A new study finds that the percentage of U.S. blood donors with evidence of hepatitis C infection is substantially lower now than in the early 1990s - in what researchers say probably reflects an overall decline in the infection rate among Americans.

The findings also hint at a link between multiple pregnancies and the virus, and of a protective effect of obesity, although it’s too soon to tell whether these associations will hold up.

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

In a minority of people, the body is able to clear the virus soon after infection. However, the infection becomes chronic 75 percent to 85 percent of the time, often leading to liver damage such as chronic inflammation, scarring of the liver tissue (cirrhosis) and, in some cases, liver cancer. The U.S. Centers for Disease Control and Prevention estimate that between 1 and 5 percent of people infected with hepatitis C eventually die of cirrhosis or liver cancer.

Since 1992, the U.S. donor-blood supply has been screened for hepatitis C, and the risk of receiving a donation carrying the virus is now estimated at one in 2 million, according to the National Heart, Lung, and Blood Institute.

Based on studies from the early 1990s, about a half a percent of U.S. blood donors between 1992 and 1993 harbored antibodies to hepatitis C - which indicates either a chronic infection or a past infection that cleared.

The rate in more recent years had been unclear, however.

In the new study, published in the Journal of Infectious Diseases, researchers analyzed samples from nearly 960,000 donors who gave blood at six U.S. blood banks between 2006 and 2007.

They found that less than a tenth of a percent of donors tested positive for antibodies to hepatitis C.

“It’s a strikingly lower prevalence” compared with 1992-1993, said lead researcher Dr. Edward L. Murphy, of the University of California, San Francisco.

The decrease, Murphy told Reuters Health, likely reflects, in part, an overall decline of hepatitis C infection among relatively younger Americans. The “Baby Boomers” born between the late 1940s and early 1960s had higher rates of injection-drug use than subsequent generations, and therefore a higher risk of carrying hepatitis C.

In support of such a “birth cohort effect,” Murphy’s team found that in 2006-2007, donors in their 40s and 50s had the highest rate of hepatitis C compared with other age groups. In the early 1990s, the highest rates had been seen among donors in their 30s and 40s.

The findings, according to Murphy, suggest that the Baby Boomer generation is likely to have the highest rate of hepatitis-C- related liver disease, and that the rate will decline among younger Americans.

“I think the message here is a positive one,” he said.

Of course, Murphy added, the rate of hepatitis C among blood donors would not be reflective of that among injection-drug users, who continue to have the highest risk of hepatitis C infection among Americans.

The study also found two “novel” factors associated with a higher risk of hepatitis C infection among blood donors.

Among women, the odds of having antibodies to the virus climbed along with the number of children they’d had. Of women who had never given birth, only about one in 3,300 had antibodies to hepatitis C, compared to one in 1,600 among women who’d had one child. The highest rate - about one in 1,000 - was seen among women who’d had five or more children.

Murphy cautioned that “you don’t want to put too much into one finding,” and said that further studies are needed to confirm the association between childbirth and hepatitis C. But the finding, he said, raises the possibility that some women acquire the infection during childbirth - via contaminated instruments or during C-section, for example.

The other new finding was that obese adults were less likely than their normal-weight counterparts to have hepatitis C antibodies. And among people with antibodies, obese individuals were less likely to have genetic material that signals the ongoing presence of the virus; the implication is that obese people may be more likely than thinner ones to clear the infection.

Murphy called that finding “surprising,” and said that, as with the findings on pregnancy, more research is needed to confirm the association.

As for why obesity would potentially protect against hepatitis C, Murphy noted that obesity is believed to involve a chronic state of low-level inflammation in the body.

That inflammation is usually considered a negative thing, as it may contribute to diseases like diabetes and heart disease. But, according to Murphy, it’s possible that when it comes to hepatitis C infection, inflammation involving increased production of certain immune-system substances is protective.

If further studies confirm the link between body weight and hepatitis C infection, he said, it will be important to weed out why.

SOURCE:  Journal of Infectious Diseases, online July 9, 2010.