Dengue

Neuronal cells in mice infected with dengue type 1 - © Institut Pasteur

Dengue, also known as dengue fever, is a viral disease transmitted to humans by mosquitoes of the genus Aedes. The incidence of dengue is currently increasing dramatically, and it is now one of the diseases said to be re-emerging. WHO estimates the annual number of cases to be 50 million, including 500,000 cases of dengue hemorrhagic fever, which has a case fatality rate of over 2.5%. Two and a half billion people live in risk areas. Although initially seen only in the world's tropical and sub-tropical regions, dengue has now reached Europe, where the first two indigenous cases were reported on the French mainland in 2010. In 2019, the mosquito vector is established in 51 French departments. There is a real risk that the disease may spread if patients infected with dengue arrive in mainland France.

50 million annual cases of dengue

Including 500,000 cases of dengue hemorrhagic fever

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Causes and origins

Dengue is an arboviral disease (a virus transmitted by insects), belonging to the Flavivirus genus of the Flaviviridae family, like West Nile virus and yellow fever. It is transmitted to humans by mosquitoes of the Aedes genus when the insects feed on human blood. The dengue virus strains are divided into four different serotypes: DENV-1, DENV-2, DENV-3 and DENV-4. Acquired immunity as a result of infection by one of the serotypes provides protection against that particular serotype, but not against the others. Consequently, a person may be infected by all four serotypes during the course of a lifetime. Subsequent infection by another serotype increases the risk of severe dengue, known as dengue hemorrhagic fever. An effective vaccine against dengue should therefore convey protective immunity against all four serotypes.

Symptoms

Classical dengue

Classical dengue begins suddenly after an incubation period of two to seven days, with the onset of a high temperature, often with headaches, nausea, vomiting, joint and muscle pain and a skin rash similar to that of measles. After three or four days there is a brief remission before the symptoms worsen, with possible conjunctival hemorrhaging, nose bleeds or ecchymosis. Symptoms then regress rapidly after another week. Recovery follows a convalescence of around two weeks. Although highly debilitating, classical dengue is not considered a serious disease, unlike hemorrhagic dengue.

Complications – dengue hemorrhagic fever

In some patients, for unexplained reasons, the clinical presentation of the disease can develop into two serious forms: hemorrhagic dengue and dengue shock syndrome, which is fatal.

The hemorrhagic form of the disease, which occurs in approximately 1% of dengue cases in the world, is extremely serious: it results in a persistent high temperature, and is often characterized by multiple hemorrhages, in particular in the gastro-intestinal tract, skin and brain. Particularly in children under 15, a state of hypovolemic shock can sometimes ensue (with a decrease in temperature, clamminess and a weak pulse, indicating circulatory collapse), causing abdominal pain and, in the absence of intravenous fluids, death. In all cases it is useful to confirm the etiology with a rapid and precise virological diagnosis, both to establish appropriate patient care and to allow public health surveillance systems to issue an alert and step up vector control measures.

Epidemiology

Dengue is currently considered to be a re-emerging disease. With economic globalization and the greater movement of people and goods, it is spreading to new geographical locations, increasingly thriving in urban environments and causing epidemics on a larger scale than ever before. The severe forms of dengue have been more and more widely reported during recent outbreaks.

Originally a tropical disease

Dengue is rife in all intertropical regions. Although until recently restricted to South-East Asia (440,000 cases in China in 1980, 200,000 cases in Thailand in 1987), it is now spreading inexorably to the Indian Ocean, the South Pacific (32,800 cases in Tahiti, Moorea and French Polynesia in 2001), the French West Indies (2003, 2006-2008 and 2009-2010), and Latin America, where the annual number of cases increased 60-fold between 1989 and 1993 in comparison to the previous period (1984-1988).

Since the end of 2009, this disease has soared to epidemic proportions in the West Indies. In 2010, 86,000 cases of dengue were reported in Martinique and Guadeloupe (figures from the Santé Publique France). In 2011 and 2012, there was no epidemic outbreak.

The first cases of hemorrhagic dengue appeared in Cuba and the Caribbean in 1981, and the disease reappeared in Cuba in 1996, after a 15-year interval. In 2013, there was an epidemic outbreak in French Guiana, and that year 2.35 million cases were reported in the Americas alone, including 37,687 cases of severe dengue. This worrying resurgence of dengue in Latin America and the Caribbean appears to be linked to the doubtful effectiveness of mosquito vector eradication programs in this region of the world. Other contributory factors appear to be demographic growth, uncontrolled urban development, natural disasters and the impoverishment of populations affected by the disease. Dengue has a major impact on the economies of the countries in which it is rife.

Moving towards colonization of temperate zones

In the last few years the secondary vector of dengue in Asia, Aedes albopictus, has colonized North America and Europe, including France. The mosquito is active in these regions between May 1 and November 30, but its ability to tolerate low temperatures and hibernate has enabled it to subsist during cold spells. The risk of dengue transmission in these regions has now been confirmed, with the detection of the first two indigenous cases in France in 2010, in Nice. In 2012, an epidemic outbreak on the Portuguese archipelago of Madeira resulted in more than 2,000 cases, and imported cases were detected in 10 other European countries outside mainland Portugal. France has been reporting recurrent indigenous cases of dengue since 2010: 1 (2013), 4 (2014), 8 (2015), 8 (2018), 9 (2019), and 12 (2020).

The latest epidemiological data

According to figures from WHO, cases of dengue were reported in the US state of Florida and in China's Yunnan province in 2013. Dengue has remained widespread in several countries in Latin America, including Honduras, Costa Rica and Mexico. In Asia, Singapore has reported an increase in cases after a hiatus of several years, and outbreaks have also occurred in Laos. In 2014, trends are pointing to an increase in dengue cases in the Cook Islands, Malaysia, Fiji and Vanuatu, and dengue type 3 virus (DENV-3) is affecting the Pacific Island countries again after a 10-year interval.

Fighting the disease

There is currently no specific treatment or vaccine available for fighting this disease. Only one . Only one vaccine is marketed: Dengvaxia, developed by Sanofi, a live, attenuated, recombinant chimeric vaccine based on the yellow fever vaccine 17D, that protects against serotypes 1, 2, 3 and 4 in subjects aged 9 to 45 years with a history of dengue virus infection and living in endemic areas. Other means of fighting the disease are the control of mosquito vectors in the regions concerned and individual measures to protect against mosquito bites.