Question
Cyst of giardia.
Answer
The cyst is the environmentally resistant, infective form of Giardia lamblia, excreted in the stool of an infected individual and responsible for transmission of giardiasis via the faecal-oral route (contaminated water/food, or person-to-person contact).
Morphology: oval to ellipsoidal in shape, measuring approximately 8–12 μm in length, with a thick, smooth, refractile cyst wall that confers marked resistance to environmental stress (including standard chlorination of water, an important factor in waterborne outbreaks). Internally, a mature cyst characteristically contains four nuclei (compared to the two nuclei of the motile trophozoite form), along with remnants of the axonemes (flagellar structures) and median bodies (curved, claw-shaped structures) visible within the cytoplasm — these internal structures are useful identifying features on microscopy.
Formation and role in transmission: as trophozoites pass through the colon (encountering a less favourable environment for continued vegetative growth), they undergo encystation, retracting their flagella and secreting the resistant cyst wall; the resulting cysts are then excreted in stool and, unlike the fragile trophozoite (which rapidly dies outside the host), are immediately infectious to a new host on ingestion and can survive for extended periods in a moist external environment (weeks in cold water), making the cyst — rather than the trophozoite — the epidemiologically important transmissible stage.
Laboratory diagnosis: cysts are typically found in formed/semi-formed stool, in contrast to trophozoites, which are more often seen in loose/watery stool — direct wet mount or concentration techniques (e.g., formalin-ether concentration) enhance detection; intermittent cyst shedding means multiple stool samples (on different days) may be needed to improve diagnostic sensitivity.