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Intestinal Nematodes: Non-Soil Transmitted Helminths 97
Fig. 10.12 Encysted
larvae of Trichinella
spiralis in striated
muscle
Morphology
Trichinella spiralis is one of the smallest nematodes infecting humans. The adult
male measures about 1.5 mm by 0.04 mm and the female about 3 mm by 0.06 mm.
The female worm is viviparous. The lifespan of the adult worm is very short. The
male worm dies soon after fertilizing the female and the female dies after discharg-
ing the larvae.
The larva is encysted in the striated muscle fibres. The larva in the cyst is coiled
two and a half times (Fig. 10.12).
Trichinella cysts are ovoid measuring 400 μm by 250 μm in size. The cyst is
formed by the tissue reaction around the encapsulated larvae. Cysts develop in stri-
ated muscles which include diaphragm, biceps, masseter and extraocular muscles.
The cyst lies longitudinally along the muscle fibres and larva remains infective
inside the cyst for years and eventually calcifies and dies.
Life cycle (Fig. 10.13)
(1) Human acquires infection via ingestion of undercooked pork containing encysted
larva. (2) Following ingestion, the larvae are released in the intestine. (3) The larvae
penetrate the mucosa of the small intestine and develop into adults. (4) After fertil-
ization, the gravid female which is viviparous deposits the larvae in the intestinal
mucosa. (5) The larva enters the blood circulation and is carried to the striated
muscles to be encysted.
The major source of human infection is ingestion of inadequately cooked pork.
Pathogenesis and Clinical Features
Trichinosis is the disease caused by T. spiralis. Intestinal invasion occurs when
the meat containing the encysted larva excysts. Larva invades the intestinal epi-
thelium of the small intestine and develops into adults. Gastrointestinal symp-
toms include nausea, diarrhoea, abdominal cramps and vomiting. The onset of
illness may last from 20–30 h after ingestion of infective meat and it mimics food
poisoning.