Travel Infections: FoodNet Casts Doubt on Residual Immunity in VFR Travelers

By Lin H. Chen, MD

Assistant Clinical Professor, Harvard Medical School and Director, Travel Medicine Center, Mt. Auburn Hospital, Cambridge, MA

Dr. Chen has reviewed research grants from the Centers for Disease Control and Prevention and Xcellerex.

Synopsis: Travel was associated with 13% of the enteric infections reported to FoodNet, and the most commonly identified pathogens were Campylobacter, nontyphoidal Salmonella, and Shigella species. Precautions to avoid consuming contaminated food and water remains highly relevant in advising travelers.

Source: Kendall ME, et al. Travel-associated enteric infections diagnosed after return to the United States, Foodborne Diseases Active Surveillance Network (FoodNet), 2004-2009. Clin Infect Dis 2012;54(S5):S480-7.

FoodNet is an active surveillance program that collects data on 9 laboratory-confirmed pathogens from 10 sites in the United States: 7 states (Connecticut, Georgia, Maryland, Minnesota, New Mexico, Oregon, and Tennessee) and certain counties in California, Colorado, and New York. Kendall et al analyzed infections reported in this network from 2004-2009 that were considered to be travel-associated and compared them to infections in non-travelers; they also estimated risks according to travel destination. The authors defined travel associations based on the intervals between return date and illness onset: <30 days for Listeria, Salmonella (typhoid and paratyphoid), <15 days for Cryptosporidium and Cyclospora, and <7 days for all other enteric pathogens.

Approximately 13% (8270/64,039) of reported enteric infections that also contained travel information were considered travel-associated. Travel-associated cases had a mean age of 33.1 years, older than nontravelers (mean age 25.5 years), especially within the group aged 18-44 years. Travel-associated cases were more likely to be Asian, less likely to be black., and less likely to be hospitalized. Five deaths were reported for travel-associated cases, attributed to Listeria (n=1), Vibrio vulnificus (n=1), and nontyphoidal Salmonella (n=3).

The most frequently identified pathogens in travelers was Campylobacter (42%), followed by nontyphoidal Salmonella (32%) and Shigella infections(13%). These organisms were also the most common and top 3 for nontravelers, although nontyphoidal Salmonella was more common in nontravelers (47% of infections), and Campylobacter was less common (27% of infections). All 3 cases of cholera were travel-associated, as well as high proportions of typhoidal and paratyphoidal Salmonella (68% and 50%, respectively). Shigella dysenteriae, S. boydii, and S. flexneri were also often travel-associated (56%, 44%, and 24%, respectively), whereas non-cholera Vibrio, Yersinia, Shiga toxin-producing Escherichia coli (STEC), and Listeria occurred more commonly in nontravelers.

The most common countries for travel-associated infections were Mexico, India, Peru, Dominican Republic, and Jamaica, and account for half of the travel-associated cases. Furthermore, race and ethnicity correlated with travel destinations. For example, 85% of Asian travelers reported travel to Asia, 95% of Hispanic travelers reported travel to Latin America and the Caribbean [LAC], and 58% of black travelers reported travel to Africa.

The authors estimated risk for each pathogen based on travel region. Africa had the highest risk for travel-associated infection (76 cases/100,000 travelers), followed by Asia (23 cases/100,000 travelers), and LAC (20 cases/100,000 travelers). Within LAC, South America had the highest rate of Campylobacter (26.4 cases/100,000 travelers). The Caribbean had the highest rate of nontyphoidal Salmonella (8.6 cases/100,000 travelers), and Central America had the highest rates of Shigella, Cryptosporidium, and STEC (8.6, 2.8, and 1.0 cases/100,000 travelers, respectively).

Europe had the lowest overall risk, and Campylobacter was the most common infection associated with Europe and Oceania.


Travelers' diarrhea (TD) is the most common ailment encountered by travelers, affecting 20-60% of travelers visiting developing countries.1 Hygiene standards at the travel destination are the usual predictors for development of TD. Ciprofloxacin continues to be effective for self-treatment of TD in many areas, but azithromycin is the drug of choice for travelers to areas where there is a high risk of fluoroquinolone-resistant Campylobacter, particularly in South and Southeast Asia.2

A GeoSentinel analysis of 6,086 travelers with gastrointestinal infections seen during 2000-2005 found highest risk to be associated with travel to South Asia, sub-Saharan Africa and South America.3 Kendall et al show similar patterns in this FoodNet analysis. Health Protection Scotland also analyzed data gathered from 2003-2007 on possible imported infectious intestinal diseases in Scotland and estimated an overall occurrence of 1.3 infections/100,000 visitors abroad, with Egypt having the highest rate at 46.7/100,000 visitors.4

Most studies to date have found enterotoxigenic Escherichia coli, enteroaggregative E. coli, and Campylobacter to be the most frequently identified pathogens, although recent studies on infections acquired in Mexico, Guatemala, and India have identified enterotoxigenic Bacteroides fragilis and Arcobacteria to be common causes of TD.5

Noroviruses are also increasingly recognized worldwide as important causes of gastroenteritis, and their prevalence as a cause of TD is beginning to be elucidated. Koo et al studied 3 cohorts of international travelers who acquired TD in Mexico, Guatemala, and India, and identified noroviruses by RT-PCR in 10.2% of cases; this was only second to E. coli in frequency.7 The prevalence of noroviruses appeared to vary by geographic location and by time of study.

The FoodNet finding that 13% of reported enteric infections were travel-related provides useful information on predicting the risk of acquiring TD as well as the etiology of TD. With a large number of travelers to Mexico, it was the most common country of acquisition although travel to Africa had the highest risk due to the relatively lower numbers of travelers to Africa. Another relevant finding is that race and ethnicity correlated with travel destinations, which suggests that many of the reported cases might have been Visiting Friends and Relatives (VFR) travelers. This refutes the general impressions and misconceptions among some VFR travelers that they may have residual immunity due to past residence in the destination country.


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