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This review summarizes the major biological, biochemical and molecular methods which have been developed during last 20 years to distinguish parasites of the genus Trichinella. From the time of the discovery of Trichinella in 1835 until the 1970, it was assumed that trichinellosis was caused by a single species of parasite, Trichinella spiralis. Many biological parameters have been compared to differentiate the parasite, such as host specificity, geographical distribution, reproductive abilities, nurse cell development and resistance to freezing. Now, investigators realize that the genus Trichinella is a much more complex group of parasites and simple biological methods are non sufficient. In order to identify and better characterize the species and genotypes of Trichinella it was necessary to develop more sensitive techniques. First, for detecting Trichinella infection immunological methods have been used, such as detection of antibodies in host blood and antigens of parasites using monoclonal antibodies against immunodominant proteins. Later, biochemical techniques have been used such as isoenzyme analysis. The main goal of these methods is to provide a simple, rapid and reproducible techniques to differentiate Trichinella parasites. For this purpose DNA-based methods appeared the best ones. Beginning with the use restriction enzymes, repetitive DNA probes for detection of parasite DNA, and later techniques based on the polymerase chain reaction (PCR), give results at the high level of sensitivity. All of this information has been used to construct a new taxonomy of the genus Thrichinella. To date, 11 taxa have been recognized in the genus: 8 species (Trichinella spiralis T1, Trichinella nativa T2, Trichinella britovi T3, Trichinella pseudospiralis T4, Trichinella murrelli T5, Trichinella nelsoni T7, Trichinella papuae T10, Trichinella zimbabwensis T 11) and additionally three genotypes whose taxonomic status is yet uncertain (T6, T8, T9). Based upon morphology, epidemiology of trichinellosis, geographical distribution and host range of the parasite, two main groups are recognized in the genus Trichinella. The first group comprises species that encapsulate in host muscle tissue, while the species of the second group do not encapsulate. The species and genotypes of the first group infect only mammals (T. spiralis, T. nativa, T. britovi, T. murrelli, T. nelsoni, T6, T8 and T9), whereas of the three species from the second group, one parasitizes mammals and birds (T. pseudospiralis) and the other two infect mammals and reptiles (T. papuae and T. zimbabwensis). Due to the big genetic differences between Trichinella isolates, investigators predict that the number of species and genotypes found within Trichinella will be increased. |
