|
You are visitor since April 1998
|
DENGUE AND AEDES by Dr Andrew Reekie, 11 Nov 98 There is some dengue around with several cases amongst expatriates. The indigenous Bangladeshi population probably have some immunity and are less susceptible. Dengue is spread by two species of the Aedes mosquito. Aedes aegypti is the famous species which the American army doctor, Walter Reed, recognised as the vector of yellow fever almost a century ago. Aedes aegypti is widely distributed throughout the tropics. A close relative, Aedes albopictus, is also involved in dengue transmission and, as it can tolerate cooler conditions, is now more important. It has a wider range and modern air and surface transport has hastened its distribution. The Aedes mosquito is popularly called the tiger mosquito for, with its very dark colour and white stripes on its legs and thorax, it is surely one of the most distinctive mosquitoes. If anyone wants to see pictures, I have nice ones in Elizabeth House, but sadly they are not reproducible. These two Aedes are essentially urban and exclusively feed on humans, living around human habitation, breeding in small pockets of water, often man-made: tree holes, bamboo stumps, leaf axils, flower pots, water jars, tin cans and water in old tyres are typical. However their eggs can survive on damp surfaces or in mud and can withstand desiccation well, eventually hatching with periods of rain. This mechanism allows them to survive in dry seasons. Hence if you have Aedes around your house, it is not enough simply to empty breeding sites, but you need to scour the surfaces briefly with something like a scrubbing brush to detach larvae and eggs. Although it has been proved that some species of mosquito can fly up to six kilometres - (How - by attaching a microtransmitter to one of its legs?!), Aedes generally remain very local, perhaps typically travelling no more than 50 metres from its breeding site. This means that often there are small pockets of a dense population in an area where it is generally much less common. In these pockets they will be very obvious to anyone who is observant. Mosqultos of all species incidentally seldom fly high. The density drops sharply above 10 or 50 metres and those of you in flats are in a much stronger position - unless you have a breeding site inside your flat: flower vases are the commonest but cisterns are worth considering. I could go on indefinitely about the fascinating insects, but will spare you more. How do you avoid dengue? Check your mosquito screening now, use knock down sprays (eg in bedrooms) concentrating on the darker area behind furniture and curtains - turn off the fans before doing so and then leave the room shut up for half an hour. They are slightly toxic. Consider mosquito coils, use repellant (not forgetting the feet) and, if outside, dress to cover up, always remembering that mosquitos will bite through loose weave material and are attracted by dark colours. Do give a passing glance at mosquitos before swatting them. You need to know if Aedes is around. Dengue is one of a large group of viruses in which arthropods are vectors. These are loosely called the 'arboviruses,' an abbreviation for arthropod borne viruses: of the arthropods involved, mosquitoes are quite the commonest, followed by ticks. The arboviruses number several hundred with yellow fever - which has never for some reason occurred in Asia - as the best known and dengue, the most important. Japanese encephalitis is another local example. Dengue has been known for centuries and there is a good description in Chinese literature of over 1 000 years ago. The name may come from Swahili but in many parts of the world it is still called breakbone fever, a reference to the severe limb pains which are usually a feature. People who have had dengue often refer to it as the worst illness they have experienced! Although there are four serotypes of the virus, there are two distinct clinical forms of the disease - simple 'classical' dengue and the less common but much publicised haemorrhagic form with its associated complication of shock - these are referred to as Dengue Haemorrhagic Fever and the Dengue Shock Syndrome and abbreviated to DHF/DSS. In these, haemorrhage occurs, commonly from sites such as the nose, gums, gut or vagina and in DSS, the blood vessels become more permeable and fluid is lost from the circulation. If managed properly, these cases only have a small mortality: In Thailand the death rate given as 2 per 1000 - and they have plenty of severe dengue there. In Classical Dengue, the onset of the illness, typically 3 to 7 days after being bitten by an infected mosquito, is sudden. I have referred to the intense limb pain, but headache, fever, loss of appetite and pain moving the eyes are common. Vomiting may occur and a fine rash sometimes appears towards the end of the illness, often affecting the palms and soles of the feet where it can be very uncomfortable. Although the fever drops suddenly about the 6 to 8th day, the patient is left exhausted and often depressed - and, for a while, more susceptible to other infections. Treatment is symptomatic but it is important to use paracetamol to treat the fever rather than aspirin which increases the tendency to bleed. Haemorrhagic dengue was first described in tropical Australia a century ago. It then reappeared in 1928 in Greece and later in Taiwan. However there have been continuous foci of it since the mid 50's especially in South-East Asia. My first experience of dengue was in Jamaica thirty years ago, but although there were many cases especially in children, almost none were seriously ill and we saw no hemorrhage. However DHF has been firmly established in the West Indies since it struck Cuba in 1981. Interestingly, it has not yet occurred in Africa although dengue has been there for centuries. I mentioned that there are four serotypes. These do not give cross immunity to each other. You could therefore conceiveably get dengue four times if you had the misfortune to be exposed to all four strains. It had been thought that DHF/DSS occurred when when a patient was infected with one type soon after an attack of another, but it is not quite as simple as this for in the past, all four types have circulated within communities without any DHF or DSS. However there are minor variations within strains themselves caused by the phenomenon of genetic drift and it seems that strains originating in South East Asia predominate in DHF/DSS. Dengue spans the globe in a belt around the tropics. Modern air travel effectively spreads the four serotypes. Last week I mentioned one of the Aedes vectors, Aedes albopictus, which is relatively tolerant of cold. This mosquito was restricted to Asia, but is now everywhere and is allowing the disease to turn up in distinctly cooler climes - currently there is danger of it extending northwards from the Caribbean into the States. A vaccine is in an advanced stage of development, but there have been technical problems and it is unlikely to be released within the next five years. A serious threat of dengue to the US might speed things up! And so you are left avoiding mosquitoes. Remember that Aedes are most active just before dusk. |
