Final answer:
The infective stage of Toxocara spp. is when their eggs have developed into larvae inside the egg, while for Toxoplasma gondii, it is the oocyst stage after sporulation in the environment, leading to toxoplasmosis when ingested by intermediate hosts or humans.
Step-by-step explanation:
The infective stage of Toxocara spp., which are parasitic roundworms, occurs when their eggs have developed into larvae inside them. These eggs are shed in the feces of infected animals, particularly cats, and must then develop in the environment. Once the eggs become embryonated, which can take about 1 to 5 days, they become capable of infecting a new host.
For Toxoplasma gondii, another parasitic organism, the infective stage is also the oocyst stage after it has sporulated. These oocysts are shed in cat feces and become infective after sporulating in the environment. Should these infective oocysts be ingested by intermediate hosts, such as birds and rodents, or even humans through various means of exposure, they can cause the disease toxoplasmosis. Intermediate hosts can develop tissue cysts, that when ingested by cats complete the life cycle of T. gondii.