Blastocystis in humans and animals: new insights using modern methodologies

Abstract

Among the waterborne protozoan parasites of medical and veterinary importance, Blastocystis is perhaps one of the less well-understood. However, in recent years, there has been a surge of interest in the organism, fueled in part by the possible association of Blastocystis infection with intestinal disorders, and its unusual taxonomic affiliations. Although there is information on the parasite's morphology, taxonomy and mode of transmission, its pathogenicity, life cycle, and function of certain organelles continue to baffle investigators. The clinical relevance of Blastocystis will be better answered once an animal model is found. Blastocystis infections have a worldwide distribution but prevalence is highest in areas with poor hygiene and deficient sanitation services and facilities. Application of modern molecular tools has advanced knowledge of the organism's genetic diversity, taxonomy and zoonotic potential.

Introduction

The appearance of an article on Blastocystis for this special issue on waterborne zoonotic parasites signifies a ‘coming of age’ for Blastocystis as this is the first dedicated review encompassing waterborne and zoonotic aspects of the parasite. Blastocystis is an ubiquitous protozoan parasite of the intestinal tract of humans and animals. Much of what we know about the organism's basic biology has been due to the research efforts of Zierdt in the 1970s and 1980s (Zierdt et al., 1967, Zierdt et al., 1983, Zierdt, 1973, Zierdt, 1988) and groups led by Boreham and Stenzel in the 1980s to 1990s (Upcroft et al., 1989, Dunn et al., 1989, Stenzel and Boreham, 1991, Stenzel and Boreham, 1996, Stenzel et al., 1991, Boreham et al., 1992, Boreham et al., 1996, Boreham and Stenzel, 1993), and in the 1990s by Yoshikawa and Singh (Teow et al., 1991, Teow et al., 1992, Ho et al., 1994, Yoshikawa et al., 1995, Singh et al., 1995, Singh et al., 1996, Tan et al., 1996a, Tan et al., 1996b; Yoshikawa et al., 1996, Yoshikawa et al., 1998, Moe et al., 1997, Moe et al., 1999). Despite being described almost a century ago (Alexeieff, 1911, Brumpt, 1912), the relatively recent and sudden surge in interest in the parasite can be attributed to Zierdt's suggestion that it causes intestinal disease (reviewed in Zierdt, 1991). However, most reports describe the morphology of the parasite, notably by transmission, freeze-fracture and scanning electron microscopy. Numerous prevalence studies have given us an appreciation of how common the parasite is in nature. Blastocystis spp. have been described in humans and in a wide variety of animal hosts including pigs, monkeys, guinea pigs, birds, reptiles and rats (Boreham and Stenzel, 1993). This review will focus on various aspects of Blastocystis in humans and animals, highlighting recent molecular evidence that strongly suggests Blastocystis’ potential as a zoonotic organism. For detailed information on Blastocystis morphology, the reader is referred to the excellent review of Boreham and Stenzel (1993).