Health officials Thursday urged 7,000 patients of an Oklahoma dentist to seek medical tests for hepatitis or the virus that causes AIDS after discovering that instruments weren't properly cleaned despite the doctor's knowing that several of his patients carried infectious diseases.

"It's uncertain how long those practices have been in place," said Kaitlin Snider, a spokeswoman for the Tulsa Health Department. "He's been practicing for 36 years."

Dr. W. Scott Harrington voluntarily closed his practice and is cooperating with investigators, she said. A hearing is set before the state Dental Board on April 19. Harrington could lose his license for "major violations" of the Oklahoma Dental Act.

Snider said letters would be sent Friday to 7,000 patients who went to Harrington's clinics in Tulsa and suburban Owasso since 2007. The letters recommend testing for hepatitis B, hepatitis C and HIV. The agencies say it is rare for infections to spread in occupational settings but that tests are important.

A complaint against Harrington said investigators were checking into a hepatitis C case when it discovered numerous safety issues at his practice, including cross-contamination of instruments and a separate set of instruments used on patients with infectious diseases. A unit that sterilizes equipment wasn't working properly, according to the complaint.

The health departments said hepatitis B, hepatitis C, and HIV are serious medical conditions and infected patients may not have outward symptoms of the disease for many years. "As a precaution, and in order to take appropriate steps to protect their health, it is important for these patients to get tested. It should be noted that transmission in this type of occupational setting is rare," their statement said.

Testing will be offered free of charge at the Tulsa Health Department's North Regional Health and Wellness Center.