The Hot Zone _ a book that tells the tale of the deadly Ebola virusof Zaire - has topped the non-fiction best-seller list for weeks andbeen on that chart for the past 35 weeks.

What with Ebola, HIV, Marburg and Lassa, it almost looks as if mostlethal viruses come out of Africa. But the jungles of Latin Americaare also in the running as foci of viral emergence, with graveconsequences for humans and animals _ and in our own geographicbackyard.

One such infective agent, thought to be extinct, has just resurfaced inVenezuela, killing horses and hospitalizing humans. A report titled"Emergence of a new epidemic/epizootic Venezuelan equineencephalitis [VEE] virus in South America" tells that story in thecurrent Proceedings of the National Academy of Sciences (PNAS),dated June 6.

Its first author is molecular virologist/epidemiologist Rebeca Rico-Hesse at Yale University in New Haven, Conn.

In her paper, she describes how DNA analysis applied to thebranching of VEE's evolutionary tree tracked mutation of the virusfrom a harmless enzootic (quiescent jungle reservoir) in rodents tosudden epidemic (human) and epizootic (animal) morbidity andmortality. VEE is a mosquito-borne arbovirus, remotely kin to therubella virus of German measles.

From the 1930s to the early 1970s, major VEE outbreaks from theU.S. to Argentina have involved "up to hundreds of thousands ofequines, tens of thousands of humans," PNAS reported. The largestflared up in Central America, lasted from 1969 to 1972, and onlystopped short in south Texas.

Its last appearance, in Venezuela 22 years ago, Rico-Hesse toldBioWorld Today, "led to speculation that epizootic VEE viruses hadbecome extinct."

From Extinction To Eruption

Then came Dec. 9, 1992. From that date to Jan. 28, 1993, equineencephalitis infected 26 horses, and killed 10, at 12 sites east ofVenezuela's Lake Maracaibo.

In June 1993, on the lake's western shore, the virus attacked wilddonkeys (for which mortality statistics are lacking).

Of 33 persons with mild flu-like symptoms, intensifying in a fewsufferers to severe headaches and high fever, 30 had their serumsampled in June 1993. Five of these human specimens (15 percent)yielded VEE antibodies, and two of them, viral isolates. "That data,"Rico-Hesse said, "allowed us to document the emergence of a newepizootic VEE, after a 20-year hiatus, and to determine its probableevolutionary origin."

Armed with these preliminary results, she was able to recommendresumption of human and equine population vaccination, with a long-extinct strain isolated in the 1940s.

Viral antigens confirmed that the infective strain was epizootic(virulent) rather than enzootic. Another such latent focus of VEEexists in the Florida Everglades, Rico-Hesse pointed out. "There,"she said, "it's silently transmitted by certain mosquitoes that can biteand infect people."

She explained that the type II Everglades virus, so far, is lessdangerous than the type I South American varieties, "but if themechanism we describe that leads the latent endemic virus to producethe more virulent form were to occur, then you'd have an epidemic inFlorida."

In Venezuela and Colombia, type I VEE virions come in threeantigenic varieties: IAB, IC and ID. The last of these, non-virulentID, lies doggo in the sylvatic jungle fauna. A Culex-species mosquitotransmits it. Whether this or other mosquito vectors spread thevirulent IAB and IC versions is still unclear.

Rico-Hesse warns: "When a virus evolves into an epidemic type, notonly does it become more virulent, but it also changes itstransmission method, so almost any mosquito can transmit it."

Working Toward An All-Strain Recombinant Vaccine

Less than 1 percent of the total VEE viral genome, Rico-Hesse andher co-authors determined, separate the relatively benign ID enzooticviruses from the virulent IC variety. But these few nucleotidemutations in both envelope glycoprotein and untranslated genomicregions, she said, "lead to drastic changes in their epidemiology _spreading beyond the jungle, producing viral build-up in the blood ofmany infected mammals, and infecting many potential mosquitocarriers."

Her ongoing research aims to evaluate these virulence-inducingmutations that accompany emergence of the virus in a yet-to-be-isolated progenitor strain.

Of five viral isolates (two from humans plus three equine), four hadidentical sequences in two regions to those of other VEE strains. Thefifth had two point mutations.

"To look at the evolutionary relationships between the 1992-93outbreak strains and previous isolates of the viruses," Rico-Hessesaid, "we selected limited areas of the viral genome, rather thandetermining its entire sequence. We explored small portions of thesevarious areas to find a region that would give us a good idea of themolecular clock _ the way the viruses are evolving.

"Now we're searching for random mutational events, changes thattake place, because when the enzymes that make copies of theseviruses are replicating, they make mistakes. These are chance errorsthat have nothing to do with the virus's virulence or transmissioncharacteristics, things like that."

These data then helped her and her co-authors create a computer-assisted retrospective family tree, of which the root strain occurred ina Florida Everglades mosquito in 1963. This branched into first- andsecond-generation VEE-ID and IAB trunks, plus several final twigsof the Venezuela 1992-93 emergence, linked to a sinister VEE-ICvariety.

With this information, Rico-Hesse and her group at Yale Universityare working to construct a recombinant vaccine based on making aDNA copy of an infectious RNA virus clone, and protective againstall VEE strains. Initially, it will protect horses rather than people.

"Equines," she explained, "have a high mortality rate; humans not ashigh. So a vaccine that controls it in horses will also protect people."

She cited as "a good example" the entirely different equine virus thatpopped up last September in Australia, and killed 14 horses plus theirtrainer. (See BioWorld Today, April 7, 1995, p. 1.) n

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.