The 77 are among 400 former patients of the facility on Shadow Lane who recently tested positive and have been interviewed by health district staff in an investigation to determine the source of infection.

With the remaining 300-plus cases, investigators identified one or more risk factors, such as a history of illicit intravenous drug use, an organ transplant or blood transfusion before 1992, hemodialysis for kidney failure, and sexual contact with a known carrier of hepatitis C.

The patients were asked about the risk factors during follow-up interviews. Because many infections might have been acquired years earlier, health officials cannot say with certainty what the source of infection is, Labus said.

"We can't make a leap that they all came from there," said Dr. Scott Holmberg of the Centers for Disease Control and Prevention about linking hepatitis C cases to the Shadow Lane center. "Some people may not tell the truth about drug use, for instance."

Others might not know, he said, whether sexual partners were infected.

Holmberg said that the number of people who have tested positive did not surprise him and that he expects more.

Health officials have estimated that 4 percent of the clinic's patient population would have been infected with hepatitis C before visiting the clinic. Laboratory testing cannot distinguish between recent and older infections, health officials said.

Genetic testing is used to identify the source of infection in acute cases and clusters, a group of cases closely grouped in time and place.

The source of chronic cases is more difficult to pinpoint. The evaluation involves examining a patient's risk over the course of his life. Acute cases are determined based on a person's risk six months before onset of symptoms.

"Genetic testing is useful in looking at clusters, but the problem with it is the virus mutates rapidly," Labus said. "Someone who was exposed a year or two ago, we would not expect them to have the same virus today."

Earlier this year, some 40,000 clinic patients were notified by health officials of possible exposure to hepatitis strains C and B and HIV because of unsafe injection practices at the center.

Labus said the health district cannot gauge how many of the 40,000 notified have undergone testing but said roughly 50,000 tests have been ordered in Southern Nevada since Feb. 27, when the health alert was announced.

Health district officials think more than 50,000 patients of the Shadow Lane facility might have been exposed to the blood-borne diseases.

Authorities are alerting patients treated at the clinic between March 2004 and Jan. 11 of this year.

"We've got many more positive test results to go," Labus said, "and we expect this investigation to last many more months to come."

Health officials previously determined that five patients diagnosed with acute hepatitis probably had contracted the disease while undergoing procedures at the clinic on Sept. 21 and that another patient contracted the disease on July 25.

In addition to the six cases, health officials more recently linked a seventh acute case to the facility from 2005. Another acute case has been linked to a sister clinic, Desert Shadow Endoscopy Center on Burnham Avenue. That person is thought to have contracted the disease in June 2006.

Health officials said the 77 recently diagnosed cases had procedures at the facility on random days, meaning they are not clustered with other cases. Labus said that some of the 77 patients do not know the exact date of their procedures, complicating investigations into possible clusters.

The affiliated clinics are now closed, and Drs. Dipak Desai and Eladio Carrera, two of the four doctors who own the Shadow Lane clinic, have had their medical licenses temporarily suspended.

In a news release, Gov. Jim Gibbons said the health district's Thursday announcement "shows the need for swift and strong action by the State Board of Medical Examiners to ensure that the physicians who played a role in infections aren't practicing medicine while these investigations are ongoing."

Hepatitis C is an inflammation of the liver from the hepatitis C virus. It is the leading cause of chronic liver disease in the United States and is usually spread by contact with an infected person's blood.

Contact reporter Annette Wells at awells@reviewjournal.com or 702-383-0283. Contact reporter Paul Harasim at pharasim@reviewjournal.com or 702-387-2908.