Feb 13, 2007 (CIDRAP News) – A team of researchers has achieved what has been until now a frustratingly elusive goal: a tissue-culture model that allows natural growth in the lab of norovirus, one of the most common and least understood causes of gastrointestinal illness worldwide.
Though it causes up to 23 million cases of illness each year, according to the Centers for Disease Control and Prevention (CDC), surprisingly little is known about how the virus attaches to and replicates within cells. The new work by Timothy Straub of the Pacific Northwest National Laboratory and collaborators from Tulane University, the University of Arizona, and Arizona State University should change that: They produced a three-dimensional culture of multiple cell types that mimics the epithelium of the human small intestine, and induced norovirus samples isolated from patients to grow and replicate in it.
"This is an important result," said Craig Hedberg, PhD, associate professor of environmental health sciences at the University of Minnesota School of Public Health. "Up until this point, we have never had a direct measure that would allow us to know how effective any of our environmental prevention measures are against norovirus. There have been a lot of studies looking at things that might be able to kill norovirus and render foods and environmental surfaces safe from contamination, but they have always used surrogates."
Norovirus has been a difficult organism to study because, like other viruses but unlike bacteria, it will not reproduce in a simple growth medium. Instead, it requires a tissue culture resembling cells in the organisms it infects.
That lack of a lab model for studying the virus has kept testing protocols, effective sanitizing and control measures, and even newer diagnostic tests out of reach. Those are important because norovirus causes such a high disease burden: It is thought to be responsible for at least half of all foodborne outbreaks of gastroenteritis every year, according to the CDC.
"Nobody knows what the incidence of this disease is in the population because it is not easy to diagnose—it is just one of the range of 'stomach flus' that people get," said David Ozonoff, PhD, emeritus chair of the department of environmental health at Boston University School of Public Health. "But it causes very substantial economic loss, because so many people stay home from work because they are sick or their kids are."
To create the model, Straub and collaborators grew human intestinal epithelial cells on collagen-coated microbeads that were tumbled in a rotating reactor vessel. They used the resulting tissues for five passages of two genotypes of norovirus that were originally isolated from patients during outbreaks on a cruise ship and in a nursing home, and proved the presence of norovirus by multiple assays following each passage.
The work, which will be published in the March edition of Emerging Infectious Diseases and was posted online ahead of print, represents the first lab model of human norovirus infection; previous models used mouse noroviruses or a related feline virus. Scientists not involved in the research said it could point the way to a better understanding of an under-appreciated pathogen.
The authors write that developing a method for culturing human norovirus in the lab was a necessary first step in the effort to understand the virus's pathogenesis. In future research with the model, they hope to identify protective immune responses and learn more about how the virus replicates, with the aim of devising better prevention measures.
Norovirus spreads through the fecal-oral route, via both food and water, but there are also indications that it can spread via environmental contamination and direct person-to-person transmission, according to the CDC.
It is fiercely contagious: Ingesting as few as 10 virus particles can cause infection, and infected persons can shed virus for up to two weeks after symptoms end, the CDC says. The illness is miserable, with nausea, diarrhea and vomiting multiple times per day. Symptoms usually last from 24 to 60 hours.
"Beyond the nuisance value to the individual, it is such a widespread illness and so persistent in institutional settings where you're dealing with immune-compromised populations that it becomes an important public health problem," Hedberg said.
The CDC does not conduct routine surveillance for norovirus, so there is no way to confirm how commonly the bug occurs. So far this year, however, large outbreaks have been reported at the Scripps Research Institute in California, at Radford University in Virginia, among customers of a south Florida restaurant, in hospitals in Saskatchewan, Massachusetts, and North Carolina, and among hundreds of passengers on the ocean liner Queen Elizabeth 2.
The Hilton Hotel near Dulles International Airport outside Washington, D.C. was hit so hard in mid-January, with 120 guests and staff sick, that it was forced to close for a floor-to-ceiling sanitizing. Some norovirus outbreaks, such as on cruise ships, have recurred despite repeated rounds of aggressive cleaning.
Straub TM, zu Bentrup KH, Coghlan PO, et al. In vitro cell culture infectivity assay for human noroviruses. Emerg Infect Dis 2007 Mar; 13(3) (early online publication) [Full text]
CDC information on norovirus