Clinical pilot study: efficacy of triple antibiotic therapy in Blastocystis positive irritable bowel syndrome patients

Blastocystis spp.are unicellular, anaerobic enteric parasites found world wide in almost all species of animals [1]. The cystic form survives for days in the environment and likely transmits infection by the faecal-oral route either by direct contact or via contaminated water supplies. Blastocystis spp. are the most common parasite found in human faeces and carriage has been reported to be associated with Irritable Bowel Syndrome (IBS) like symptoms [1]-[3]. IBS is a chronic heterogenous disorder present in up to 10% of the global population characterised by a symptom complex of abdominal pain and variable bowel habit in the absence of gross morphological, histological or inflammatory diagnostic markers [4]. Although IBS does not increase mortality the disease impairs quality of life and costs billions of dollars annually in direct and indirect health care costs [5]. Genetic, environmental, gender and psychological factors are known to be important in the development of the disease but the pathophysiology is unknown. As many as 10% of cases of IBS are recognised to develop after a bout of acute infectious gastroenteritis and this has focused research on microbial interactions with the luminal gut mucosa [6].

Faecal microbiota profiles are reported to be significantly different in IBS patients, most notably in the diarrhoea predominant IBS (D-IBS) subgroup [7]. IBS patients have diminished faecal bacterial diversity with temporal instability, fewer aerobic bacteria and decreased Bifidobacterium spp./Enterobacter spp. ratio [7],[8]. Altered intestinal motility and abdominal hypersensitivity are typically seen in IBS, and many researchers have reported subtle alterations of the enteric immune and nervous system of IBS patients [9]. Increased numbers of mast cells secreting serotonin that are closely aligned to mucosal dendritic cells are consistently observed in the gastrointestinal mucosa of patients with IBS [10]. Changes in cytokine secretion are reported not only with many luminal enteric microbial infections but increased levels of pro-inflammatory cytokines (interleukin (IL)-6, IL-8, and tumour necrosis factor-? (TNF-?)) have been reported in the plasma and peripheral blood mononuclear cells of IBS patients [6]. Many of these cytokines increase epithelial permeability by disrupting the mucosal tight junctions [11]. Proteases are also reported to be elevated in the faeces of IBS patients [12],[13] although it is not clear if these proteases derive from enteric microorganisms, human intestinal cells or both. Blastocystis organisms have been shown to activate IL-8 gene expression in human colonic epithelial cells in vitro[14], increase epithelial permeability [15] and degrade intestinal Immunoglobulin A (IgA) [16], mediated possibly by action of cysteine proteases [17] reported to be present on the cell surface [18].

The prevalence of Blastocystis carriage is increased in all IBS patients but highest in D-IBS patients (73% compared to 27% in healthy control patients) [19], suggesting that this subgroup may provide a more homogenous IBS patient group to study. Seventeen different subtypes (ST) of Blastocystis spp. have been described to date [20] revealing moderate host ST specificity and regional ST variation in humans. Early reports that particular subtypes of Blastocystis may be associated with clinical pathogenicity in humans have not been confirmed [1]. Treatment of Blastocystis spp. with antimicrobials is problematic and insensitive diagnostic methods and possible re-infection complicates assessment of successful eradication [21],[22]. Monotherapy with metronidazole is the most commonly recommended drug and published eradication rates vary from 0-100% [21]. If Blastocystis spp. could be predictably successfully eradicated in symptomatic patients then it may be possible to more easily evaluate whether infection is associated with clinical symptoms. In this pilot study we treated ten patients with diarrhoea-predominant IBS (IBS-D) who were positive for Blastocystis carriage with triple antibiotic therapy (diloxanide furoate, trimethoprim-sulfamethoxazole, secnidazole) and assessed clinical status, ST and clearance rates.

Successful faecal eradication of Blastocystis spp.occurred in 60% of patients at the end of this study following fourteen days of triple antibiotic therapy using diloxanide furoate, trimethoprim/sulfamethoxazole and secnidazole (TAB). Our previous study [22] used metronidazole as a monotherapy and achieved 0% successful faecal eradication, suggesting that this TAB regimen may be useful to treat Blastocystis infection. Although six patients in our study had already received at least one course of metronidazole therapy previously, three of these patients proceeded to clear the organism after TAB therapy indicating that triple therapy is still a worthwhile alternative in this group. One recent study has reported 100% failure to clear in five patients with Blastocystis ST3 infection treated with TAB combination [26]. However, these five patients had faecal specimens retested 12 months following the TAB therapy and it was not possible to exclude reinfection in this group. Triple antibiotic therapy has been shown to be useful in treating infecting organisms such as Helicobacter pylori and Mycobacterium spp. and it may be necessary to explore combinations of antibiotics and methods of delivery to further improve the clearance rate of Blastocystis infection.

Widely varying rates of eradication (0-100%) of Blastocystis using a variety of antibiotics including ketoconazole, ornidazole, tinidazole, rifaximin and paromomycin have been reported [21] and some of this apparent inconsistency may be due to using insensitive faecal smears as a sole means of diagnosis. Diagnostic methods such as faecal XIVC or PCR have been reported to increase accuracy by 30-50% for detection of Blastocystis spp. [10]. Metronidazole resistance has also been reported [27] and differences in antimicrobial sensitivities to metronidazole and emetine have been demonstrated between axenic ST4 and ST7 cultures [28]. Resistance may therefore contribute to regional variation in eradication rates. A number of antimicrobials have been tested for activity against Blastocystis spp. in vitro, in both axenic and xenic conditions [29]-[31] and this data is a useful starting point for choice of antimicrobial. The Blastocystis organism appears to establish in the anaerobic environment of the human ileum and caecum, thrives in the presence of bacteria, and only in extremely rare circumstances is invasive to the gastrointestinal mucosa [1]. An intrinsic difficulty in treating Blastocystis spp. may be delivering the appropriate drugs to the intraluminal site of infection or understanding how to alter the faecal microbiota to produce an environment that is unsupportive or hostile to the parasite.

Secnidazole, similar to metronidazole, is a 5-imidazole drug substituted at position 1 and has been shown to have significant (although 50% less than metronidazole) inhibitory activity against Blastocystis spp. in vitro tested on axenic cultures [31]. All nitroimidazole drugs are rapidly absorbed from the small bowel, metabolised in the liver and excreted primarily in the urine. They exert their antimicrobial action in an anaerobic intracellular environment after reduction to a toxic metabolite that disrupts DNA linkage [32]. Metronidazole is the most common empiric therapy used against Blastocystis[33] and as many patients that present for therapy have already failed metronidazole we felt it was useful to use a different imidazole particularly as resistance to metronidazole has been shown not to extend across the entire class of 1-sustituted 5-imidazoles [28]. The pharmacodynamics of secnidazole are not as well defined as those of metronidazole but it is known that secnidazole has reduced excretion of parent drug and metabolites in the urine (50% compared to 90% with metronidazole) and therefore may have increased faecal excretion. It also has more than double the excretion half-life of metronidazole at 29 hours [28]. These pharmacologic properties may be an advantage in treating Blastocystis.

Trimethoprim/sulfamethoxazole duotherapy is often proposed as second line therapy for Blastocystis spp. infection and eradication rates of 22-100% have been reported for this combination therapy [21]. These two drugs act synergistically to interfere with the production of dihydrofolate reductase that is required for purine and including DNA synthesis. Blastocystis ST7 does not take up ^Hthymidine [18],[31], is likely to synthesize purines de novo, and likely to have a dihydrofolate reductase that can be targeted. Trimethoprim is rapidly absorbed from the small bowel [34] is lipophilic with wide tissue distribution and 80% is excreted in the urine unchanged. Trimethoprim therapy alone has been shown to have inhibitory activity similar to secnidazole in vitro[31]. Sulfamethoxazole similarly is rapidly absorbed in the small bowel [34] metabolised in the liver, and over 90% excreted in the urine. They both have a half-life of around 10 hours. Although sulfamethoxazole alone was found to be non-inhibitory in vitro[28] a combination of trimethoprim:sulfamethoxazole (in the ratio of 1:5, Bactrim’) was found to be much less effective at inhibiting Blastocystis growth in axenic cultures than a trimethoprim/sulfamethoxazole 1:2 ratio which may indicate that the synergistic effect of the sulfamethoxazole plays an important role in directly inhibiting Blastocystis spp. growth. Enteric coating this duotherapy compounded in a 1:2 ratio may be a useful therapeutic avenue to explore.

An alternative therapeutic approach would be to use an antimicrobial drug that has already been described to have intraluminal amoebocidal properties. A number of these drugs tested in vitro such as emetine, furazolidone, nitazoxanide, mefloquine, quinicrine, quinine, iodoquinol, clioquinol (Entero-Vioform’) and chloroquine showed moderate to high inhibition on Blastocystis cell growth [28]-[31]. Emetine and clioquinol therapy have serious adverse reactions reported and are not used currently in clinical practice. Other intraluminal drugs such as diloxanide furoate, paromomycin, doxycycline and pyrimethamine showed no inhibitory effect on Blastocystis[29],[30]. Paromomycin is a relatively safe, poorly absorbed aminoglycoside with activity against not only gram-negative but many gram-positive bacteria, including Mycobacterium tuberculosis, and protozoa such as Leishmania spp. [35]. Interestingly two recent studies have reported high Blastocystis clearance rates (77-100%) [26],[36] using paromomycin and if confirmed this would suggest the broad spectrum aminoglycoside is achieving success by an indirect effect on the faecal microbiota. The largest study [36] retrospectively analysed faecal microscopy results on 52 symptomatic Blastocystis carriers treated with paromomycin alone and clearance rates were estimated to be 77%. Rifaximin is an oral broad-spectrum antibiotic derived from rifamycin that is not absorbed systemically (97% excreted in the faeces) [37]. It has been used successfully to treat hepatic encephalopathy, travellers diarrhoea [37] and non-constipated IBS patients [38]. Only one case report of its successful use in Blastocystis infection has been published [39]. Diloxanide furoate was chosen for our study as it had a good safety profile, the drug remains intraluminal until it is hydrolysed to diloxanide, absorbed and subsequently excreted in the urine. It is known to effectively clear intestinal amoebiasis although the mechanism of action is not known [40]. It was not possible to assess whether the addition of diloxanide furoate increased the efficacy of the regimen in this study. The addition of an intraluminal antimicrobial to a therapeutic regimen seems logical and paromomycin may be the most appropriate choice to add to a regimen at the moment. Unfortunately monotherapy is usually a cheaper option (the cost of 14 days of appropriate doses of metronidazole, trimethoprim/sulfamethoxazole, diloxanide furoate, paromomycin, secnidazole and rifaximin being AUS $24, $24, $55, $70, $80, $80 respectively) and the cost of TAB rising to AUS$159. If an effective drug combination could be identified it is likely the costs of a combination therapy would reduce.

This small pilot study was not designed to interrogate specific variables that might be relevant to Blastocystis spp. symptoms, clearance or clinical outcome. The literature is inconsistent but some studies have suggested that particular subtypes, namely ST 1, 3 and 4 of Blastocystis may be pathogenic [1]. In contrast, and in accord with recent studies [22],[26] our patients demonstrated a diverse range of subtypes and no association was seen between subtype and faecal clearance or resolution of symptoms. Although previous studies have reported mixed infection rates of 5-30% [22],[41] almost half of the patients (40%) in this study demonstrated mixed infections. The use of ST specific primers and XIVC techniques that allow ST’s present in small numbers to grow to detectable numbers are factors likely to increase identification of mixed infections.

It is well recognised that many Blastocystis carriers are healthy. Much research has been directed towards identifying genetic characteristics specific to the parasite, such as ST, that may indicate pathogenicity. Some studies have investigated host factors that may allow Blastocystis spp. to cause clinical symptoms. Epidemiological studies conducted in patients with compromised immune function (HIV carriage, chronic renal failure, haematological malignancies with immunosuppressant therapy [42],[43]) have shown increased rates of Blastocystis carriage but no clear consensus has emerged linking carriage to symptoms. IBS [6] and Blastocystis infection [14] have both been linked to changes in inflammatory cytokine expression. The prevalence of single nucleotide gene polymorphisms present in IL-8 (pro-inflammatory) and IL-10 (anti-inflammatory) cytokines is reported to be significantly increased not only in IBS patients but also in IBS patients positive for Blastocystis infection [44] suggesting that factors regulating the immune response in the host are important in expression of clinical disease.

IgA is also an important part of host mucosal immunity. Blastocystis proteases have been reported to cleave human secretory IgA [16]. Interestingly 40% of our group of IBS-D Blastocystis had low serum IgA and this was much higher than the 1.4% (1 in 19) recorded in our healthy adult volunteers negative for Blastocystis spp. (unpublished observations). The levels of serum IgA recorded in our patients are compatible with partial selective IgA deficiency but did not reach <50% of the lower limit of normal defined as selective IgA deficiency. Selective IgA deficiency (prevalence of 0.17% in Australia) is usually asymptomatic, but may be associated with an increased risk of respiratory, urinary and gastrointestinal (including giardiasis) sepsis, and allergic and autoimmune diseases [45]. There was no apparent association between IgA levels and subsequent clearance. Serum IgA levels recorded in children are generally lower than adults [46] and if IgA levels influence the risk of carriage of Blastocystis then children may be more susceptible.

In this paper we describe successful clearance of the Blastocystis organism from faeces after testing at day 15 and week 4 after antibiotics. Blastocystis organisms may be excreted irregularly [47] so it was useful to have had two clear faecal specimens 4 weeks apart to confirm faecal clearance. We used three methods of detecting Blastocystis in the stool, taking care to include XIVC and PCR amplification of Blastocystis DNA as the latter two methods have approximately a 90% sensitivity detection rate compared with only around 50% for microscopy alone [48]. Nevertheless false negative results may occur in a small percent of cases, where complete clearance of Blastocystis spp. in the ileum may not have occurred. It would be useful to be able to distinguish symptomatic and asymptomatic patients positive for Blastocystis faecal carriage. Although Blastocystis spp. are non-invasive organisms serum antibodies against Blastocystis spp. have been reported in humans infected with the parasite [49]. The presence of antibodies often only signifies exposure to that Blastocystis antigen some time in the past. However one study has reported that an antibody reacting to a 29 kDa protein, thought to be a cysteine protease present on the plasma membrane of Blastocystis spp., is more commonly seen in symptomatic patients [50]. If confirmed this finding and similar approaches may allow us to utilise the expression of host response to infection to determine need for therapy.

The only variable that was found to be significant was duration of GIT symptoms and eventual clearance of the parasite. If we assume that the GIT symptoms were related to the Blastocystis carriage this finding may suggest that Blastocystis infection that persists in the host for years may be more difficult to eradicate, particularly as spontaneous clearance has been reported to occur in only 19% of symptomatic patients over 12 months [36].

The patients in this study were a careful selection of D-IBS patients with no other identifiable enteric pathogen or other cause for symptoms. Nevertheless only 50% of patients who reported clinical improvement had cleared the organism. Perhaps reducing parasite load was enough to achieve clinical improvement in these patients. The two patients who reported dramatic clinical improvement (>4 points) both cleared the organism. In contrast 50% of the patients who did not report clinical improvement cleared the organism, suggesting that Blastocystis carriage was not the cause or not the only cause of their D-IBS. IBS is a heterogenous clinical condition and many diseases masquerade as IBS. Over time definitive specific diagnostic tests or therapies that allow us to discriminate other causes of IBS may become available. Although Giardia duodenalis was identified microscopically in the 1700 s acceptance of its pathogenicity only occurred in the last forty years when effective therapy with imidazoles (>90% successful eradication) demonstrated clinical improvement [51]. Although 60% patients cleared in this study clinical cause and effect will be able to be more easily evaluated when we can more reliably and effectively eradicate Blastocystis spp.

Successful faecal eradication of Blastocystis spp. occurred in 60% of D-IBS patients at the end of this study following fourteen days of triple antibiotic therapy using diloxanide furoate, trimethoprim/sulfamethoxazole and secnidazole (TAB). Metronidazole monotherapy achieved 0% successful eradication in our previous study and as some isolates of Blastocystis spp. have also been shown to be resistant to metronidazole, this drug should no longer be recommended as first line therapy. Surprisingly paromomycin, a drug that exhibits little inhibitory activity in vitro, has been reported to have eradication rates above 75%. If these results are confirmed in future studies using sensitive diagnostic markers, paromomycin may prove to be a useful basis for a double or triple antibiotic regimen. Predictable successful eradication (>90%) of Blastocystis spp. will expedite investigation of the clinical significance of Blastocystis infection. No meaningful analysis of clearance and symptoms was possible in this trial but 40% Blastocystis patients demonstrated low serum IgA levels. It may be worth focusing future studies on investigating immune profiles in the host and including host IgA levels.

RN carried out the clinical investigations, microbiology and molecular genetic studies, sequence alignment, statistical analysis and drafted the manuscript. RN, HBO and RT conceived the study design, participated in the design and coordination and helped draft the manuscript. All authors read and approved the final manuscript.