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Each year about this time, a vagabond virus begins a trek across the continent on a course that will, by spring, carry it from Mexico to Maine.

In its travels, the rotavirus annually causes severe diarrhea among 3.5 million youngsters, hospitalizes 70,000, and kills as many as 125. Nationwide, the disease rings up bills of more than $200 million for hospitalization.Researchers from Mexico, the United States and Canada say a five-year study of the recurring epidemics provides solid evidence that the virus moves across the United States in a ``unique pattern' - the first virus known to spread along a distinct, predictable geographic route.

Each fall, the virus begins its five-month odyssey in Mexico. Each year it dies out in Canada's Maritime Provinces.

``Regional peaks of the annual rotavirus epidemic appear to be recurrent and predictable, following a specific season,' the researchers reported in the Journal of the American Medical Association.

One of the researchers, Dr. Roger I. Glass, chief of the Centers for Disease Control's viral gastroenteritis unit, said no one knows why the virus follows the same pattern year after year.

But the network set up to track its travels could someday help monitor the effectiveness of experimental rotavirus vaccines, now being tested in 20 U.S. medical centers. Although vaccine tests now involve more than 1,000 children, no commercial vaccine is likely for at least two years.

A similar tracking system monitors emerging influenza strains, so that researchers can make vaccines to combat them. Unlike the flu, however, rotavirus does not produce new strains each year - and it emerges in a predictable pattern.

``Flu, when it comes into the United States, could come in through New York or Los Angeles,' Dr. Glass said. Rotavirus always comes in through Los Angeles and always exits through Boston and the Maritime Provinces. That's unique.'

Until recently, rotavirus was overlooked as a cause of significant illness in the United States. A 1985 Institute of Medicine assessment of the nation's vaccine development priorities ranked it as a low priority.

But in 1988, the CDC concluded that rotavirus was a possible cause of one-third of the 200,000 hospitalizations for diarrhea each year - especially those that occurred during the winter in children under the age of 3.

Up to 500 children die from complications of severe diarrhea each year and while doctors haven't confirmed that any deaths are due to rotavirus, one-third of them coincide with regional peaks in the rotavirus epidemic.

``No one considered rotavirus to be a severe disease, at least in this country,' said Dr. Glass. ``We now have epidemiological evidence that these bugs do kill American children.'

CDC epidemiologist Dr. Charles LeBaron, lead author of the study, said rotavirus commonly affects children in day-care centers, who excrete the virus before and after getting diarrhea. That means infected children can infect others before anyone recognizes they are sick.

The new evidence of rotavirus's predictability comes from a network of 104 medical centers in the United States, Canada and Mexico, which reported 34,644 cases of rotavirus-induced diarrhea.

Each year, the first surge in cases begins showing up in northern Mexico in September and spreads steadily across the southwestern United States. Cases peak in southern California by December. Georgia's rotavirus season peaks in February and tapers off in March. In Boston, New York and Canada's Maritime Provinces the illness peaks in March and April.

Despite its apparent predictability, Dr. Glass said researchers still must answer two pressing questions regarding rotavirus.

``The bottom line is, 'how does this bug spread?' ' he said. ``And why does it occur every winter?'

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