Introduction to Diagnostic Medical Parasitology
- Giardiasis occurs worldwide with highest prevalences in areas with poor sanitation
- Persons of all ages can be infected; in endemic areas the infection is more frequent in children
- Person-to-person transmission occurs by hand-to-mouth transfer of cysts in the faeces of an infected individual (family members) or via contaminated water or food
- There is still debate about the extent to which Giardia is a zoonotic infection (new molecular methods might give more precise insights)
- Many asymptomatic cyst passers!
- Mainly self-limiting diarrhoeas (for some days)
- Chronic diarrhoeas exceptional (e.g. in patients with hypogammaglobulin disorders)
- Modifications of the intestinal epithelium may be seen in symptomatic cases (atrophy of villi, hypertrophy of crypts and cellular infiltration of the mucosa)
The incubation period is variable (generally 1 week).
In symptomatic cases, the following findings have been reported:
- Acute and chronic diarrhoeas
- Abdominal cramps, discomfort
- Frequent loose or pale and greasy stools
- Weight loss
- Epigastric pain
Giardia cysts and trophozoites can be detected in faecal samples. Since Giardia is not regularly found in the stools, multiple samples have to be analysed.
As a supplement, duodenal samples can be analysed (in aspirates or using the “Entero-Test”).
Real-time PCR or nested PCR amplifying the small subunit ribosomal RNA gene to detect and quantify G. lamblia trophozoites. The detection limit for the rt-PCR is 50 trophozoites per gram stool.
Several commercially available test kits detect with a high sensitivity Giardia copro-antigens in faecal samples by immunochromatographic, immunoenzyme or direct fluorescent antibody assays.
Not of diagnostic relevance since it cannot distinguish past from acute infection.
- To confirm a Giardiasis infection
If one has to rely on microscopy, at least three independent stool samples should be screened before one rules out the diagnosis! As an alternative strategy, one could use an antigen detection assay analysing one sample. If the test result is negative a second sample should be screened.
- To screen patients with diarrhoea for protozoa infections
At least three stool samples should be taken. Microscopic analyses allow detection of several pathogens. This may also be realized using a multiple antigen detection assay (e.g. Triage Micro Parasite assay). In the near future, a multiplex rt-PCR might be the optimized method, depending on its validation.
- For epidemiological studies
For convenience one may rely on antigen detection, avoiding laborious microscopy.
Prevention and control
Education in public hygiene
- Protection of the public from faecal contamination
- Sanitary disposal of human faeces
- Training in thorough hand washing after defecation
Protection of water supplies from faecal contamination
- Problems with surface water
- Sedimentation and filtration of water is preventive but chlorination is not