Scientists at the National Institute of Allergy and Infectious
Diseases (NIAID) in Bethesda, Md., have identified a strain of
hepatitis E virus in pigs that is very similar to the strain that causes
disease in humans. However, there is no evidence that the pig virus
causes disease in either humans or pigs. The finding, published in
the Sept. 2, 1997 issue of the Proceedings of the National Academy
of Sciences, should help advance studies of hepatitis E disease in
humans and eventually could lead to the development of a vaccine.
"This is a very interesting finding that will open new avenues
of research, and contribute to strategies to treat or prevent hepatitis
E disease," says Robert H. Purcell, M.D., chief of the hepatitis viruses
section in NIAID's Laboratory of Infectious Diseases (LID) and senior
author of the study. "Unlike hepatitis A, B and C, hepatitis E disease
almost never occurs in the United States. However, epidemics of the
disease do occur periodically in developing nations in Africa and
Hepatitis E virus is most commonly transmitted to people
through contaminated drinking water in areas with poor sanitation.
The 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 acute hepatitis E in the United States
have occurred among travelers returning from areas where hepatitis
E disease is endemic. Nevertheless, recent studies have shown that
upwards of 20 percent of healthy people in this country -- even those
who have not traveled abroad -- have antibodies to hepatitis E virus
or related agents in their blood. Similar evidence of exposure to
hepatitis E virus or related agents also has been documented in
primates and swine.
To explore the nature of these infections in pigs, Xiang-Jin
Meng, M.D., Ph.D., working with Dr. Purcell and their LID colleague
Suzanne U. Emerson, Ph.D., screened swine blood samples with an
assay designed to detect antibodies to strains of human hepatitis E
virus. Most of the samples, taken from swine herds in the Midwestern
United States, tested positive for hepatitis E virus antibodies.
In a separate analysis, piglets born to antibody-negative sows
were found to seroconvert (develop antibodies to hepatitis E virus)
when raised in large pens with other piglets. None of the piglets,
however, showed any clinical signs of disease after seroconversion.
Using polymerase chain reaction (PCR) techniques, the LID
scientists isolated putative hepatitis E virus genetic material from
swine blood samples and compared its genetic sequence to that of
human hepatitis E virus. They found that the swine virus was closely
related to, but distinct from, human strains of the virus.
"At the amino acid level, the swine and human strains are
about 90 percent alike," explains Dr. Meng. Amino acids are the
molecules from which proteins are made. "Among most human
strains of hepatitis E virus, amino acid identity is between 97 and 99
percent." The researchers say their findings strongly suggest that a
previously unrecognized strain of hepatitis E virus circulates in the
"It's important to remember that the virus strain isolated from
the swine in this study is distinct from the strains known to cause
disease in humans," explains Dr. Meng. "Still, further studies are
needed to determine whether swine hepatitis E virus is species-
specific or is circulating in the human population without causing
disease. These subclinical infections of humans with swine hepatitis
E virus might explain the relatively high prevalence of hepatitis E
antibodies in healthy individuals in the United States."
If that were the case, says Dr. Meng, the strong immunologic
cross-reactivity of the swine and human strains suggests that swine
hepatitis E virus could prove useful as a vaccine against the human
virus. The similarities between the swine and human viruses also
suggest that pigs might provide an alternative animal model for
studying hepatitis E virus infection. Currently, scientists must use
expensive primate models to study the virus.
"The possibility that swine hepatitis E virus may infect humans
also raises a public health concern regarding the use of pig organs in
human transplantation," cautions Dr. Purcell. "Nonpathogenic pig
viruses could possibly become pathogenic in human transplant
recipients, particularly since transplant patients receive immune-
Apart from these concerns, Dr. Purcell adds, there is no
evidence that the pig virus poses any threat to healthy humans or
"Swine hepatitis E virus is probably common throughout the
world," he says. "Antibodies to hepatitis E or related agents have
been found in healthy swine as well as in several other species of
domesticated and wild animals in a number of countries. Similarly,
such antibodies have been found in most human populations, even
where hepatitis E disease does not occur. Furthermore, the degree
of genetic divergence of the swine virus from human hepatitis E virus
suggests that it has been around for a long time."
In addition to the NIAID scientists, collaborators on this study
include Patrick G. Halbur, D.V.M., Ph.D., of the Iowa State University
College of Veterinary Medicine; Dale M. Webb, D.V.M., Ph.D., of the
Illinois Department of Agriculture; James R. Lehman, D.V.M., of
Atlanta, Ill.; and other veterinarians in Iowa and Illinois.
NIAID, a component of the National Institutes of Health (NIH),
supports research on AIDS, malaria and other infectious diseases, as
well as allergies and immunology. NIH and CDC are agencies of the
U.S. Department of Health and Human Services.
Press releases, fact sheets and other NIAID-related materials are
available on the Internet via the NIAID home page at