Andrew C Thompson

Zoonotic parasites are important causes of endemic and emerging human disease in northern North America and Greenland (the North), where prevalence of some parasites is higher than in the general North American population. The North today is in transition, facing increased resource extraction, globalisation of trade and travel, and rapid and accelerating environmental change. This comprehensive review addresses the diversity, distribution, ecology, epidemiology, and significance of nine zoonotic parasites in animal and human populations in the North. Based on a qualitative risk assessment with criteria heavily weighted for human health, these zoonotic parasites are ranked, in the order of decreasing importance, as follows: Echinococcus multilocularis, Toxoplasma gondii, Trichinella and Giardia, Echinococcus granulosus/canadensis and Cryptosporidium, Toxocara, anisakid nematodes, and diphyllobothriid cestodes. Recent and future trends in the importance of these parasites for human health in the North are explored. For example, the incidence of human exposure to endemic helminth zoonoses (e.g. Diphyllobothrium, Trichinella, and Echinococcus) appears to be declining, while water-borne protozoans such as Giardia, Cryptosporidium, and Toxoplasma may be emerging causes of human disease in a warming North. Parasites that undergo temperature-dependent development in the environment (such as Toxoplasma, ascarid and anisakid nematodes, and diphyllobothriid cestodes) will likely undergo accelerated development in endemic areas and temperate-adapted strains/species will move north, resulting in faunal shifts. Food-borne pathogens (e.g. Trichinella, Toxoplasma, anisakid nematodes, and diphyllobothriid cestodes) may be increasingly important as animal products are exported from the North and tourists, workers, and domestic animals enter the North. Finally, key needs are identified to better assess and mitigate risks associated with zoonotic parasites, including enhanced surveillance in animals and people, detection methods, and delivery and evaluation of veterinary and public health services.

The taxonomy of Giardia has been controversial for well over 100 years, resulting in a confusing nomenclature with different names often being used for the same species. This has led to uncertainty in our understanding of the epidemiology of Giardia infections, and particularly the question of host specificity and zoonotic transmission. The lack of morphological characters on which to base a species level taxonomy for the forms of Giardia that Infect mammals has not allowed these Issues to he resolved. It is only recently that PCR-based tools have been developed and applied directly to isolates of Giardia from a range of mammalian species. As a consequence, the taxonomy or Giardia can now be revised providing a more effective platform for epidemiological studies and importantly, improving communication between researchers in the held.

Cryptosporidium is an apicomplexan parasite that has gained much attention as a clinically important human pathogen since the late 1980s; however, little is known regarding the developmental biology of this parasite. Recent molecular and biological studies provide evidence that Cryptosporidium should be placed in a taxonomic group separate from the coccidia and closer to the gregarines (reviewed in Barta and Thompson, 2006). Furthermore, novel extracellular gregarine-like life cycle stages have been described. In addition to these findings, Hijjawi et al. (2004) reported the cell-free propagation of the life cycle of C. parvum, which also led to the identification of developmental stages similar to those observed in some gregarine species. The completion of the life cycle of Cryptosporidium in the absence of host cells raises many questions about the developmental nature of this parasite and its relationship to lower species of apicomplexans such as gregarines. This chapter covers recent observations on the developmental biology and life cycle of Cryptosporidium in an attempt to highlight more facts on the evolutionary biology of this unique parasite. Similarities between Cryptosporidium and some gregarine species are also included.

An increasing number of parasites are being added to the list of those that can be transmitted via food or water and that pose a risk to human health if ingested. These zoonotic infections usually have complicated life cycles requiring a number of hosts for completion or a diversity of cycles of transmission that may interact. The challenge in all control efforts is to break the cycle of transmission that may lead to human infection, which requires the ability to detect and characterize the relevant parasite life cycle stage in food or water. This requires tools that are both sensitive and specific, and often beyond the limitations of conventional techniques such as microscopy.

Determining the source of infection is central to an understanding of the epidemiology of giardiasis. In this respect, the role of zoonotic transmission has been a matter of controversy for many years. This has been complicated by the fact that the causative agent of giardiasis, Giardia duodenalis, is a common parasite of people, domestic animals and wildlife. The development and application of molecular epidemiological tools has now made it possible to directly genotype Giardia isolated from animals and environmental samples. These studies have shown that many species of mammals are susceptible to infection with zoonotic and host-adapted genotypes of G. duodenalis and that they are often present in the same endemic foci. Recent studies have also demonstrated that zoonotic transmission does occur in nature. However, available data suggests that zoonotic transmission does not appear to play a major role in waterborne outbreaks of giardiasis. More studies are required on the molecular epidemiology of Giardia infections in order to more accurately determine the frequency of zoonotic transmission in localised endemic foci and in outbreak situations.

This chapter presents an overview on the successful cultivation of Cryptosporidium. The successful cultivation reveals previously undescribed gregarine-like developmental stage. s. Cryptosporidium shows peculiarities that separate it from other coccidia, such as two morphofunctional oocysts, a multi-membranous feeder organelle, and endogenous development in the microvilli of epithelial surfaces. Cryptosporidium has a closer affinity with the gregarines than with the coccidian. This chapter presents a study in which at least two developmental stages in Cryptosporidium are found to be similar to those of some gregarines. These are an extracellular trophozoite/gamont-like stage and a gametocyst-like stage. Both stages were detected in vitro and in vivo. Furthermore, surface association of gamont-like stages as well as oocyst formation by budding from gametocyst stages was observed in this study.

This chapter focuses on the high resolution genotyping of Cryptosporidium by mutation scanning. A range of different molecular approaches has been developed to characterize Cryptosporidium species and genotypes. Some of these approaches may not necessarily precisely resolve sequence variation because they depend on the size-separation of DNA molecules. Some methods are laborious and time-consuming to perform when the analysis of large numbers of samples is needed. Such limitations may be overcome by employing mutation detection methods, such as Single-Strand Conformation Polymorphism (SSCP). This chapter presents a study, the aim of that was to evaluate an SSCP approach for the genotyping of Cryptosporidium oocyst isolates. The present SSCP methods have advantages over some approaches to screen for genetic variation in Cryptosporidium. In contrast to arbitrarily primed-PCR, SSCP employs amplicons produced at higher stringency using specific primers, thus minimizing the co-amplification of extraneous DNA and maximizing reproducibility. SSCP can be used to screen relatively large numbers of samples for variation prior to selective DNA sequence analysis, which reduces considerably time, labor, and expense, and in contrast to PCR-coupled RFLP, which screens for sequence variation at a small number of endonuclease restriction sites, SSCP scans the entire length of an amplicon for variability. The results presented in this chapter indicate that the present approach can be used as a diagnostic tool to identify any of the currently recognized species and genotypes of Cryptosporidium. Combined with DNA sequencing, the approach also provides a powerful analytical tool.

More than 200 compounds have been tested for activity against Cryptosporidium parvum, both in vitro and in vivo, there is still no effective treatment. Previous studies have revealed the anticryptosporidial effect of the tubulin specific herbicides, the dinitroanilines. The in vitro activities of two members of this class of compounds, oryzalin and trifluralin have been demonstrated against Cryptosporidium. Recent studies exposed IC50 values for oryzalin and trifluralin against Cryptosporidium of 750 and 800 nM, respectively. This chapter presents a study, the aim of which on-going study is to examine tubulin as an effective target both in vivo and ex vivo.