Why a travel history matters: absent vectors and long incubation
Some infections depend on vectors (carriers such as a mosquito) absent from the UK, and long incubation (the delay before symptoms) means patients may not link illness to the trip.
Remember
Ask unwell patients about recent travel: tropical infections are rare in the UK, can have long incubation periods and may be contagious.
Memory aid
A good aid is to ask where, when and what, then check the incubation period before ruling anything out.
Use it to remember to ask where the patient went, when, and what they were exposed to, and to allow for a long incubation period.
Many travel-related infections depend on a vector, which is a carrier that transmits the organism, such as a mosquito, that is native to certain countries. The female Anopheles mosquito is the vector for the Plasmodium parasites that cause malaria. It is common in sub-Saharan Africa, whereas the main tropical vectors are not established in the UK. Where a vector does not live, the disease it carries cannot spread locally, so the risk arises mainly when someone travels to the region where the vector or source lives. Other infections come from environmental sources such as contaminated water.
The incubation period is the time between catching an infection and the first symptoms. It can be days for some infections but many months for others. Some malaria infections take close to a year to cause symptoms, so a trip taken months earlier can still be relevant. Patients often do not link their illness with a trip, and may be surprised when asked, so the history has to be prompted.
Tropical infections are rare in the UK, so they are easily missed unless the clinician asks about travel. An easily treatable infection can then develop complications because it is not in the differential diagnosis, the list of possible causes under consideration. If the infection is contagious, which means it spreads between people, the delay can also allow spread within a hospital or the community. A good travel history therefore helps prevent complications and spread, and allows containment (isolation and infection control) and treatment to be set up.
So ask directly, because patients do not always realise that a trip is relevant to their symptoms. Ask about all recent trips rather than just the latest, and record where, when, for how long, what was eaten and drunk, and any bites.
Key facts
- Vector
- many travel-related infections depend on a carrier (a vector), for example a mosquito, that is native to certain countries.
- Malaria example
- the female Anopheles mosquito carries the Plasmodium parasites that cause malaria, and is common in sub-Saharan Africa, whereas the main tropical vectors are not established in the UK.
- Long incubation
- some malaria infections take close to a year to cause symptoms, so a trip made months ago can still be relevant.
- Rare at home
- tropical infections are rare in the UK, so they are easily missed unless the clinician asks about travel.
- Cost of delay
- a missed infection can cause complications, and if it is contagious it can spread within a hospital or the community.
- Practical rule
- ask about travel, with dates and places, in anyone with an unexplained fever or acute illness.
Exam traps
- Getting home more than two weeks ago does not make malaria unlikely, since some infections cause symptoms only many months later.
- These infections are not caught from recycled cabin air: they come from organisms or vectors at the destination, so flight duration is not the exposure that matters.
- Symptoms do not usually follow within a couple of days of exposure: some infections, including malaria, can cause symptoms many months later, so the history has to look further back than the trip just finished.
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