hemorrhagic form of dengue infection, since just a few children have dengue antibodies that trigger the autoimmune process.
Effects Of The Fever
It is known that not all the patients suffering from dengue fever will develop the hemorrhagic complication. There are many factors related to both patients and the causative organism that increase the risk of developing DHF. They include the following:
- Patients already having antibodies for a particular serotype of virus. Women are more often affected as compared to men.
- Whites more often affected than blacks.
- Malnutrition is thought to offer protection.
- Children below 12 years of age are more likely to contract the disease.
- The strain of serotype 2, found in South-east Asia is thought to have more potential of causing DHF.
The key to control both dengue fever and DHF is to control the spreading mosquito which also reduces the risk of other diseases. Control efforts have been handicapped by the presence of non-degradable tyres and long-lasting plastic containers in trash repositories. Moreover, insecticide resistance, urban poverty and the inability of the public health community to mobilise the populace to help out in eliminating mosquito breeding sites further add to the problem.
Dengue vaccines are in the late stages of their development. Whether or not vaccines can provide a safe and durable immunity in immunological diseases like dengue is something that will have to be tested. However, it is hoped that the vaccination will reduce transmission to negligible levels.
It can never be over-emphasised that there is a need for the public, in general, and doctors, in particular, to recognise the symptoms of dengue fever and the associated hemorrhagic syndrome, so that time and money is not wasted on unnecessary tests. What has been most disappointing is the hype caused by the media and the apathy from general public, especially community welfare systems. One has yet to see an anti-mosquito drive initiated by any of the community and other welfare organisations present in the city.
One feels that the community of scientists has a responsibility towards the problem. Much work is needed by the research community to devise safer and more effective methods of mosquito eradication.