R E F L E C T I O N S O N B I O D I V E R S I T Y
Frog legs and parasite tales
What do amphibians, Asian folk medicines, ozone-layer damage, and new antibiotics have in common? Columbia╠s tropical medicine specialist sees portents and possibilities in nature's infinite variety
By DICKSON D. DESPOMMIERTHAT A TREATMENT can be worse than the disease is more than a cliché. Healers in Hainan, China, and several southeast Asian nations have long known that poultices made from a frog's leg muscles effectively treat open wounds and eye diseases, yet the same muscle tissues may harbor parasites that replace the original disease with another, perhaps worse. It is far from rare for someone with an eye infection, for example, to regain vision after such treatment, only to lose it again after the same eye develops the tightness and swelling that characterize an endemic disease known as sparganosis.
If humans are sometimes fatally clever in adapting natural substances for healing purposes, our resourcefulness pales in comparison to other organisms' efforts at adaptation. Parasitologists regard the life cycle of the tapeworm Spirometra mansonoides, the cause of sparganosis, as one of the most complex in nature. This organism requires several intermediate animal hosts to complete the journey from egg to adult parasite: It starts as a non-parasitic microscopic larva, swimming free (for about an hour, while its own stored energy lasts) in stagnant fresh water, then is sequentially consumed to infect (and, in turn, consume portions of) a crustacean, a tadpole and its adult equivalent the frog, or alternatively a snake or fish, finally ending its journey as a mature worm in the small intestine of a cat, tiger, leopard, panther, or other felid. With each host, it undergoes transformations culminating in adulthood--much like a butterfly, except that the parasite depends on other life forms.
Within each of its host animals, S. mansonoides confronts a bewildering array of physiological challenges. It has been selected for life first in a hypotonic environment (with lower salt concentration than its own) in fresh water, then a hypertonic environment in the blood space and muscle tissue of the crustacean, and yet another hypertonic environment in the tadpole and frog, all the while enduring a fluctuating temperature gradient until it infects the cat, where it enters another hypotonic environment with an elevated constant temperature but a fluctuating food supply (the adult parasite, lacking a digestive tract, doesn't eat until the cat takes its own meal). Despite such tortuous biology, this infectious agent has succeeded on a grand scale. The prevalence of the adult tapeworm in wild and domestic cats reaches 100 percent in some areas, while aberrant larval infection in humans can be as high as 30 percent in the mountainous regions of Hainan. What appear as weak links in Spirometra's life cycle may actually turn out to be points of reinforcement.
One of the larva's more interesting features is its ability to secrete a hormone-like protein that induces uncontrolled cellular growth. This factor has yet to be fully characterized but has been isolated in semi-pure form. When a larva or the growth hormone alone is placed under the skin of laboratory-reared mice, all tissues become affected; the rodents grow to immense proportions, often achieving three times their normal size within a month. Under natural conditions within the frog, this increased growth can spell disaster, since obese frogs are sluggish and are easier prey for cats who hunt them. Humans cannot be infected with the adult worm, due to differences in intestinal physiology between the two mammals to which the parasite is exquisitely adapted--but if frog, snake, or fish tissue containing the larva is used as a poultice, the parasite's highly evolved nervous system can detect heat, triggering a thermotactic behavior: It crawls out of the cold-blooded vertebrate tissue into the open wound or under the eyelid of the unsuspecting human. The growth hormone elicits cellulitis, leading to blindness if an eye is infiltrated. Not uncommonly, the parasite seeks other venues, such as the optic nerve or brain, usually causing loss of sensory or motor function. Rarely, death may ensue. When it enters a human host, Spirometra is a stranger in a strange land.
An unexpected natural shield
IN ANOTHER STRANGE land, the laboratories of the National Institutes of Health, a different change of contexts has spawned a different relationship between humans and frogs. Gene therapy researcher Michael Zasloff, after harvesting ovaries from the African clawed frog Xenopus laevis to splice human gene segments into the oocytes and study the resulting proteins, noted that the mortality rate of the frogs was astonishingly low. After a quick and rather unsanitary ovariectomy--which, if performed in humans, would kill them with peritonitis within a week--80 percent of his frogs survived. Something must be protecting them.
Within six months of turning his attention to his tenacious egg donors, Zasloff was ready to resign from the NIH, change specialties, and form a biotechnology company. He had discovered a new class of polypeptide compounds with outstanding antimicrobial properties; Xenopus laevis females secrete at least two of these substances from skin glands. Magainins (named for the Hebrew word magain, or shield) are produced by virtually all frog species examined to date; with different related magainins produced by each species and about 3,800 frog species known in the world, it is possible that hundreds of thousands of antimicrobials are waiting to be discovered. Magainin Pharmaceuticals quickly attracted venture capital, and Zasloff won a Kilby Prize for scientific innovation in 1994.
Magainins are polymerized linear compounds made from a unique mixture of amino acids. In a solution of water, each molecule assumes a corkscrew-like shape that aggregates into a barrel-shaped conglomerate that can create a structural defect in the outer membrane of most living cells, thus destroying them on contact. Magainins are effective in vitro against a broad range of infectious agents, from bacteria to fungi to protozoans. In mammals they are quite toxic, but even this may turn out to be an advantage; initial trials of direct injection into certain solid tumors have been encouraging. To date, magainins have been only partially successful when applied experimentally to infectious diseases. When we know more about how to chemically modify them and optimize their route of injection, magainins may be effective against tuberculosis and malaria.
It's the ozone layer, stupid
AT THE UNLIKELY intersection between the disciplines of herpetology and atmospheric studies lies information that underscores the linkages between our species's well-being and that of others. Oregon State University's internationally recognized herpetologist Andrew Blaustein, who has studied the effect of ultraviolet-B radiation (with a wavelength of 280-320 nm) on amphibians, particularly frogs and toads, has expressed concern over ozone depletion and disappearance of species often enough that some skeptical colleagues have labeled him "Chicken Little." The joke--like the ozone layer itself, as data from polar orbiting satellites and Finnish weather stations confirm for both the South and North Poles--has worn thin over the years.
Ozone (O3) has the property of intercepting and neutralizing UV-B, protecting life forms that are susceptible to DNA damage by this form of radiation. Blaustein's preliminary experiments in lakes of Oregon's Cascade range point to UV-B radiation as potentially the sole cause of a decline in populations of the Western toad Bufo boreas and other species, especially those that deposit fertilized eggs in shallow waters; amphibians that breed in water too deep for UV-B to reach are unaffected. Certain species such as the Pacific tree frog appear to be lucky exceptions resistant to the effects of UV-B, probably because of high levels of an enzyme (photolyase) that repairs radiation damage to DNA. The work of Blaustein and colleagues is providing and testing a working hypothesis that may explain the decline of some amphibian species. And if ozone depletion is already threatening these species with extinction, other elements of worldwide food chains are at risk--from phytoplankton (already shown under controlled experiments to be vulnerable to ozone effects) to the krill that feed on plankton to the whales and penguins that feed on krill....
The message from frogs to us
WHY SHOULD ANYONE care if frogs and toads disappear? People who have suffered from sparganosis might even welcome the change. But to most of the human race, reduction in biodiversity could be tragic news. Antibiotic development is constantly just a step ahead of--and sometimes behind--the capacity of pathogens to resist these drugs. Treatment for drug-resistant tuberculosis, hospital-acquired Staphylococcus aureus infection, and penicillin-resistant syphilis will rely on new generations of natural antibiotics derived from soil-dwelling organisms. Other antimicrobials are derived from large plants; quinine, for example, the parent drug of agents still used against malaria, originally came from cinchona bark. Paclitaxel (Taxol), a proven antineoplastic, is derived from the bark of the Pacific yew. Where will future generations of antibiotics come from when all the easily isolated ones have run their course and the coral reefs, temperate and tropical rain forests, and other natural resources we still take for granted are damaged or gone?
Natural-products chemists have rarely been able to take advantage of the hints given to them through chance encounters with traditional medicines. On the other hand, the magainin story is a superb example of Pasteur's admonition that luck favors the prepared mind.(1) Continued research could lead to practical application of these molecules, provided their toxicity can be tamed. Yet even here the task of drug development is not easy. A random search through the 3,800 or so species of frogs in search of the next effective magainin would interest few drug companies; even if such a search were conducted, it might be too late, with amphibians dying off at an alarming rate. How, then, can we take advantage of the world as it exists and shorten the time between discoveries of new treatments?
A survey of human populations that use treatments like amphibian-tissue poultices may well lead to new therapies against illnesses that have outpaced standard Western antibiotics. This approach requires an intimate knowledge of human diseases and behavior and most importantly an open mind toward cultures that have developed their pharmacopoeias by trial and error and are often reluctant to share this information with outsiders. Future generations of scientists must be willing to pursue knowledge through rigorous scientific inquiry across any boundary if we are to survive the next onslaught of infectious diseases--entities predictably lurking around the next dark corner of this crowded earth.
Natural selection, driven by punctuations of environmental upheaval, produces a series of correct answers to questions of how animals and plants survive in their complex niches, inventing new defense mechanisms and dispensing with ineffective ones. These mechanisms include the synthesis of antibiotics, the most familiar of which are produced by soil organisms. An antibiotic is in a real sense one way for an organism to assert its territorial imperative, producing a zone of growth inhibition for other competing microbes. (Gramicidin, recognized as the world's first antibiotic to be isolated from the plentiful resource of soil, turns out to be related to the magainins in both structure and activity. It is indeed a small world, even at the chemical level.) The task researchers face is to recognize a biological answer when we find one in its original setting and to learn to do so in whatever short time we have.
Pasteur L. Oeuvres de Pasteur reunies. 7 vols (Paris: Masson, 1922-1939). Vol. VI, p. 348, and vol. VII, p. 131.
DICKSON D. DESPOMMIER, Ph.D., is professor of public health (parasitology and environmental sciences) and microbiology at Columbia's School of Public Health and College of Physicians and Surgeons.
PHOTO CREDITS: © Zoltan Takacs (frog); Dickson D. Despommier (S. mansonoides, deforestation); © Molecular Simulations, Inc. (magainin).