Why Does Norovirus Spread So Easily?

Patient Presentation
A 19-month-old male came to clinic with a 24-hour history of emesis and diarrhea. He has vomited 3 times but was having frequent watery stools that were non-bloody. His mother said she was changing his diaper as soon as she was able to and would estimate that the diaper was 1/4-1/2 soaked with fluid. His mother wasn’t sure if he was having wet diapers but said that she thought she could smell urine with some of the diapers. He was still drinking and she had started offering him flavored oral rehydration solution. He did attend daycare and the mother said she knew of several other children with diarrheal illnesses. The past medical history was non-contributory.

The pertinent physical exam showed a slightly tired boy. His weight was up 350 grams from a weight 6 weeks previously, and his vital signs were normal. His capillary refill was 1-2 seconds with moist lips and tearing. His examination was otherwise unremarkable with a soft abdomen but increased bowel sounds.

The diagnosis of acute gastroenteritis was made. The mother was counseled to monitor him closely for potential dehydration and increased symptoms. A stool culture was performed to identify the potential outbreak and norovirus was identified the following day. The mother was contacted and said that the diarrhea had already markedly decreased and he had not had any more emesis. Public health was contacted about the norovirus in the daycare setting.

Norovirus is a Caliciviridae family member. They were first observed by electron microscopy during a 1968 outbreak in Norwalk Connecticut (hence the alternative name of Norwalk virus). There are 3 genogroups and many genotypes within each genogroup. “Noroviruses are the leading cause of acute gastroenteritis in people of all ages worldwide, and are estimated to cause 12-24% of community-based or clinic-based cases…, 11-17% of emergency room or hospital cases, and approximately 70,000-200,000 deaths annually.” Seventy percent of cases occur in the 6-23 month age range. They have caused pandemics usually at 2-3 year intervals over the past 20+ years.

Clinically patients may be asymptomatic and it not be recognized and they can also have several infections overtime as the active immunity wanes overtime. Patients often have non-bloody diarrhea, emesis, abdominal cramps and fever. Severe diarrhea can lead to dehydration and its severe complications of hypotension and shock. Incubation period is 12-72 hours with a duration of symptoms being self-limited (12-48 hours) but up to 2-5 days in immunocompromised individuals. However it can last much longer. Norovirus cannot be specifically differentiated without testing, usually by direct viral detection.

Treatment includes prevention measures including hand-hygiene and general sanitation. Disinfection of surfaces can be accomplished with a chlorine bleach solution of 1:50-1:10 dilution of household bleach (of 5-25%). Oral rehydration is a mainstay to prevent or treat diarrhea. Intravenous fluids may also be needed for fluid resuscitation. Again hygiene procedures and exclusion of ill persons are important. As food preparation is a common transmission method, use of gloves for preparing and distributing food in public settings is important. Immunity to infection does occur but usually is limited. Culturing of the virus has also been difficult until more recently. Therefore it is has been difficult to develop vaccines. However, candidate vaccines are being evaluated in clinical trials.

Learning Point
As norovirus is spread in feces, the transmission is through the fecal-oral route, including hands, utensils, ready-to-eat foods, and it is thought that oysters can concentrate norovirus from contaminated water.
Norovirus is highly contagious because:

  • Low numbers of 18-1000 particles can cause an infection
  • Extended time period of viral shedding (usually 2-5 days but up to weeks)
  • Particles are hearty and can remain in the environment including a wide range of temperatures (0 – 60°C)
  • Virus is mutable so new strains can emerge

Questions for Further Discussion
1. What are other common causes of acute gastroenteritis?
2. What infectious causes of acute gastroenteritis are reportable to public health in your area?
3. What electrolyte balance is recommended for oral rehydration solutions?

Related Cases

To Learn More
To view pediatric review articles on this topic from the past year check PubMed.

Evidence-based medicine information on this topic can be found at SearchingPediatrics.com and the Cochrane Database of Systematic Reviews.

Information prescriptions for patients can be found at MedlinePlus for these topics: Norovirus Infections and Gastroenteritis.

To view current news articles on this topic check Google News.

To view images related to this topic check Google Images.

To view videos related to this topic check YouTube Videos.

Shah MP, Hall AJ. Norovirus Illnesses in Children and Adolescents. Infect Dis Clin North Am. 2018;32(1):103-118. doi:10.1016/j.idc.2017.11.004

Banyai K, Estes MK, Martella V, Parashar UD. Viral gastroenteritis. Lancet. 2018;392(10142):175-186. doi:10.1016/S0140-6736(18)31128-0

Cates JE, Vinje J, Parashar U, Hall AJ. Recent advances in human norovirus research and implications for candidate vaccines. Expert Rev Vaccines. 2020;19(6):539-548. doi:10.1080/14760584.2020.1777860

Donna M. D’Alessandro, MD
Professor of Pediatrics, University of Iowa