Travel to tropical countries aids in spread of influenza
International travel, especially to subtropical countries, appears to contribute to the spread of influenza worldwide, researchers from the Netherlands have found.
“WHO estimates that 5% to 15% of the worldwide population is affected by seasonal influenza viruses annually,” the researchers wrote in Emerging Infectious Disease. “Outbreaks of influenza associated with travel by air, ship or train indicate that international travelers are at risk for this infection and may introduce novel strains into domestic populations.”
The researchers conducted a prospective study that included 1,190 participants who attended the travel clinic of the Public Health Service Amsterdam from October 2006 to October 2007. All of the travelers were aged at least 18 years, immunocompetent and planning to travel for 1 to 13 weeks to one or more subtropical countries. All participants provided blood samples before departure and 2 to 6 weeks after return. They also kept a travel diary that included itinerary, symptoms of disease and any treatment. The blood samples were tested for influenza antibodies.
Of the 1,190 travelers, 86 had confirmed influenza upon return, with an attack rate of 7%. The incidence rate was 8.9 per 100 person-months (95% CI, 7.1-10.9). The highest attack and incidence rates were among travelers to south-central and western Asia. Among those with confirmed influenza, 66 had a rise in antibody titer against one influenza virus; 11 against two viruses; four against three viruses and five against four viruses.
Among all of the travelers, 117 developed fever and 40 had influenza-like illness. Influenza was confirmed in 15 of the travelers who developed fever and nine of the travelers with influenza-like illness. The attack rate for influenza-like illness was 0.8% and the incidence rate was 0.9 per 100 person-months (95% CI, 1.2-3.2).
Risk factors for confirmed influenza infection included those aged 55 to 64 years, born in a non-Western country and experiencing influenza-like illness.
“Short-term travelers to tropical regions contract influenza regularly, which is probably a major factor in the epidemiology of influenza,” the researchers wrote. “Because travelers often visit tropical regions, where influenza viruses continuously circulate, after contracting the disease they become vectors that further spread the virus worldwide.”
Disclosure: The researchers report no relevant financial disclosures.