A study led by researchers at the National Institutes of Health (NIH)
suggests that hepatitis E virus (HEV) is common among wild rats in the
United States. Although hepatitis E disease is very rare among people in
the United States, many have HEV antibodies in their blood - evidence that
they were once infected with the virus, even though it did not make them
sick. The finding raises questions about whether there is any connection
between rats and HEV infection in humans. A report of the study appears in
the August 1999 issue of the American Journal of Tropical Medicine and
Scientists analyzed blood samples from 239 rats captured in alleyways in
Baltimore, MD, along the Mississippi River levee in New Orleans, LA, and in
urban and rural areas of Hawaii. Tests revealed that more than 80 percent
of the rats had HEV antibodies in their blood, evidence of prior HEV
infection. Yamina Kabrane-Lazizi, Ph.D., of the National Institute of
Allergy and Infectious Diseases (NIAID) was lead author of the study. Her
collaborators included scientists from the National Institute of
Neurological Disorders and Stroke, the Johns Hopkins School of Public Health
in Baltimore, and the John A. Burns School of Medicine in Honolulu, HI.
Hepatitis E disease generally affects young adults and usually is not
life-threatening, except in pregnant women infected with the virus where
fatality rates of 15 to 20 percent have been reported. According to the
Centers for Disease Control and Prevention (CDC), virtually all cases of
hepatitis E disease in the United States have occurred among travelers
returning from developing countries, where the disease is endemic and spread
through contaminated drinking water. Nevertheless, tests show that between
1 and 5 percent of healthy blood donors in the United States have HEV
antibodies in their blood.
"Since hepatitis E is so rare in this country, it's puzzling that so many
people have antibodies to the virus in their blood," says Robert H. Purcell,
M.D., chief of the hepatitis viruses section in NIAID's Laboratory of
Infectious Diseases and senior author of the study. "It may be that the
high prevalence of HEV antibodies in the absence of disease is due to
infection with a strain of HEV that doesn't cause disease. Another
possibility is that animals serve as reservoirs of HEV and pass a weakened
form of the virus on to humans."
Dr. Purcell and his colleagues recently isolated HEV from swine in the
United States, suggesting one possible source of exposure. However, HEV
antibodies are detected primarily among residents of urban areas in the
United States, where people rarely encounter pigs. Rats, on the other hand,
have been ubiquitous city-dwellers for hundreds of years.
"Our data strongly suggest that many wild rats in the United States are
naturally infected by HEV," says Dr. Purcell. "A major question is whether
the HEV strain infecting rats is a new strain unique to rats or a variant of
already recognized HEV strains."
To answer that question the researchers will need to isolate and
characterize the rat virus, something they were unable to do in the current
study. Their data suggest that rats become infected with HEV early in life.
Other studies have shown that HEV disappears from the blood soon after HEV
antibodies are produced. The researchers' best hope for isolating the
virus, therefore, lies in trapping young rats that recently have been
infected with HEV.
Dr. Purcell and his colleagues will attempt to pass the virus from infected
wild rats to rats raised in the laboratory. In addition to providing clues
about the source of HEV antibodies in humans, their efforts could result in
an inexpensive new animal model for studying HEV infection, says Dr.
Purcell. Currently, scientists must use expensive primate models to study
"Rat HEV has probably been around a long time," says Dr. Purcell. "We still
do not know the source of the high prevalence of HEV antibodies in humans.
The possibility that the rat virus plays a role bears further study. It's
important to remember, however, that there is no evidence that rat HEV poses
a threat to human health."
NIAID is a component of the National Institutes of Health (NIH). NIAID
conducts and supports research to prevent, diagnose and treat illnesses such
as HIV disease and other sexually transmitted diseases, tuberculosis,
malaria, asthma and allergies. NIH is an agency of the U.S. Department of
Health and Human Services.
Press releases, fact sheets and other NIAID-related materials are available
on the NIAID Web site at http://www.niaid.nih.gov.