Page 220 - parasitology for medical and clinical laboratoryprofessionals

200 CHAPTER 9

INTRODUCTION given rise to the Mosaic and Islamic traditions of avoid-
ing pork. Eating pork is a practice that has also been at-
Three major parasitic infections invade the major tis- tributed to tapeworm infection (see following sections
sues and organs of the human body. One infection of this chapter). Five species of Trichinella, a nematode
is trichinosis, caused by the roundworm Trichinella characterized by its invasive larval stages, exist, includ-
spiralis, which was discussed in Chapter Five of this ing T. spiralis, T. native, T. nelsoni, T. britoui, and
book. The other two major species, Echinococcus gran- T. pseudospiralis. Of these five species, T. spiralis is
ulosus and Echinococcus multilocularis, are organisms most important as a parasite of humans as it does not
that cause a major disease called echinococcosis. Both have specific hosts that it infects, and the disease is dis-
similarities and differences exist between the trichinella tributed throughout the world.
organism and those of the genus Echinococcus. Both The association between trichina infections and
E. granulosus and E. multilocularis cause a serious dis- pigs has been long recognized but the encysted larvae
ease where hydatiform cysts form, whereas T. spiralis in the muscle were not seen until 1821. Even after the
causes single cyst forms in muscles, and is a self-limiting cysts in the infected muscle were observed, the condi-
condition. E. granulosus and E. multilocularis are ces- tion was not associated with disease in humans. The
todal organisms, but unlike the cestodal diseases dis- discovery of the worm in humans was made in 1835
cussed earlier, these species are found only as organisms by James Paget, who was then a medical student at
with only a few proglottids. The entire echinococcal St. Bartholomew’s Hospital in London. Sir James Paget
organism may consist of only three segments, whereas was later knighted for his accomplishments in this and
other cestode species may grow to perhaps several meters. other areas of medicine. Another important medical
One major difference that exists between the diseases finding attributed to this observant physician is the bone
of trichinosis and echinococcosis relates to the differing disease osteitis deformans, or softening of the bones,
genera of the two, where the organism causing trichinosis, which is a condition called Paget’s disease. The defini-
T. spiralis, is a roundworm, and the other two echinococ- tive report of trichinellosis was written by Richard Owen
cal organisms are classified as cestodes. Echinococcal who attempted to minimize Paget’s role, although he
infections begin initially as a tapeworm in the digestive sys- failed to associate that the wormlike organism embedded
tem, and cysts may form in vital organs as a result of this in human muscle was the larval stage of a nematode.
primary infection. Therefore, for these reasons this disease Religious proscriptions relating to eating pork by
is also treated as a tissue- invasive disease and is discussed Muslims and Jews perhaps stem from basic knowledge of
along with trichinosis. The two genera of these parasites cestodal infections and well as Trichinella spiralis. Certain
are similar in that they may be contracted through eating ritualistic practices in food preparation in some religions
infected or poorly cooked meat. Another difference is that and cultures almost certainly relate to avoiding the eating
T. spiralis is found in pork primarily, although other meats of meat contaminated by microorganisms. Many centu-
have been implicated, and E. granulosus and E. multilocu- ries ago the Jewish rabbis were known to inspect meat
laris may be found in a variety of meats. and other foods before they were pronounced edible, as
evidenced by writings of that era. It could reasonably be
concluded that some of these practices led eventually to
TRICHINOSIS, AN INFECTION the establishment of governmental offices responsible for
OF TRICHINELLA SPIRALIS ensuring food safety.

Trichinosis is also known as trichinellosis or as a trichina Morphology
infection and is caused by the intestinal nematode worm
Trichinella spiralis. This organism requires two hosts The adult worms were discovered by Rudolf Virchow in
in its life cycle. The female worms produce larvae that 1859 and Friedrich Zenker in 1860. Zenker finally rec-
encyst in muscle upon which the new host becomes ognized the clinical significance of the infection and con-
infected when the encysted muscle is eaten. Because cluded that humans became infected by eating raw pork.
human infections are usually acquired by eating pork The importance of these studies lies not only in the field
containing the encysted larvae, this might have initially of human parasitology but also in the more general field of