Giardia lamblia is a protozoal parasite which infects the upper small intestine causing inflammation. It is a common parasite in the Western hemisphere as well as the developing world, and in Britain is present as dormant cysts, which do not cause symptoms, in up to 10% of the population. It is transmitted when cysts from the faeces of an infected person are ingested by another person. This can happen when drinking water is contaminated, or when food is contaminated due to poor hygiene, or during oral-anal sexual contact (rimming).

Symptoms and diagnosis

If Giardia does cause symptoms, they may get better without treatment. The symptoms, known as giardiasis, are diarrhoea, wind, abdominal cramps (which can by very painful), bloating and malabsorption.

Active Giardia organisms, called trophozoites, can sometimes be detected in samples of faeces, but these tests may have to be repeated several times because the parasite can be difficult to detect even when it is present. The most positive way of diagnosing giardiasis is by testing a sample of duodenal fluid, which may be tried if tests on faeces are repeatedly negative. The fluid is obtained by passing an endoscope through the mouth and stomach and into the small intestine.

Treatment

Giardia responds well to standard anti-protozoal drugs, and is no harder to treat in HIV-positive people than in uninfected people. The commonest drug used is metronidazole (200-400mg after food three times a day for ten days). This may cause nausea and diarrhoea as well as a nasty metallic taste in the mouth. Tinidazole (2g after food followed by a repeat dose after two weeks) is also effective and may cause fewer side-effects. It is important to avoid alcohol whilst taking either drug.