Irritable Bowel Syndrome with predominant diarrhea and giardiasis: Is it one or the other?

Diarrhea-predominant Irritable Bowel Syndrome (IBS-D) is where more than a quarter (25%) of bowel movements are type 6 or 7 on the Bristol Stool Form Scale, and less than a quarter are type 1 or 2. The symptoms must meet the corresponding Rome IV criteria: recurrent abdominal pain, at least one day per week (on average) in the last three months, associated with at least two of the following criteria: (1) Related to defaecation; (2) Associated with a change in stool frequency; and (3) Associated with a change in stool form (appearance).¹

Giardiasis is a common cause of infectious gastroenteritis worldwide, being associated with poverty, with a prevalence that varies from 2% (high-income countries) to 30% (low-income countries).²

The typical symptoms of giardiasis which include diarrhea, often explosive, especially in the morning, without blood or mucus, flatulence, abdominal pain and swelling,² often suggest IBS-D, with which a differential diagnosis must be made. However, with certain frequency, this can become difficult with the usual procedures and end up going unnoticed.

We present the case of a 29-year-old woman who reported having travelled to Mexico over a year beforehand, where she had acute gastroenteritis that took a long time to resolve. She reported weight loss, abdominal distension and pain, and chronic explosive diarrhea, with numerous soft-liquid stools, urgency and relief of pain when expelling wind and/or faeces, since her return from the trip. All tests, including coeliac serology, thyroid function, serial stool cultures/ova and parasite exams and a colonoscopy, were completely normal, and her symptoms were interpreted as a post-infectious-type IBS-D, probably in relation to some viral or parasitic infection acquired in Mexico.

The conventional treatment provided no improvement. With the abdominal pain and diarrhea persisting, treatment was recommended with paroxetine 20 mg/day.

As there was only slight improvement after several months of treatment, the patient was advised to increase the dose to 50 mg/day (30-0-20). After another two months, still with no improvement, further stool cultures/ova and parasite exams were performed, which were again negative, and it was decided to try treatment with rifaximin (400 mg/twice daily, one week a month, repeating for another month), with no results.

Finally, an endoscopy was performed with duodenal biopsies to rule out another malabsorption disorder (biochemically, there were no data to suggest that), and possible duodenal giardiasis, although the stool cultures/ova and parasite exams had been repeatedly negative, with no eggs or trophozoites detected. Duodenal biopsies were also normal, with no evidence of atrophy or giardiasis. We requested that a PCR for Giardia duodenalis also be performed on the biopsies and this came back positive, clarifying a diagnosis which had been eluding us for months. The patient was treated with tinidazole 50 mg/kg body weight, in a single oral dose, which led to rapid improvement in symptoms within a few weeks.

There are studies³ that find a strong association between giardiasis and post-infectious IBS (PI-IBS) in young people. D’Anchino et al.⁴ studied 100 patients with symptomatic giardiasis and found that in 82 of them IBS had previously been identified, suggesting that the symptoms attributed to giardiasis may, in fact, be the result of pre-existing IBS, exacerbated by Giardia infection.

Moreover, individuals with giardiasis are approximately four times more likely to be diagnosed with IBS 90 days after the diagnosis of giardiasis than those without giardiasis.³

In terms of the relationship between the two disorders, giardiasis may be a trigger for the exacerbation of IBS, but the parasitic infection is no longer necessary for the symptoms to persist once they have become established.⁴

From our experience, we recommend performing a PCR assay on a stool sample in patients with symptoms suggestive of IBS-D who have travelled abroad, particularly to low-income areas (risk factor for giardiasis), who have persistent diarrhea, even if stool investigations, including stool cultures and ova and parasite exam, are persistently negative.