Quick: Name an infectious pathogen that can develop in thebody for years without any symptoms, can't be cultured invitro, lacks an effective animal model and often shows no signsof responding to drug treatment even after it's cured.
AIDS? No. TB? No -- but close. The disease is leprosy, and itselusive, evasive, invasive pathogen, Mycobacteria leprae.belongs to the same genus as M. tuberculosis.
Leprosy is endemic in 93 countries, according to the WorldHealth Organization. WHO estimates that M. leprae affects 11million or 12 million people throughout the world, withperhaps as many more concealing the stigma of their disease.The agency has set a goal of eliminating leprosy as a publichealth threat.
Compared to better-known, lethal Third World afflictions suchas malaria or schistosomiasis, which afflict tens or hundreds ofmillions, leprosy (or Hansen's disease) infects a relatively smallnumber of victims. But the human and financial cost of theirlifelong medical management is several logs higher than theirpopulation statistics would suggest.
Leprosy is debilitating, and the major etiology of infection-caused blindness, but it is not a killer. Its victims die of otherdiseases or of old age.
Known by name since Biblical times, leprosy still outfoxesmedical science. Modern antibiotics -- dapsone, rifampin andclofazimine -- make short work of M. leprae until the microbestrikes back with drug resistance or relapse. Even whenchemotherapy works, it leaves leprologists in the dark as towhether or not the patient is cured. So clinicians eithercontinue maintenance drug therapy for years or stop it whenthe patient's symptoms begin to clear up, even though anunseen fraction of live bacilli lurk unseen within body tissues.
A major problem in treating Hansen's disease is that deadorganisms may persist undetected in the healing tissues foryears.
The highest-tech drug monitoring in use today involvesinjecting the foot pad of a thymectomised, irradiated mousewith skin smears or biopsies and watching the slow-growingbacteria multiply to a point where they can be observedmicroscopically. This viability-testing process takes 46 weekson average.
The foot pads are several degrees cooler than rodent body heat,a temperature gradient that stimulates the fastidious pathogento proliferate.
But Hansen's disease research teams on several continents noware beginning to co-opt a higher technology, polymerase chainreaction (PCR). The latest report on this gene-amplifyingsystem is in the current issue of Lancet, titled "Use ofpolymerase chain reaction to assess efficacy of leprosychemotherapy."
The work is a cooperative project between two British centers,the London School of Hygiene and Tropical Medicine andUniversity College Medical School in London, and two Pakistanicenters, Karachi's Agha Khan University and Marie AdelaideLeprosy Centre.
Their method relies on the fact that the DNA of dead cells ismore subject to degradation than are other cell components.
The team performed PCR amplification on 93 skin biopsysamples from 13 Pakistani patients with varying types andstages of leprosy, repeated three, six, 12 and 24 months afterthe start of their drug treatment.
To construct leprosy-specific PCR primers, the scientistsextracted M. leprae nucleic acid from the spleen of an infectedarmadillo. (Armadillos are primitive mammals that avidlyharbor the leprosy bacterium, presumably because their bodytemperature is several degrees cooler than other mammals.)
Testing in the PCR with DNA from 14 mycobacterial speciesother than M. leprae confirmed the leprosy-specific sequenceof their primers.
As the Lancet paper points out, "the use of PCR as a measure ofviability depends upon how quickly DNA is degraded after celldeath."
Unlike standard methods of tracking viability with therapy,"PCR clearly showed a decrease in the number of genomespresent, especially during the first three months of treatment."This finding," the paper said, "accords with the observations of(Diana) Williams et al of little or no detectable M. leprae DNA inpatients treated for two months."
Williams is the molecular biologist at the Laboratory ResearchBranch of the Gillis W. Long Hansen's Disease Center atLouisiana State University. The geographical location of thisranking U.S. facility for leprosy science is no accident. Louisianaand Texas report a majority of the 12 to 20 indigenous cases ofleprosy diagnosed annually in the U.S., according toepidemiologist David Sniadack of the Centers for DiseaseControl in Atlanta. He told BioWorld that these two states havehigh populations of armadillos, a living reservoir of high-titerM. leprae infection. But Sniadack added that no cause-and-effect conclusion can be drawn from this guilt by association,which is under active study at the Louisiana research facility.
Williams noted that in remote, poverty-line rural areas, peopleeat the meat of armadillos, but not all who do contract leprosy.Cooking kills the teeming pathogens, she pointed out, butkilling and dressing the animal may transfer live microbes toan open cut or abrasion.
Williams and the Louisiana group have been using PCR todetect and monitor leprosy since 1990. She is familiar with theBritish/Pakistani work reported in the current Lancet, butbelieves it's far too soon to consider the DNA amplificationmethod ripe for field application. "Due to the expense andexperimental nature of this technology," she told BioWorld, "itis presently not a clinically applicable tool for such use inendemic, developing countries, where the cost of skin biopsiesalone -- basis of the Lancet-reported work -- is prohibitive."
She has developed nasal mucosa and small slit-skin harvestingfor less costly sampling. Williams sees PCR rather as a tool forresearching various aspects of the disease, screening new drugsfor efficacy "and monitoring the potential for relapse inselected controlled studies." She predicted that "as thetechnology evolves and becomes more simplistic and cost-effective, PCR will become more applicable for a clinicalenvironment."
Molecular biologist Jacquie Keer, a co-author of the Lancetreport, told BioWorld that she and her colleagues are nowdeveloping a colorimetric process for reading the PCR viabilitytest as an alternative to agarose gel electrophoresis, which isnot available to some Third World countries. They hope tobring their PCR and colorimetric assays to Karachi "some timenext year" for preliminary clinical testing.
Williams has done the same, but is concentrating on a non-radioactive probe for the PCR product "very similar tocolorimetry," again to make the test more usable in the areas ofthe world where it might eventually be employed for leprosydiagnosis and monitoring.
"PCR technology, as reported in the Lancet article," shecommented, "is part of a powerful tool with potentialapplications in many diseases, not just infectious ones, not justleprosy, but for detection of AIDS, TB, genetically inheriteddisorders, even prenatal diagnosis."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.