Viral evolution and possible genotype-specific waning of immunity following natural infection place adult travelers, and non-travelers alike, at risk of acquiring novel norovirus infections and subsequent illness (withstanding the possibility for a certain level of innate protection against specific norovirus genotypes due to genetic determinants of histo-blood group antigens (HBGAs) on the gut epithelial surfaces). Sporadic/endemic norovirus-related illness is common, as shown through cohort-based community studies, as is viral detection in the environment. Norovirus transmission in such situations can be difficult to control due to its low infective dose, environmental hardiness, and multiple routes of transmission (person-to-person as well as through contaminated food, water, and/or fomites). Further, vomiting may facilitate transmission in contained settings, such as cruise ships, where norovirus has been identified as the responsible pathogen in approximately 97% of AGE outbreaks reported to the US-based Vessel Sanitation Program. It is important to recognize that norovirus causes vomiting in a substantial proportion of cases in the absence of diarrhea, thus the term AGE is most relevant for describing clinical symptoms related to this infection. Of the seven recognized norovirus genogroups, genogroup I (GI) and II (GII), cause the majority of human illness with genotype GII.4 responsible for most outbreaks in recent years. While norovirus incidence rates among travelers are rarely reported, prevalence estimates among individuals with TD and/or AGE range from 3 to 65% depending upon factors such as diagnostic methods, population, and viral activity level, and co-infections are commonly reported. Noroviruses are a leading cause of AGE globally across all age groups and have, further, been identified as an important cause of non-bacterial TD and AGE among travelers. However, under-ascertainment remains a concern because of limited post-travel healthcare-seeking behavior due to factors such as differences in the clinical course of infection (i.e., relatively milder severity and chronicity of infection) and limited viral testing in both clinical practice and research. Awareness of viral causes of travel-acquired AGE has increased along with improved and more readily accessible diagnostic methods. Parasitic infections, such as Giardia lamblia, are generally less frequent but important causes of persistent gastrointestinal symptoms and may contribute a relatively larger proportion of post-travel diarrhea cases. have lately become less frequent in on-site assessment, but may persist in returning travelers. coli (EAEC), Campylobacter jejuni (particularly in Southeast Asia), are still identified most frequently in patients abroad with AGE/TD, while Shigella spp. Bacterial pathogens, such as Enterotoxigenic Escherichia coli (ETEC), Enteroaggregative E. Our understanding of their individual contribution reflects endemic circulation within the travel destination, as well as study design and diagnostics such as choice of study population, targeted pathogens, test performance for targeted pathogens, and adequacy of biological sample(s) with respect to optimal detection. Pathogens implicated in travel-acquired AGE include bacteria, parasites and viruses. AGE - usually described as diarrhea - is still the leading diagnosis of ill returning travelers seeking medical care, as well as illness while abroad among cohorts of travelers from high-resource settings. TD results in a substantial proportion of incapacitation among patients, but there are limited data in many areas. While there is evidence that TD incidence has declined in many regions, reported incidence rates for travelers to resource-limited destinations still exceed 20% in the initial 2 weeks. In AGE as a broader term, vomiting and other acute abdominal symptoms may be the leading symptoms with or without diarrhea. abdominal cramps, nausea, vomiting) has been estimated to occur in up to 50% of international travelers during the initial 2 weeks of travel, depending upon study methods, population, and destination(s). Travelers’ diarrhea (TD) (loose/watery stools as dominate symptom while traveling or upon return which may include other symptoms, e.g. In international travelers from high- to low-resource settings, poor local sanitation and hygiene result in transmission of enteric pathogens to susceptible travelers. Acute gastroenteritis (AGE) remains an extremely common problem among the general population and among international travelers.
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