ORLANDO, Fla. - Scientists at the Centers for Disease Control and Prevention (CDC) have found Hantavirus activity to be much more prevalent in some of America's eastern national parks than previously believed.
An outbreak in 1993 of Hantavirus pulmonary syndrome (HPS) in the southwestern United States led epidemiologists to initially identify Sin Nombre virus (SNV) as the leading cause of HPS in this country, and the deer mouse as its principal reservoir.
However, with the discovery of additional western Hantaviruses, like El Moro canyon virus, several rodent species identified to be Hantavirus reservoirs and the rarity of cases east of the Rockies, the virus was thought to be predominantly a western problem. But scientists really couldn't be certain of this in part because there was no information on what the prevalence of Hantavirus was or even if it was actually present among rural rodent populations in the eastern half of the United States.
So, with this in mind, CDC scientists devised the U.S. National Park study with three goals:
"We chose the 39 national parks in the east because we thought they were a very good medium for this type of study," said James Mills, PhD, chief of medical ecology of the Special Pathogens Branch at the CDC and lead author of the study. "[The parks] have very well categorized flora and fauna as well as millions of visitors each year who have potential contact with rodents. So Hantavirus prevalence in rodent populations was something that had to be studied."
Scientists divided the parks into five areas based on their geographic proximity to each other (northeastern, Great Lakes, central, western and southeast) with each area being assigned one team of epidemiologists. The various teams visited their assigned sites from May through August 1994. Setting 23,000 traps over a three-month period, the teams captured a total of 1,900 rodents, representing 41 different species.
Blood and tissue samples were collected from the captured rodents and analyzed using an enzyme immunoassay test to see if antibodies they harbored reacted against various SNV antigens. The deer mouse and white-footed mouse were the most dominant species, accounting for close to two-thirds of all those tested. Other species rounding out the remaining one-third included other mice, voles and chipmunks, said Mills.
Deer mice were observed in 11 of the 39 national parks; 45 out of 649 captured were found to have Hantavirus antibodies. Of the 590 white-footed mice trapped by researchers, only 24 carried Hantavirus antibody. Out of 12 rice rats, two were identified with Hantavirus antibody, as well as four of 31 cotton rats and eight of 18 western harvest mice captured. One chipmunk out of 33 trapped tested positive for Hantavirus antibody.
Mills and his team found evidence of Hantavirus infection in rodents in states where human disease had never been observed before (Tennessee, Michigan, Iowa, Oklahoma, Nebraska and Washington, D.C.), hinting at the potential for future human cases. Although researchers found evidence of Hantavirus in only 11 of the 39 parks studied, this did not mean the remaining parks were free from infection, said Mills.
"Our study did not intensively focus on any one particular park," he said. "Antibody was the only marker of infection we used - we had to ignore the temporal component of the infection even though we knew that prevalence of infection is indeed variable in time. So, it's very likely we may have missed some parts that had Hantavirus because prevalence was so low." Mills added that it's also possible that, on a local scale, the virus is subject to periodic `extinctions' and `re-introductions' from adjacent reservoirs, such that infection is always present on a regional scale but may or may not be found within a local population at a given time.
Hantavirus antibody prevalence
|Species||Number captured||Antibody prevalence|
|Western harvest mouse||18||8|
The highest regional Hantavirus antibody prevalence in the study area for both deer and white-footed mice was seen in the northeastern U.S., with prevalence strikingly high in the Appalachian Mountain parks.
Hantavirus antibody seroprevalence for deer mice was found to be high at sites like the Shenandoah National Park, which registered 13% infection among captured rodents. One of only three white-footed mice captured at Rock Creek Park, near Washington D.C., had antibody to Hantavirus. Fort Larned Historic Park in Kansas had a 16% antibody prevalence rate among trapped deer mice and Allegheny Portage Railroad National Historic Site in Pennsylvania saw a 19% prevalence rate of both species. No Hantavirus antibody was detected among the 67 white-footed mice trapped in southeastern U.S. parks.
Looking at Hantavirus antibody prevalence by habitat, most of the infected species were found to be in the coastal wetland and forested parks. But no seropositive mice were found in mixed pine and hardwood forests. But Mills said other scientists should not read too much into the virus' perceived choice of habitats, stressing the prevalence was due more to region than whether mixed forests affected Hantavirus prevalence.
"More study is warranted," said Mills. "The state of New York did a very intensive study that detected a high prevalence of Hantavirus on Long Island. I think every state should do this systematically to get an accurate assessment of how the prevalence of infection in reservoir populations varies in space and time. There is still a lot left to do."
For more information:
- Mills JN. Prevalence of Hantavirus antibody in small rodents from U.S. national parks. Presented at the 46th Annual Meeting of the American Society of Tropical Medicine and Hygiene. Dec. 7-12. Orlando, Fla.
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