Reusing syringes is “something that shouldn’t happen anywhere,” Mr. Labus said. “It is not acceptable. What we are focused on now is what was going on and how do we stop it.”

The outbreak, which began in February, has attracted the attention of federal health officials and law enforcement authorities, including the Nevada attorney general and the Federal Bureau of Investigation.

It has also embarrassed the state’s governor, Jim Gibbons, who originally derided news of the outbreak as overstated and a creation of news media “buffoonery.” (Mr. Gibbons, a Republican, has since called for the resignation of state health officials, and on Thursday he called the newly revealed cases “heartbreaking and disturbing.”)

Since February, the county health department has notified 40,000 patients who had visited the clinic that they might have been at risk for infection with the hepatitis B and C viruses or H.I.V. and should be tested. About 50,000 test panels of blood from patients have been conducted in laboratories around the region, although multiple tests may have been done on some patients and some of those tested might never have visited the medical practice.

Of those who tested positive for hepatitis C, Mr. Labus said, eight clearly contracted the illness at the two clinics, and the latest 77 were likely to have been exposed at one of them as well, because the patients had no other known risk factors.