Among the estimated 600,000 to 700,000 Rwandan refugees at theGoma encampment in Zaire, the cholera epidemic reached its peak inlate July and early August.A 10-person team from the U.S. Centers for Disease Control andPrevention (CDC) in Atlanta reached the scene on July 27, medicalepidemiologist Brent Burkholder told BioWorld Today. He was amember of the medical surveillance mission from CDC's InternationalHealth Programs Office.Between July 14 and Aug. 14, Burkholder said, "48,347 bodies of thedead _ probably an under-coat _ were collected." The causes of theirdeath were not reported, but a subsequent survey of surviving familymembers found that 88 percent had died of diarrheal diseases. Of these,57 percent with watery diarrhea were presumed caused by cholera.Critical loss of body fluids in the form of watery diarrhea is what killscholera victims."A number of rehydration camps were set up to treat the choleracases," Burkholder recalled, "with either oral or intravenousreplenishment of water and electrolyte salts purged by the disease."He added that, "Probably all of the 600,000 to 700,000 were exposedto the Vibrio cholerae bacillus, with an estimated one in 10 to one in 20becoming symptomatic." The large majority of people infected by V.cholerae show no ill effects and develop immunity.The team helped immunize the helpless population against variousinfections, but not cholera, for which no effective vaccine exists, andfor which in any event there was no time to build up immunity.Ancient Plague, Modern ChallengeCholera goes back in recorded history at least 2,000 years, writesmolecular geneticist John Makelanos of Harvard Medical School. Hisarticle in this week's issue of Science, (Sept. 2) is one of seven"Perspectives" prefacing the journal's theme issue on vaccines.Makelanos' paper, "Cholera Vaccines: Fighting an Ancient Scourge,"recalls that seven pandemics of the lethal disease have struck the earthsince 1871. The latest, named the El Tor biotype, began in 1961 andcontinues today.El Tor is the strain that afflicts the Rwandan refugees, says Paul Blake,who heads CDC's Food-Borne and Diarrheal Diseases Branch."El Tor is the name of a place in Saudi Arabia," Makelanos toldBioWorld Today. "In the year 1910, it was a quarantine station,examining pilgrims on their way to Mecca, to keep the sick from goingon."He continued: "From some of these patients the examiners isolated thefirst El Tor cholera bacilli. These quarantine officers includedcolleagues of Robert Koch (1843-1910), the pioneer Germanbacteriologist who had discovered the Vibrio cholerae bacillus."El Tor is the predominant type of cholera in the world today. It took thestrain 80 years to go round the world. "In the early 1970s," CDC's PaulBlake told BioWorld Today, "there was a tremendous El Tor epidemicin Africa, but it didn't reach Latin America until 1991." In that year ithit Peru and an estimated 1 million Latin Americans have sincecontracted El Tor cholera.139 Serotypes And Still CountingThis strain's surface antigen, target of most efforts to produce a choleravaccine, is designated O1. Since it surfaced in Saudi Arabia over eightdecades ago, cholera-watchers have counted 138 more antigenicserotypes, most of which variants caused only sporadic cases. But thencame the latest strain, number O139, or "Bengal." It is closely relatedto El Tor, but not close enough.In October 1992, O139 launched a massive cholera attack on SouthAsia, sending vaccinologists back to square one. It had picked up newgene sequences unknown to El Tor, which defied all existing vaccineson the drawing boards.Makelenos' laboratory at Harvard is developing live attenuated oralvaccines for both O1 and O139 vaccines. "They appear to be safe andprotective," he told BioWorld Today, "and are proceeding down thepath to formulation and more extensive development."His construct is based on deleting the vibrio's genes that encodevirulence factors, such as cholera toxin, "and other things that enhancethe organism's ability to multiply in the gut. Thus attenuated," heexplained, "the pathogen is unable to cause a severe diarrhea, but isstill able to multiply in the intestine, and produce immunity to othercomponents that haven't been deleted."Because V. cholerae multiplies in the gastro-intestinal tract, Makelanossaid, "the diarrhea it causes is teeming with these bacilli, probably 10to 100 billion in a liter of fluid." His prototype vaccine can achieveimmunity "with only 1 million live attenuated organisms given to aperson." He pointed out that a single liter of culture medium in the labcan grow up 1013 organisms, "enough to immunize ten millionpeople."Field Tests, Present And Future"Part of the beauty of a live vaccine," Makelanos added, "is that a verysmall number of organisms, given once, create immunity."Another part of the beauty is that these living V. cholerae can be quick-frozen into a lyophilized powder that needs no refrigeration. Stirredinto a glass of water and swallowed, it confers lifelong immunity byway of the body's mucosal immune system. (See "Tolerance" article inBioWorld Today, Aug. 29, p. 1.)When Harvard's bivalent O1-plus-0139 vaccine "is at an appropriatestage," Makelanos said, it will be clinically tested by the U.S. ArmyMedical Research Institute of Infectious Disease, Fort Detrick, Md.,with supplies manufactured by the Virus Research Institute,Cambridge, Mass.Meanwhile, the only current large-scale field trial of a cholera vaccineunder way today is testing 60,000 volunteers in Djakarta, Indonesiawith a different recombinant attenuated oral cholera vaccine. The trialis under the direction of vaccinologist Myron Levine, who heads theCenter for Vaccine Development at the University of Maryland,Baltimore.Makelanos said Levine is "the first person to get live recombinantvaccines into large-scale field trials of any sort, as with their presentvaccine in Indonesia."Levine's vaccine immunizes against a vibrio strain called "Classical,"now relatively remote, with pockets of it in East Asia."In this vaccine," said vaccinologist Robert Edelman, deputy directorof the Maryland Center, "the toxin has been genetically manipulated sothat it lacks the A subunit, which is responsible for the actual inductionof the adverse chemical events within the cell, leading to theoutpouring of the electrolytes. But it still retains the B portion of thetoxin," Edelman told BioWorld Today, "which allows it to bind to theintestinal cells, and immunize the person. It fools his immune systeminto thinking it's going to make antibodies against the whole toxin, butit's not."Edelman added that the Classical strain is partially protective against ElTor cholera, completely stopping diarrhea in 60 percent of patients, andrendering it less severe in the remainder.Some scientists suspect Classical of being the scourge that drove thefirst six cholera pandemics between 1871 and the seventh, El Tor in1910. n
-- David N. Leff Science Editor
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