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
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
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
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,"
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."