OREGON STATE UNIVERSITY

Report: Emerging viruses merit more concern

02/04/1998

CORVALLIS - The recent conversion of a Hong Kong "chicken flu" virus into one that proved infectious and deadly to humans may have provided a sneak preview of other emerging viruses around the world which merit more attention and concern than they are getting, one expert says.

The necessary slaughter of about 1.5 million chickens in Hong Kong and deaths of several people - from a virus that experts previously thought could never be transmitted to humans - ought to be a wake-up call for the risks posed by a different viral family called caliciviruses, says Dr. Alvin Smith, a professor of veterinary medicine at Oregon State University.

The most immediate concern, he says, is the intentional spread in Australia and New Zealand of a calicivirus linked to a fatal hemorrhagic disease in rabbits. He outlines those concerns, the history of known caliciviral diseases and emergence of new ones in two professional articles just released, including one in the journal Emerging Infectious Diseases.

"I see little difference scientifically between what happened in Hong Kong with that flu virus and what may be evolving right now with caliciviruses," said Smith, who directs the OSU Laboratory for Calicivirus Studies and this week will testify to a New Zealand legislative committee that is considering legalizing release of the rabbit virus. "The biggest difference is that in Hong Kong they took aggressive steps to control a virus deadly to animals, whereas in New Zealand they are proposing to spread one."

Smith has criticized movements in both Australia and New Zealand to openly encourage use of this rabbit disease as a biological control agent, which proponents hope will help rid the two nations of unwanted rabbit populations. And some of the science done in this area has been questionable, he says - in one letter to New Zealand officials he questioned a statistical analysis, about which he said "the numbers were treated as political prisoners and simply tortured until they gave the needed answers."

A veteran of more than two decades of study on caliciviruses, Smith says the governments, scientists and people of these two countries do not yet appreciate the risks posed by this viral family. Caliciviruses can mutate, cause different diseases, often infect multiple animal species, lurk in ocean reservoirs and suddenly emerge, he said, to cause health problems such as pneumonia, encephalitis, hepatitis, diarrhea, abortion and hemorrhage - ranging in severity from no illness to rapid death.

Cats, dogs, cattle, pigs, fish, marine mammals, birds, rabbits, primates and humans are among the hosts of caliciviruses, whose trademark is the ease with which they can jump species boundaries. One calicivirus first identified in seals is known to cause infections in fish, other marine mammals, pigs, cattle, monkeys and people.

In his latest publication, Smith and four co-authors challenge the assumption that the virus which causes rabbit hemorrhagic disease - a severe illness which causes death within a day or two from internal bleeding and blood clotting - cannot be transmitted to other animals or humans.

There are five groups within the viral family Caliciviridae, Smith said, and viruses from at least four of those groups are known to infect humans and other species. The fifth group, which includes the rabbit hemorrhagic disease virus, supposedly does not infect humans or any other non-rabbit species.

The inability to culture this virus in a laboratory setting makes it difficult to prove a causative relationship in human illness, Smith said. But a recent epidemiological study done in Australia looked at illness reported by two groups over a period of time - including 153 people who for various reasons had high exposure to rabbit hemorrhagic disease, and 116 people with no known viral contact.

When adjusted for seasonal differences, the people with high exposure reported almost twice the overall rate of illnesses; about double the amount of flu, fever, or diarrhea; and triple the neurologic symptoms or skin rashes. The odds of this occurring by chance are about one in 200, Smith said.

"We can no longer say there's no evidence that rabbit hemorrhagic disease virus can infect humans," Smith said. "We're moving closer to the conclusion that indeed it does affect humans, not further away from that conclusion. If nothing else I think the studies that have been done should shift the burden of proof to those people who maintain this virus is specific only to rabbits."

Smith said an infective calicivirus may cause either the same, or quite different disease symptoms when it shifts from one species to another. Practically no monitoring is done of caliciviruses thought to infect animals - one exception was a study done with samples from several hundred blood donors from Portland, Ore., in which about 19 percent showed antibodies indicating caliciviral exposure.

Caliciviruses first became prominent in the United States in the 1930s when one strain caused a major disease epidemic in the American swine industry. That virus was eventually linked to human disease and to marine origins, which appear to harbor a reservoir of infection and caliciviral types.

"What we need at this point is not intentional spread but more research, better assays, vaccines, analysis of the impact of caliciviruses on animal agriculture, and an understanding of how this viral family can mutate and cause disease," Smith said.