Which parasite uses the domestic cat specifically as its definitive host?

• A. Giardia lamblia
• B. Isospora rivolta
• C. Toxoplasma gondii
• D. Balantidium coli

Answer: (B)

In this context, the scientific idea is that the definitive host is the one in which the parasite completes its sexual life cycle and produces transmissible sexual stages. Isospora rivolta fits this for the domestic cat because, in the cat’s intestinal tract, it undergoes sexual development and forms oocysts that are shed in the feces. Those oocysts are the infectious stage that can continue the cycle when ingested by another animal, so the cat is the host where the parasite’s sexual reproduction takes place.

Giardia lamblia, by contrast, reproduces primarily by asexual division inside the host, and its transmission relies on cysts rather than a cat-specific sexual lifecycle, so the cat is not the definitive host in the same sense. Balantidium coli is associated mainly with pigs, with humans as incidental hosts, not cats. Toxoplasma gondii also uses cats as a definitive host, but in this set the example most classically taught for a feline definitive host among the coccidian parasites listed is Isospora rivolta.