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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.
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