Giardiasis

Giardiasis is caused by the single celled parasite Giardia intestinalis. Infection occurs following the ingestion of cysts in contaminated water, food or via direct person to person contact. Infection may be asymptomatic or present with abdominal pain, bloating, nausea, flatulence, diarrhoea and steatorrhoea. Giardiasis may also lead to temporary lactose intolerance. It is the most commonly diagnosed intestinal parasite in the UK.

Drug-resistant Giardia intestinalis is increasingly recognised as a cause of treatment refractory disease. If treatment failure due to drug resistance is suspected, please discuss investigation and management with the Parasitology consultant or registrar.

See also Intestinal protozoa.

Detection of Giardia by microscopy

• Sample type: Standard stool sample (for cyst examination) – this does not need to be fresh as cysts are robust. Giardia trophozoites are only detectable when stools are examined within 4 hours of voiding. Giardia cysts are frequently excreted intermittently so that a minimum of six stools may be required for microscopic exclusion.
• Sample type: Duodenal or jejunal aspirate – Giardia trophozoites may be demonstrated in aspirated duodenal or jejunal fluid if examined within 4 hours (cysts will last longer but may not be present).

Detection of Giardia by PCR

• Sample type: Standard stool sample – molecular diagnosis via the multiplex stool parasite PCR (including Cryptosporidium species, Giardia intestinalis and Entamoeba histolytica) offers several advantages over standard microscopy based diagnostics. The assay is significantly more sensitive (greater than ten fold improvement in the limit of detection for some species) than light microscopy.
  
  Stool samples for the molecular test must NOT be in any fixative as this may cause false negatives.

• Sample type: Biopsies, duodenal or jejunal aspirate fluid and other sample types (following discussion with staff) – divergent sample types may be analysed for the presence of Giaridia DNA where microscopy would be inappropriate. Please contact Clinical Parasitology staff for further information. Please note that we have not validated this test for sample types other than stool.
  
  These samples must NOT be in any fixative as this may cause false negatives.

Detection of Giardia by serology

• Giardia serology is no longer available, please send an unfixed stool sample for microscopy and PCR.