EXETER — Four patients have been diagnosed with the same strain of hepatitis C, and hundreds more will be tested as Exeter Hospital and state health officials investigate what is being called the state's first such outbreak of the disease in a medical facility.

At a news conference Thursday afternoon, several representatives from the hospital and the state's Division of Public Health confirmed that four patients were diagnosed over the past two weeks and that they share an identical strain, meaning they were infected by the same source, officials said.

Hepatitis C is a potentially fatal liver disease commonly spread through exposure to blood, said Dr. Jose Montero, the state's public health director.

A joint investigation revealed that those infected were all patients who received care in the hospital's cardiac catheterization lab and its recovery unit, which was shut down on May 25 after officials determined that the area may have been the location where the infections occurred.

Officials said they're still trying to figure out exactly how it was spread.

While rare outbreaks of hepatitis C have occurred in medical facilities in other parts of the country, Montero said this is the first incident in New Hampshire.

“Despite the low number of cases we have identified to date, we are taking this issue extremely seriously. … We are doing everything we can to quickly identify and eliminate the common source of infection. We are taking measures to identify those individuals that could possibly be affected and care for those patients and individuals that we have identified,” said Dr. Richard Hollister, chairman of the hospital's Department of Medicine.

Signs of the outbreak began May 14 when the hospital launched an investigation into three cases of hepatitis C possibly linked to the facility. A fourth case was then discovered, Hollister said, and on May 15, the hospital reported the cases to state health officials. The state began testing the patients on May 17 and later determined that the virus that infected the four patients had a “common genetic fingerprint, suggesting a common source of infection,” Hollister said.

The only common connection found between the four infected patients to date is the cardiac catheterization lab, Hollister said.

Hospital administrators voluntarily decided to suspend operations of the laboratory and its recovery unit on May 25 “out of an abundance of caution,” Hollister said.

On May 29, the hospital began testing all staff and physicians who had any possible connection to the cardiac-catheterization lab.

Some 28 staff members have been tested so far, Hollister said, and testing will continue until all staffers are cleared to return to work and resume operations.

Officials said 651 patients who were treated in the cardiac catheterization lab and its recovery unit between Aug. 1, 2011, and May 25 have been identified and are being contacted by phone and letter and urged to receive free hepatitis C testing at the hospital.

Positive test results will be reported to the patient's primary care physician and checked by the state lab to see if they share the strain found in the original four infected patients, Hollister said.

Montero said some people may not show symptoms right away and that 15 to 25 percent clear the virus without treatment. The majority of those infected develop chronic infection, which can become a lifelong illness that leads to serious liver problems, officials said.

Symptoms may include loss of appetite, abdominal pain, fatigue, nausea, and jaundice.

Patients who have been identified as possibly being exposed are being urged to call the hospital's Information and Referral center at 580-6124 to schedule a blood test. Staff will be available Monday through Friday from 7 a.m. to 7 p.m.