Sources of Airborne Norovirus in Hospital Outbreaks

Abstract

Background: Noroviruses are the major cause of viral gastroenteritis. Disease transmission is difficult to prevent and outbreaks in health-care facilities commonly occur. Contact with infected persons and contaminated environments are believed to be the main routes of transmission. However, noroviruses have recently been found in aerosols and airborne transmission has been suggested. The aim of our study was to investigate associations between symptoms of gastroenteritis and the presence of airborne norovirus, and to investigate the size of norovirus-carrying particles.

Methods: Air sampling was repeatedly performed close to 26 patients with norovirus infections. Samples were analyzed for norovirus RNA by reverse transcription quantitative polymerase chain reaction. The times since each patient's last episodes of vomiting and diarrhea were recorded. Size-separating aerosol particle collection was performed.

Results: Norovirus RNA was found in 21 (24%) of 86 air samples from 10 different patients. Only air samples during outbreaks, or before a succeeding outbreak, tested positive for norovirus RNA. Airborne norovirus RNA was also strongly associated with a shorter time period since the last vomiting episode (odds ratio 8.1; P = .04 within 3 hours since the last vomiting episode). The concentrations of airborne norovirus ranged from 5-215 copies/m3, and detectable amounts of norovirus RNA were found in particles <0.95 µm and >4.51 µm.

Conclusions: The results suggest that recent vomiting is the major source of airborne norovirus and imply a connection between airborne norovirus and outbreaks. The presence of norovirus RNA in submicrometre particles indicates that airborne transmission can be an important transmission route.

Keywords: airborne transmission; bioaerosol; hospital; norovirus; vomiting.

Figure 1.

Percent of NoV-positive air samples in relation to time since (A) last vomiting episode and (B) last diarrhea. The area of each bubble is proportional to the total number of air samples within each time interval. The values above each bubble represent the number of positive and total air samples. Abbreviation: NoV, norovirus.

Figure 2.

Odds ratios for positive air samples within 3 hours from last symptom versus no symptom within 3 hours. The sensitivity analysis was performed using logistic regression with a random effects model to account for dependency between samples from the same individuals. Abbreviation: CI, confidence interval.

Figure 3.

Size ranges of norovirus-positive aerosol samples (dark green) from the cascade impactor during 3 different outbreaks (negative stages are colored gray). During the fourth outbreak, all stages were negative. Note that the width of the stages is on a log scale.