Foodborne Animal Parasites 697
Phylum Nematoda
Class Adenophorea (= Aphasmidia)
Order Trichinellida
Genus Trichinella
Class Secernentea (= Phasmidia)
Order Rhabditida
Genus Turbatrix
Order Ascaridida
Genus Ascaris
Subfamily Anisakinae
Genus Anisakis
Genus Pseudoterranova
(Phocanema)
Genus Toxocara
Trichinosis
Trichinella spiralis is the etiological agent of trichinosis (trichinellosis), the roundworm disease that
has been of greatest concern from the standpoint of food transmission, especially in the United States.
The organism was first described in 1835 by J. Paget in London, and the first human case of trichinosis
was seen in Germany in 1859.
61 Although most flatworm and roundworm diseases of humans are
caused by parasites that require at least two different host animals, the trichinae are transmitted from
host to host; no free-living stages exist. In other words, both larval and adult stages of T. spiralis are
passed in the same host. It is contracted often from raw or improperly cooked pork or bear meats.
The adult forms of T. spiralis live in the duodenal and jejunal mucosas of mammals such as swine,
canines, bears, marine mammals, and humans that have consumed trichinae-infested flesh. The adult
females are 3–4 mm long, and adult males are about half this size. Although they may remain in the
intestines for about a month, no symptoms are produced. The eggs hatch within female worms, and
each female can produce around 1500. These larvae, each about 0.1 mm in length, burrow though the
gut wall and pass throughout the body, ultimately lodging in certain muscles. Only those that enter
skeletal muscles live and grow; the others are destroyed. The specific muscles affected include those
of the eye, tongue, and diaphragm. When assaying for trichinae larvae in pork, the U.S. Department of
Agriculture (USDA) employs diaphragm muscle or tongue tissues. In a recent study, the Crus muscle
of the diaphragm was found to yield more larvae per gram than several others.
64 As the larvae burrow
into muscles several weeks later, severe pain, fever, and other symptoms occur, which sometimes lead
to death from heart failure (see below). The larvae grow to about 1 mm in muscles and then encyst by
curling up and becoming enclosed in a calcified wall some 6–18 months later (Figure 29–2). The larvae
develop no further until consumed by another animal (including humans), but they may remain viable
for up to 10 years in a living host. When the encysted flesh is ingested by a second host, the encysted larvae are freed by the enzymatic activities in the stomach, and they mature in the lumen of the intestines.
Prevalence
About 75 species of animals can be infected by T. spiralis, but avians appear to be resistant.
69 During
the 1930s and 1940s, about 16% of Americans were infected.
69 For the period 1966–1970, 4.7% of
Phylum Nematoda
Class Adenophorea (= Aphasmidia)
Order Trichinellida
Genus Trichinella
Class Secernentea (= Phasmidia)
Order Rhabditida
Genus Turbatrix
Order Ascaridida
Genus Ascaris
Subfamily Anisakinae
Genus Anisakis
Genus Pseudoterranova
(Phocanema)
Genus Toxocara
Trichinosis
Trichinella spiralis is the etiological agent of trichinosis (trichinellosis), the roundworm disease that
has been of greatest concern from the standpoint of food transmission, especially in the United States.
The organism was first described in 1835 by J. Paget in London, and the first human case of trichinosis
was seen in Germany in 1859.
61 Although most flatworm and roundworm diseases of humans are
caused by parasites that require at least two different host animals, the trichinae are transmitted from
host to host; no free-living stages exist. In other words, both larval and adult stages of T. spiralis are
passed in the same host. It is contracted often from raw or improperly cooked pork or bear meats.
The adult forms of T. spiralis live in the duodenal and jejunal mucosas of mammals such as swine,
canines, bears, marine mammals, and humans that have consumed trichinae-infested flesh. The adult
females are 3–4 mm long, and adult males are about half this size. Although they may remain in the
intestines for about a month, no symptoms are produced. The eggs hatch within female worms, and
each female can produce around 1500. These larvae, each about 0.1 mm in length, burrow though the
gut wall and pass throughout the body, ultimately lodging in certain muscles. Only those that enter
skeletal muscles live and grow; the others are destroyed. The specific muscles affected include those
of the eye, tongue, and diaphragm. When assaying for trichinae larvae in pork, the U.S. Department of
Agriculture (USDA) employs diaphragm muscle or tongue tissues. In a recent study, the Crus muscle
of the diaphragm was found to yield more larvae per gram than several others.
64 As the larvae burrow
into muscles several weeks later, severe pain, fever, and other symptoms occur, which sometimes lead
to death from heart failure (see below). The larvae grow to about 1 mm in muscles and then encyst by
curling up and becoming enclosed in a calcified wall some 6–18 months later (Figure 29–2). The larvae
develop no further until consumed by another animal (including humans), but they may remain viable
for up to 10 years in a living host. When the encysted flesh is ingested by a second host, the encysted larvae are freed by the enzymatic activities in the stomach, and they mature in the lumen of the intestines.
Prevalence
About 75 species of animals can be infected by T. spiralis, but avians appear to be resistant.
69 During
the 1930s and 1940s, about 16% of Americans were infected.
69 For the period 1966–1970, 4.7% of