Researchers have discovered that a tiny parasite that eludes many routine diagnostic tests may cause a mysterious wasting illness in AIDS patients.

Although there are only scattered reports of the protozoan microsporidia, it may be widespread, according to Dr. Robert Owen, professor of medicine at the University of California at San Fransisco.

Owen reported that microsporidia were linked to severe diarrhea and weight loss in a patient at the Veterans Administration Medical Center in San Francisco who was later diagnosed with AIDS. Many AIDS patients suffer from severe wasting illness and the federal government has recently expanded its definition of the disease to include that aspect.

Early evidence suggests that microsporidia may infect many people with normal immune systems, perhaps causing no symptoms, Owen reported in a university news release.

Standard laboratory exams used to detect intestinal invaders usually miss microsporidia. The organisms are distant relatives to pond creatures often examined by biology students under simple microscopes.

But microsporidia protozoa are much smaller -- about the size of the smallest bacteria -- and they fail to absorb most stains that make bacteria visible under the microscope.

Once inside living cells, they multiply freely and can easily be mistaken for normal cell structures. Currently, only electron microscopes can detect microsporidia in the tissue from an intestinal biopsy.

Many types of microsporidia have been associated with animal diseases. One causes brain inflammation in rabbits. Another destroyed the silkworm industry. A third decimated shipments of honeybees before beekeepers realized that feeding the insects during shipment bolstered their resistance to the infection.

"Virtually all animal groups from protozoa to primates harbor microsporidia," Owen said. "So it would be remarkable if such a ubiquitous pathogen did not occur in human beings."

Because they are so difficult to detect, however, they are rarely suspected in human disease. Only four species have been associated with isolated cases of hepatitis, diarrhea, muscle inflammation and eye infections.

Owen said the source of the human infections remain unclear, but microsporidia are commonly found in insects and their spores have been found in urine, feces and the uncooked tissues of animals.

The organism invades the gut wall by shooting its nucleus through a coiled tubule into an intestinal cell. As they multiply and damage cells, the microsporidia hinder the absorption of nutrients. Infected cells die, releasing mature spores into the gut to infect other cells or pass out of the body.

Owen said that medicine's understanding of microsporidia will probably parallel its awakening to parasites called cryptosporidia. Larger relatives of microsporidia, they first were noticed in 1981 when they were found in a few AIDS patients. Improved methods to detect them were soon developed and revealed that cryptosporidia infect about 4 percent of all AIDS patients.