Parasite Eccentricities

PERSPECTIVES IN BIOLOGY AND MEDICINE Volume 30 ¦ Number 1 ¦ Autumn 1986 PARASITE ECCENTRICITIES JERRY Y. NIEDERKORN* Among the diverse life forms that inhabit our planet, the parasitic organisms demonstrate some of the most fascinating and spectacular examples of adaptation. From the 3-mm pinworm to the 33-foot-long tapeworm Diphyllobothrium latum, the diversity among parasites is striking . Our relationship with parasites is as old as humanity itself. Paleopathological studies have demonstrated the presence ofblood fluke eggs in the livers of 2,000-year-old Egyptian mummies [I]. Even individual parasite partners can form long-term relationships with their hosts as is found with the blood fluke Schistosoma, in which a single worm can reside in its hapless human host for up to 3 decades [2]. Fecundity is another remarkable feature of many worm parasites. A single D. latum tapeworm can produce up to 1 million eggs per day [2]. Although less prolific, the intestinal nematode Ascam lumbricoides releases in the neighborhood of 250,000 eggs each day. To offset this modest egg production, the ova of this worm are among the most resistant biological products found in the animal kingdom [I]. Ascaris eggs can retain their infectivity even after being subjected to formalin fixation [3]. Other parasites increase their chances for transmission by the continuous asexual production of infectious larvae. For example, the tapeworm Echinococcus granulosus develops fluid-filled hydatid cysts that contain infective larvae. Although a relatively rare occurrence, hydatid cysts can reach enormous sizes. A case reported from Australia recorded the removal of a hyatid cyst This essay is dedicated to the memory ofDr. David A. Becker, friend and mentor, who rekindled the author's interest in the world of worms. The author is indebted to Dr. Rupert Billingham for his encouragement to write this paper. The manuscript was carefully prepared by Ms. Sara Howard. *Department of Ophthalmology, University of Texas Health Science Center at Dallas, 5323 Harry Hines Boulevard, Dallas, Texas 75235-9057.© by The University of Chicago. AU rights reserved. 003 1-5982/87/300 1 -0506$0 1 .00 Perspectives in Biology and Medicine, 30, 1 ¦ Autumn 1986 \ 1 containing over 15 quarts of fluid [I]. In other words, approximately 32 pounds of the patient's body weight was attributable to parasite material. In addition to sheer mass, longevity, and reproductive potential, parasites have developed incredible strategies for ensuring both their longterm survival within the host and the perpetuation of the parasite species. Finding a Host: The Trojan Horse Strategy Perhaps the most common mode of parasite transmission is through the ingestion of infectious larvae or ova by the unsuspecting host. There are countless examples of parasite life cycles in which the host unwittingly consumes plant or animal stuffs contaminated with parasite larvae or eggs. However, a number of ingenious parasite species have refined this strategy to truly remarkable degrees. One fascinating example of this charade is found in the trematode Leucochloridium macrostomum [4]. This flatworm parasite utilizes songbirds as its definitive hosts. However, the worm larvae must first undergo a period of development within a terrestrial snail. Terrestrial snails normally avoid light, exposed areas and dwell under the cover of leaves and decaying wood—a behavior that lessens the likelihood of prédation by birds. However, when infected with L. macrostomum larvae, the snails undergo striking changes not only in their appearance but also in their behavior. Larval worms migrate to the snail's tentacles which, in turn, swell enormously and become bright green with yellow and brown rings. For reasons that are unknown, the snail then becomes attracted to brightly lit areas, where the tentacles begin to pulse at 50—100 beats per minute. The brightly colored grubshaped tentacles do not escape the keen eye of songbirds which happily swoop down and feast on the Trojan horse meal. Although less sophisticated, other larval parasites alter the behavior of their intermediate hosts as a means of ensuring their transmission to the definitive host. The larval tapeworm Ligula intestinalis infects the peritoneal cavity of small shoal fish such as rudd. The enormous mass of the larval worms fills the body cavity of the fish, which then is unable to swim in its normal fashion...