Washington University researchers are the first to grow a common cause of diarrhea, vomiting and other stomach problems in a lab, a move that a North Carolina expert said could speed along development of a vaccine, the university said Monday.
Food poisoning caused by the norovirus sickens an estimated 23 million Americans each year, according to the Atlanta-based Centers for Disease Control and Prevention. About 50,000 people are hospitalized and an estimated 400 die, epidemiologists believe.
Washington University scientists showed that the mouse norovirus MNV-1 could be grown inside cells from mice with defective immune systems. The university said those findings could ease the way for further research about the mouse virus and may help researchers seeking to duplicate the accomplishment with human forms.
Details are published this week in the online journal Public Library of Science-Biology.
The Washington University finding "allows you to study everything about the basics of virus replication, the role of the various genes and how the affected host responds to infection," said Ralph Baric, a professor of epidemiology at the University of North Carolina's School of Public Health and a leading researcher into the norovirus.
"It allows you to use it as a model system to study vaccine efficacy. Culturing norovirus is a big deal."
In fact, scientists who developed the technique say they may already have a clue into vaccine development.
"By looking at the mouse virus we'd grown in the lab, we were able to identify a part of the capsid, the virus' protective shell, that is essential to its ability to cause disease," said Skip Virgin, professor of pathology, immunology and molecular microbiology.
"If this part of the capsid has an equivalent in human noroviruses, altering or disabling it may give us a way to produce forms of the viruses that are weak enough to serve as vaccines."
The norovirus earned its name after an outbreak in 1968 at a school in Norwalk, Ohio. In recent years, it gained attention due to repeated outbreaks on cruise ships. This summer, 134 people became ill in an outbreak at Yellowstone National Park.
It is most commonly spread through eating contaminated food, touching contaminated surfaces and having direct contact with infected people. The illness tends to last 24 to 48 hours.
Though deaths are rare in industrialized nations, infections spread rapidly, are difficult to prevent from spreading, and can create significant discomfort. In developing nations, noroviruses are a major cause of human illness.
Last year, Christianne Wobus and Stephanie Karst, postdoctoral fellows in Virgin's lab, identified MNV-1, the first known mouse norovirus. In tests in mice, researchers found that the virus thrived in macrophages — immune system cells that normally engulf and destroy pathogens — and in dendritic cells, or sentry-like cells that pick up and display proteins from pathogens.
"We think there may be dendritic cells just beneath the lining of the human gut that are providing the gateway the virus needs to cause disease," Virgin said.
Comparisons of MNV-1 and human noroviruses have revealed similarities in gene sequence, structure and overall arrangement of the genome, researchers said.
But Virgin said differences between mouse and human physiology may impact MNV-1's interactions with its host. Mice, for example, apparently cannot vomit. And researchers aren't sure if MNV-1 can make mice with normal immune systems sick.