Parasite is a bad word with negative connotations. Yet, "bad things" can be good for you - and every situation is different.
About one third of people in the world carry at least one parasite in their gastrointestinal tract (the numbers reported vary from 2-4% in Japanese residing in developing countries to 6% in patients of a specialized private hospital in Saudi Arabia, 7% among individuals with mental retardation in New York, 17% in HIV patients with low CD4 counts in France, 30% in households of USA, Saudi Arabia and Turkey, 40% in Pakistan and Brasil, to over 50% in Nigeria, 50-70% in Morocco and almost 100% in Senegal). Prevalence varies between countries and within communities. Women and children appear to harbor larger numbers of parasites. But men, in general, may be less healthy than women, so the relationship between health and intestinal parasites is not as straightforward as one might think.
Let's take a look at three studies published this year.
Paper by Lukeš and co-authors (2014) suggests that intestinal parasites such as Blastocystis (single celled protozoa closely related to algae and molds) and nematodes (e.g., tapeworms or hookworms) can be actually beneficial to human health. To prove the point, Julius Lukeš even ingested a handful of tapeworms called Diphyllobothrium latum. After more than a year with the tapeworms, which might have grown to be as long as four meters each by now, he still feels healthy and convinced that we should rethink our views of organisms that live off our bodies.
According to a Danish study (Krogsgaard et al, 2014), Blastocystis could be rare in individuals with low microbial diversity, disturbed by antibiotic treatment, inflammation, infection and diet, while common in the healthy population. Healthy individuals are more likely to carry intestinal parasites (50% vs 36%) than those with IBS and IBD. Protozoa Blastocystis and Dientamoeba were the most common parasites found. D fragilis was detected in a greater proportion of fecal samples from controls than cases (35% vs 23%; P = .03), and so was Blastocystis (22% of controls vs 15% of cases; P = .09), and combinations of parasite species (16% of controls vs 8% of cases; P = .05). D fragilis infection was more likely among those with low frequency of defecation and those having children 5 to 18 years old in the household. Blastocystis was associated with high income, increasing age, no animals in the household and drinking bottled water. These results are drawn from analyzing hundreds of individuals - 124 cases/204 controls.
Smaller and more focused studies, however, do find association between Blastocystis and IBS.
A recent French study compared the prevalence of Blastocystis among 56 IBS patients and 56 control and found that Blastocystis species are likely to couse IBS symptoms in men (prevalence was 37% in IBS sufferers vs 5% in healthy men; difference in women was not statistically significant). One of the older smaller studies that indicated possible relation between Blastocytes and IBS (Funda Dogruman-Al et al., 2009) suggested that Blastocystis don't really attack the body, it's the human organism that attacks the microbes first and the outcome is the result of defensive actions by the microbes. Obviously, symptoms do depend on many other health factors. Nourrisson and co-authors identified them as the prevalence of certain "good" bacteria. Men are more likely to get constipated when their Blastocystis leads to a significant decrease in Bifidobacterium species. On the other hand, they are doing just fine if Blastocystis causes decrease in Faecalibacterium prausnitzii - relatively good bacteria known for its anti-inflammatory properties,
Another interesting insight into the impact of a complex interplay of environmental factors on our health. Still a long way to go to understand how we interact with microbes that call us home.
Lukeš J, Kuchta R, Scholz T, & Pomajbíková K (2014). (Self-) infections with parasites: re-interpretations for the present. Trends in parasitology, 30 (8), 377-85 PMID: 25033775
Krogsgaard LR, Engsbro AL, Stensvold CR, Nielsen HV, & Bytzer P (2014). The Prevalence of Intestinal Parasites Is Not Greater Among Individuals With Irritable Bowel Syndrome: a Population-Based Case-Control Study. Clinical gastroenterology and hepatology : the official clinical practice journal of the American Gastroenterological Association PMID: 25229421
Nourrisson C, Scanzi J, Pereira B, NkoudMongo C, Wawrzyniak I, Cian A, Viscogliosi E, Livrelli V, Delbac F, Dapoigny M, & Poirier P (2014). Blastocystis Is Associated with Decrease of Fecal Microbiota Protective Bacteria: Comparative Analysis between Patients with Irritable Bowel Syndrome and Control Subjects. PloS one, 9 (11) PMID: 25365580
El Safadi D, Gaayeb L, Meloni D, Cian A, Poirier P, et al. (2014) Children of Senegal River Basin show the highest prevalence of Blastocystis sp. ever observed worldwide. BMC Infect Dis 14: 164 doi:10.1186/1471-2334-14-164.
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