Sporadic Pediatric Norovirus Cases May Predict Broader Outbreaks

Seasonal increases in sporadic pediatric cases of norovirus gastroenteritis correlate with norovirus outbreaks among older populations, a new study found.

The study, published in Clinical Infectious Diseases, analyzed seasonal patterns and genotypic characteristics of norovirus cases between December 2012 and June 2016 in middle Tennessee.

“Sporadic case surveillance and outbreaks followed very similar patterns geographically and temporally,” John R. Dunn, DVM, PhD, state epidemiologist with the Tennessee Department of Health, told Contagion®. “These commonalities in the different surveillance systems indicate that opportunities may exist to slow or prevent outbreaks when sporadic cases start to increase in the community.”

During the study period, 755 pediatric sporadic norovirus cases and 45 outbreaks involving 1924 people were reported.

The mean age of sporadic pediatric cases was 2.9 years, 81.3% were among children younger than 5, and 30% reported attending childcare facilities.

Among 740 outbreak cases with reported ages, 61.6% were in people older than 50, and 42% of outbreaks occurred in long-term care facilities. Childcare facilities and restaurants each accounted for 8.9% of outbreaks. Person-to-person transmission was reported in 80% of outbreaks and 8.9% were reported as foodborne transmission.

“Predominant genotypes explained the majority of both surveillance cases and outbreaks,” Dunn said.

Most cases were attributed to GII genogroup noroviruses, including 90.1% of sporadic cases and 83.3% of reported outbreaks, with GII.4 being the most prevalent genotype, reported among 39.1% of sporadic cases and 52.8% of outbreaks.

The proportion of cases infected with GII noroviruses was significantly higher in the winter than in the summer. Seasonality wasn’t reported among cases involving GI noroviruses, which are more strongly associated with foodborne and waterborne transmission.

The study found that most norovirus infections occurred between November and April, including 74.9% of sporadic cases and 86.8% of reported outbreaks.

“During the winter months, providers and clinicians should consider norovirus in patients with acute gastroenteritis and counsel patients regarding preventing transmission in settings where vulnerable populations reside, eg, long-term care facilities,” Dunn said.

The study used Markov modeling to examine whether detecting peaks in norovirus transmission could forecast norovirus outbreaks in the wider community. Results suggested that the 3-day moving average of sporadic cases was positively associated with the probability of an outbreak. However, data for outbreaks was insufficient to draw a statistically significant conclusion.

“Further studies to describe the interrelationship of sporadic norovirus and outbreaks are needed,” Dunn said. “These studies will help elucidate transmission dynamics and potential populations for candidate vaccines.”

Norovirus infections, which are the leading cause of gastroenteritis, caused about 900 deaths, 109000 hospitalizations, 465000 emergency department visits, and 2.27 million ambulatory clinic encounters annually from 2001-2015, with annual health care costs estimated at $430 million to $750 million, according to a US Centers for Disease Control and Prevention study published earlier this year.

Several norovirus vaccine candidates are in development, boosted by the discovery of an antibody that inhibits multiple strains of norovirus. The antibody, A1431, is able to neutralize numerous strains of the virus.

Advancements in detection also have supported the fight against the virus. Last year, investigators at the University of Arizona unveiled a smartphone-based device that uses a fluorescent microscope assay to detect norovirus in water.