Basic facts about dengue
p. 129-134
Texte intégral
1Dengue is currently one of the most widespread arbovirus diseases in the world. Two-fifths of the world population, or about 2.5 billion people, are at risk. In the year 2001 alone, there were over 609,000 cases of dengue in the Americas, including 15,000 cases of the hemorrhagic form. Worldwide, the prevalence of dengue has increased spectacularly in recent decades. The illness is now endemic in more than a hundred countries of Africa, the Americas and the Caribbean, the eastern Mediterranean, Southeast Asia and the western Pacific.
The infectious agent
2Four serotypes, or sub-species, of the dengue virus have been identified: they are antigenically closely related, and have been named dengue 1 (DEN-1), dengue 2 (DEN-2), dengue 3 (DEN-3) and dengue 4 (DEN-4). There is no cross-immunity between subspecies: recovery gives life immunity against the serotype that caused the infection, but not against the other three. The virus is transmitted to man by a mosquito of the genus Aedes. Humans are the main known reservoir for the dengue virus.
The mosquitoes vector
3The vector mosquitoes for dengue are Aedes aegypti in Africa, the Caribbean and the Americas, Ae. aegypti and Ae. albopictus in Southeast Asia, and Ae. aegypti and Ae. polynesiensis in the Pacific. Since 1986, Ae. albopictus has also made its appearance in Central America, the southern United States, Cuba, San Domingo and Brazil, mainly via used tyres imported from Asia to the United States.
4In all the Caribbean countries, Ae. aegypti is what is called a "domestic" mosquito, using man-made breeding sites such as flower vases, drinking water storage containers, coconut shells, poultry drinkers, various water-containing trash, etc.
5Like all mosquito species, Aedes has a four-stage life cycle. The pre-imago stages (egg, larva and pupa) are all aquatic, while the adult, or imago, is winged. From egg-laying to the emergence of the adult takes seven days, while the adult mosquito can live for a month or two.
6Only the female bites. She mates only once, after which she needs blood proteins to bring her eggs to maturity. She takes several blood meals at regular intervals (every four or five days on average), and during these meals she may absorb an infectious agent such as the dengue virus. The virus has to replicate (multiply) in the mosquito to be transmitted to a healthy subject during another blood meal. An infected female remains infected throughout her life and can even transmit the infection to her offspring (called transovarian or vertical transmission).
7The Ae. aegypti female bites mainly in the daylight hours of early morning and evening. She generally takes her blood meals from humans.
The illness
8In humans, dengue takes two main forms: the “benign” form characterized mainly by a sudden high fever, headaches, aching muscles and painful joints. The severe forms (hemorrhagic, neurological etc.) can be fatal. The hemorrhagic form was described for the first time in 1953 in young children in Manilla, Philippines. It is now one of the main causes of infant mortality in most Southeast Asian countries.
9There is controversy over the mechanisms involved in the occurrence of hemorrhagic dengue. Some writers suggest that "sequential infections" are the cause, i.e. a succession of infections by different serotypes in the same person may favour haemorrhaging. Others think it is a matter of differences between strains of the different serotypes.
10Diagnosing dengue with certainty requires laboratory analysis of a blood sample.
Dengue in the Caribbean and the Americas
11The symptoms of an illness corresponding to the "classic" form of dengue, and known as dengue fever, have been known in the Caribbean for at least two hundred years. The French priest Father Dutertre described denguelike epidemics in the Antilles in 1635. However, most of the symptoms described at the time could be attributable to malaria, or even yellow fever, as often as to dengue. Given the state of medical knowledge at the time, we should not place too much faith in the epidemiological value of these chronicles.
12The dengue virus (serotype 2) was isolated for the first time in the Caribbean in 1952, in Trinidad. In 1963, serotype 3 was isolated in Porto Rico, where the outbreak hit 27,000 people. That epidemic then spread to other parts of the region: 15,000 cases were reported in Jamaica and 30,000 in Venezuela.
13In 1977, serotype 1, first isolated in Jamaica, caused another large-scale epidemic in the Caribbean: 35,000 cases in Porto Rico, 47,000 in Martinique.
14The hemorrhagic form appeared in the region in 1981, with an outbreak in Cuba: 344,203 cases of classic dengue, 116,151 people hospitalized in under six months, 24,000 cases of hemorrhagic dengue, 158 deaths including 101 children under 15 years of age. That year, serotype 4 was isolated for the first time on Saint-Barthélémy and St. Martin.
15Since 1982, more than 80,000 cases of classic dengue have been recorded yearly in the Caribbean and the Americas. In late 1989 and early 1990, an epidemic of hemorrhagic dengue broke out in Venezuela, with nearly 9,000 cases of classic dengue, 3,000 of hemorrhagic dengue and 73 deaths. From 1995, outbreaks of hemorrhagic dengue spread like wildfire around Central America, the Caribbean and North America.
16Of the French départements of America, French Guiana saw its first hemorrhagic dengue epidemic in 1991-1992 (44 cases, six deaths). In 1995, seven cases of hemorrhagic dengue with two deaths were recorded in Guadeloupe, and three, with one death, in Martinique. Martinique saw its first major epidemic of hemorrhagic dengue in 1997 (52 cases, including nine fatal). In 2001, serotype 3 was circulating widely in the Caribbean including the FDAs, where it was the only serotype that had not yet been recorded. Another epidemic hit Martinique that year, with nearly 24,000 cases of classic dengue and four deaths.
17In recent years dengue, and hemorrhagic dengue in particular, have been spreading fast to new areas in the Americas. With outbreaks becoming almost yearly events in the FDAs, effective mobilization against the disease seems to be an urgent task.
Socio-economic consequences of dengue
18Dengue has serious social and economic consequences. A survey in Japan showed that the 1977 epidemic there caused morbidity rates of 10 to 20 % in schools and workplaces. In Porto Rico, in the same epidemic, absenteeism from schools and workplaces increased by 30 to 95 % compared to a normal period, and the direct and indirect costs of the epidemic were estimated at 10 million dollars.
Prevention and community participation
19At present there is no vaccine against dengue and no specific medical treatment. The only way to prevent an epidemic is to control the vector mosquito: in the Caribbean and the Americas, this is Ae. aegypti.
20In the present stage of knowledge we have to set aside the possibility of changing the aggressive behaviour of biting populations or their ability to transmit the virus. This leaves us with the following possibilities:
- Physically eliminate the mosquitoes with chemical or biological insecticides:
- in their aquatic larval sites, using products as remanent as possible;
- in places frequented by the adults, indoors or out.
- Prevent mosquitoes from settling in an area:
- by drying out or destroying potential breeding sites;
- by preventing larvae from developing in existing sites using remanent chemical larvicides, as in the first option.
21Local residents can therefore usefully contribute to the vector control drive with a few simple acts:
- around the home, prevent or remove anything in which unwanted water can collect;
- change water in flower vases at least twice a week;
- keep mosquitoes out of water reserves by taking care to cover all water storage recipients.
Vector control measures in the FDAs
22The FDAs were included in the Pan-American Ae. aegypti eradication campaign that began in 1947. In 1958, following local operations under the responsibility of the Pasteur Institute in Cayenne, the Pan-American Health Organisation (PAHO, regional office of the WHO) declared the species eradicated in French Guiana. However, it reappeared there in 1964. The French Ministry of Health set up its own campaign against Ae. aegypti in the FDAs in 1968, as a provisional action. Initially, in line with the ideas of the times as expressed in WHO statements, it was a matter of eradicating the mosquito. Thirty years on, it is recognized that eradication is impossible, and the byword now is "control".
23Daily mosquito control tasks are carried out by over two hundred officers in the FDAs, but organised in different ways in each case. In Guadeloupe, vector control work depends entirely on a State body, the Direction de la santé et du développement social (DSDS). In French Guiana, field operations are conducted by the département Council with the Pasteur Institute supervising and conducting entomological research. In Martinique, dengue vector control is part of the general mosquito control drive, i.e. against all mosquito species that feed on humans, involving the DSDS and the département Council under an agreement to pool human and material resources.
24The teams' daily work is essentially home visits focusing on larval control: identifying larval sites, chemical treatment of sites needed by the residents (water reserves, for example), elimination of unnecessary larval sites, and recording entomological parameters. In Martinique, five years of observations in five localities with monthly visits to the same homes showed the limitations of such routine visits.
25In all three départements, space spraying against adult mosquitoes is regularly carried out from trucks, but the effects are very limited spatially and very short-lived.
*
26Dengue is a real public health problem in Guadeloupe, French Guiana and Martinique as in all Caribbean countries: the severe forms (hemorrhagic or other) carry a risk of mortality, and the periodic epidemics have serious social and economic repercussions.
27The disease can strike anyone, and the human population maintains the vector breeding sites; everyone, therefore, should be involved in prevention.
28Prevention and vector control, as designed and carried out so far, have failed to end the epidemics, despite the precise, useful recommendations drawn up in 1998 by a multidisciplinary group of specialists following the 1997 epidemic (see Guide de la surveillance de la lutte contre la dengue dans les départements français d'Amérique cited above).
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