Norovirus That Wouldn't Leave: Successive Outbreaks at a Nebraska Event Center, 2017

Summary

• Norovirus is an extremely contagious cause of gastroenteritis. Infected food workers can spread it, and it is hard to remove from contaminated surfaces.
• In October–November 2017, 159 people who worked at or attended nine events at a Nebraska event center (facility A) fell ill — three confirmed and 156 probable cases.
• The facility didn't follow the first recommendations, and transmission went on for about 2 weeks. It stopped only after thorough decontamination and strict exclusion of ill employees.

How it started

On October 30, officials learned that about 30 people had fallen ill after a wedding reception at facility A on October 27 (event 1), with diarrhea, vomiting, cramps and fever some 12–48 hours later. On November 6 they heard of similar illness at five later events. Investigators then sent an online questionnaire to employees and to attendees of these and later events, and ran a case-control study.

• Probable case: diarrhea (≥3 loose stools within 24 hours) or vomiting plus at least one other symptom, starting 6–72 hours after a facility A event on or after October 27. Confirmed: the same, plus norovirus found in stool by RT-PCR.
• Responses: 378 people from nine events — 18 of 25 employees (72%) and 360 of 1,383 attendees (26%). A tenth event returned only one response among 70 attendees and was excluded.
• Result: 159 cases (six employees, 153 attendees) and 186 controls. The only items served at all nine events were water, ice and drink garnishes, and no item was linked to illness.
• Attack rates at the first six events, before any intervention, ranged from 7% to 35% (median 18.5%).

Two routes

• A vomiting episode: at the start of event 1, an attendee vomited on the lobby carpet at the entrance to the event hall — possibly how the virus arrived. The carpet was vacuumed and cleaned with a product that doesn't work against norovirus. Vacuuming can also throw the virus into the air.
• A sick food worker: an employee fell ill at work on November 7 with nausea, vomiting, fever, headache and muscle aches, and was back preparing and serving food on November 8, <24 hours later.

On November 7, investigators advised cleaning with chlorine bleach or a disinfectant the Environmental Protection Agency lists as effective against norovirus, and keeping ill employees home until ≥48 hours after symptoms end. The facility did minimal cleaning and didn't exclude ill staff, and illness continued: attack rates were 4% (six of 150) at event 7 on November 10 and 15% (53 of 360) at event 8 on November 11.

The lab: norovirus genogroup II was found in all three stool samples tested. Sequencing by the Nebraska Public Health Laboratory and CDC showed they were the same genotype, GII.P12-GII.3 — two from event 1 and one from event 8.

Cases by event and date of illness onset. Figure from the CDC report.

What finally worked

Before a planned event on November 18 (event 9), officials repeated the call for strict exclusion on November 15 and suggested ways to take the pressure off employees to work while sick — paid sick leave, or bringing in staff from another location. Postponing or moving the event was also considered. Facility A hired a professional cleaning service experienced with norovirus to sanitize the building on November 16 and 17. At event 9, the attack rate was 1% (three of 350), and no further illness was reported.

Why norovirus is so stubborn

• It's the most common cause of outbreak-associated acute gastroenteritis worldwide, and as few as 18–2,800 virus particles can cause infection.
• Infected people — even those without symptoms — can shed large amounts of virus.
• It resists many common disinfectants and can persist on surfaces for up to 2 weeks.
• It spreads through food, person to person, airborne particles and contaminated surfaces, often by several routes at once.

Limits: attendance at each event was the host's estimate, and hosts passed on the questionnaire, so exact attack rates couldn't be calculated. No environmental samples were taken: by the time officials learned the outbreak was ongoing, 10 days had passed since the vomiting episode, and testing wouldn't have changed the advice to clean thoroughly.

The lesson

For an ongoing norovirus outbreak in a similar setting, act early against every route: extensive sanitation and excluding all ill food handlers for at least 48 hours after their symptoms end. Health officials can also check that facilities are actually carrying out the recommendations.

Sources

Based on Rebecca J. Free, Bryan F. Buss, Samir Koirala and others (CDC, the Nebraska Department of Health and Human Services and the Sarpy/Cass Health Department), "Successive Norovirus Outbreaks at an Event Center — Nebraska, October–November, 2017," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain.