History

Trichinella spiralis is a parasitic nematode belonging to the order Enoplida (Figure 21.1). The suborder Dorylaimina is small with Trichuris species as the closest relative (Storer et al. 1973).

Figure 21.1 Classification of Trichinella. These data were obtained from Storer et al. (1973) and Lichtenfels et al. (1994).

Table 21.1 The distribution of Trichinella species/phenotypes and some characteristics

Species/phenotype

Distribution

Characteristics

Trichinella spiralis s. str. (Owen Cosmopolitan 1835)

Trichinella nativa (Britov and Boev Arctic and subarctic areas 1972)

Trichinella britovi (Pozio et al. 1992) Temperate areas of Palaearctic regions

Trichinella pseudospiralis (Garkavi Australian and Palaearctic regions 1972)

Trichinella nelsoni (Britov and Boev In Africa, south of the Sahara 1972)

Trichinella T5 Trichinella T6

Trichinella T8

Temperature areas of North America (USA)

Montana and Pensylvania in USA

In Africa, south of the Sahara

High infectivity to pigs and rats Highest female fertility of the genus No resistance to freezing of muscle larvae

High resistance to freezing of muscle larvae

Widespread in wildlife Low infectivity to pigs and rats Low resistance to freezing of muscle larvae

Muscle larvae can survive freezing for some months

Non-encapsulation in host muscle Low infectivity for swine Slow nurse cell development Low infectivity to pigs and rats No resistance to freezing of muscle larvae

Widespread in wildlife Considered a phenotype of T. britovi

Considered a phenotype of T. nativa Less resistant to freezing compared to T. nativa muscle larvae Considered a phenotype of T. britovi

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Trichinella spiralis was first discovered in 1835 when a medical student, James Paget, at St Bartholomew's Hospital, London, saw white particles in a cadaver (reviewed by Campbell and Denham 1983). The particles were analyzed by microscopy and the results published as a description of a new parasite species named Trichina spiralis (Owen 1835). It then took another 25 years before Friedrich Albert von Zenker, 1860, established the lifecycle and documented that transmission could occur to humans from infected pig meat (Zenker 1860). Zenker's association of a defined pathogen with a defined disease was a milestone in medical microbiology, though it rarely receives the recognition it deserves, being overshadowed by later discoveries in bacteriology.

The name of the parasite was later changed to Trichinella spiralis since the name Trichina had already been used for a genus of a fly (Railliet 1896). Over the years it was believed that only one species of Trichinella occurred. Today five sibling species and three phenotypes of uncertain taxonomic level have been identified (Table 21.1; Pozio et al. 1992). Of the various species T. spiralis has been most studied, both in animals and in humans, as being the most significant for causing disease and economic losses.

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