In the last century, U.S. armed forces serving in areas wheretropical diseases are rampant have lost more man-days tomalaria than to bullets. Even in Somalia, which is on the fringeof the endemic malaria zone of East Africa, "there were morecases of malaria than bullet wounds," said tropical-medicinespecialist Stephen Hoffman.
Hoffman, who directs the Naval Medical Research Institute'smalaria program in Rockville, Md., is principal author of apaper in Saturday's issue of Lancet titled "Diagnosis of malariaby detection of Plasmodium falciparum HRP-2 antigen with arapid dipstick antigen-capture assay."
Hoffman explained to BioWorld the need for such diagnosis."Suppose you come back from Africa with chills and fever, andwalk into a hospital emergency room," he said. "It would bevery difficult for them to diagnose malaria in you."
What's involved is taking blood, smearing its red cells on amicroscope slide and counting cells infected with visible P.falciparum merozoites. This requires a trained lab technicianand 20 to 30 minutes or more. A single dipstick assay can bedone in 20 minutes; a group of 10 in half an hour.
It can take weeks to train a blood-smear reader, Hoffman said,but only hours for a dipstick reader. "So having a reasonablyinterpretable dipstick assay would be a major advance," headded, "so long as it had good sensitivity and specificity, whichthis one does."
Hoffman and his collaborators field-tested their prototypedipstick in western Kenya on 40 adult male volunteers andabout 160 boys and girls at four primary schools. The adultsgave blood by finger prick daily for six days during the 1992rainy season, when malaria prevalence was expected to exceed70 percent. The children participated during the 1993 springrainy season.
Both groups received six- or seven-day courses of radical drugtherapy immediately after first giving blood and again dailyduring the treatment.
Meanwhile, in Baltimore, 20 men and women taking part in amalaria vaccine trial at Johns Hopkins' School of Hygiene andPublic Health allowed themselves to be bitten by fiveAnopheles stephensi mosquitos infected with P. falciparumparasites. Before this challenge, they gave blood drawn byfinger prick, as did the Kenyans, to assess the dipstick'sdiagnostic reliability.
Overall results showed the dipstick to be 95.5 to 100 percentsensitive for detecting more than 60 parasites per microliter ofblood. For fewer than 60 but more than 11 parasites, the scorewas 70 to 81 percent, and for fewer than 10 it was 11 to 67percent.
Among the American volunteers, specificity reached 95 percentand 98 percent for those exposed to the malarial mosquitobites.
"The dipstick assay's sensitivity, specificity, simplicity andspeed may make it an important tool in the battle againstmalaria," the Lancet paper concluded.
Getting the Dipstick to First- and Third-World Markets
Joseph Perrone, business director of tropical disease diagnosticsat Becton Dickinson's Advanced Diagnostics Division inBaltimore, is already poised to commercialize that tool. "We'reprobably going to be selling the dipstick initially for exportonly to Europe, to get it on the market while it goes throughFDA clearance," he told BioWorld. "Eventually, we obviouslywant to market this in all the endemic areas of the world."
But will the price be right? Medical epidemiology parasitologistPeter Blowland at the Center for Disease Control's (CDC)National Center for Infectious Diseases, told BioWorld he hearda Becton Dickinson representative speaking at a conference saythe company intends to price the dipstick at $1. "This would beoutrageously expensive," said Blowland, noting that manyAfrican countries can budget only $1 a year per capita for allhealth care.
However, Becton Dickinson's Perrone said, "We hope to make itaffordable for lesser-developed countries, for which it wasprimarily designed. I say that because we made great efforts toget stability so the product could be used in those areas. Iwould challenge you to find any other immunodiagnostic teston the market that has one-year stability at 37 degrees C.Everything else needs 4 degrees C for six months."
Becton Dickinson holds a number of patents on the malarialdiagnostic dipstock (covering the chromatographic assay) andthe liposome used as a detector. The sticks are made ofcellulose fiber and contain an immobilized IgG1 monoclonalantibody directed against a synthetic peptide from theparasite's antigenic HRP-2 (histidine-rich protein-2). Theantigen-detecting reagent contains polyclonal antibodiesagainst the falciparum-specific protein, conjugated to liposomescontaining pink dye. A thin pink band on the dipstick denotes amalaria-positive test result.
Asked why his company employed a polyclonal as well as amonoclonal antibody, Perrone said, "There is a patent fromHybritech that prohibits, without licensing, the use of amonoclonal-to-monoclonal system."
CDC's Blowland allowed that "depending on the situation, thisvery intriguing type of test would be exceedingly useful." Heobserved that people living in a Kenyan village are unlikely tohave access to the "gold standard" blood-smear diagnosis,which requires microscopes, refrigeration, electricity andtrained technicians.
However, he sees the dipstick's immediate usefulness indeveloped nations, notably the U.S., to diagnose malaria amongreturning travelers, Peace Corps volunteers and the military.Besides individual patient care, he expects that such widelyaccessible diagnosis could reduce the needless consumption ofanti-malarial drugs and thus decrease acquired reistanceamong the parasites.
And Now for the Bad News
"But there are down sides to the dipstick, too," Blowland added.Paradoxically, it's all too sensitive. "Nearly 100 percentsensitivity can pick up a very small number of parasites inareas with fairly high levels of immunity to malaria," he said.
He paints the picture of a Kenyan man who walks into thevillage health facility with a fever, and the dipstick reads 100parasites per cubic millimeter. "It takes a big stretch," Blowlandsaid, "to assume he's sick because of those 100 parasites. Fromour CDC malaria studies in Kenya, clinical definition requires atleast 3,000."
He warns that relying on the dipstick in such endemic areascould be potentially very problematic. "You would be treatingpeople unnecessarily, thus increasing parasite drug resistanceand not looking for other possible causes of the fever PP earinfection or pneumonia, for example."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.