[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 8 The Nematodes
FOCUSING IN
This chapter begins with the discussion of the
helminths. These organisms differ from the previously
described single-cell protozoa in the fact
that they are multicellular and contain internal
organ systems. The first group of helminths discussed
are the nematodes, commonly known as
the intestinal roundworms.
MORPHOLOGY AND LIFE
CYCLE NOTES
Members of the class Nematoda (multicellular
parasites that appear round in cross section) may
assume three basic morphologic forms: eggs
(female sex cells after fertilization), juvenile
worms known as larvae, and adult worms. The
eggs vary in size and shape. In the appropriate
environment, developing larvae located inside
fertilized eggs emerge and continue to mature.
These larvae are typically long and slender. The
growing larvae complete the maturation process,
resulting in the emergence of adult worms. Sexes
are separate. The adult female worms are usually
larger than the adult males. The adults are
equipped with complete digestive and reproductive
systems. Specific features vary with the individual
species.
The life cycles of the individual nematodes are
similar but organism-specific. An intestinal nematode
infection may be initiated in several ways.
In the case of pinworms, for example, ingestion
of the infected eggs transmits the disease. The
hookworm larvae, on the other hand, burrow
through the skin of the foot and make way into
the intestinal tract. The exact means whereby
each organism enters the host and migrates into
the intestinal tract varies by species. The eggs or
larvae, depending on the species, continue to
develop into adulthood. The resulting adult
worms reside in the intestine, where they concentrate
on obtaining nutrition and reproduction.
Fertilized adult female nematodes lay their
eggs in the intestine. These eggs may be passed
into the stool. Once outside the body, the larvae
located inside the eggs require warm moist soil
and 2 to 4 weeks for continued development.
The developed egg is then ready for infection into
a new host and the cycle is repeated. It is important
to note that this description of a nematode
life cycle is only basic and general. In two of the
nematode life cycles, Trichinella and Dracunculus,
tissue becomes involved, serving as the
primary residence of the organisms. It is interesting
to note here that in general terms, most
members of the nematode group have the ability
to exist independent of a host (i.e., they are freeliving).
The specifics of each nematode organism’s
life cycle are addressed on an individual
basis.
Quick Quiz! 8-1
The junvenile stage of developing nematode worms
is referred to as: (Objective 8-1)
A. Eggs
B. Cysts
C. Larvae
D. Adults
LABORATORY DIAGNOSIS
The laboratory diagnosis of nematodes may be
accomplished by the recovery of eggs, larvae, and
occasional adult worms. The specimens of choice
vary by species and include cellophane tape preparations
taken around the anal opening, stool
samples, tissue biopsies, and infected skin ulcers.
In addition, serologic test methods are available
for the diagnosis of select nematode organisms.
Representative laboratory diagnosis methodologies
are located in Chapter 2 as well as in each
individual parasite discussion, as appropriate.
Quick Quiz! 8-2
Adult nematodes are never recovered in clinical
samples. (Objective 8-6)
A. True
B. False