IDENTIFICATION of Giardia lamblia

Giardiasis is diagnosed by finding cysts or trophozoites in the feces, and both life cycle stages have a characteristic appearance.

Giardia lamblia - trophozoite
10-20 µm long by 5-15 µm wide
1. Note sucking disk.
2. Iron-hematoxylin stain

The trophozoites average 10-20 μm in length, 5-15 μm in width, have a distinct “teardrop” or “pear” shape and two nuclei at the anterior end . The two nuclei, laterally located in the bilaterally symmetrical trophozoite, have central karysomes present; nuclei are not visible in unstained preparations.

Four pairs of flagella arise from basal bodies clustered between the two nuclei. The broad anterior end of the trophozoite contains a concave area which covers half the ventral surface. This is the adhesive or sucking disc that allows the parasite to attach to the mucosa of the patient’s small intestine. When viewed from the side G. lamblia is spoon shaped.
Microscopists often say that G. lamblia trophozoites are "staring back at them and smiling." The trophozoites also contain an axostyle (consisting of two axonemes) or dark transverse rod, which may be a supportive element. Two curved median or parabasal bodies cross the axoneme at an oblique angle giving the parasite its “smile”.

 The cysts average 8-19 μm in length, 7-10 μm in width, are typically oval, but also ellipsoidal, or round and contain 4 nuclei and remnants of the axostyle. The 4 nuclei are usually located on one end; there is no peripheral chromatin and the karyosomes are smaller than in the trophozoites. Staining may cause shrinkage making the cytoplasm pull away from the cyst wall. Many fibrils (flagellar remnants) and median bodies may be seen in the cytoplasm. Because of these unique characteristics, when found, G. lamblia is one of the easiest intestinal protozoans of humans to identify.

Giardia lamblia cysts
8-19 µm long by 7-10 µm wide