By analyzing DNA from a strange mass of tissue found in a man's abdomen, researchers have discovered a previously unknown parasite that can infect and kill humans. The researchers have yet to name the parasite or determine what it looks like, but they believe that it may be in the same class as tapeworms--although it appears far more aggressive.
The parasite's mode of transmission and natural host remain unclear, the researchers said.
The organism has been detected in only one person, a man with AIDS who lived in the San Francisco Bay area. The man died at age 44 from the parasitic infection. After his death, researchers found that the parasite had formed two large masses composed of many sacs of unusual cells surrounded by fibrous tissue. One mass was in the man's intestine and adjacent tissues; the other was in his liver.
The researchers reported their findings in the June 29 issue of The Lancet. The authors include Dr. Monica Santamaria-Fries, a pathologist at the Kaiser-Permanente Medical Center in Santa Clara, California; Dr. Luis Fajardo, a professor of pathology at Stanford; and Dr. David Relman, an assistant professor of medicine at Stanford. Fajardo and Relman work at both the Stanford University School of Medicine and the Veterans Affairs Palo Alto Health Care System.
Discoveries of human parasites are quite rare. The most recent was the discovery of a parasite called Cyclospora about 3 years ago, according to Dr. Paul Basch, a professor of health research and policy who teaches medical parasitology at Stanford.
"If this new finding is confirmed it would certainly be a significant addition to medical knowledge," said Basch, who was not involved in the new research.
"Now that this has been reported, it alerts people to its existence. That could lead to more reports of the parasite in the future," he added.
"The organism very likely has existed in nature for a long time. We don't know why this disease and pathology have never been described in the scientific literature before," said Fajardo, chief of the pathology service at the Veterans Affairs Palo Alto Health Care System.
"One possibility is that the parasite has only recently acquired the opportunity or the ability to infect humans and invade visceral tissues. Another possibility is that in the past, the mass of tissue the parasite creates had been mistaken for something else--perhaps a lymphoma, which grossly, it very much resembles," Fajardo said.
Also, the parasite might behave unusually in people with weakened immune systems, such as this man with AIDS. "It's possible that in people with healthy immune systems, the parasite is unable to cause such invasive disease," Fajardo said.
In March 1994, the patient was admitted to the Kaiser center with a 2-month history of abdominal and lower back pain, vomiting, weight loss, night sweats, and fever. After finding a large mass in his abdomen, physicians performed a biopsy. Despite treatment, the man died 9 weeks later. The unusual tissue had spread to his liver and caused his death, Fajardo said.
When Kaiser pathologist Santamaria-Fries examined the tissue sample, she was unable to explain what she saw. She sent a sample to Stanford's Fajardo, a specialist in identifying parasitic infections, and asked his opinion. Fajardo did not know what to make of the tissue.
"Usually it takes me about an hour to figure out which microorganism is in a tissue section," he said. "In this case it took us about a year."
The cells making up the tissue were too small to be human cells, but were unlike the cells of any parasite known to Fajardo or his colleagues. Fajardo consulted with pathologists throughout the country and got conflicting responses.
"Some specialists in parasites said the tissue was from a tumor. The experts in tumors thought it was a parasitic infection. And no one could identify the parasite," he said.
PCR Technique Needed to Identify New Pathogen
Convinced that he was looking at a new species, Fajardo asked Relman to help solve the mystery by using a strategy Relman has developed to characterize new or unrecognized disease-causing organisms.
Relman's strategy uses a molecular technique called the polymerase chain reaction (PCR) to create millions of copies of a particularly useful DNA sequence, amplified directly from the organism within the infected tissue. Because this DNA sequence varies from one species to another, it can help scientists classify unknown organisms. This approach has led to the identification of several important disease-causing organisms.
In the recent case, Relman found that the DNA sequences from the tissue sample were unlike any stored in genetic data bases. The organism's closest relatives seem to be in the class Cestoda, which includes tapeworms, another type of intestinal parasite.
"This finding illustrates the power and usefulness of molecular techniques for identifying unknown organisms, especially when traditional methods fail," Relman said.
If the man's physicians had known the mass was due to infection by a tapeworm-like organism, they might have attempted to treat him with drugs known to fight such infections, Relman said. "That very well might have stopped this new parasite, but of course, this is just speculation," he said.
"If more clinicians were aware that this type of [PCR] analysis is possible, other cases and other pathogens might be identified," he added.
Although only a few researchers are able to conduct this analysis, more are becoming proficient, Relman said. The Unexplained Deaths Project of the US Centers for Disease Control and Prevention is working with Relman to apply the technique more widely, he added.
In addition to the technique's diagnostic value, it provides a tool for exploring the diversity of organisms that live within humans and other animals. Some of these, like the newly identified parasite, may be harmful, but others may prove beneficial.
"We're on the threshold of discovering a whole new world of microbial diversity," Relman said. "By using technology that wasn't available previously, we may discover whole new families of organisms within the human body."