Summary: Blastocystis is an unusual enteric protozoan parasite of humans and many animals. It has a worldwide distribution and is often the most commonly isolated organism in parasitological surveys. The parasite has been described since the early 1900s, but only in the last decade or so have there been significant advances in our understanding of Blastocystis biology. However, the pleomorphic nature of the parasite and the lack of standardization in techniques have led to confusion and, in some cases, misinterpretation of data. This has hindered laboratory diagnosis and efforts to understand its mode of reproduction, life cycle, prevalence, and pathogenesis. Accumulating epidemiological, in vivo, and in vitro data strongly suggest that Blastocystis is a pathogen. Many genotypes exist in nature, and recent observations indicate that humans are, in reality, hosts to numerous zoonotic genotypes. Such genetic diversity has led to a suggestion that previously conflicting observations on the pathogenesis of Blastocystis are due to pathogenic and nonpathogenic genotypes. Recent epidemiological, animal infection, and in vitro host-Blastocystis interaction studies suggest that this may indeed be the case. This review focuses on such recent advances and also provides updates on laboratory and clinical aspects of Blastocystis spp.

New insights on classification, identification, and clinical relevance of Blastocystis spp.

Blastocystis is a common parasite of the human large intestine but has an uncertain role in disease. In this review, we appraise the published evidence addressing this and its weaknesses. Genetic diversity studies have led to the identification of numerous subtypes (STs) within the genus Blastocystis and, recently, methods for studying variation within STs have been developed, with implications for our understanding of host specificity. The geographic distribution of STs is summarised and the impact this may have on investigations into the role of the organism in disease is discussed. Finally, we describe the organelle and nuclear genome characteristics and look to future developments in the field.

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Recent developments in Blastocystis research.

Summary: Globally, the number of immunosuppressed people increases each year, with the human immunodeficiency virus (HIV) pandemic continuing to spread unabated in many parts of the world. Immunosuppression may also occur in malnourished persons, patients undergoing chemotherapy for malignancy, and those receiving immunosuppressive therapy. Components of the immune system can be functionally or genetically abnormal as a result of acquired (e.g., caused by HIV infection, lymphoma, or high-dose steroids or other immunosuppressive medications) or congenital illnesses, with more than 120 congenital immunodeficiencies described to date that either affect humoral immunity or compromise T-cell function. All individuals affected by immunosuppression are at risk of infection by opportunistic parasites (such as the microsporidia) as well as those more commonly associated with gastrointestinal disease (such as Giardia). The outcome of infection by enteric protozoan parasites is dependent on absolute CD4+ cell counts, with lower counts being associated with more severe disease, more atypical disease, and a greater risk of disseminated disease. This review summarizes our current state of knowledge on the significance of enteric parasitic protozoa as a cause of disease in immunosuppressed persons and also provides guidance on recent advances in diagnosis and therapy for the control of these important parasites.

https://cmr.asm.org/content/cmr/22/4/634.full.pdf

Clinical Significance of Enteric Protozoa in the Immunosuppressed Human Population

Irritable bowel syndrome: A review on the role of intestinal protozoa and the importance of their detection and diagnosis

Blastocystis subtypes detected in humans and animals from Colombia

The infectivity of two Blastocystis hominis strains, RN94-9 and NIH:1295:1, was examined in 3-week-old SPF Wistar rats. The NIH:1295:1 strain, originally isolated from a guinea pig, was only able to infect rats via intracecal inoculation of the cultured organisms, while the RN94-9 strain, originally isolated from a laboratory rat, was able to infect rats by oral inoculation of the cultures due to the presence of a cystic form in the in vitro culture. Since many cysts were discharged in the feces of the infected rats, the infectivity of the concentrated cysts was compared between the two strains. Successful oral infection was observed in rats inoculated with 1×102–1×106 cysts of the RN94-9 and NIH:1295:1 strains. The infectivity of the ten cysts varied in the three experiments of ten rats, being 20–100% and 30–100% in the RN94-9 and NIH:1295:1 strains, respectively. When an uninfected normal rat was housed with five experimentally inoculated rats, the normal rat became infected, demonstrating the fecal-oral transmission of the cyst form of this parasite. These results show that the Wistar rat is an ideal host for the propagation of strains RN94-9 and NIH:1295:1 of B. hominis, and demonstrate that the cyst form is the only transmissible form of this parasite.

Fecal-oral transmission of the cyst form of Blastocystis hominis in rats

Infectivity of different genotypes of human Blastocystis hominis isolates in chickens and rats.

The infectivity of six Blastocystis isolates obtained from two domestic chickens, two Japanese quails and two domestic geese, were examined in 1-week-old male chicks. All six isolates were able to infect the chicks via the intracecal inoculation of 1×106 cells of cultured organisms. Since the infected chicks discharged many cysts in their feces, the infectivity of the concentrated cysts in chicks was compared among three isolates from different bird species. The CK86-1 and QQ93-3 isolates, which were obtained from a chicken and a quail, respectively, were successfully infected in chicks by orally inoculating with 1×102–1×106 cysts. On the other hand, the AC03-1 isolate from a goose required more cysts to infect the chicks, from 1×103 cysts to 1×106 cysts. In addition, when an uninfected normal chick was housed with five experimentally inoculated chicks with cysts of the QQ93-3 isolate, the normal chick became infected, indicating the fecal–oral transmission of the cyst form among the birds. These results show that the transmission of Blastocystis infection occurs easily between the same or different bird species. Therefore, the proposal of new Blastocystis species on the basis of different avian host species is problematic.

Satoru Iwatani

Papers

Fecal-oral transmission of the cyst form of Blastocystis hominis in rats

Infectivity of different genotypes of human Blastocystis hominis isolates in chickens and rats.

Infectivity of Blastocystis isolates from chickens, quails and geese in chickens