Trichinella spiralis was first seen by James Paget but was named and described by his professor Richard Owen.   It is a parasite of carnivorous animals and is especially common in rats and in swine fed on uncooked garbage and slaughter house scraps Humans become infected by eating raw pork. It is a cosmopolitan parasite and prevalent in many European countries with the highest interest being in China.

Trichinella spiralis spiralis - temperate zone -

Trichinella spiralis nelsoni - Tropics - low infectivity for pigs and rats and high infectivity for lions, hyenas

Trichinella spiralis nativa - Arctic - low infectivity for pigs, found in polar bears, resistant to freezing

Humans become infected by eating raw meat containing encysted ]arvae.  The cyst becomes digested and releases the larvae which invade the intestinal mucosa. They develop and mate in the second day.  By the 6th day of infection, the female adults deposit motile larvae which are carried by the intestinal lymphatics or mesenteric venules to other tissues in the body.  The very active muscles are invaded and the larvae become encapsulated. The cyst wall is derived from the host's muscle fibre and the larvae remain viable for many years.

Clinical Disease

Symptoms during the intestinal phase may go unnoticed or may be severe.  Epidemics can result in

outbreaks of gastro-enteritis, 2 to 7 days after the ingestion of raw pork.   Diarrhoea with or without

abdominal pain may last for several weeks.  Eosinophilia and fever occur in most cases.  Leucocytosis is common and hyperglobulinaemia is characteristic.  Myocytosis and circum orbital oedema are classic signs.  There can also be central nervous system involvement

Pathogenicity

The primary pathogenic effect of Trichinella comes from the destruction of the striated muscle fibres in which it encysts.   There can be neurological manifestations of trichinosis and death may be ascribed to myocarditis, encephalitis or pneumonitis.