A hitherto-unknown lung infection struck 42 healthy adults inthe Western U.S. this summer and killed 26 of them. Molecularepidemiologists at the Centers for Disease Control (CDC) inAtlanta identified the mystery pathogen as a novel U.S. strainof a familiar foreign virus by using polymerase chain reaction(PCR) and automated gene-sequencing technologies.
The scientists report their feat in a 10-page series of papers intoday's issue of Science, led off by an editorial titled"Hantavirus Outbreak Yields to PCR."
The outbreak first flared last spring in the remote "fourcorners" region, where the borders of Arizona, Colorado, NewMexico and Utah come together. In mid-May, a young NavajoIndian distance runner died of acute respiratory diseasesyndrome (ARDS) a few days after attending the funeral of hisfiance, who had succumbed to the same sudden pulmonaryfailure.
As more cases arose, mainly in New Mexico, baffled statehealth authorities sent autopsy tissue samples to the CDC foranalysis of their nucleic acid. The unusual suspects the agencyscreened included a family of Hantaviruses, known since the1950s in East Asia and Europe to cause flu-like illness and amodicum of deaths by kidney failure.
The first Hantavirus was found along Korea's Hantaan river inthe early 1950s, when many U.N. troops in the Korean warcaught the infection.
At CDC during the last week in May, ELISA tests using severalof these Hantaviruses (HVs) as antigens detected some cross-reactivity in the antibodies present.
"This surprising immunodiagnostic hint led us to quickly lookat the gene-bank data base, pull out all of the known HVsequences and embark on a polymerase chain reaction fishingexpedition," Stuart Nichol, chief of molecular biology at CDC'sspecial pathogens branch, told BioWorld."
The PCR primers they constructed used as bait a conservedregion of the known HV genomes, with a maximum predictedhomology among the four strains they tested. (HV is an RNAvirus with three segments, encoding respectively polymeraseprotein, two glycoproteins and nucleocapsid protein.)
Nichol and his co-workers reasoned that if the "Four Cornersvirus," as it was then dubbed, belonged to the Hanta family, itwas likely to share the same common sequence. It did.
By the first week in June, Nichol said, "we had nested-PCR-amplified pieces of HV genetic material out of the autopsytissues. And we had directly sequenced, by automatedthermocycling, the gene sequences of these fragments, whichallowed us to compare the genome of this virus with all theknown HVs.
"We immediately realized that we were dealing with a uniqueHantavirus, previously unidentified," he said.
Knowing HVs to be rodent-borne, (the usual suspects inspreading diseases in the region), CDC moved quickly todispatch trappers to the infection-focus area to find the culpritanimal harboring this new virus as primary reservoir. Rodentsamples came back, including some from houses where peoplehad died.
Combining serological and genetic techniques, the CDCinvestigators soon determined a direct link between the virusand a six-inch, gray to reddish-brown rodent, the deer mouse(Peromyscus maniculatus). Consequently, this small animal,ubiquitous in North America, had undergone a populationexplosion in May that coincided with the outbreak, and HV RNAturned up profusely in its tissues.
"So within a couple of weeks, we went from dealing with amystery disease with a high lethality -- and a lot of anxiety --to a point where we had a good idea what we were dealingwith," Nichol observed. And they were able to place rodent-control measures in houses.
Subsequently, the CDC molecular virologists extended theknown genetic sequences of the new virus directly from theclinical samples by extracting RNA from the lung tissues, thenmade clones and expressed them in E. coli or baculovirus hosts.The proteins they produced will serve as antigens for improveddiagnosis in patients and screening deer mouse populationsacross the U.S.
Some Asian and European HV epidemics rise and fall in a twin-peaks pattern, in spring and autumn. Not knowing if the FourCorners eruption in May was typical, health authorities worrythat the November-January peak might bring back high ratesof morbidity and mortality.
So far, said Nichol, the numbers of clinical cases do not appearto be on the rise. "We hope it will be a single peak," he said,"because an autumn upsurge would come at the same time asthe annual large number of influenza cases. It would be amessy situation trying to differentiate Hantavirus pulmonarysyndrome (HVPS) from flu. By the time a microbiological testcould tell them apart, the HVPS patient might be dead.
Nichol explained that the name HVPS has replaced Four Cornersvirus because "the disease, we now know, is not confined to thefour corners states; we're finding cases across the entirecountry, from Louisiana to California to North Dakota."However, the numbers reported in Science -- 42 stricken, 26dead -- have not yet increased.
"In the age of PCR, a lot of virology can clearly be conductedwithout a virus," wrote Science editorialist Eliot Marshall. Henoted that to this day, the pulmonary syndrome HV has notbeen isolated in culture, but has firmly established its identityand is eliciting countermeasures.
As Nichol told BioWorld: "The rapidity with which this diseasedetective story has unfolded is due in large part to thebiotechnology-type approaches that can be applied to theseproblems nowadays. In the past it would take many months oryears to connect a viral or bacterial agent with a specificinfection." As an example, he recalled the Lassa virus, firstrecognized in 1969, but which "took quite a few years to sortout."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.