Which parasite only uses the cat as its definitive host?

Which parasite only uses the cat as its definitive host?

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

Answer: (D)

The important idea here is the definitive host—the creature in which the parasite completes its sexual life cycle. Toxoplasma gondii carries out its sexual reproduction in the intestines of felines, so cats are the definitive hosts. In intermediate hosts (including humans), the parasite exists as asexual stages and does not complete the sexual cycle. That makes Toxoplasma gondii the parasite among the options that uses cats as its definitive host. The other organisms do not rely on cats for their sexual cycle (they either have no feline sexual phase or have a different primary host for that stage).

The important idea here is the definitive host—the creature in which the parasite completes its sexual life cycle. Toxoplasma gondii carries out its sexual reproduction in the intestines of felines, so cats are the definitive hosts. In intermediate hosts (including humans), the parasite exists as asexual stages and does not complete the sexual cycle. That makes Toxoplasma gondii the parasite among the options that uses cats as its definitive host. The other organisms do not rely on cats for their sexual cycle (they either have no feline sexual phase or have a different primary host for that stage).