Fragments of the worm were found in an AIDS patient who died from the infestation, but they were not identified until a year later. The worm had molecular biology resembling that of a Tapeworm but acted more aggressively in the body than most tapeworms.
Doctors initially thought the patient was suffering from a rapidly growing cancer in his abdomen. But a biopsy and the use of new genetic laboratory techniques found that the large mass of tissue was from the worm's fast growth, which caused scar tissue.
In reporting the discovery in the international journal The Lancet, Stanford University scientists said they wanted to alert doctors to the worm's existence so they might treat anyone suspected of having it with drugs known to be effective against similar parasites.
The scientists who solved the puzzle are from both Stanford and the Kaiser-Permanente Medical Center in Santa Clara, Calif. The authors said they did not find the entire worm, which has not been named, but identified it from fragments by using the genetic techniques.
The scientists do not know how often the new worm causes disease in humans and where the microbe exists in nature, Dr. David Relman, a co-author, said in an interview.
In recent years, scientists have identified a number of new microbes, including one called cyclospora, which is causing intestinal illness in at least 11 states. Health officials have tentatively linked many such cases to eating fresh fruits like raspberries and strawberries.
The man who died from the worm infection was a 44-year-old accountant from the San Francisco Bay area who had been infected with the AIDS virus for five years. He often went camping in California and had two dogs but had never traveled outside the United States.
In early 1994 he began suffering pain in his abdomen and back, weight loss, sweating at night and fever. After the man was admitted to the hospital in Santa Clara in March, his abdomen swelled as the worm invaded and destroyed parts of his intestine and liver. Doctors initially thought he had developed one of the many types of cancer that often complicate the course of someone with AIDS. Shortly before his deaths nine weeks later, doctors performed a surgical procedure to remove tissue from his abdomen.
Pathologists headed by Dr. Luis Fajardo examined slides containing thin slices of the man's tissue under a microscope. Some were stained with chemicals to help identify a known microbe. But the experts could not determine precisely what it was in the tissue that caused the man's death, although the damage seemed to be caused by a parasitic infection. It then took about a year to solve the mystery.
Slides were sent to pathologists and experts in cancer and infectious disease around the country. Some infectious disease specialists thought it was a cancer. But some cancer experts thought it was an infection.
The cells were too small to be of human origin. But they were unlike those of any known parasite.
Eventually a chemist at Stanford found unusually large amounts of silicon in the tissue, a finding inconsistent with human tissue.
About 10 months after the man's death, Dr. Fajardo asked a Stanford colleague, Dr. David Relman, to help solve the mystery by using a strategy he had developed and used to identify a new microbe as the cause of a rare illness called Whipple's disease a few years ago.
Dr. Relman used a molecular technique, polymerase chain reaction (P.C.R.), to create millions of copies of some particularly useful sequences of DNA from the suspected microbe. Since such DNA blueprints vary in various species they are used to help classify microbes.
But the sequences from the man's tissue did not match any stored in genetic data bases. The sequences were distinct from human DNA and placed the microbe in the Tapeworm class, Dr. Relman said. Sequences have been determined for far fewer disease-causing Tapeworms than for disease-causing bacteria.
"We don't know whether this is an organism that has been recognized previously and may be quite well known but has not had its sequence determined, or is truly novel," Dr. Relman said. "But we think it is a truly novel organism because the pathology is unusual and has not been published before."
It is not known whether the AIDS-weakened immune system had any effect on the infestation's course.