Description Of The Organisms

Trichinellosis, a nematode infection with worldwide distribution, is caused by tissue-dwelling nematodes of the species Trichinella. The infection is acquired by eating raw or inadequately cooked meat products containing encysted larvae (Figure 19b.1). Formerly, the etiologic agent was considered a monotypic nematode species, Trichinella spiralis. However, accumulating evidence of variation in transmission cycles, infectivity to experimental hosts and biochemical and genetic characteristics has led to taxonomic revision. Currently, seven genetically distinct species exist that vary according to major reservoir hosts and geographic distribution; three other forms exist whose classifications remain to be determined (Table 19b.1). Each species has been characterized by a combination

Principles and Practice of Clinical Parasitology

Edited by Stephen Gillespie and Richard D. Pearson Published 2001 by John Wiley & Sons Ltd

Fig. 19b.1 Life-cycle of Trichinella spiralis. US Department of Agriculture, by courtesy of Dr H. Ray Gamble Table 19b.1 Known hosts and geographic distribution of Trichinella spp.

Species

Hosts

Geographic distribution

Trichinella spiralis Domestic swine, rats and other scavening carnivores Wild carnivorous mammals including polar, grizzly and black bears, foxes, and dogs Wild carnivorous mammals including red fox, racoon dog, wild boar and other Wild carnivorous mammals Wild carnivorous mammals

Raptorial birds, marsupials, rodents and wild canids

T. nativa

T. britovi

T. nelsoni T. murrelli T. pseudospiralis

T. papuae T6 T8 T9

Domestic and sylvatic swine Wild carnivorous mammals Wild carnivorous animals Wild carnivorous mammals

Cosmopolitan

Arctic and subarctic areas throughout Holarctic regions

Palearctic region south of — 6°C (Europe, North

Africa, Asia Minor, India) Sub-Saharan Africa Temperate North America Caucasia, Central Asia, North America and

Tasmania Papua New guinea Rocky Mountains, North America Southern Africa Japan of DNA typing and isozyme patterns (Bandi et al., 1995; Pozio and La Rosa, 1998; Appleyard et al., 1999); a single polymerase chain reaction (PCR) performed in a single muscle larva distinguishes all currently recognized genotypes

(Zarlenga et al., 1999; Pozio and La Rosa, 2000). All recognized variants are adapted to survival in life-cycles involving various species of carnivorous hosts. T. spiralis is adapted to the common domestic pig and is historically responsible for most of the infections in the USA and Europe and, most likely, in other regions as well. T. nelsoni, transmitted to humans through wild pigs, is found in Africa and southern Europe. T. nativa is maintained in Arctic and sub-Arctic scavenging wildlife (e.g. polar and grizzly bears, arctic and red foxes). T. britovi occurs in a variety of carnivores in northern Europe, Asia Minor and India. T. pseudospiralis, a distinct non-encapsulating species, has a sylvatic life-cycle, primarily involving small mammalian and marsupial predators and raptorial birds but also infects swine and has become increasingly recognized as a cause of human disease (Ancelle et al., 1985, 1998; Andrews et al., 1994; Jongwutiwes, 1998; Ranque et al., 2000). A second non-encapsulating species, T. papuae, was recently described in domestic and sylvatic swine in Papua New Guinea (Pozio et al., 1999). The taxonomic status of other variants (Trichinella T6, T8 and T9), also adapted to scavenging wild carnivores, is currently under review. The sylvatic animal Trichinella species and variants show distinct characteristics of adaptive advantage for survival in nature, such as variable degrees of resistance to temperature extremes. Most if not all of these known variants are capable of infecting humans when ingested. Evidence suggests that the frequency, duration and severity of clinical signs may be related in part to the infecting species (Murrell and Bruschi, 1994; Pozio and La Rosa, 2000).

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