Meanwhile, the president of the American Postal Workers Union said he planned to tell all its members to refuse to work in buildings where any trace of anthrax remains.
Two Washington postal workers have died since a newspaper photo editor in Florida fell victim to inhalation anthrax in October. The postal workers are believed to have developed the disease after coming in contact with a potent anthrax-laced letter addressed to Sen. Tom Daschle last month.
Aside from the case of Kathy Nguyen, a 61-year-old New York hospital worker who succumbed to the disease on Oct. 31, and Ludgren, all other inhalation anthrax cases have been restricted to people associated with members of the government and the media.
But Bill Burrus, president of the 360,000-member American Postal Workers Union said, "it's a continuing concern that so much uncertainty continues to exist regarding the source of these infections."
"I'm telling my members we will not work in contaminated facilities," he said, because experts differ widely on how much anthrax is needed to cause an infection.
Postal facilities in New Jersey and Washington remain closed for decontamination. Nationwide, the U.S. Postal Service has tested 278 facilities for anthrax and found some contamination at 21 of them. Nineteen have been decontaminated and reopened.
Anthrax Goes International
As the postal service and its workers debated the presence of contamination in their facilities, a doctor at a children's hospital in Santiago, Chile, appeared to be the target of the first case of anthrax by mail outside the United States.
A letter tainted with anthrax was received by Dr. Antonio Banfi, a pediatrician at a children's hospital in Santiago, the CDC said on Thursday.
The Chilean Health Ministry said Banfi, who opened the envelope, and 12 others nearby were tested for exposure to anthrax spores, but the tests came back negative.
They were nevertheless being treated for the disease as a precaution.
Chilean officials said Banfi became suspicious because the letter was postmarked in Zurich but marked with a Florida return address. No other details were made available.
There have been other reports of anthrax-tainted letters being received around the world, but most have turned out to be false alarms — including ones in Kenya, the Bahamas and at Pakistan's largest newspaper.
In Argentina, anthrax spores were discovered in mail, but tests determined that they were a harmless strain of the bacteria.
See a timeline of the anthrax attacks and the investigation.