Giardia

Giardia is one of the most common intestinal parasites of cats, yet it is easy to miss because its signs are non-specific and some infected cats appear perfectly healthy while still shedding the parasite. It is not a worm but a single-celled protozoan that lives in the small intestine, where it interferes with the normal absorption of nutrients.

The classic sign is diarrhoea that is soft, pale, greasy and remarkably foul-smelling, often coming and going over days or weeks. Because the parasite hampers digestion, affected cats — especially kittens — may steadily lose weight or fail to thrive even though they are eating well. Other cats carry Giardia silently, which is exactly why it spreads so persistently through multi-cat homes and shelters.

The parasite has two forms: an active feeding stage that lives in the gut, and a tough, walled cyst that is passed in the faeces and can survive for weeks in a cool, damp environment. A cat becomes infected by swallowing these cysts — from contaminated water, from a shared litter tray, or simply from grooming cysts off its own coat. This environmental toughness is the reason treatment so often fails on medication alone: unless the surroundings are cleaned and the cat's fur is washed, it promptly re-infects itself.

Diagnosis requires a laboratory faecal test, because the tiny cysts are invisible to the naked eye and the symptoms overlap with many other conditions. Once confirmed, a combination of the right anti-parasitic drugs, environmental decontamination, and bathing usually clears the infection — though a second course is sometimes needed. Owners should also practise good hygiene, as certain strains of Giardia can, uncommonly, be passed from cats to humans.