SIR, Blastocystis hominis is a common human parasite with a worldwide distribution. Prevalence rates in developed countries vary from 1.5% to 10%, with rates up to 50% in developing countries.1 Although the numbers of B. hominis reported to the UK Public Health Laboratory Service (PHLS) Communicable Diseases Surveillance Centre (CDSC) are substantially lower, we feel this is due largely to a lack of awareness. Consequently, the true incidence of B. hominis in England and Wales remains unknown.
Over the past 100 years, B. hominis has had a varied taxonomic history, being described as a yeast, a degenerate or cyst form of a flagellate, a protozoan and a sporozoan.2 In 1996, Silberman et al.,3 using phylogenetic analysis of ribosomal RNA, classified it as the only member of the Stramenopiles (a diverse group of eukaryotes that include kelp, diatoms, slime nets and water moulds) found in humans.
B. hominis has been reported in patients with intestinal symptoms,4'5 intestinal obstruction due to carcinoma,6 in AIDS patients with diarrhoea,' and in patients with irritable bowel syndrome.8 In addition, it also has been reported in asymptomatic individuals;9,10 therefore, its exact role in human disease remains controversial. However, many important questions regarding possible pathogenic subgroups of B. hominis and associated pathogenic mechanisms remain unanswered.
Over the past decade, the UK NEQAS Parasitology Scheme has been instrumental in educating laboratories in the identification and possible significance of potential enteric pathogens such as B. hominis and Dientamoeba fragilis, mainly by circulation of stained faecal smears. Paradoxically, B. hominis was present in three samples of unstained faecal material distributed during 2000, but was only mentioned once and not scored. Whilst we recognise the difficulties of ensuring an even distribution of B. hominis, failure to acknowledge the presence of this potential pathogen in NEQAS specimens may give the message that it is not significant and could discourage laboratories from reporting clinical isolates of B. hominis to CDSC.
In fact, the numbers reported to CDSC have fallen dramatically, from 551 in 1996 to 293 in 1999. As B. hominis is the most common faecal parasite seen at both Aberystwyth PHL and Swansea PHL11 in the UK, we feel that the CDSC figures do not reflect the true incidence of B. hominis in England and Wales. Indeed, all 139 reports of B. hominis reported to CDSC Wales in 2000 were detected by our two laboratories (unpublished data). We believe that this can be attributed to laboratory awareness and the use of suitable methodologies.11
Although B. hominis is a controversial parasite in terms of pathogenicity, largely it has been overlooked in the UK. The UK NEQAS Parasitology Scheme has an important role to play in the continued education of laboratories with regard to the identification of B. hominis; therefore, we feel it important to acknowledge its presence in future distributions, both in stained smears and unstained preparations. Increased laboratory awareness should result in more accurate data on the incidence of this parasite in England and Wales.
J. J. Windsor*, L. Macfarlane*, T. M. Whiteside*, R. M. Chalmers t, A. L. Thomas and
D. H. M. Joynson t
*Aberystwyth Public Health Laboratory, Bronglais Hospital, Caradoc Road, Aberystwyth, Ceredigion SY23 1ER
t Cryptosporidium Reference Unit
Swansea Public Health Laboratory, Swansea, Wales, UK References
1 Stenzel DJ, Boreham PFL. Blastocystis hominis revisited. Clin Microbiol Rev 1996; 9: 563-84.
2 Zierdt CH. Blastocystis hominis - past and future. Clin Microbiol Rev 1991; 4: 61-79.
3 Silberman JD, Sogin ML, Leipe DD, Clark CG. Human parasite finds taxonomic home. Nature 1996; 380: 398.
4 Sheenan DJ, Raucher BG, McKitrick JC. Association of Blastocystis hominis with signs and symptoms of human disease. J Clin Microbiol 1986; 24: 548-50.
5Udkow MP, Markell EK. Blastocystis hominis: prevalence in asymptomatic versus symptomatic hosts. J Infect Dis 1993; 168: 242.
6 Horiki N, Kaneda Y, Maruyama M, Fujita Y, Tachibani H. Intestinal blockage by carcinoma and Blastocystis hominis infection. Am J Trop Med Hyg 1999; 60: 400-2.
7 Corte L, Rabodonirina M, Piens MA, Perreard M, Mojon M, Trepo C. Prevalence of intestinal protozoans in French patients infected with HIV. J Acquir Immune Defic Syndr 1993; 6: 1024-9.
8 Giacometti A, Cirioni O, Fiorentini A, Fortuna M, Scalise G. Irritable bowel syndrome in patients with Blastocystis hominis infection. Eur J Clin Microbiol Infect Dis 1999; 18: 436-9.
9 Shlim DR, Hoge CW, Rajah R, Rabold JG, Echeverria P. Is Blastocystis hominis a cause of diarrhoea in travellers? A prospective controlled study in Nepal. Clin Infect Dis 1995; 21: 97-101.
10 Hellard ME, Sinclair MI, Hogg GG, Fairley CK. Prevalence of enteric pathogens among community-based asymptomatic individuals. J Gastroenterol Hepatol 2000; 15: 290-3.
11 Windsor JJ, Jones SK, Macfarlane L, Chalmers R, Thomas AL, Joynson DHM. Enhanced detection of Blastocystis hominis [Abstract]. 25`" PHLS Annual Scientific Conference, University of Warwick, UK. September 18-20 2000, Poster no 177.
SIR, We are grateful to Windsor et al. for highlighting the achievements of UK NEQAS Parasitology in educating laboratories in the identification of various enteric parasites, but were surprised to find them holding us responsible for under-notification of Blastocystis hominis in the UK.
Whether or not this organism is a pathogen is not an issue for NEQAS, as our terms of reference simply are to address proficiency in laboratory examination. Furthermore, failure of laboratories to report the organism is outside our control, as we have no authority over the internal operating procedures of our participants. We can and do respond to requests for advice on matters relating to parasite diagnosis, but do our best not to exceed our remit.
UK NEQAS Parasitology was designed to improve the diagnosis of parasites by the examination of clinical material (almost all of our distributions are sourced from human cases) and to provide teaching material that illustrates unusual or uncommon parasites. In order to develop diagnostic skills, participants are encouraged to report all parasites, even those considered to be non-pathogenic. However, parasites may not be scored or mentioned in the summary for the following reasons:
1 A mean of less than five ova per deposit or less than one cyst per five fields (x40 objective) means that the parasite cannot be guaranteed to be present in every sample.
2 The request is to examine for ova or cysts only any parasites falling outside these criteria will not be included in the summary, even if pathogenic.
3 There are more than three different parasites per specimen - only the three most numerous parasites are scored and the others are mentioned in the summary. Participants are not penalised for listing more than three parasites, provided they are present in the distribution, and, as many of the specimens used in the schemes come from the tropics, it is not uncommon for them to contain more than three parasites.
The intended results issued are based both on a thorough in-house predistribution examination of the sample and on the comments from 20 referee laboratories.
Over the past year, the fact that B. hominis was neither scored nor reported in the summary was due to the complex nature of the specimens distributed and the nature of the scoring system, and not a failure to acknowledge the presence of a potential pathogen in the specimen concerned. Indeed, as Windsor et al. state, the role of this organism in human disease remains controversial.
UK NEQAS Parasitology will continue to send out, and score, distributions containing B. hominis, and, in fairness to all laboratories taking part in the scheme, will continue to follow the policy that organisms are scored only when we are sure that the specimen represents a fair test of diagnostic ability.
Peter L. Chiodini and Monika M. Kettelhut
Department of Clinical Parasitology (QC) Hospital for Tropical Diseases Mortimer Market, Capper Street London WC1E 6AU, UK
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