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Scientists Have New Evidence on Viruses

October 30, 1986

WASHINGTON (AP) _ Scientists say they have the first direct evidence that two mild viruses can combine in a simultaneous infection to produce a virulent disease virus within an animal.

Researchers at the University of California, Los Angeles, Medical School say the findings could help explain how some disease-causing viruses originated, as well as how infection with weak viruses could lead to serious disease within a host.

Dr. Jack G. Stevens, chairman of microbiology and immunology at the school, said in a telephone interview that the work shows that the viral combinations seen in laboratory experiments also can occur naturally in mammals.

″It’s possible that this same thing could play a role in natural human disease,″ Stevens said. ″At some point in the past, some benign viruses could have exchanged genes in this way to produce a different virulent virus that causes disease today.″

However, he noted, while this kind of viral mixing may be responsible for potent disease germs in an historical sense, it is highly unlikely that any individual could get a simultaneous infection in such a way as to have a deadlier virus form in his or her body.

In a report to be published in the Nov. 7 issue of the journal Science, the UCLA researchers say they injected mice with strains of herpes simplex virus 1 (HSV-1) to see if they could duplicate laboratory findings in animals. In cell cultures, HSV-1 readily exchanges genetic information to produce different strains.

When two mild virus strains were injected at the same site simultaneously, the report said, the combination proved 100 times more lethal to animals than either virus alone.

HSV-1 is a common human disease virus that causes cold sores. In this country, it also is the leading cause of blindness by infection as well as the most common cause of sometimes-lethal brain inflammation called viral encephalitis, Stevens said.

Most strains of herpes viruses invade the nervous system and cause disease, the researchers said. When HSV-1 is injected into the footpads of mice, it enters peripheral nerves, travels through the central nervous system and generally causes a fatal encephalitis.

However, two HSV-1 strains, called ANG and KOS, are atypical, the report said. While these viruses invade nerves, they do not readily cause fatal disease.

Stevens, working with graduate student Ron T. Javier and Dr. Farhad Sedarati, used the viruses in two experiments with 26 mice.

When an equal mixture of the two viruses was injected into the rear footpads of mice, 62 percent died, the study found. No mice died when inoculated in the same way with either virus alone or in separate feet simultaneously with different strains.

The scientists injected 14 viruses isolated from the brains of 10 of the dead mice into other mice and 4 of these viruses produced lethal infections.

Studies of the genetic structure of these new deadly viruses proved that at least 3 of the 4 were so-called recombinant viruses, meaning that they contained genes combined from those of the two original, non-lethal viruses, the report said.

″To our knowledge, this is the first evidence that during a mixed infection, two avirulent viruses can interact in an animal to produce disease,″ the report said.

Stevens noted that the experiment did not produce a super virus.

″We made a virus that is no more virulent than one you could find in nature,″ he said. ″But we showed that you could get a virus different from the originals.″

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