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

Efficacy of maslinic acid and fenbendazole on muscle larvae of Trichinella zimbabwensis in laboratory rats

Trichinellosis is a zoonotic disease caused by nematode species of the genus Trichinella. Anthelmintics targeting the intestinal adults and muscle-dwelling larvae of Trichinella spp. have been tested, with limited success. This study was aimed at determining the efficacy of maslinic acid and fenbendazole on muscle larvae of Trichinella zimbabwensis in laboratory rats. Forty-two Sprague–Dawley rats, with an average weight of 270 g and 180 g for males and females respectively, were infected with T. zimbabwensis larvae. Infected rats were randomly assigned to three groups which were subjected to single treatments with each of maslinic acid, fenbendazole and a combination of both on day 25 post-infection (pi), and three groups which were subjected to double treatments with each of these drugs and a combination on days 25 and 32 pi. The untreated control group received a placebo. In single-treatment groups, the efficacy of each treatment, measured by rate of reduction in muscle larvae, was significant (P< 0.001) for both drugs compared to the untreated control group. There was no apparent synergistic effect on the combination of the two drugs in reducing the muscle larval burden, either in single or double treatments. In all the treatment regimens, the reductions were significant (P< 0.001) when compared to the untreated control and not significant when the single treatments were compared with the double treatments (P>0.05). We conclude that the efficacy of maslinic acid against larval stages of T. zimbabwensis in rats was comparable to that of fenbendazole, with no side-effects observed, making maslinic acid a promising anthelmintic against larval stages of Trichinella species.

Effect of methanolic extract of Balanites aegyptiaca fruits on enteral and parenteral stages of Trichinella spiralis in rats

There is a considerable interest in developing new anthelmintic drugs including those from medicinal plants due to increasing evidence of parasitic resistance against present anthelmintic drugs and decreasing activity against encapsulated larval stages of parasites. This study was carried out to assess, for the first time, the effectiveness of methanolic extract of Balanites aegyptiaca (BAE) fruits against different stages (pre-adult, migrating larvae, and encysted larvae) of Trichinella spiralis in rats compared with commonly used anthelmintic albendazole. Oral administration of BAE at a dose of 1,000 mg/kg b.wt. for five successive days throughout the parasite life cycle led to a marked reduction of migrating and encysted larval rate by 81.7% and 61.7%, respectively, in the muscular tissue. This treatment was less effective against adults in the gut (47.8%). Albendazole treatment at a dose of 10 mg/kg b.wt. for five successive days resulted in a marked eradication of T. spiralis adult worms (94.4%) and less reduction of migrating and encysted larval infections of skeletal muscles (62.2% and 26.4%, respectively). BAE-treated groups showed marked decreases in serum-glucose levels, triglyceride concentrations, aspartate aminotransferase (AST), creatinine phosphokinase (CPK) activities, and lipid peroxide products (malondialdehyde, MDA) as well as an increase in glutathione level in both serum and muscular tissue compared to albendazole-treated- and infected-untreated groups. This result was confirmed by few numbers of living- and dead-encysted larvae and less destruction of the diaphragm and skeletal muscle tissues in BAE-treated groups compared to other treated groups. It can be concluded that the methanolic extract of B. aegyptiaca fruits has high effectiveness against parenteral stages of T. spiralis than albendazole. Albendazole is more effective against enteral stage of T. spiralis than the extract.

Genetic diversity among isolates of Trichinella spiralis from the Province of Buenos Aires, Argentina

Random Amplified Polymorphic DNAs, (RAPDs) are used to study the occurrence of Trichinella britovi and T5 among domestic animals in the Province of Buenos Aires, Argentina and to assess the genetic diversity among isolates of T. spiralisfrom this area in a number of infected hosts. All the local isolates proved to be T. spiralis. Six of the eight primers used indicate that the Buenos Aires isolates are distinct from each other as they produce a considerable number of polymorphic bands. Our overall estimates are relatively higher than other intraspecific distances previously estimated within species of this genus and among T. spiralis isolates. Such high degrees of variability observed among local isolates and between isolates from Buenos Aires and Spain should be taken into account when defining isolates within this species, and considering differences in the epidemiology of T. spiralis.

Molecular characterization of Trichinella genotypes by inter-simple sequence repeat polymerase chain reaction (ISSR-PCR)

A bulk analysis of inter-simple sequence repeat-polymerase chain reaction (ISSR-PCR) provides a quick, reliable, and highly informative system for DNA banding patterns that permit species identification. The present study evaluates the applicability of this system to Trichinella species identification. After a single amplification carried out on a single larva with the primer 816([CA]nRY) under high stringency conditions, which provide high reproducibility, we were able to identify by consistent banding patterns 5 sibling species: Trichinella spiralis (ISS48), 2 Trichinella britovi isolates (ISS11 and ISS86), Trichinella murrelli (ISS35), Trichinella nativa (ISS71), Trichinella nelsoni (ISS29); 3 additional Trichinella genotypes: T8 (ISS149), T9 (ISS408 and ISS409), and T6 (ISS34); and the nonencapsulated species Trichinella pseudospiralis (ISS13). Moreover, 33 new Trichinella isolates from 2 zoogeographical regions were unequivocally identified. All Trichinella isolates have shown an identical pattern with those produced by the reference strain. According to these data, we have demonstrated that ISSR-PCR is a robust technique that emerges as a useful new application for the molecular identification of Trichinella isolates in epidemiological studies.

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.

Variation and immunity to intestinal worms

Genetically determined variation in host capacity to express resistance to a given parasite plays a major role in determining the outcome of infection. It can be assumed that the same is true of variation in parasites, but very much less is known of its influence on the host-parasite relationship. Phenotypic and genotypic variation within species of intestinal worms is now well documented, detailed studies having been made of parasites such as Ascaris in humans and trichostrongyles in domestic animals. However, the extent to which this variation affects the course of infection or the host immune response in these hosts is limited. Of the nematodes used as experimental models in laboratory rodents, detailed data on phenotypic or genotypic variation are limited to Strongyloides and Trichinella. Parasite variation is known to be subject to host-mediated selection, the emergence of anthelmintic resistance being a good example. Repeated passage has been used to select lines of parasite that survive in abnormal hosts or which show adaptation to host immunity. Experimental studies with Trichinella genotypes in mice have demonstrated the extent to which parasite variation influences the nature and degree of the host's immune and inflammatory responses, the complex interplay between immunogenicity and pathogenicity influencing both partners in the relationship. Recent studies with isolates of Trichuris muris have shown how parasite variation influences the capacity of mice to express the T helper cell responses necessary for resistance. Molecular differences between T. muris isolates have been shown in their excreted/secreted products as well as at the level of their DNA. Knowledge of the functional consequences of parasite variation will add to our understanding of host-parasite evolution as well as providing a rational basis for predicting the outcome of controls strategies that rest on the improvement of host resistance through vaccination or selective breeding.

CCR3 is required for tissue eosinophilia and larval cytotoxicity after infection with Trichinella spiralis

The CCR3 binds at least seven different CC chemokines and is expressed on eosinophils, mast cells (MC), and a subset of Th cells (Th2) that generate cytokines implicated in mucosal immune responses. Using mice with a targeted disruption of CCR3 (CCR3(-/-)) and their +/+ littermates, we investigated the role of CCR3 in the amplification of tissue eosinophilia and MC hyperplasia in the mouse after infection with Trichinella spiralis. In CCR3(-/-) mice, eosinophils are not recruited to the jejunal mucosa after infection and are not present in the skeletal muscle adjacent to encysting larvae. In addition, the number of cysts in the skeletal muscle is increased and the frequency of encysted larvae exhibiting necrosis is reduced. The CCR3(-/-) mice exhibit the expected MC hyperplasia in the jejunum and caecum and reject the adult worms from the small intestine at a normal rate. This study is consistent with distinct functions for MC (adult worm expulsion) and eosinophils (toxicity to larvae) in immunity to a helminth, T. spiralis, and defines the essential requirement for CCR3 in eosinophil, but not MC recruitment to tissues.

Predilection muscles and physical condition of raccoon dogs (Nyctereutes procyonoides) experimentally infected with Trichinella spiralis and Trichinella nativa

The predilection muscles of Trichinella spiralis and T. nativa were studied in 2 experimental groups of 6 raccoon dogs (Nyctereutes procyonoides), the third group serving as a control for clinical signs. The infection dose for both parasites was 1 larva/g body weight. After 12 weeks, the animals were euthanized and 13 sampling sites were analysed by the digestion method. Larvae were found in all sampled skeleton muscles of the infected animals, but not in the specimens from the heart or intestinal musculature. Both parasite species reproduced equally well in the raccoon dog. The median density of infection in positive tissues was 353 larvae per gram (lpg) with T. spiralis and 343 lpg with T. nativa. All the infected animals had the highest larvae numbers in the carpal flexors (M. flexor carpi ulnaris). Also tongue and eye muscles had high infection levels. There were no significant differences in the predilection sites between these 2 parasite species. Trichinellosis increased the relative amount of fat, but not the body weight in the captive raccoon dogs. Thus, Trichinella as a muscle parasite might have catabolic effect on these animals.

The effects of mating probability on the population genetics of nematodes

We review how constraints on the mating probability of female worms by segregation within individual host guts form a critical element in the parasite population structure of nematodes. We consider the effects of these constraints on the population genetics of nematodes under various assumptions regarding worm competition as reflected in the relationship between the abundance of a certain genotype within the gut and the probability of reproductive success. The consequences for the emergence of resistance to drugs and immunotherapy and implications for host-parasite coevolution are discussed. We also review evidence for genetic heterogeneity in parasite populations as a necessary prerequisite for the applicability of mating probability models designed to assess the population genetics of nematodes.

Helminth immunogenetics: Why bother?

The importance of host genotype as a determinant of protective responses against helminth infection is well established. In contrast, there have been relatively few investigations of the role of helminth genotype, despite the importance accorded to the genetics of other disease-causing organisms. Here, Andrew Read and Mark Viney discuss the reasons for this oversight. They argue that it is not for any compelling empirical reason: there is at least as much evidence that worm genetics affects host protective responsiveness as there is that it does not.

Immunology and genetics of zoonotic infections involving parasites

Zoonoses involving parasites are both common and important, some causing serious diseases. The abilities of such parasites to pass between humans and a variety of vertebrates facilitates their use as experimental model systems. Much can be learned about the host-parasite relationships of zoonotic infections from laboratory-based studies, particularly where it is possible to make use of defined parasite isolates in a variety of host species or in genetically defined strains of a given host. This review focuses on our current understanding of the immunological influences upon host-parasite interactions involving zoonotic parasites and on how such influences are modified by the genetic variability found n populations of both hosts and parasites. Particular emphasis is given to data arising from work with the intestinal parasitic nematode Trichinella spiralis and other isolates of the genus Trichinella.

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.

Induction of differential T-helper-cell responses in mice infected with variants of the parasitic nematode Trichuris muris

The resistance or susceptibility of mice to infection with the intestinal nematode parasite Trichuris muris is closely correlated with polarization of T helper (Th) cell responses to the type 2 (Th2) or type 1 (Th1) subset. Comparison of infections with three isolates of T. muris (E/K, E/N, and S) in three inbred strains of mice (CBA, C57BL/10, and B10.BR) has shown that host Th response phenotype can be parasite determined. Although the mouse strains used show genetically determined variation in ability to respond to T. muris (CBA > C57BL/10 > B10.BR), the speed of worm expulsion in a given strain depended upon the isolate used for infection (E/K > E/N > S). The two isolates that induced the most effective resistance (E/K and E/N) elicited parasite-specific host antibody responses that were dominated by immunoglobulin G1 (IgG1), and antigen-stimulated T cells from infected mice released interleukin-5 in vitro. With the isolate that induced the least host resistance (S), the dominant antibody response was IgG2a, and T cells released gamma interferon in vitro. These data show clearly that parasite variant-specific factors play a major role in Th subset polarization during infection.

Oral and parenteral vaccination against Trichinella spiralis infections in high- and low-responder mice

Vaccination by different routes and with different adjuvants is known to influence profiles of immune responses and may be used to overcome genetically determined low-responsiveness to infection. A mouse model of infection with the intestinal nematode Trichinella spiralis was used to investigate the effect of mode of vaccination upon immune responsiveness and worm expulsion phenotype in high- (NIH) and low- (C57 BL/10) responder strains of mice. Muscle larval homogenate antigen was given subcutaneously in Freund's complete adjuvant (FCA) to induce a systemic immune response or with cholera toxin (CT) orally to stimulate mucosal immunity. Both approaches significantly protected NIH mice. Vaccination with FCA was correlated with elevated serum IgG after infection, whereas oral CT-vaccination resulted in increased levels of intestinal IgA. Neither type of vaccination successfully protected the low-responder C57 BL/10 strain and there were no effects on the low antibody levels that infection induced in this strain.

Efficacy of oral vaccination against the murine intestinal parasite Trichuris muris is dependent upon host genetics

Oral vaccinations with Trichuris muris adult worm homogenate antigen with cholera toxin as the adjuvant were successful in both high-responder BALB/c and low-responder C57BL/10 mice, resulting in high levels of protection against subsequent infection, but were ineffective in the low-responder B10.BR mice. Subcutaneous vaccination with antigen in Freund's complete adjuvant resulted in protection of all of these strains but was most effective in high-responder BALB/c and least effective in B10.BR mice. Oral vaccination resulted in a T. muris-specific intestinal immunoglobulin A response only in the two protected strains. High levels of serum immunoglobulin G1 antibody were induced by Freund's complete adjuvant vaccination in all cases. A relationship between vaccine efficacy, expulsion phenotype, and induced T-helper subset-associated cytokines (interleukin-5 and gamma interferon) was noted. It was concluded that effective vaccination against T. muris requires the induction of Th2 responses and that this can be achieved by both oral and parenteral administration of antigens.

Immune response profiles in vaccinated and non-vaccinated high- and low-responder mice during infection with the intestinal nematode Trichinella spiralis

NIH and C57 BL/10 (B10) mice show genetically determined differences in their response to Trichinella spiralis infection. This study examines the influence of these on parameters of the immune response to infection after vaccination using muscle-larval excretory-secretory antigen in Freund's complete adjuvant. Serum antibody levels were greatly elevated when mice of both strains were vaccinated prior to infection; however, NIH produced significantly higher-level antibody responses than B10. Vaccination accelerated and increased the capacity of mesenteric lymph node T-cells to proliferate in vitro in response to specific antigen stimulation in both mouse strains but, in general, the stimulation indices of NIH cells were higher than those of the B10. The capacity of mesenteric lymph node cells (MLNC) and spleen cells (SC) to produce IL-5 and gamma IFN was measured after specific in vitro stimulation and early gamma IFN secretion was noted in the supernatants of NIH MLNC and SC, but not in B10 SC. Concentrations of IL-5 rose steadily over the first 10-14 days after infection in cell cultures from both strains. Prior vaccination of these animals appeared to enhance cytokine levels. It is postulated that the efficacy of vaccination in NIH mice is a consequence of their genetically determined capacity to produce early and high-level responses to the antigens of T. spiralis and to express these in intestinal effector mechanisms.

Influence of variation in host strain and parasite isolate on inflammatory and antibody responses to Trichinella spiralis in mice

Variation in the immunogenicity of 3 isolates of Trichinella spiralis was assessed by the parameters of adult worm recovery, mast cell, eosinophil and antibody responses in mice of defined response phenotype. The levels of the protective, inflammatory and immune responses induced by infection differed between the isolates. Isolates showed considerable variation in the capacity to elicit mast cell and eosinophil responses. All induced increases in parasite-specific antibody, levels of total (IgGAM) antibody and of IgM and IgG isotypes rose steadily after infection, but there were significant differences in levels of response. The IgGAM response was correlated with the number of worms present, i.e. the greatest response was seen in low responder (C57BL/10) mice infected with the longest-surviving isolates. All isolates elicited specific IgG1 and IgG2a antibodies after infection, although, again, there were isolate-specific differences in the levels and kinetics of response. Levels of these isotypes were always higher, although not significantly so, in high-responder NIH mice. Low-responder mice showed higher IgE serum levels than high-responder mice after infection, one isolate giving much higher IgE values than the other two.

Molecular variation in Trichinella

The taxonomic status of variants within the genus Trichinella is problematical. Some authors recognise no fewer than four species (Trichinella spiralis, T. pseudospiralis, T. nativa and T. nelsoni), others regard T. nativa and T. nelsoni as strains of T. spiralis (T. spiralis var nativa or sylvatica), while others consider the genus to be monospecific, with a variety of more or less well defined isolates. Much of the current evidence adduced to support these various positions is similar to that used pre-1983. It derives from studies of the incidence of Trichinella infections in wild and in domestic animals, comparisons of infectivity of different isolates in laboratory animals and studies of immunity. However, it has become clear that infectivity and epidemiological studies are unreliable tools for discriminating between isolates of Trichinella and it has been shown that differences in the elicitation of immune responses are as much a function of the host as of the parasite. The introduction of monoclonal antibody technology has, however, permitted the identification of specific antigens in different isolates. The information is as yet scant, and one antigen does not a species make. Isozyme analysis provides some support for separating the various isolates of Trichinella into distinct groups, but cannot of itself shed light on the species problem until certain conditions are met. These conditions are difficult to achieve even in organisms abundantly available and without the baggage of the parasitic habit. Isozyme analysis is probably best used to support the newer studies of genomic DNA. Recent analyses of DNA by restriction endonucleases and dot-blot hybridisation techniques show ample promise of insights into speciation, and a new technique for amplifying the DNA from a single larva by the polymerase chain reaction offers exciting prospects. However, the position yet remains as stated in the first section of this abstract.

Immunization against geographical isolates of Trichinella spiralis in mice

Partially purified antigen preparations from six isolates of Trichinella spiralis were used to immunize mice. Immunogenicity of the antigens was assessed in terms of antibody and lymphocyte responses and ability to stimulate protective immunity against challenge. Isolate antigens showed considerable cross-reactivity, and all elicited protective responses. Two major patterns of immunizing ability could be distinguished: (a) isolates that immunized well against heterologous challenge and elicited good immunity in London isolate-immunized mice (C-76 and Laso), and (b) isolates that immunized poorly against heterologous challenge and were least effective in London isolate-immunized mice (GM-1 and Mad-83). The immunogenicity of one isolate (C-76) was markedly greater than the others, inducing rapid loss in unvaccinated mice, almost complete protection in London isolate-immunized mice, and immunity in London isolate-challenged mice equivalent to the homologous antigen. These variations in immunogenicity, cross-reactivity and immunizing ability are discussed in terms of constraints that may operate against the development and use of vaccines against parasites that are widely distributed geographically.

Infectivity, antigenicity and host responses to isolates of the genus Trichinella

Comparisons were made of the infectivity and antigenicity of 4 Trichinella spiralis isolates (S, D, Y, W), of quite different geographical origins, and T. pseudospiralis (P) in rapid- and slow-responder inbred mice. Infectivity was measured by the Index of Reproductive Capacity (ICR) expressed as the ratio between the number of muscle larvae recovered on day 30 post-infection (p.i.) and the numbers of larvae given at infection. Antigen recognition was measured by the degree of proliferation of mesenteric lymph node cells (MLNC) to in vitro stimulation with crude muscle larvae antigen (CMLA) and by the total antibody responses to CMLA at day 25 p.i. as measured by ELISA. Regarding infectivity the isolates fell into two groups, high infectivity (S, D and Y) and low infectivity (W and P). Analysis of CMLA, detergent-stripped (CTAB) and 125I-labelled surface larval proteins was made by SDS-PAGE under reducing conditions. Differences in antigen profiles were seen in all antigen preparations, being most noticeable in CTAB and 125I-labelled proteins from W and P isolates. Antigen recognition by polyclonal infection-derived antisera and by monoclonal antibodies raised against the T. spiralis London strain (L) was studied in the W (Arctic) and S (Spanish) isolates. Polyclonal antisera recognized different antigens in the S and W isolates, as did the monoclonal antibody, although recognition was more restricted. Neither antibody recognized a 64 kDa band in the W isolate which was clearly visible in the others tested.