New infections of the liver with hepatitis C virus are far less common these days in the U.S. compared to the early 1980s, a new government study finds.

According to the new study by researchers at the U.S. Centers for Disease Control and Prevention (CDC), in the mid-1980s roughly 70 of every million Americans developed acute hepatitis C each year. Between 1994 and 2006, that rate was 90 percent lower: only 7 per million per year.

Over the years, however, injection drug users have accounted for a growing proportion of cases, and their risk of infection remains an important public health problem, the CDC researchers said in the Archives of Internal Medicine.

New infections with the hepatitis C virus usually aren't obvious. In most cases there are no symptoms at first; hepatitis C quietly damages the liver for years until it's finally diagnosed. But about two or three out of every 10 newly infected patients have what's called "acute hepatitis." They have fever and nausea, their skin gets yellow from jaundice because the liver isn't working properly, and in general they feel very sick.

Tracking rates of acute hepatitis C as they rise or fall gives researchers an idea of whether rates of silent new infections are rising or falling, too.

Hepatitis C is passed through contact with infected blood -- most often by sharing tainted needles -- although a small number of cases are sexually transmitted or passed from mother to baby during childbirth.

So far, efforts to curb hepatitis C transmission among drug users "have had success to some degree," said Dr. John Ward, director of the CDC's division of viral hepatitis, in an interview. Ward didn't work on the current new study, which was led by Dr. Ian T. Williams.

Dr. Williams and colleagues looked at rates of acute hepatitis C reported in six U.S. counties between 1982 and 2006.

In addition to finding that the rate of new infections had dropped, they made some discoveries about certain high-risk groups.

Among people receiving transfusions, for example, the risk of infection from hepatitis C virus in the blood has steadily declined over time, with only five possible cases identified between 1994 and 2006. Since 1992, all donated blood in the U.S. has been tested for hepatitis C, and the risk of receiving a donation carrying the virus is now estimated at one in 2 million, according to the CDC.

The number of cases reportedly related to injection drug use also declined over time. But drug abuse accounted for a growing proportion of acute hepatitis C infections, rising from about 32 percent of cases in the 1980s to at least 46 percent for the years 1994 through 2006.

In another third of cases, there was no clear risk factor, but most of those people reported past drug abuse.

Efforts to curb HIV transmission among injection drug users -- through education and needle-exchange programs, for example -- have been very effective, according to Ward.

But it's proven harder to battle hepatitis C, he explained, because it spreads more easily than HIV. Even a faint amount of blood on a shared needle, for example, might be enough to transmit hepatitis C.

The knowledge of what to do to prevent hepatitis C among injection drug users "is just not as deep as it is for HIV," Ward said.

The other ongoing public health concern with hepatitis C is the large number of Americans with chronic infection who may develop serious liver disease in the future. It's estimated that 3.2 million Americans are living with chronic hepatitis C, Ward said, and about half are unaware of it.

These people often develop inflammation of the liver, scarring of the liver tissue (cirrhosis) and, in some cases, liver cancer. In the U.S. and many other countries, hepatitis C is the most common reason why people need liver transplants.

The CDC recommends that people with risk factors for the infection -- including anyone with a history of injection drug use, and those who received a blood transfusion or organ transplant before 1992 -- be tested for hepatitis C.

Once an infection is discovered, doctors can do regular blood tests of liver function to catch early signs of liver disease. And there are drugs for hepatitis C that can clear the virus in some people.

Regarding the lower rates of new infections, Ward told Reuters Health, "That's great news."

"It shows that prevention can work," he added. "But our work is not done."