Abstract
The transmission of human norovirus excreted from infected persons occasionally causes sporadic infections and outbreaks. Both symptomatic patients and asymptomatic carriers have been reported to contribute to norovirus transmission, but little is known about the magnitude of the contribution of asymptomatic carriers. We carried out a 1-year survey of residents of a district of Bangkok, Thailand to determine the percentage of norovirus transmissions originating from asymptomatic individuals. We screened 38 individuals recruited from 16 families from May 2018 to April 2019 for GI and GII genotypes. Norovirus was detected every month, and 101 of 716 stool samples (14.1%) from individuals with no symptoms of acute gastroenteritis were norovirus-positive. The average infection frequency was 2.4 times per person per year. Fourteen genotypes were identified from the positive samples, with GII.4 being detected most frequently. Notably, 89.1% of the norovirus-positive samples were provided by individuals with no diarrhea episode. Similar to cases of symptomatic infections in Thailand, asymptomatic infections were observed most frequently in December. We detected 4 cases of NV infection caused by household transmission, and 3 of the 4 transmissions originated from asymptomatic individuals. We also identified a case in which norovirus derived from an asymptomatic individual caused diarrhea in a family member. These results suggest that asymptomatic individuals play a substantial role in both the maintenance and spreading of norovirus in a community through household transmission.
Highlights
Human norovirus (NV) is a major causative agent of nonbacterial acute gastroenteritis (AGE)
3 of the 4 cases of transmission were observed during Thailand’s winter season. These results suggest that NV may be transmitted by asymptomatic individuals in households under the influence of unknown seasonal factor(s), and that NV derived from asymptomatic individuals can occasionally induce diarrhea in another individual
We carried out a community-based survey to determine the extents of asymptomatic infection of NV and NV transmissions originating from asymptomatic individuals
Summary
Human norovirus (NV) is a major causative agent of nonbacterial acute gastroenteritis (AGE). NVs infect humans orally through direct or indirect contact with stool or vomit excreted from infected individuals [3, 8]. It has been suggested that contact with symptomatic individuals is a major route for NV transmission in a given community [9]. Outbreaks of NV often occur in semi-closed environments (e.g., nursing homes and hospitals) that facilitate viral transmission through person-to-person contacts [4, 8, 10]. Specific characteristics of NV, such as its low infectious dose and high shedding titer, have contributed to its maintenance in human communities [10, 11]. Because humans are a primary source of NV, it is assumed that NVs are retained in human communities throughout the cycle of infection and transmission
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