- The Washington Times - Thursday, November 1, 2001

Top scientists yesterday declared that no one knows the number of anthrax spores that must be inhaled to cause death, although government officials and the media repeatedly have put the number at 8,000 to 10,000.
Knowing the lethal dosage has become "critical," says Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
That's because trace amounts of anthrax have turned up at increasing numbers of postal facilities and government offices and because of the mysterious death from inhaled anthrax of a New York hospital worker who likely encountered just small amounts of the germ.
"I'm, quite frankly, uncomfortable with this empirical number of 8,000 to 10,000," Dr. Fauci said yesterday on NBC's "Today" show. "Certainly the studies that were done the experimental studies in animal models indicate … that's the critical point in number … [but at] this point in time, you have to keep an open mind. … If you have 50,000 spores, there's no question that that could do it. If you have one, or two, or three, most people feel that won't do it. But it's the area in between that's the real gray zone now, and that has people scratching their heads."
Harvard microbiologist Matthew Meselson, one of the world's leading authorities on human anthrax outbreaks, says the number is "just plain wrong."
"No one knows how many anthrax spores it takes to kill a small percent of monkeys much less humans."
Mr. Meselson points out that no studies of humans have been done.
Kathy Nguyen, 61, who died in New York yesterday, was not involved in sorting or handling large amounts of mail and worked in an area said to be anthrax-free.
Despite the mystery, her case provides a dramatic illustration of the difference between the anthrax to which victims of bioterrorist attacks are exposed and the "natural" type found in farmland that could be expected to routinely infect farmers and ranchers.
"Anthrax is not prevalent in the United States, but you could say it's not uncommon in a few states where it affects cattle," said Gary Weber of the National Cattleman's Beef Association. "I suspect the main reason it doesn't affect humans is that spores in soil are not in a form that would predispose them to be inhaled."
The anthrax that killed Mrs. Nguyen was engineered to be readily inhaled. It was finely milled sliced and diced to be from one to two microns in size. It apparently was treated with some special additive that reduced the spores' natural tendency to bunch together and enabled them to hang unseen in the air.
That much about the weapons-grade anthrax is known, and study of the recent and unprecedented anthrax assaults eventually may reveal the lethal dose of inhaled anthrax.
Meanwhile, scientists can only guess. As Mr. Meselson explains, the 8,000-spore figure is based on almost irrelevant experiments conducted at Maryland's Fort Detrick in the 1950s.
The military wanted to know how much inhaled anthrax was required to kill 50 percent of the 1,000 monkeys in its trial. It came up with the 8,000 to 10,000 number.
"But they couldn't tell how much would be required to kill a smaller percent of the monkey population, because finding the smaller percentage actually required using a greater number of monkeys," Mr. Meselson says. "You can't get enough monkeys to calculate the amount of anthrax needed to kill 10 percent, for instance."
Mr. Meselson adds: "We want to know about humans. And which humans? It may take more or less anthrax to kill older persons than younger ones, or persons of different races and ethnic groups."
Mr. Meselson studied a famous 1979 anthrax release that occurred downwind of the Soviet Union's biological warfare center at Yekaterinburg, which was then known as Sverdlovsk. At least 66 persons died. He says, "What we found in Russia was that nobody died below age of 24. Which already tells you not all people are the same. So far, no one infected here has been young. What we see in the United States begins to look the same."


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