Comparative studies on geographical strains of Trichinella spiralis

Zoonotic helminths - why the challenge remains

Helminth zoonoses remain a global problem to public health and the economy of many countries. Polymerase chain reaction-based techniques and sequencing have resolved many taxonomic issues and are now essential to understanding the epidemiology of helminth zoonotic infections and the ecology of the causative agents. This is clearly demonstrated from research on Echinococcus (echinococcosis) and Trichinella (trichinosis). Unfortunately, a variety of anthropogenic factors are worsening the problems caused by helminth zoonoses. These include cultural factors, urbanization and climate change. Wildlife plays an increasingly important role in the maintenance of many helminth zoonoses making surveillance and control increasingly difficult. The emergence or re-emergence of helminth zoonoses such as Ancylostoma ceylanicum, Toxocara, Dracunculus and Thelazia exacerbate an already discouraging scenario compounding the control of a group of long neglected diseases.

Systematics and epidemiology of trichinella

In this review, we describe the current knowledge on the systematics, ecology and epidemiology of Trichinella and trichinellosis, and the impact of recent research discoveries on the understanding of this zoonosis. The epidemiology of this zoonosis has experienced important changes over the past two decades, especially with regard to the importance of the sylvatic cycle and the sylvatic species. Outbreaks of trichinellosis due to Trichinella spiralis from domestic swine, while still frequent, increasingly are caused by other Trichinella spp. infecting hosts such as horses, dogs, wild boars, bears and walruses. The latter revelations have occurred as a result of a series of discoveries on the systematics of Trichinella spp., facilitated by new molecular tools. As a consequence, the genus is now composed of two clades, an encapsulated group (five species and three genotypes) and a non-encapsulated one (three species). This has sparked renewed investigations on the host range of these parasites and their epidemiological features. Most dramatic, perhaps, is the recognition that reptiles may also serve as hosts for certain species. This new knowledge base, in addition to having an important relevance for food safety policies and protection measures, is raising important questions on the phylogeny of Trichinella spp., the ecological characteristics of the species and their geographic histories. Answers to these questions may have great value for the understanding of the evolutionary biology for other parasitic helminths, and may increase the value of this genus as models for research on parasitism in general.

Biological variation in Trichinella species and genotypes

At present, the genus Trichinella comprises seven species of which five have encapsulated muscle larvae (T. spiralis, T. nativa, T. britovi, T. nelsoni and T. murrelli) and two do not (T. pseudospiralis and T. papuae) plus three genotypes of non-specific status (T6, T8 and T9). The diagnostic characteristics of these species are based on biological, biochemical and genetic criteria. Of biological significance is variation observed among species and isolates in parameters such as infectivity and immunogenicity. Infectivity of Trichinella species or isolates is determined, among other considerations, by the immune status of the host in response to species- or isolate-specific antigens. Common and particular antigens determine the extent of protective responses against homologous or heterologous challenge. The kinetics of isotype, cytokine and inflammatory responses against T. spiralis infections are isolate-dependent. Trichinella spiralis and T. pseudospiralis induce different dose-dependent T-cell polarizations in the early host response, with T. spiralis initially preferentially promoting Th1-type responses before switching to Th2 and T. pseudospiralis driving Th2-type responses from the outset.

The systematics of the genus Trichinella with a key to species

The authors review the major biological, biochemical, and molecular characters that are used to distinguish the seven Trichinella species (T. spiralis, T. nativa, T. britovi, T. pseudospiralis, T. murrelli, T. nelsoni, T. papuae) and three genotypes whose taxonomic status is yet uncertain (T-6, T-8, T-9). A comparison of host specificity, morphology, reproductive abilities, nurse cell development and freeze resistance is presented, along with useful biochemical and molecular markers. Finally, this information is used to construct a diagnostic key for the species. A phylogenetic classification of the species is needed.

Trichinella isolates: parasite variability and host responses

The genus Trichinella has the widest geographical distribution and the largest range of host species of all parasitic nematodes. It remains a significant human pathogen. More than 300 isolates of the genus are now available for laboratory study. The taxonomy of the genus, which has only recently achieved some stability, depends upon the use of a variety of biological, biochemical and genetic criteria. The biological characteristics of isolates, particularly those relating to infectivity and pathogenicity, can show considerable variation because they are subject to strong host influences, notably those associated with immune and inflammatory responses. Comparative studies of different isolates in different hosts, particularly strains of inbred mice, have helped to define these influences and to identify the relative contributions of isolate immunogenicity and host immune response capacity to the outcome of infection. Data from such studies can not only contribute to a better understanding of the biology of this genus but can also throw light on fundamental aspects of host-parasite interactions.

Infectivity of Trichinella isolates in mice is determined by host immune responsiveness

A comparison was made of the development, survival and reproduction of 5 isolates of Trichinella spiralis in inbred mice. Low responder C57BL/10 mice allowed worms of all isolates to survive longer and reproduce more successfully than did high responder NIH mice, suggesting that host immunity exerted a dominant influence upon infectivity. One isolate (Is-5 (W) -an arctic isolate) had a markedly lower infectivity than all other isolates, and was selected for more detailed study, together with isolate Is-1 (S) (a temperate isolate) which showed high infectivity. The lower infectivity of Is-5 (W) reflected a more rapid onset of immunity in mice infected with this parasite, immunity reducing the reproductive potential of female worms and causing an early expulsion from the intestine. No evidence of a dose-dependent suppression of immunity was found to explain the higher infectivity of Is-1 (S). Is-5 (W) provided a very rapid mucosal mastocytosis Is-1 (S) elicited higher levels of circulating parasite-specific antibodies than did Is-5 (W). These results are discussed in relation to the interplay of parasite immunogenicity and host immune responsiveness in determining infectivity, and point to the importance of identifying those immunodominant parasite molecules which control the balance of the host-parasite relationship.

Comparative studies on soluble protein profiles and isozyme patterns of seven Trichinella isolates

Soluble protein profiles and isozyme patterns of eight enzymes were compared for extracts of muscle stage larvae of the seven Trichinella isolates, using isoelectric focusing in polyacrylamide gel. Soluble protein profiles and isozyme patterns of four enzymes: malic enzyme, glucosephosphate isomerase, phosphoglucomutase, superoxide dismutase of them were clearly divided into four types. T. pseudospiralis from a racoon and the Polar strain from a polar bear formed type 1 and type 2. The Iwasaki strain from a Japanese black bear and the Yamagata strain from a racoon dog, both from Japan, were type 3. Type 4 consisted of three remaining strains, the Polish strain from a wild pig, the USA strain from a pig and the Thai strain from a human case, which have similar infectivities to pigs. The Thai strain varied a bit electrophoretically from other members of type 4. Zymograms of adenylate kinase and malate dehydrogenase were similar in types 2 and 3. The 6-phosphogluconate dehydrogenase zymogram of type 3, similar to that of type 4, was different from that of type 2. It is assumed from the data that type 3 (Japanese strain) was genetically intermediate to types 2 and 4. T. pseudospiralis and the Polar strain had a common main isozyme of 6-phosphogluconate dehydrogenase. The zymogram of lactate dehydrogenase was common except for T. pseudospiralis.

Enzyme polymorphism in Trichinella

The isoenzyme profiles of five isolates of the supposed 'species' of Trichinella, T. nativa, T. spiralis and T. nelsoni were compared. Four enzymes (AK, PGM, MPI and GPI) gave good resolution and clearly differentiated T. Spiralis from the other two species. T. nativa and T. nelsoni had similar isoenzyme patterns; the two separate isolates of T. nativa and T. spiralis used gave similar results, thus indicating the validity and the reproducibility of the technique. The value of enzyme electrophoresis for specific and subspecific classification of Trichinella is discussed and compared with the more traditional methods of taxonomy which have failed to resolve the controversy surrounding speciation.

The significance of infra-specific variations of hosts and parasites in the epidemiology of helminths of medical importance

The epidemiology of parasitic infections may be profoundly influenced by infra-specific variations in infectivity for either intermediate or definitive hosts. Such variations will decide the basic ecology of the host-parasite relationship, and variations in virulence and pathogenicity therefore determine different patterns of disease. While some of the evidence of infra-specific variations has been derived from experimental studies, much of it has been recorded by those concerned with investigations of the epidemiology and transmission of parasitic infections and the practical aspects of their control. The unequivocal demonstration of genetic heterogeneity of certain parasites of medical importance has therefore resulted in a better understanding of their epidemiology and, in some cases, is leading to greater precision in attempts to control them.