Dengue viruses (DENV-1, DENV-2, DENV-3, and DENV-4) are
the most common cause of arboviral disease in the world and are mainly transmitted
by bites of the mosquito vector Aedes aegypti
. In recent decades,
the incidence of the severe form of dengue virus infection, dengue haemorrhagic
fever, has risen dramatically, mainly in South East Asia and the Americas.
According to the World Health Organization, the viruses cause an estimated
50-100 million illnesses worldwide each year, including 250 000 to 500 000
cases of dengue haemorrhagic fever (DHF) and 24 000 deaths (1).
Classical dengue fever is characterised by fever, headache, retro-orbital
pain, myalgia/arthralgia, skin rash, and leucopenia, usually with no deaths
occurring. In contrast, DHF is clinically characterised by haemorrhagic
diathesis and vascular leakage (manifesting as a rise in haematocrit), ultimately
resulting in hypovolaemic shock. The pathogenesis of DHF is not fully understood,
but it has been well documented that secondary dengue infection is a major
risk factor of the disease (2, 3). As a consequence, and maybe also as a
result of genetic factors, travellers rarely develop DHF (4). A high percentage
of dengue infections in travellers may be asymptomatic (5, 6).
It has, however, been recognised that travellers have an important role
in introducing more virulent dengue strains into endemic areas where usually
only mild disease occurs (7), or into non-endemic areas where the mosquito
vector is common (8). The recent introduction of Aedes albopictus
to Europe, notably Italy, France, and Albania, may serve as a warning of
things to come (9).
The European Network on Imported Infectious Disease Surveillance (TropNetEurop,
was founded in 1999 to detect emerging infections of potential regional,
national, or global impact at their point of entry into the European population.
The network currently consists of 46 collaborating centres in 15 European
countries. Annually, the collaborating centres give approximately 220 000
consultations prior to travel, and treat 63 000 patients post-travel (10,
Within this network, the numbers of reported dengue cases increased from
64 in 1999 to 218 in 2002. The increase has been attributed primarily to
the increased number of reporting sites of the network rather than to an
increased incidence of dengue infections in travellers. The median age in
our population was 32 years (range 1-69 years). Males were slightly more
frequently affected with a male to female ratio of 1.2:1. The median duration
of travel during which patients acquired the dengue infection decreased
from 38 days in 1999 to 23 days in 2002.
Forty five per cent of dengue cases were acquired by patients who had travelled
in countries in South East Asia, 19% were imported from South and Central
America, 16% from the Indian subcontinent, 12% from the Caribbean, and 8%
from Africa. This distribution mainly reflects worldwide dengue activity,
as well as the popularity of these countries as tourist destinations. Thailand,
Vietnam, and Indonesia are not only highly endemic areas for dengue viruses;
they are also countries with an expanding tourism sector. Thailand alone
was the place of acquisition for 134 cases (28%) of all travel associated
dengue infections in our network over the past four years. In 2002 there
was a proportional rise in the number of infections acquired in South East
Asia compared with 2001. Reporting increased most notably from Thailand,
Indonesia, and Brazil, while India and the Philippines saw a decline in
numbers. There was a clear seasonal pattern in the reporting of dengue with
peaks during the travel seasons in summer and winter.
Most patients were European-born travellers (94%), but we noted that immigrants
born outside Europe and foreign visitors had a 4.3 times higher risk of
developing DHF when compared with those born in Europe. The reporting system
does not include information on the countries of residence of non-European
travellers, but many are likely to be from dengue-endemic countries. Overall,
13 cases of dengue DHF have been reported (2.7% of all dengue infections).
The travel history of these patients showed that infections were acquired
in all the world's endemic regions: seven in South East Asia, two in Central
America, two in South America, one on the Indian subcontinent, and one in
Central Africa. The median travel duration in patients with DHF was 29 days
and was not significantly longer compared with patients who developed classical
dengue fever (26 days, p-value 0.38). No deaths occurred among the reported
dengue patients (DF and DHF).
Three fatal cases of dengue in Europe have been reported to the network
from sources outside the immediate membership. The first case occurred in
1997 in the United Kingdom (UK) and was not published. The other two patients
died of DHF in 2002; one was a Bangladeshi immigrant living in the UK who
returned from visiting Bangladesh (12), the other was a young Finnish journalist
travelling in South East Asia (publication in progress).
Overall, in our network we saw an increase in the total number of reported
dengue cases, an increase in cases with more severe disease (DHF), and an
increase in cases with unusual clinical findings (several cases presented
with liver involvement and two cases with affected optic nerves(13)). As
long as no effective vaccine is available, dengue viruses will present an
increasingly serious threat to European travellers and an even greater threat
to those living in the countries to which they travel.