The objective of this study is to determine whether in the setting of primary health care it is effective to treat with metronidazole returning travellers with gastrointestinal symptoms and B. hominis in the stool or not.

Prevalence of B. hominis is between 30-50% in developing countries. Many travellers visit developing countries and are therefore at risk to be infected by this parasite. It's frequent that travellers return from developing countries with gastro-intestinal symptoms and approximately 10% of them have B. hominis as the sole parasite identified in the stools. Some anti-infective drugs, including metronidazole, trimethoprim-sulfamethoxazole and nitazoxanide, have shown to have activity against B. hominis, but there is still controversy about the pathogenic potential of B. hominis and there is no consensus about the indications for treatment.

It is hypothesised that metronidazole is more effective than placebo in returning travellers with gastrointestinal symptoms and B. hominis as the sole intestinal parasite identified in the stool.

Inclusion Criteria:

• ≥ 18 years
• Gastrointestinal symptoms for more than 10 days
• B. hominis in any quantity in at least one stool specimen out of 3 examined
• No other pathogenic micro-organism identified

Exclusion Criteria:

• Fever > 37.5°
• bloody diarrhoea
• weight loss > 10% of usual body weight
• significant decrease of general condition
• oncological diseases
• immune deficiencies
• known chronic intestinal diseases
• use of anti-protozoan drugs in the last 2 weeks
• use of anti-coagulant treatment or antabuse
• pregnant and lactating women