Chapter 3 - Taxonomy of the Trichinella genus

Abstract

For more than 100 years, the genus Trichinella was considered monospecific and Trichinella spiralis was considered the only species with a cosmopolitan distribution, infecting a very large number of mammals including humans. The domestic cycle involving grazing domestic pigs and farm rats was considered the predominant mode of circulation for T. spiralis; surrounding wildlife were considered as an ancillary part of on-farm transmission. The expansion of helminthological investigations to wild animals starting from the 1950s highlighted the presence of nematodes of the genus Trichinella with biological characteristics different from those of T. spiralis, but without discernable morphological characteristics at any of the developmental stages that could be used to differentiate species. The switch from a classical taxonomy to new taxonomic studies first based on crossing–breeding experiments in laboratory animals, and then on biochemical (allozymes) and molecular characters, has allowed the identification of 10 sibling species and three genotypes divided into two clades based on the presence or absence of a collagen capsule around the muscle larva. The clade in which the muscle larvae (ML) do not induce capsule formation infects mammals and birds (Trichinella pseudospiralis) or mammals and reptiles (Trichinella papuae and Trichinella zimbabwensis). The clade characterized by the presence of a collagen capsule around the nurse-cell–larva complex infects only mammals. This clade is comprised of seven species (T. spiralis, Trichinella nativa, Trichinella britovi, Trichinella murrelli, Trichinella nelsoni, Trichinella patagoniensis, and Trichinella chanchalensis) and three genotypes (Trichinella T6, T8, and T9). Each taxon is characterized by a well-defined distribution area; however, there are two species which, for different reasons, have a cosmopolitan distribution: T. spiralis spread by the passive introduction of infected swine to most continents and T. pseudospiralis spread by birds worldwide. Carnivores are the main hosts of Trichinella species and genotypes, with the exception of T. spiralis which is also very well adapted to suidae and consequently represents the main causative agent of human trichinellosis. This taxonomic scheme is supported by the analysis of more than 10,000 Trichinella isolates from different host species and continents. However, there remain some geographical areas from which no Trichinella parasites have been collected and identified at the species/genotype level such as Central America and India. The phylogeny of the genus showed that the encapsulated and nonencapsulated clades diverged from their most recent common ancestor in Asia about 21 million years ago (mya) with taxon diversifications commencing about 10–7 mya. Herein we present the most up-to-date information on the taxonomy of Trichinella.