[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 8 The Nematodes
191
PATHOGENESIS AND CLINICAL
SYMPTOMS
In general terms, three possible factors may contribute
to the ultimate severity of a nematode
infection: (1) the number of worms present;
(2) the length of time the infection persists; and
(3) the overall health of the host. Infections with
nematodes have been known to last for up to 12
months or longer (some infections may last 10
to 15 years or more), depending on the specific
species involved. The occurence of reinfections
and/or autoinfections may increase the infection
time up to several years and beyond; some infections
persist indefinitely. In most cases, an infection
of a small number of worms in a relatively
healthy individual may remain asymptomatic or
cause minimal discomfort. However, patients
who have a heavy worm burden, particularly if
combined with other health problems, are probably
more likely to experience severe symptoms
and/or complications.
The life cycle of each of the nematodes
involves the intestinal tract. With one exception,
all the nematodes may cause intestinal infection
symptoms at some point during their invasion
of the host. These typically include abdominal
pain, diarrhea, nausea, vomiting, fever, and
eosinophilia. Skin irritation, the formation of
skin blisters, and muscle involvement may also
be present.
Quick Quiz! 8-3
These individuals are prone to experiencing severe
nematode infections: (Objective 8-7)
A. Completely healthy individuals with a heavy worm
burden
B. Unhealthy individuals who are asymptomatic
C. Healthy individuals with a light worm burden
D. Unhealthy individuals with a heavy worm burden
NEMATODE CLASSIFICATION
The nematodes belong to the phylum Nemathelminthes.
Recent investigation of current
taxonomy classification terms revealed that the
placement of the term Nematoda varies by
source, listing it as a phylum, order or class. For
the purposes of this text and to maintain taxonomic
consistency, Nematoda is considered as a
class just like all of the helminth groups covered
in this text. The nematode species may be divided
into two groups, those primarily involved with
the intestinal tract, termed intestinal species, and
those that migrate into the tissues following
initial contact with the intestinal tract, termed
intestina tissue species. The species discussed in
this chapter under each category are listed in
Figure 8-1.
Phylum
Nemathelminthes
Class
Nematoda
Intestinal Species
Enterobius vermicularis
Trichuris trichiura
Ascaris lumbricoides
Necator americanus
Ancylostoma duodenale
Strongloides stercoralis
Intestinal-Tissue Species
Trichinella spiralis
Dracunculus medinensis
FIGURE 8-1 Parasite classification: The nematodes.