Wild rats have hepatitis E history.

In research that could help scientists ascertain how people contract hepatitis E, a new study indicates that more than 80 percent of U.S. wild rats might carry antibodies to the virus.

Hepatitis E infection is widespread in many developing countries, and up to 5 percent of people in the United States carry antibodies to the virus, indicating that they have been infected in the past. Infection can cause flulike symptoms or can be more severe, especially in pregnant women.

Earlier research suggested that pigs carry a strain of hepatitis E virus that they may transmit to humans. However, infection rates in the United States are highest in urban centers, where people are unlikely to encounter farm animals. Some researchers, therefore, suspected that other animals, such as city-dwelling rats, might carry the virus, as well.

In the new study, Yamina Kabrane-Lazizi of the National Institute of Allergy and Infectious Diseases (INLAID) in Bethesda, Md., and her colleagues captured 239 rats in alleyways, along rivers, and at other spots. In the August AMERICAN JOURNAL OF TROPICAL MEDICINE AND HYGIENE, the team reports that 77 percent of captured rats from Maryland, 90 percent from Hawaii, and 44 percent from Louisiana carried antibodies to the virus in their blood.

Because hepatitis E infection often doesn't last long, the researchers were not able to isolate the virus from the captured rats and determine whether the strain that had infected them is one that also infects swine and people.

"There's no direct evidence for transmission from rats to people," says study coauthor Robert H. Purcell, also of NIAID. "We'd like very much to recover the virus [from rats] and compare it with human and swine strains to see where it fits into the ecology."