Dengue, epidemiology

The disease occurs everywhere in the tropics but is especially frequent India, Southeast Asia, Central America and the Caribbean. In 1927-28 there was a large epidemic in Greece in which approximately 2000 people died. Until recently, the infection had been very rare in Africa, except for the east coast (sporadic cases) and parts of the Sahel but the epidemiology of dengue is changing and we can expect the disease to increase in other parts of Africa. The disease is not stable in other parts of the world either. After an absence of 20 years, dengue virus type 3 broke out in the Caribbean in 1994. Dengue can occur as an epidemic, as was recently seen in parts of Central and South America and in 1997 in the islands of the Pacific Ocean. The disease occurs more frequently in the rainy season. In 1998 there was a massive increase in the number of cases in Southeast Asia and South America. In Brazil alone a total of 234,828 cases were reported in the first 4 months of 1998 (almost as many as for the whole of 1997).

Man is probably the main reservoir of the disease. The significance of a sylvatic reservoir in wild monkeys has not yet been established. There is transovarial transmission of dengue virus in the mosquito and this can be repeated for a few generations without the adults feeding on an infected host.

Dengue virus has 4 serotypes and belongs to the Flaviviridae. Each serotype contains a number of subtypes. The viruses are transmitted by infected female Aedes mosquitoes, of which Aedes aegypti is the most important. The virus multiplies in the mosquito and is introduced into the skin via the insect’s saliva. The mosquito develops a life-long non-cytocidal infection. The virus has an RNA genome. This codes for three structural and seven non-structural proteins. Initially a large precursor protein is made which is then cleaved by host-cell and viral proteases.

Aedes aegypti is the most important vector in Africa, America and Asia. This mosquito prefers a peridomestic setting (near houses). In Africa there is a lightly-coloured peridomestic form and a dark-coloured sylvatic form. A. aegypti has two longitudinal dorsal white stripes and an arched stripe (“lyre”), making the insect easily recognisable. These are insects which prefer to bite during the day (unlike Anopheles). Aedes aegypti not only transmits arboviruses but the mosquitoes also have an annoying biting habit ("nuisance mosquito”). They bite during the day, mainly in the late afternoon. They buzz a little but do not keep people awake with the noise they make (unlike Culex mosquitoes). Traditionally it was thought that Aedes aegypti had limited flying ability (100 m). This was called into question by more recent data. Studies with labelled mosquitoes revealed an area of ± 840 meters in diameter in which eggs were laid. The insects are labelled by allowing them to feed on blood to which rubidium chloride (RbCl) had been added as a marker. Rubidium is an alkaline metal which is very similar to potassium and is not toxic in low concentrations. The element is then present in the eggs and can be detected by atom-absorption spectroscopy. It is also possible to trap adult mosquitoes, label them (e.g. with fluorescent dyes), release them and then recapture them later (“mark-release-recapture”). Adult mosquitoes can be trapped for epidemiological reasons, on a person for example (“human bait”) or through various techniques such as insect traps with bait such as CO₂, octenol, light, heat and/or mechanical suction. It is however possible that the sample of insects which is caught in a certain way, differs in composition from the population which is responsible for transmission under natural conditions. In order to study the density of vectors in an area, entomological surveys are used. A frequently used index is the number of positive water containers per 100 houses (“Breteau index”). The “house index” is also frequently used (percentage of houses which are positive for Aedes larvae). If the density is low (e.g. Breteau lower than 5) “ovitraps” can be used to collect the eggs (usually black glass beakers with some clean water and a piece of cardboard, hay or twig in it). These can also be used for surveillance e.g. around seaports and airports.

Category: Medicine Notes