Catching Norovirus: Transmission on an Airplane
Catching Norovirus: Transmission on an Airplane
Abstract & Commentary
By Paul Trowbridge, MD, and Maria D. Mileno, MD
Dr. Trowbridge is a resident, Internal Medicine, The Miriam Hospital, Brown University, Providence, RI. Dr. Mileno is Director, Travel Medicine, The Miriam Hospital, Associate Professor of Medicine, Brown University, Providence, RI
Dr. Trowbridge and Dr. Mileno report no financial relationships relevant to this field of study.
Synopsis: Members of a New England bus tour group returning to California by plane showed symptoms consistent with norovirus infection, and several cases were confirmed by reverse-transcription polymerase chain reaction (RT-PCR).
Source: Kirking HL, Cortes J, Burrer S, et al. Likely transmission of norovirus on an airplane, October 2008. Clin Infect Dis. 2010;50(9):1216-1221.
Of 41 members of a New England bus tour group returning to California by plane, 15 had symptoms consistent with norovirus infection and 6 cases were confirmed by reverse-transcription polymerase chain reaction (RT-PCR) and found to be the same strain. Six members were symptomatic during the course of the flight, with vomiting or diarrhea. There was an increased relative risk for developing similar symptoms associated with sitting near tour group members or in an aisle seat. Eight percent of other passengers responding to the survey developed a norovirus-like illness in subsequent days, with 1 case confirmed.
This small retrospective investigation was performed with the passengers of a transcontinental flight from Boston to Los Angeles in October 2008. Telephone surveys were conducted on two groups of people on that flight: members of a New England bus tour who were flying back home to the West Coast, and the other passengers who flew along with them. The first part of the surveys sought to establish whether the tour group members had symptoms consistent with norovirus infection, when they had these symptoms in relation to the flight, and what seats and bathrooms people used. The other passengers were asked similar questions in a separate telephone survey. Additionally, this second group was asked about people around them in their work or family environments who went on to develop similar symptoms over the days following the flight. For both of these groups, if they had symptoms consistent with norovirus infection, broadly defined as vomiting or diarrhea (≥ 3 loose stools/24 hrs), they were asked to submit stool samples for norovirus testing with RT-PCR. If positive and in sufficient quantity, these stool samples were sent to the Centers for Disease Control and Prevention for genetic sequencing. Patients with symptoms consistent with norovirus infection, but either without confirmatory tests or with negative stool samples were considered probable cases. Patients with symptoms and positive stool samples were considered confirmed cases.
Within the bus tour group, some of whom instigated this study by developing severe vomiting and diarrhea while in-flight, 36 of the total of 41 people completed the telephone survey. Additionally, hospital records were available on 1 other person from the group. Of these 37 people (90% of the tour group), 15 were probable cases, notably one having diarrhea in the aisle of the first-class section, and 5 having vomiting while in their seats. Six of the tour members who were probable cases submitted stools; all were positive by RT-PCR for norovirus and confirmed to be of the same strain. The peak date of illness for this group was the day of the flight, but multiple probable, and at least one confirmed case of norovirus infection were present before the flight as well.
Norovirus infections have a mean incubation period of 24 to 48 hours, and the peak date of illness in other passenger populations was 2 days after the flight. Of this group, eighty-five people (80%) completed the survey, either by phone or by email. Seven of these passengers (8%) met criteria for probable infection, although only 1 of the 5 stool samples submitted was positive by RT-PCR, and that one was insufficient for genetic sequencing to confirm the strain. Analysis of this group's seating positions in relation to the tour group members showed a high relative risk of becoming ill if passengers were seated near a tour group passenger (defined as sitting within 1 row) or in an aisle seat; these were statistically significant. Even sitting next to an asymptomatic tour group member was associated with an increased risk of later developing symptoms. In-flight bathroom use was not associated with an increased relative risk, although this was difficult to analyze, as the timing of bathroom use was not known.
Commentary
Despite not actually providing definitive evidence of norovirus transmission, this study effectively demonstrates likely transmission. Finding the same strain of the virus in both the originally infected and the subsequently ill passengers on the plane, along with culture data showing contamination of food or fomites would have been more convincing. The findings are not surprising since norovirus is highly contagious, and airplanes are closed, crowded environments. Previous studies, as cited in this article, have also associated air travel with outbreaks of norovirus-like illnesses. However, this study went further and documented the distribution of passengers within the plane, then correlated proximity to known ill and other potentially exposed passengers with likeliness of developing symptoms. This correlation was present even for asymptomatic tour group members, suggesting possible asymptomatic transmission, which is known to occur with norovirus infections.
Based on a high correlation between sitting near a tour group member and later becoming symptomatic, the authors speculated on the mode(s) of transmission. Aerosolization, particularly of vomited viral particles that later could have been ingested, may have played a role in the spread of this infection. While interesting to speculate about in the setting of a closed, circulating air system, there are no data provided to support this. Further research could be useful, given the millions of passengers each year who board these closed aircraft systems.
Not investigated was whether passengers on subsequent flights in the same plane developed symptoms.Norovirus can persist in the environment for weeks, is resistant to many disinfectants, and has a very low infectious inoculum. In addition to suggesting general infection control practices, such as motion-sensing or foot-operated faucets, soap dispensers, and drains, in order to limit fomite-mediated transmission, it would be important to determine if general disinfection practices need revision as well. The investigators inquired about further transmission to passengers' family members and coworkers in their survey, but did not provide any additional data regarding such.
As many interesting points as this investigation raises, it should be noted, and is by the authors themselves, that this study was fairly limited. While transmission of norovirus in-flight seems likely to have occurred, "probable" cases were very broadly defined, making the study sensitive but far from specific. Even if cases had negative RT-PCRs, as was true for all but one of the cases in the second group who submitted a stool sample, they were still considered probable infections. What made this study useful was what also limited its utility: how quickly it was performed.
Two other norovirus investigations of importance to travel medicine providers have been published recently. Ajami et al. detected norovirus RNA in 30 of 320 (9.4%) acute diarrheic stool samples obtained from students who traveled to Mexico. Prevalence was higher in the winter than the summer season.1 Of particular relevance to immunocompromised transplant recipients, Schorn et al. showed that norovirus infection persisted in nine consecutive adult renal allograft recipients with or without clinical symptoms.2 Persistent viral shedding and intermittent diarrhea lasted for 97-898 days. However, unlike the study of airline passengers in close proximity, no evidence for a likely route of transmission was found.
References
- Ajami N, Koo H, Darkoh C et al. Characterization of norovirus-associated traveler's diarrhea. Clin Infect Dis 2010;51(2):123-130.
- Schorn R, Höhne M, Meerbach A. Chronic norovirus infection after kidney transplantation: Molecular evidence for immune-driven viral evolution. Clin Infect Dis 2010;51(3):307-314.
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