Incidence and risk factors for infectious diseases in long-term travelers: a prospective study.

Guidelines for travellers' vaccinations, malaria prevention, and other
(behavioural) health advices are based on a combination of theoretical risks,
prevalence of symptomatic disease in returned travellers, and surveillance data
of notifiable diseases. These data mainly give insight in the prevalence of
symptomatic infectious diseases in short-term travelers. From previous
prospective studies conducted by the MHS Amsterdam in 1991/1992 among 600
short-term travellers, we know that the overall attack rate of diarrhoea was
52%. Certain groups of travellers were at significant higher risk of diarrhoea
e.g., travellers to the Middle East and the Indian subcontinent, those with no
history of previous long-term travel, with history of gastro-intestinal
treatment, and self arranged travel (as opposed to organised travel). The
attack rate of a falciparum malaria infection was 1.1%, with a significantly
higher rate in West Africa than in East Africa. For dengue the overall attack
rate in short-term travellers visiting areas where dengue is endemic was 3.6%,
with a significantly lower risk in those with a history of travel to tropical
countries. It is likely that long-term travelers in general are at higher risk
of infection. Risks increase with duration of stay. Infection rates in long
term travellers, however, might be influenced by other risk factors: for
malaria e.g., long-term travellers tend to be less compliant with preventive
measures than short-term travellers. Backpackers might be at higher risk than
expatriates who live in their own house. Long-term travellers may be involved
in different activities that can be of influence on infection risks, such as
sexual or other close contacts with local populations (hepatitis B, TB,
syphilis, hiv). For some infectious diseases, on the other hand, risks might be
smaller because long-term travellers are adviced vaccinations against more
diseases than short-term travellers (e.g. rabies, hepatitis B, meningitis). In
long-term travelers, data on symptomatic and asymptomatic infections are scarce
because data on hospital admissions and diagnoses abroad are not routinely
collected, and often unreliable. Available data are estimates based on
seroprevalence studies in returning travellers and a few prosprective studies
in some specific groups, such as Peace Corps Volunteers or United Nation
groups, are available. The currently given advices on vaccinations, malaria
prophylaxis and general preventive measures are mostly based on theoretical
risks and extrapolations of risks in short-term travelers, but not evidence
based. There may be large differences between certain groups of long-term
travellers such as backpackers and expatriates. The results of this study will
also be used to develop post-travel screening policies: is post travel
screening necessary, which type of long term traveller should be screened, and
for which infections?

What is the attack rate of symptomatic and asymptomatic (tropical) diseases in
long-term travelers.
Which behavioral risk factors are of influence on the attack rates? (compliance
with preventive measures, other risk behaviour)
Are certain groups of long-term travellers at higher risk than others? (e.g. to
certain destinations, expatriates, backpackers)
Are attack rates for the infectious diseases under study different between
long-term and short-term travellers?
Is there a need for post-travel screening and if so, for which infections?

Prospective observational mono-centre study among long-term travellers who plan
to travel to (sub)tropical countries for more than 13 weeks and less than 52
weeks. These countries include all countries where Plasmodium falciparum
malaria is endemic according to the World Health Organization guidelines for
travellers (see annex 1 of the protocol).
At the travel clinic of the MHS, during 24 months, all travellers of 18 years
and older, who speak english or dutch, who are born in a Western country, and
who intend to travel to a (sub) tropical area for more than 13 weeks and less
than 52 weeks, will be asked to participate.
All travellers are asked to keep a diary while abroad. All travellers will
provide a blood sample at inclusion (before departure) and one after travel
(within 6 weeks after return).
To determine the attack rate of infections they are tested for seroconversion
for falciparum malaria, dengue fever, schistosomiasis, filariasis,
strongoloidiasis, toxocariasis, hepatitis E, syphilis, hiv and TB.
After an estimated 24 months of inclusion, 12 months are needed for follow up.
The fouth year is used to perform laboratory tests, analyse data end write the
results.

The only risk for the participant is the drawing of a bloodsample, which can
cause local pain, fainting, haematoma or a mild inflammatory reaction. All
these possible side effects are expected to be mild and not long-lasting.