Giardia intestinalis

Pathogenic, cosmopolitan parasite with higher prevalence in developing countries. The vegetative form lives in the small intestine (duodenum), free in the duodenal secretions or adherent to the mucosal epithelium by a ventral adhesive (sucking) disc. Giardia intestinalis reproduces asexually by binary fission.


The vegetative form is very fragile and displays a particular motility, seen in fresh but not fixed specimens, in which flagella rapidly vibrate while the body of the trophozoite moves like a falling leaf. This movement is short-lived and the trophozoite thereafter becomes immobile, with only the caudal flagella continuing to undulate, often for an extended period of time.

Size: 10-20 μm long, 5-12 μm wide.

Morphology: flattened, pear shaped (piriform), bilaterally symmetrical, with a convex dorsal surface and a concave ventral one. The ventral surface, in the wider anterior end, displays a kidney-shaped hollow with a raised, stiff rim that acts as an adhesive (sucking) disc. Inside there are two large nuclei that can often be seen by direct mount examination. In the posterior part, within the cytoplasm, there is a pair of transverse curved (comma-like) rods known as median bodies or parabasal bodies: their presence indicates the pre-encystment phase of the parasite. Seen in profile, the trophozoite is spoon-shaped.

Nuclei: two nuclei, without peripheral chromatin; the dot-like karyosome can be centrally or eccentrically located. Permanent staining shows the structure of the nuclei.

Cytostome: absent.

Flagella: four pairs, one anterior, two lateral (on either side) and one caudal. The latter pair contains intracytoplasmic microtubular rods (axonemes) that are straight and parallel and provide support along the longitudinal axis. The flagella can be stained to some extent, but only with Giemsa, May-Grünwald-Giemsa or Field stains.


This is the infective stage and the most frequently seen diagnostic form.

Size: 8-13 μm (usual range, 8-11 μm).

Morphology: mature cysts have a fairly regular oval shape; in recently formed cysts one end is broader than the other and the shape is similar to that of a trophozoite.

Nuclei: mature cysts have 4 nuclei, immature ones have 2. They are usually located at one end of the cyst. Peripheral chromatin is absent, the karyosome is usually dot-like and can be located in different positions.

Cytoplasm: in non-stained specimens, besides nuclei, the following structures can be seen:

-     An S-shaped structure located longitudinally and composed of remnants of flagella and axonemes;

-     Comma-shaped, refractile median bodies (parabasal bodies).

The typical shape of the cyst and the appearance of its contents are usually easy to identify, especially in fresh specimens. Cystic morphology is often altered by long-term storage in formalin solution, which causes the parasite to shrink away from the cystic wall and makes the diagnosis of this species difficult.

Generally, Giardia cysts are numerous in stool but their formation varies over time, for which, in some samples, they can be rare or completely absent; this is called the “negative period”. In this case, as well as after effective drug treatment, stool specimens may contain rare oval-shaped elements of the size of a Giardia cyst. These elements, which contain a small, round or oval granular mass, are altered cysts; they are almost empty and not viable. With Lugol’s iodine solution, these elements stain grayish-blue.