Deadly Human Parasite Discovered

August 1, 1996

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

By analyzing DNA from a strange mass of tissue found in a man'sabdomen, researchers have discovered a previously unknown parasitethat can infect and kill humans. The researchers have yet to namethe parasite or determine what it looks like, but they believethat it may be in the same class as tapeworms--although it appearsfar 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 withAIDS who lived in the San Francisco Bay area. The man died atage 44 from the parasitic infection. After his death, researchersfound that the parasite had formed two large masses composed ofmany sacs of unusual cells surrounded by fibrous tissue. One masswas in the man's intestine and adjacent tissues; the other wasin his liver.

The researchers reported their findings in the June 29 issue ofThe Lancet. The authors include Dr. Monica Santamaria-Fries, apathologist 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 Schoolof Medicine and the Veterans Affairs Palo Alto Health Care System.

Discoveries of human parasites are quite rare. The most recentwas the discovery of a parasite called Cyclospora about 3 yearsago, according to Dr. Paul Basch, a professor of health researchand policy who teaches medical parasitology at Stanford.

"If this new finding is confirmed it would certainly be asignificant addition to medical knowledge," said Basch, whowas not involved in the new research.

"Now that this has been reported, it alerts people to itsexistence. That could lead to more reports of the parasite inthe future," he added.

"The organism very likely has existed in nature for a longtime. We don't know why this disease and pathology have neverbeen described in the scientific literature before," saidFajardo, chief of the pathology service at the Veterans AffairsPalo Alto Health Care System.

"One possibility is that the parasite has only recently acquiredthe opportunity or the ability to infect humans and invade visceraltissues. Another possibility is that in the past, the mass oftissue the parasite creates had been mistaken for something else--perhapsa lymphoma, which grossly, it very much resembles," Fajardosaid.

Also, the parasite might behave unusually in people with weakenedimmune systems, such as this man with AIDS. "It's possiblethat in people with healthy immune systems, the parasite is unableto cause such invasive disease," Fajardo said.

In March 1994, the patient was admitted to the Kaiser center witha 2-month history of abdominal and lower back pain, vomiting,weight loss, night sweats, and fever. After finding a large massin his abdomen, physicians performed a biopsy. Despite treatment,the man died 9 weeks later. The unusual tissue had spread to hisliver 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'sFajardo, a specialist in identifying parasitic infections, andasked his opinion. Fajardo did not know what to make of the tissue.

"Usually it takes me about an hour to figure out which microorganismis in a tissue section," he said. "In this case it tookus 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 orhis colleagues. Fajardo consulted with pathologists throughoutthe country and got conflicting responses.

"Some specialists in parasites said the tissue was from atumor. 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 askedRelman to help solve the mystery by using a strategy Relman hasdeveloped to characterize new or unrecognized disease-causingorganisms.

Relman's strategy uses a molecular technique called the polymerasechain reaction (PCR) to create millions of copies of a particularlyuseful DNA sequence, amplified directly from the organism withinthe infected tissue. Because this DNA sequence varies from onespecies to another, it can help scientists classify unknown organisms.This approach has led to the identification of several importantdisease-causing organisms.

In the recent case, Relman found that the DNA sequences from thetissue sample were unlike any stored in genetic data bases. Theorganism'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 moleculartechniques for identifying unknown organisms, especially whentraditional methods fail," Relman said.

If the man's physicians had known the mass was due to infectionby a tapeworm-like organism, they might have attempted to treathim with drugs known to fight such infections, Relman said. "Thatvery 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] analysisis 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 DeathsProject of the US Centers for Disease Control and Prevention isworking with Relman to apply the technique more widely, he added.

In addition to the technique's diagnostic value, it provides atool for exploring the diversity of organisms that live withinhumans and other animals. Some of these, like the newly identifiedparasite, may be harmful, but others may prove beneficial.

"We're on the threshold of discovering a whole new worldof microbial diversity," Relman said. "By using technologythat wasn't available previously, we may discover whole new familiesof organisms within the human body."