The Journal of Health, Population and Nutrition icddr,b ISSN: 1606-0997 EISSN: 2072-1315 Vol. 25, Num. 4, 2007, pp. 387-391

Journal of Health, Population and Nutrition, Vol. 25, No. 4, December, 2007, pp. 387-391

Editorial

Human Intestinal Parasites

Rashidul Haque

Scientist and Head of Parasitology Laboratory, Laboratory Sciences Division, ICDDR,B, GPO Box 128, Dhaka 1000, Bangladesh, Email: rhaque@icddrb.org
Correspondence and reprint requests should be addressed to: Dr. Rashidul Haque, Scientist and Head of Parasitology Laboratory, Laboratory Sciences Division, ICDDR,B, GPO Box 128, Dhaka 1000, Bangladesh, Email: rhaque@icddrb.org

Code Number: hn07051

Parasitic infections, caused by intestinal helminths and protozoan parasites, are among the most preva­lent infections in humans in developing coun­tries. In developed countries, protozoan parasites more commonly cause gastrointestinal infections compared to helminths. Intestinal parasites cause a significant morbidity and mortality in endemic countries.

Helminths are worms with many cells. Nematodes (roundworms), cestodes (tapeworms), and trema­todes (flatworms) are among the most common helminths that inhabit the human gut. Usually, helminths cannot multiply in the human body. Protozoan parasites that have only one cell can multiply inside the human body. There are four spe­cies of intestinal helminthic parasites, also known as geohelminths and soil-transmitted helminths: Ascaris lumbricoides (roundworm), Trichiuris trichiu­ria (whipworm), Ancylostoma duodenale, and Neca­tor americanicus (hookworms). These infections are most prevalent in tropical and subtropical regions of the developing world where adequate water and sanitation facilities are lacking (1,2). Recent esti­mates suggest that A. lumbricoides can infect over a billion, T. trichiura 795 million, and hookworms 740 million people (3). Other species of intesti­nal helminths are not widely prevalent. Intestinal helminths rarely cause death. Instead, the burden of disease is related to less mortality than to the chronic and insidious effects on health and nutri­tional status of the host (4,5). In addition to their health effects, intestinal helminth infections also impair physical and mental growth of children, thwart educational achievement, and hinder economic development (6,7).

In an article published in this issue of the Journal, Jacobsen et al. looked at the prevalence of intes­tinal parasites in young Quichua children in the highland or rural Ecuador (13). They have found a high prevalence of intestinal parasites, especially the intestinal protozoan parasites. They have used the traditional microscopic technique to diagnose intestinal parasitic infections. In total, 203 stool samples were examined from children aged 12-60 months and found that 85.7% of them had at least on parasite. The overall prevalence of in­testinal protozoan parasites were: E. histolytica/E. dispar 57.1%, Escherichia coli 34.0%, G. intestinalis 21.1%, C. parvum 8.9%, and C. mesnili 1.7%, while the prevalence of intestinal helminthic parasites in this study were: A. lumbricoides 35.5%, T. trichiura 0.5 %, H. diminuta 1.0%, and S. stercoralis 0.7%. A recent study in Nicaragua in asymptomatic indi­viduals found that 12.1% (58/480) were positive for E. histolytica/E. dispar by microscopy, but E. his­tolytica and E. disapr were positive by polymerase chain reaction (PCR) only in three and four stool samples respectively among the microscopic posi­tive samples (Unpublished data). This study proves again that the diagnosis of E. histolytica/E. dispar is neither sensitive nor specific when it is done by mi­croscopy. To understand the real prevalence of E. histolytica-associated infection, a molecular method must be used for its diagnosis.

Over the last several years, we have seen new ap­proaches to the diagnosis, treatment, and preven­tion of intestinal protozoan parasites. However, the diagnosis and treatment of intestinal helminth infections have not been changed much, and the traditional microscopic method can be used for their diagnosis. Antigen-detection tests are now commercially available for the diagnosis of all three major intestinal protozoan parasites. Diagno­sis of E. histolytica cannot be done any longer by microscopy, since this parasite is morphologically similar to the non-pathogenic parasite E. dispar. E. histolytica-specific antigen-detection test is now commercially available from TechLab, Blacksburg, Virginia, for the detection of E. histolytica antigen in stool specimens (14,15). In several studies, this E. histolytica-specific antigen-detection test has been used for the specific detection of E. histolytica (16,17). These studies have found that this antigen-detection test is sensitive and specific for the de­tection of E. histolytica. In a study in Bangladesh, E. histolytica-specific antigen-detection test identi­fied E. histolytica in 50 of 1,164 asymptomatic pre-school children aged 2-5 years (18). In a study in Nicaragua among patients with diarrhoea, where E. histolytica-specific test has been used, found that the prevalence of E. histolytica was 0.5% (19). In a study conducted in a cohort of Bangladeshi chil­dren found that the prevalence of E. histolytica in diarrhoeal stool samples was 8.0% (20). No studies that have been carried till date using E. histolytica­specific diagnostic test reported the prevalence of E. histolytica more than 10%. In addition to the antigen-detection test, several PCR-based tests specific for E. histolytica have been developed and used for specific detection of E. histolytica (21,22). Rapid diagnostic test for the detection of E. histolytica antigen in stool specimens has also been reported (23).

Diagnosis of giardiasis is best accomplished by de­tection of Giardia antigen in stool, since the classic microscopic examination is less sensitive and spe­cific. A recent comparison of nine different antigen-detection tests demonstrated that all had high sensitivity and specificity, except one (24). Giardia-specific antigen-detection tests are now also commercially available from several diagnostic companies, and their performance is quite good, except a few. In addition to antigen-detection tests, PCR-based test for the detection of G. intesti­nalis has also been reported (25). The population genetics of Giardia are complex. However, a recent genetic linkage study has confirmed the distinct grouping of Giardia in two major types (26). These two main genotypes/assemblages of G. intestinals are commonly known as: assemblage A and assem­blage B of G. intestinalis. Differentiation of these two assemblages of G. intestinalis can only be done by PCR-based tests. Findings of the largest case-control study conducted to date on the relationship between genotypes of G. intestinalis and symptoms of patients have been published (27). This study has shown that the Giardia assemblage A infection is associated with diarrhoea. In contrast, Giardia assemblage B infection is significantly associated with asymptomatic Giardia-associated infection, which was found to occur at a significantly higher rate (18.0%) as detected by the antigen-detection test (27). The PCR-based approach allowed resolu­tion of infection to the genotype level and brought some clarity to the findings of asymptomatic giar­diasis. Similar large-scale case-control studies need to be carried out in other continents to understand more on the association of Giardia assemblages with diarrhoea/dysentery.

Diagnosis of cryptosporidiosis is also best accom­plished by detection of Cryptosporidium spp. antigen in stool samples, since classic microscopic ex­amination is less sensitive, and modified acid-fast staining is required. Cryptosporidium spp.-specific antigen-detection test has been used in several studies and has been found to be sensitive and spe­cific compared to classic microscopic examination and PCR-based test (28,29). There are two main species of Cryptosporidium that infect humans: C. hominis (genotype I) and C. parvum (genotype II). The PCR-based test is required for differentiation of these two species of Cryptosporidium spp. (30). Both C. hominis and C. parvum have been found in humans. There are a few other species of Cryptos­poridium that also can be found in humans (31-33). Rapid diagnostic tests for the detection of G. lamblia and Cryptosporidium spp. have also been reported (34,35). Multiplex PCR-based test for the detec­tion of E. histolytica, G. intestinalis, and Cryptosporid­ium spp. has already been reported, and the deve­lopment of multiplex antigen-detection test for these three common and pathogenic intestinal proto­zoan parasites is underway at TechLab, Blacksburg, Human intestinal parasites Virginia (36, Herbain J. Personal communication, 2007). These modern antigen-detection tests and PCR-based tests need to be used for understanding the actual prevalence and epidemiology of these protozoan parasites.

Soil-transmitted helminth infections are invariably more prevalent in the poorest sections of the popu­lations in endemic areas of developing countries. The goal is to reduce morbidity from soil-transmit-ted helminth infections to such levels that these in­fections are no longer of public-health importance. An additional goal is to improve the developmental, functional and intellectual capacity of affected chil­dren (37). Highly-effective, safe single-dose drugs, such as albendazole, now available, can be dis­pensed through healthcare services, school health programmes, and community interventions direct-ed at vulnerable groups (38). As these infections are endemic in poor communities, more permanent control will only be feasible where chemotherapy is supplemented by improved water supplies and sanitation, strengthened by sanitation education. In the long term, this type of permanent transmis­sion control will only be possible with improved living conditions through economic development. Intestinal protozoa multiply rapidly in their hosts, and as there is a lack of effective vaccines, chemo-therapy has been the only practised way to treat individuals and reduce transmission. The current treatment modalities for intestinal protozoan parasites include metronidazole, iodoquinol, diloxanide furoate, paromomycin, chloroquine, and trimeth-oprim-sulphamethoxazole (39). Nitazoxanide, a broad-spectrum anti-parasitic agent, was reported to be better than placebo for the treatment of cryp­tosporidiosis in a double-blind study performed in Mexico (40). Genomes of these three important protozoan parasites have already been published (41-43), and studies are underway to understand protective immunity to these protozoan parasites to develop vaccines for them.

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