Founder of American Parasitology
Nematode parasite from a bullfrog
Joseph Leidy's contribution to American Parasitology (1) is perhaps second only to his contribution to Vertebrate Paleontology. Although several European scientists were active during the 19th Century, Leidy was essentially the only American researcher in the field. His work was highly regarded in Europe, but it wasn't until the early part of the 20th Century that it was broadly appreciated by his American peers (2).
Leidy's parasitological studies were concentrated into two separate periods. The first period (1846-1858) began early in Leidy's scientific career, while the second (1874-1891) occupied the latter part. In between, Leidy's research efforts were concentrated on Vertebrate Paleontology.
Early studies included many short articles and notices covering parasites from a wide variety of animals. Notable among these are his discoveries of canine heartworm (3) and of the human parasite Trichina spiralis in pork (4). This period is also notable for two larger works. In 1853 Leidy published a groundbreaking monograph titled "A Flora and Fauna Within Living Animals." In 1856 he published a synopsis (summary) of his studies on parasites (5). This synopsis contains brief descriptions and host information of a total of 172 species of parasitic protozoans, flukes, tapeworms and nematodes (6).
Distoma horridus, a parasitic fluke from a snake
In 1874, after nearly 16 years during which his efforts were devoted to paleontology, Leidy resumed his studies in parasitology. In addition to numerous shorter publications, he published longer articles on the tapeworms in birds, parasites of shad and herring, leeches (7) and the symbionts of termites. He also revised a section on medical parasitology ("Parasites and the Diseases They Produce") for an American edition of the British medical publication, System of Surgery, and wrote a treatise on human intestinal parasites in Dr. William Pepper's A System of Practical Medicine (8). His work in Pepper's publication represents the first comprehensive presentation of human parasites ever published in America.
Leidy's mastery of comparative anatomy and microscopic observation were manifest in his study of parasites. He reported on microscopic features undetected by leading European scientists (9). He also differed with his European peers in the classification of many of the minute animals he discovered. The Europeans commonly shoehorned new organisms into "time honored and universally accepted" genera (groups of related species). Leidy was also generally inclined to assign new species to existing groups, but he often found new species that that were significantly different. So he found himself erecting many new genera (10). His European peers sometimes rejected his findings, but Leidy characteristically avoided the challenge and concluded that time will tell who was right. In most cases, Leidy's work was ultimately confirmed by later researchers (both European and American).
His European colleagues (and millions of other people) should have also heeded Leidy's recommendations for the prevention of parasitic diseases by thoroughly cooking meat. As early as 1853 he wrote:
"Cooking food is of advantage in destroying the germs of parasites, hence man, notwithstanding his liability to the latter, is less infested than most other mammalia."
Websites:
- HealthCentral.com's web page on trichinosis: www.healthcentral.com/mhc/top/000631.cfm
- Tree of Life's web page parasitic protist:
tolweb.org/accessory/Parasitic_Protists?acc_id=53 - U.C.
Museum of Paleontology's web page on termite symbionts:
www.ucmp.berkeley.edu/protista/termiteprotists.html
Notes:
-
Parasitology is the study
of parasites. A parasite is now defined as an organism living in or on another
organism (the host) from which it obtains food. A parasite is harmful (but not
necessarily fatal) to its host.
Not all of the "parasites" studied by Leidy and other 19th Century parasitologists meet this modern definition of parasites. Instead, they used the term "parasite" to mean one organism that lived on or in another. They also used the term "entozoan" (= inside animal) to refer to an animal that lived inside another and endophyte (=inside plant) for algae, fungi or bacteria that lived inside an animal.
Modern scientists use the term symbiosis (=living with) for the intimate association of different organisms. (Symbionts are the organisms that partake in the association.) Three broad categories of symbiosis are recognized: Commensalism is a symbiosis in which neither organism is harmed or helped. Mutalism is a symbiosis in which both are helped (an often where both are dependent upon each other). Parasitism is a symbiosis in which one symbiont (the parasite) is harmful to the other (its host).
[go back] - The
lack of appreciation by American parasitologists was largely a consequence of
Leidy's lone status in the field. He was essentially the only American working
in the field until the turn of the century. Another factor was the change in scientific
methods. Leidy was primarily a describer of nature. The next generation of parasitologists
depended heavily on experimental methods. By the 1920's however, they began to
realize the breadth and scope of his contributions.
[go back] - Canine
heartworm is a common parasitic disease of dogs caused by the nematode Dirofilaria
immitis. The parasites lives in the heart and large blood vessels of the lungs
and cause heart and lung failure in the hosts. Leidy discovered and described
this parasite in 1856.
[go back] - Trichinia
spiralis is a parasite of humans and the cause of trichinosis. The disease
had been a major public health problem for many years in both Europe and North
America. The adult worms were first described by British scientists in 1835, but
no one knew how the parasite infected humans.
Leidy first reported on finding the parasite in pork in the October 1846 issue of the Proceedings of The Academy of Natural Sciences of Philadelphia. One year later, this report was reprinted in a British journal, the Annals and Magazine of Natural History. The world's leading expert in parasitic worms, Karl Moritz Deising, incorporated Leidy's findings in his classic and influential monograph "Systema Helminthum" in 1851. Unfortunately, Deising referred to Leidy's parasite as Trichinia affinis ( a parasite that does not infect humans) rather than Trichinia spiralis.
Consequently, European scientists investigating the life history of Trichina sprialis were unaware of (or dismissed) Leidy's discovery. Leidy again reported on the relationship between the parasite and pork in 1853 and 1866; both times he stated that thorough cooking killed the parasite and prevented infection.
These later reports were apparently unknown or ignored by the Europeans. It would take nearly two decades following Leidy's first report to learn of the connection between the parasite and pork.
[go back] - "A
Synopsis of Entozoa and Some of Their Ectocogneners". Proceeding of the Academy
of Natural Sciences of Philadelphia. Vol. 10. 1856.
[go back] - Protozoans
are unicellular (or acelluar) organisms that were once classified as animals.
Their current classification is a matter of debate, but most biologists recognize
that they represent several, widely divergent evolutionary lineages. Most protozoans
are not parasitic, but some parasitic forms are responsible for some of our most
serious diseases. Giardiasis (Giardia), sleeping sickness (Trypanosoma) and malaria (Plasmodium) are three examples of protozoan parasitic diseases.
Flukes (trematodes) and tapeworms (cestodes) are two groups of exclusively parasitic flatworms (Platyhelminthes) that are distantly related to the common flatworm, Planaria.
Nematodes (roundworms) comprise a diverse and extremely abundant group of worm-like animals found in virtually all habitats. Most are free-living, but many are parasites of plants or animals (including humans).
[go back] - Leeches
(Hirudinea) comprise a group of segmented worms related to earthworms. Most live
in freshwater and prey on small invertebrates, but some are bloodsucking parasites
of vertebrates.
[go back] - A former student of Leidy,
William Pepper was the driving force in modernizing medical education at the University
of Pennsylvania in the second half of the 19th Century. He was also instrumental
in diversifying the university through the creation of the Schools of Dentistry,
Biology and Veterinary Medicine.
[go back] - Perhaps the most notable
case where Leidy outdid his European peers involved gregorines. Leidy detected
minute fribrillae (hair-like structures) that he concluded to be the first traces
of muscular structures. This structure had been missed by Carl T. von Siebold,
a leading European parasitologist. (Gregorines are microscopic parasites which
belong to either the Sporozoa or the Apicomplexa. The most infamous apicomplexan
is Plasmodium, the parasite that causes malaria.)
[go back] - Leidy
named at least 806 new genera of animals, fungi and protozoans. The largest share
were vertebrates, but he named at least
113 protozoans and 339 invertebrates, many of which were parasites.
[go back]