In addition to the behavioural and exposure risks already described in different groups of travellers, the mode of travel predisposes to specific medical problems. Immobility due to prolonged travel is likely to predispose to venous thrombosis and pulmonary embolism, especially in patients with pre-existing risk factors. It is possible that specific factors associated with air travel, such as low air pressure, hypoxia and dehydration, exacerbate this, but the evidence base is poor and few scientific data have been published to quantify such an increase in risk (Kesteven, 2000; Geroulakos, 2001). Venous thrombosis and pulmonary embolus should be considered in the differential diagnosis of recent travellers, particularly in those with leg pain, fever or dyspnoea.
In-flight medical emergencies affect about 1/11000 passengers and comprise a full range ofmedical problems, some of which need further attention when the patient arrives, including the effects of overindulgence in alcohol (Beighton, 1967; Dowdall, 2000; Goodwin, 2000). Recirculation of air leads to sharing of pathogens, and the transmission of influenza (Klontz et al., 1989) and tuberculosis between air passengers is recorded (Driver et al., 1994; Kenyon et al., 1996). Although the risk of tuberculosis transmission is low and is limited to passengers near to the index case, it generates considerable concern (Or-merod, 2000).
Passengers and crew on cruise ships are also exposed to conditions of crowding, and respiratory symptoms are the most common reason for consultation (29%) during cruises. Although tuberculosis transmission has not been reported in this setting, influenza A outbreaks are well described and in the last decade several serious outbreaks of Legionnaires' disease have been reported on cruise ships (Minooee and Rickman, 1999). Outbreaks of gastroenteritis are also commonly recorded, including bacterial infections caused by pathogens such as Shigella spp, Salmonella spp and Vibrio spp. More devastating are the frequently recorded explosive outbreaks of small round structured viruses, Norwalk and similar agents, particularly in an elderly population, although the majority of passengers so affected will be treated on ship and recover before returning to shore.
Expedition travel is less hazardous than one might expect. A recent questionnaire survey of 246 expedition leaders revealed only 835 medical incidents in 130 000 person-days of travel, of which 33% were gastrointestinal and 21% were 'general medical', including 23 cases of proven or suspected malaria, most of which were dealt with by local doctors (Anderson and Johnson, 2000). Of 206 expedition participants treated by a doctor, only 10 saw their general practitioner and only five needed to see a hospital doctor after their return to the UK.
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