Hunting for the elusive cure for the common cold, scientists have decoded the genomes of all known strains of the human rhinovirus, the main cause of the malady that makes millions miserable each year.
But don't toss out the chicken soup yet.
There is so much diversity among the strains that hopes for a vaccine or a treatment that would prevent or cure every cold are slim, according to the scientists' study, published online Thursday in the journal Science.
The 138 samples -- collected from human noses around the world -- offer insight into the viruses' construction and evolution, scientists said, and could eventually lead to better drugs for fighting colds.
A key finding is that rhinoviruses can swap genetic material. That means two cold strains infecting the same person may recombine to form a new strain with new properties, complicating the quest for a medicine or vaccine that would remain effective.
Researchers also found that some strains were closely matched in certain regions of the genome. These regions could make good targets for therapies, the authors said.
"We may have to have four or five drugs, and you'd need a test at your doctor's office to know which drug will work," said the senior author, Dr. Stephen B. Liggett, director of cardiopulmonary genomics at the University of Maryland School of Medicine.
Rhinoviruses -- rhinos is Greek for nose -- cause about half of all colds. (Other culprits include coronaviruses, respiratory syncytial viruses and adenoviruses.) Viruses survive by hijacking other cells to serve as hosts, and in the case of rhinoviruses, the cells they invade line the nose.
The study, by researchers at the University of Maryland, the University of Wisconsin in Madison and the J. Craig Venter Institute, analyzed about 99 known strains that had been collected from noses and stored, frozen, in a virus laboratory. The scientists also examined fresh samples collected from the field.
The common cold causes more than mere discomfort. Liggett said he got involved in the rhinovirus genome project out of frustration over a lack of progress in treating asthma, his main focus of research. Rhinoviruses exacerbate asthma attacks, and in very young children can even program the immune system to develop the disorder, he said.
Rhinoviruses are also a leading cause of children's ear infections and can be debilitating in the very elderly.
The U.S. cost of hospitalizations, doctors visits, loss of work productivity, skipped school days and over-the- counter "remedies" is estimated at $60 billion.
"From a medical standpoint, a suffering standpoint and a cost standpoint, it touches all the bases as something we should take care of," Liggett said. "But it gets short-shrifted."
Over the next three years, the researchers plan to sequence 1,000 additional rhinovirus genomes collected from noses around the country to determine which strains aggravate asthma and cause other serious infections.
Those, and not the strains that cause mere head colds, are likely to be candidates for treatment, said Ann C. Palmenberg, chairwoman of the Institute for Molecular Virology at the University of Wisconsin in Madison and the study's lead author.
But we will never eradicate the common cold, said Ronald Eccles, director of the Common Cold Centre at Cardiff University in Wales, who was not involved in the study. "Even if we knocked out the rhinoviruses, there are still many more types to take their place," he said. "As long as there are human noses, there will be viruses to infect them."