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At least 919 records · Page 51Linked to original sources
PubMed1995

Modelling patterns of parasite aggregation in natural populations: trichostrongylid nematode-ruminant interactions as a case study.

The characteristically aggregated frequency distribution of macroparasites in their hosts is a key feature of host-parasite population biology. We begin with a brief review of the theoretical literature concerning parasite aggregation. Though this work has illustrated much about both the sources and impact of parasite aggregation, there is still no definite analysis of both these aspects. We then go on to illustrate the use of one approach to this problem--the construction of Moment Closure Equations (MCEs), which can be used to represent both the mean and second moments (variances and covariances) of the distribution of different parasite stages and phenomenological measures of host immunity. We apply these models to one of the best documented interactions involving free-living animal hosts--the interaction between trichostrongylid nematodes and ruminants. The analysis compares patterns of variability in experimental infections of Teladorsagia circumcincta in sheep with the equivalent wildlife situation--the epidemiology of T. circumcincta in a feral population of Soay sheep on St Kilda, Outer Hebrides. We focus on the relationship between mean parasite load and aggregation (inversely measured by the negative binomial parameter, k) for cohorts of hosts. The analysis and empirical data indicate that k tracks the increase and subsequent decline in the mean burden with host age. We discuss this result in terms of the degree of heterogeneity in the impact of host immunity or parasite-induced mortality required to shorten the tail of the parasite distribution (and therefore increase k) in older animals. The model is also used to analyse the relationship between estimated worm and egg counts (since only the latter are often available for wildlife hosts). Finally, we use these results to review directions for future work on the nature and impact of parasite aggregation.

Age Factors↗
PubMed2002

Parasites and marine invasions.

Introduced marine species are a major environmental and economic problem. The rate of these biological invasions has substantially increased in recent years due to the globalization of the world's economies. The damage caused by invasive species is often a result of the higher densities and larger sizes they attain compared to where they are native. A prominent hypothesis explaining the success of introduced species is that they are relatively free of the effects of natural enemies. Most notably, they may encounter fewer parasites in their introduced range compared to their native range. Parasites are ubiquitous and pervasive in marine systems, yet their role in marine invasions is relatively unexplored. Although data on parasites of marine organisms exist, the extent to which parasites can mediate marine invasions, or the extent to which invasive parasites and pathogens are responsible for infecting or potentially decimating native marine species have not been examined. In this review, we present a theoretical framework to model invasion success and examine the evidence for a relationship between parasite presence and the success of introduced marine species. For this, we compare the prevalence and species richness of parasites in several introduced populations of marine species with populations where they are native. We also discuss the potential impacts of introduced marine parasites on native ecosystems.

Animals↗
PubMed2003

Cortisol influences the host-parasite interaction between the rainbow trout (Oncorhynchus mykiss) and the crustacean ectoparasite Argulus japonicus.

The host-parasite interaction between the rainbow trout Oncorhynchus mykiss and the fish louse Argulus japonicus was investigated by administering low levels of dietary cortisol before infecting the fish with low numbers of the parasite. After 24 h, the dietary cortisol treatment elevated blood cortisol and glucose levels and stimulated the synthesis of secretory granules in the upper layer of skin cells. Infection with 6 lice per fish caused skin infiltration by lymphocytes, also in areas without parasites. The lymphocyte numbers in the blood at 48 h post-parasite infection were reduced. Other changes, typical for exposure to many stressors and mediated by cortisol, were also found in the epidermis of parasitized fish, although neither plasma cortisol nor glucose levels were noticeably affected. Glucocorticoid receptors were localized immunohistochemically and found in the upper epidermal layer of pavement and filament cells, and in the leucocytes migrating in these layers. Cortisol-fed fish had reduced numbers of parasites and the changes in the host skin are likely involved in this reduction. Thus a mild cortisol stress response might be adaptive in rejecting these parasites. Further, the data suggest that this effect of cortisol is mediated by the glucocorticoid receptor in the skin epidermis, as these are located directly at the site of parasite attachment and feeding in the upper skin cells that produce more secretory granules in response to cortisol feeding.

Animals↗
PubMed2006

Alternative life-history and transmission strategies in a parasite: first come, first served?

Alternative transmission strategies are common in many parasitic organisms, often representing discrete phenotypes adopted in response to external cues. The facultative truncation of the normal 3-host life-cycle to a 2-host cycle in many trematodes provides an example: some individuals mature precociously, via progenesis, in their intermediate host and produce eggs without the need to reach a definitive host. The factors that determine how many and which individuals adopt the truncated life-cycle within a parasite population remain unknown. We investigated the occurrence of progenesis in the trematode Stegodexamene anguillae within its fish intermediate host. Location within the host was a key determinant of progenesis. Although the size and egg output of progenetic metacercariae encysted in host gonads did not differ from those of the few progenetic metacercariae in other host tissues, the likelihood of metacercariae becoming progenetic was much higher for those in the gonads than those elsewhere in the host. Progenetic parasites can only evacuate their eggs along with host eggs or sperm, providing a link between the parasite's transmission strategy and its location in the host. Host size and sex, and the presence of other parasite species in the host, did not affect the occurrence of progenesis in S. anguillae. However, the proportion of metacercariae in host gonads and the proportion of progenetic metacercariae both decreased with increasing numbers of S. anguillae per host. These results suggest that progenesis is adopted mostly by the parasites that successfully establish in host gonads. These are generally the first to infect a fish; subsequent arrivals settle in other tissues as the gonads quickly become saturated with parasites. In this system, the site of encystment within the fish host both promotes and constrains the adoption of a facultative, truncated life-cycle by the parasite.

Animals↗
PubMedDate not supplied

[Proteases in helminthic parasites].

Proteases catalyse the cleavage of internal peptide bonds within peptides and proteins. They are classified into four major classes and are involved in a broad range of eukaryotic processes. Proteases have also been found to play a number of critical roles in the virulence of pathogenic agents, particularly of nematode parasites. Parasitic proteases are involved in different aspects of host-parasite interactions. They facilitate the invasion of host tissues and allow nutrition as well as the survival of the parasite in its host. Proteases also participate in the parasite's evasion from the host's immune response. The functional diversity and complexity of these enzymes are described in this review, with a particular focus on the principally identified proteases of four helminths: Schistosoma sp., Fasciola sp., Taenia sp. and Haemonchus sp. Some of these proteases, especially the cysteine proteases secreted by the parasitic trematode Fasciola hepatica, have been successfully tested in experimental immunodiagnosis. Proteases identified in different parasites are currently under study for a use as recombinant vaccines. In this respect, proteases are proposed as major potential targets for immunotherapy and chemotherapy against parasitic diseases.

Animals↗
PubMed2001

Does clutch size evolve in response to parasites and immunocompetence?

Parasites have been argued to influence clutch size evolution, but past work and theory has largely focused on within-species optimization solutions rather than clearly addressing among-species variation. The effects of parasites on clutch size variation among species can be complex, however, because different parasites can induce age-specific differences in mortality that can cause clutch size to evolve in different directions. We provide a conceptual argument that differences in immunocompetence among species should integrate differences in overall levels of parasite-induced mortality to which a species is exposed. We test this assumption and show that mortality caused by parasites is positively correlated with immunocompetence measured by cell-mediated measures. Under life history theory, clutch size should increase with increased adult mortality and decrease with increased juvenile mortality. Using immunocompetence as a general assay of parasite-induced mortality, we tested these predictions by using data for 25 species. We found that clutch size increased strongly with adult immunocompetence. In contrast, clutch size decreased weakly with increased juvenile immunocompetence. But, immunocompetence of juveniles may be constrained by selection on adults, and, when we controlled for adult immunocompetence, clutch size decreased with juvenile immunocompetence. Thus, immunocompetence seems to reflect evolutionary differences in parasite virulence experienced by species, and differences in age-specific parasite virulence appears to exert opposite selection on clutch size evolution.

Animals↗
PubMed2003

Parasites and immune responses: memory illusion?

Immunological memory responses to intracellular protozoa and extracellular helminths govern host resistance and susceptibility to reinfection. Humans and livestock living in parasitic disease endemic regions face continuous exposure from a very early age that often leads to asymptomatic chronic infection over their entire lifespan. Fundamental immunological studies suggest that the generation of T-cell memory is driven by tightly coordinated innate and adaptive cellular immune responses rapidly triggered following initial host infection. A key distinguishing feature of immune memory maintenance between the majority of parasitic diseases and most bacterial or viral diseases is long-term antigen persistence. Consequently, functional parasite immune memory is in a continuous, dynamic flux between activation and deactivation producing functional parasite killing or functional memory cell death. In this sense, T-cell immune memory can be regarded as "memory illusion." Furthermore, due to the finite capacity of memory lymphocytes to proliferate, continuous parasite antigen stimulation may exceed a threshold level at some point in the chronically infected host. This may result in suboptimal effector immune memory leading to host susceptibility to reinfection, or immune dysregulation yielding disease reactivation or immune pathology. The goal of this review is to highlight, through numerous examples, what is currently known about T-cell immune memory to parasites and to provide compelling hypotheses on the survival and maintenance of parasite "memory illusion." These novel concepts are discussed in the context of rationale parasite vaccine design strategies.

Animals↗
PubMed1985

Parasites and asthma--predictive or protective?

Most prevalence surveys (figures 1 and 2) suggest that asthma is less common in heavily parasitized countries, but case-control studies either show no association or an increase in parasitism in asthmatics. Studies which included egg counts suggest that asthmatics have a lower parasite burden than normals, compatible with the notion that asthma protects against parasitic infection, or vice versa. The completely contradictory findings of serum IgE in three studies do not help to elucidate any association or its mechanism. The available epidemiologic data neither refute nor support the theory that parasitic disease protects against or causes asthma. A more definitive answer might come from a comparison of asthma prevalence in heavily parasitized and parasite-free subjects, or from a prospective study of asthma incidence in two or more comparable communities where one population is naturally or therapeutically free of intestinal parasites. If an effective antihelminthic vaccine is developed, it would also be interesting to watch vaccinees for development of, or changes in, asthma symptoms, and to determine whether these findings correlate with vaccine-induced changes in IgE.

Adult↗
PubMed2026

Virulent Parasites Emerge in Hosts With Rising Temperatures.

Climate change is increasing the risk of emerging parasites. However, whether more virulent variants will spread during climate-driven outbreaks remains unclear. Here, we aimed to explore the short-term trajectory of parasite evolution-at the phenotypic and genomic scales-across environmentally relevant temperatures in a thermally mismatched host-parasite interaction. We experimentally evolved a wild parasitic bacterium (Leucobacter musarum), across the thermal range (20°C-30°C) and extremes (35°C) of Cabo Verde-the site of field collection-in a Caenorhabditis elegans host strain. Starting from a single bacterial isolate, we then tracked phenotypic and de novo genomic changes that arose across replicate populations following ten passages of experimental evolution. We found that at 25°C, warm for the host but an average temperature for the parasite, host-mediated selection favoured higher virulence and genomic diversification by the end of the experiment. At hot temperatures, towards the limit of host survival, virulence was maintained across all parasite populations. Parasites evolved at hot temperatures also displayed a latent virulence boost, deadlier once hosts experienced a heatwave. Patterns of molecular evolution were constrained to parallel changes in fewer loci at extreme temperatures. Our findings suggest that shifting environmental temperatures will leave phenotypic and genomic signatures on evolving parasites.

Animals↗
PubMedDate not supplied

[The parasitic communities of the minnow (Phoxinus phoxinus L>)].

Phoxinus phoxinus being a widely distributed through Palaearctic and ecologically specialised species is a good model to study parasite communities. 72 specimens of P. phoxinus from 5 localities of the Siberia and Mongolia has been studied by the author and 651 specimens--from 5 localities in Kola Peninsula by the other researches. 595 specimens were studied by B. S. Shulman (1983) year around. To describe infra- and component parasite communities the Brillouin, Shannon, Berger-Parker indexes were used. Communities within individual fish were compared within, and between localities with quantitative percentage similarity index (Esch e. a., 1988). The Brillouin, Shannon and Berger-Parker indexes can be calculated based on abundance or total number of parasites of each species for component or compound communities. Intensity cannot be used. Significant negative correlation was observed between the Brillouin and Berger-Parker dominance indexes (r = -0.984, p < 0.01, n = 66) in infracommunities. The Brillouin index was significant positive correlated with the number of parasites in infracommunity (r = 0.407, p < 0.01, n = 66) and with the number of species in infracommunity (r = 0.92, p < 0.01, n = 66). Berger-Parker dominance index was significant negative correlated with the number of parasite in infacommunity (r = -0.815, p < 0.01, n = 66) and with the number of species in infracommunity (r = -0.268, p < 0.05). Total number of parasites and the number of species in infracommunity was positive correlated significantly (r = 0.565, p < 0.01, n = 66). These correlations remain the same at the component community level for the Shannon/Berger-Parker indexes (r = -0.965, p < 0.01, n = 10) and total number of parasites/number of species (r = 0.789, p < 0.01, n = 10) only. Similarity index was not correlate with any other indexes and parameters. No significant correlation was found between the Brillouin, Berger-Parker indexes and geographical characters of localities (longitude, latitude, altitude above sea-level). Weak correlation (r = 0.551, p < 0.098, n = 10) was observed only between the Berger-Parker dominance index and the latitude. 37 metazoan parasites have been found in all localities (specialists--40%, allogenic species--22%, rare species--40%). Allogenic generalist species of the genus Diplostomum (D. phoxini in most cases) were dominant in 60% of the component communities and 36% of infracommunities on average. 20% of the component communities were dominant by autogenic specialists Gyrodactylus magnificus and only 12% of the infracommunities were dominated by this species. Trematode and nematode larvae were core species in most localities. Frequency distribution of infracommunity richness varied from locality to locality being overdispersed in combine sample. Parasite communities at least on the infracommunity level demonstrate high variability of all parameters. Some tendency to decrease the diversity seems to be present depending on remoteness from prospective centre of origin of the genus Phoxinus.

Animals↗
PubMed1984

Implications of parasite-nutrition interactions from a world perspective.

Parasitic intestinal infections are among the most common in Africa, Asia, and Latin America. Although the mortality and morbidity caused by intestinal parasitic infections are relatively low, the absolute number of deaths and cases of disease is rather high in relation to other bacterial and viral infections. Scientific evidence for a causal relationship between intestinal parasitic infections and malnutrition is only fragmentary. The overdispersed frequency distribution of helminth parasites in the human population and the stability of giardiasis in some individuals may help only to define the potential target groups for immediate medical intervention, i.e., those patients heavily parasitized and/or severely malnourished. Long-term preventive intervention should include prenatal control of malnutrition, breast feeding and proper weaning food practices, oral rehydration for acute diarrhea, and immunization and possibly control of parasitic infections. Solid arguments in favor of the latter are expected to emerge from the field studies on intestinal parasitic infections and nutrition designed in 1981 by the World Health Organization and the United Nations Children's Emergency Fund. The results of these studies that will be carried out in several countries should serve as a guide for future parasite control projects and nutritional policies, including supplementary feeding programs.

Intestines↗
PubMed1994

Parasites, immunology of hosts, and host sexual selection.

Parasite-mediated sexual selection is reviewed with special emphasis on the bird literature. Choosy females may benefit from choosing parasite-free mates if such males provide better parental care, do not transmit contagious parasites, or provide resistance genes to offspring. There is evidence in support of each of these mechanisms. The immunocompetence handicap hypothesis posits that secondary sexual characters reliably reveal the ability of males to resist parasites due to the immunosuppressive effects of testosterone and other biochemicals. Several aspects of these negative feedback mechanisms are supported by laboratory studies, but evidence from free-living animals is almost completely absent. Corticosterone rather than testosterone may potentially mediate the immunocompetence handicap mechanism. A simple version of the immunocompetence handicap is developed suggesting that body condition of male hosts is a sufficient mediator of the handicap mechanism of reliable sexual signaling. Sexual selection appears to be more intense in sexually dichromatic bird species, and comparative studies using pairwise comparisons of closely related taxa reveal that sexually dichromatic bird species have larger spleens, larger bursa of Fabricius, and higher concentrations of leukocytes than monochromatic species. Parasite-mediated sexual selection is proposed to affect parasite biology by increasing (1) the variance-to-mean ratio in parasite abundance, (2) variance in the intensity of natural selection affecting hosts, and (3) speciation rates among parasites exploiting hosts subject to intense sexual selection as compared to those subject to less intense selection.

Animals↗
PubMed1997

Transmission dynamics and host-parasite interactions of Trichostrongylus tenuis in red grouse (Lagopus lagopus scoticus).

Two components of the transmission dynamics of Trichostrongylus tenuis in red grouse are examined and quantified, namely parasite transmission rate and density-dependent reductions in egg production. Age-intensity data for birds of known age suggest that the rate of parasite uptake increases during the first 6 mo of a bird's life and this increase reflects an increase in feeding rate with age and exhibits no signs of self-cure. Analysis of these age-intensity curves permits us to estimate the transmission rate of the free-living infective stages. Reinfection rates of adults treated to reduce parasite intensities were not significantly different from infection rates of naive immature grouse. Secondary infections continued to rise over a period of 18 mo and this suggests that there is no strong host-mediated response against the parasite. Any density-dependent reduction in parasite fecundity is probably very weak and would act through interspecific competition between parasites. Initial analysis of worm egg production in relation to the intensity of worm infection found weak evidence of density-dependent suppression of egg production at high worm intensities. However, a more rigorous analysis found that such a relationship suffered from Type I errors and was a consequence of the aggregated distribution of the parasites. Any density-dependent suppression of parasite egg production is too weak to be detected and would only occur at high worm intensities. The potential density-dependent reductions in fecundity on the population dynamics of T. tenuis and red grouse are examined using a mathematical model. The model suggests that the presence of density-dependent reductions in worm fecundity could produce significant reductions in the propensity of the grouse-nematode system to exhibit population cycles. The sustained cycles observed in the long-term dynamics of the grouse populations in the study area suggest that density-dependent reductions in worm fecundity and establishment are either absent or only operating at levels that are not detectable in field studies.

Age Factors↗
PubMed1994

Niche restriction in parasites: proximate and ultimate causes.

Hutchinson's (1957) definition of an ecological niche as a multidimensional hypervolume determined by a number of physical and biotic variables is adopted. The number of niche dimensions is very great, but as a working hypothesis it is assumed that a few are sufficient to characterize the niche of a parasite species to a high degree of accuracy. They are host species, microhabitat(s), macrohabitat(s), geographical range, sex and age of host, season, food and hyperparasites. Methods to measure niche width, in particular specificity indices, are discussed, and some examples of niche restriction are described. Proximate and ultimate causes of niche restriction are discussed, mainly using marine parasites as examples. Among proximate causes of one niche dimension, host specificity, are ecological factors restricting exposure to infection to certain host species; host-specific chemical factors that induce hatching, direct infective stages to a host and bring about settlement of a parasite; factors that lead to mortality in or on the wrong host; morphological adaptations that guarantee survival in or on the 'correct' host; and availability of suitable hosts. Many factors are likely to be responsible for microhabitat specificity, but have been little studied, except for some physiological and morphological adaptations to particular microhabitats. Macrohabitats and geographical range may be determined by the distribution of intermediate hosts and certain food items, and by a variety of chemical and physical factors. Hosts of different sexes may differ in feeding habits and the composition of the skin, and thus acquire parasites differentially. Hosts of different age may be differentially infected due to accumulation of parasites with age, loss of parasites due to developing resistance (or immunity), and different size and feeding habits. Among ultimate causes of niche restriction and segregation are avoidance of competition, predation and hyperparasites; facilitation of mating; reinforcement of reproductive barriers; and adaptations to environmental complexity. Few studies permit a decision on which factor or factors are responsible in particular cases. Interspecific competition may play a greater role in helminth communities of some host groups than of others, but it seems that, overall, its role has been exaggerated at least for marine parasites. Some 'classical' examples of microhabitat segregation explained by interspecific competition can also be explained by reinforcement of reproductive barriers. There is evidence for the importance of facilitation of mating in microhabitat restriction, and the availability of many vacant niches indicates that competition, overall, is not of great importance.

Animals↗
PubMed2005

Ecological implications of parasites in natural Daphnia populations.

In natural host populations, parasitism is considered to be omnipresent and to play an important role in shaping host life history and population dynamics. Here, we study parasitism in natural populations of the zooplankton host Daphnia magna investigating their individual and population level effects during a 2-year field study. Our results revealed a rich and highly prevalent community of parasites, with eight endoparasite species (four microsporidia, one amoeba, two bacteria and one nematode) and six epibionts (belonging to five different taxa: Chlorophyta, Bacillariophyceae, Ciliata, Fungi and Rotifera). Several of the endoparasites were associated with a severe overall fecundity reduction of the hosts, while such effects were not seen for epibionts. In particular, infections by Pasteuria ramosa, White Fat Cell Disease and Flabelliforma magnivora were strongly associated with a reduction in overall D. magna fecundity. Across the sampling period, average population fecundity of D. magna was negatively associated with overall infection intensity and total endoparasite richness. Population density of D. magna was negatively correlated to overall endoparasite prevalence and positively correlated with epibiont richness. Finally, the reduction in host fecundity caused by different parasite species was negatively correlated to both parasite prevalence and the length of the time period during which the parasite persisted in the host population. Consistent with epidemiological models, these results indicate that parasite mediated host damages influence the population dynamics of both hosts and parasites.

Animals↗
PubMed1994

A tropical assemblage of ectoparasites: gill and head parasites of Lethrinus miniatus (Teleostei, Lethrinidae).

Aspects of the community ecology of metazoan ectoparasites of Lethrinus miniatus (Teleostei. Lethrinidae) from 3 localities on the Great Barrier Reef were examined. Twelve species of Monogenea, 6 of Copepoda, 2 species of adult and several larval Isopoda and 1 leech were found. Total parasite numbers reach 3500 per host, and prevalence of infection with a particular parasite species reached 100% in some species. Numbers of parasite species per fish varied from 5 to 11. Four parasite species were most dominant on different fish if intensities of infection were used to calculate dominance, one species representing about 80% of all parasite individuals on a particular fish at 2 localities and about 50% at the third locality. Ten parasite species were most dominant on different fish, if volume (biomass) was used for calculating dominance, one species representing about 55% at one and 80% of total parasite volume at 2 localities. Parasites included at least 12 congeneric species of 4 or more genera, and those (of 2 genera) occurring on the gills showed much overlap in their distribution. Congeneric Monogenea of the families Dactylogyridae and Diplectanidae occurring in the same sites differ markedly in the size and structure of their copulatory sclerites. Positive associations between species were much more common than negative ones. The bony parts of the gills (gill arches) were never found to be infected, indicating the availability of empty niches.

Animals↗
PubMed2004

Phenyl substitution of furamidine markedly potentiates its anti-parasitic activity against Trypanosoma cruzi and Leishmania amazonensis.

Furamidine (DB75) and related unfused aromatic diamidines have proven useful for the treatment of parasitic infections. These compounds were primarily developed to combat infections by Pneumocystis carinii and African trypanosomes but they are also active against other parasites. Here we have investigated the in vitro effects of DB75 and its phenyl-substituted analog DB569 on two kinetoplastid haemoflagellates Trypanosomatidae: Trypanosoma cruzi and Leishmania (L) amazonensis. The phenyl-amidine compound DB569 has equivalent DNA binding properties compared to DB75 but it was selected on the basis of its distinct tumor cell distribution properties. We found that DB569 is significantly more potent than DB75 at reducing the proliferation of the parasites, using either isolated parasites in cultures or with cardiomyocyte and macrophage host cells. DB569 is effective towards the intracellular forms of T. cruzi (IC(50) in the low-micromolar range) and it exhibits trypanocidal dose-dependent effects against trypomastigote forms of T. cruzi parasites obtained from the Y strain and Dm28c clone, which belong to two different biodemes. Fluorescence microscopy experiments indicated that both diamidines were mostly localized in the nucleus of the mammalian host cells and within the nuclei and kinetoplast of the parasites. Electron microscopy studies showed that the treatment of the parasites with DB75 and DB569 induces important alterations of the parasite nucleus and kinetoplast, at sites where their DNA target is localized. Altogether, the data suggest that the phenyl-substituted furamidine analogue DB569 is a potential new candidate for the treatment of the Chagas' disease and Leishmaniasis.

Animals↗
PubMed2004

Parasite cryopreservation by vitrification.

Parasitic protozoa and helminths and parasitic/vector insects each have distinct requirements for cryopreservation. Most parasitic protozoa respond to cryopreservation stresses similarly to other single cell suspensions, but few species are currently routinely cryopreserved by protocols specifically designed for vitrification. With slow equilibrium cooling, some protozoa osmotically dehydrated by solutes concentrated in the residual unfrozen fraction will survive by vitrifying. Several species of helminths, together with insect embryos cannot be cryopreserved by slow cooling protocols and have an absolute requirement for vitrification. Studies incorporating slow cooling and stepped cooling of both protozoa and helminths, particularly the intraerythrocytic stages of malaria and the schistosomula larvae of Schistosoma mansoni have aided in the design of vitrification protocols for parasites. For helminths, the most widely used cryopreservation protocol, originally successful for cryopreserving S. mansoni schistosomula, consists of the addition of ethanediol in two steps, followed by rapid cooling (approximately 5100 degrees C min(-1)) to -196 degrees C. This technique exploits the temperature-dependent differential in permeability of the cryoprotectant additive (CPA) to first permeate into the organism at 37 degrees C followed by a dehydration-mediated internal CPA increase in concentration resulting from incubation in a second higher CPA concentration at 0 degree C. Samples are rapidly warmed/diluted (approximately 14,000 degrees C min(-1)) to recover the organisms from liquid nitrogen storage. Variations on this technique have also been successful in cryopreserving the larvae and adult worms of filariae, muscle stage larvae of Trichinella spp., the infective stages of gastro-intestinal nematode parasites and insect embryos. Other protocols where the dehydration step precedes CPA addition have been used to cryopreserve entomogenous nematode larvae by vitrification. Techniques that utilize high concentrations of CPA cocktails and slower cooling, developed for the vitrification of mammalian embryos, have been applied to the cryopreservation of parasitic protozoa, but with limited success to date. Where cryopreservation by classical slow cooling methods is possible, vitrification has enhanced the levels of survival obtained. And vitrification has enabled the successful cryopreservation of those parasitic species not able to be cryopreserved by traditional methods. Since a limited number of parasitic organisms has been cryopreserved using vitrification protocols, there is considerable scope for further improvement in the cryopreservation techniques used for many parasitic species.

Animals↗