Final answer:
Trichuris vulpis becomes infective when eggs containing the larvae have fully developed. This occurs after the eggs have been shed in feces and embryonated in the environment.
Step-by-step explanation:
Trichuris vulpis, also known as the canine whipworm, becomes infective at the stage where the eggs have fully developed into infectious larvae. In the lifecycle of Trichuris vulpis, the infective stage is reached after the eggs are shed in the feces of an infected host. These eggs then undergo embryonation in the environment, which can take about 2 to 4 weeks, depending on environmental conditions. Once embryonated, the eggs contain the infective larvae. When another host ingests these embryonated eggs, typically through soil or fecal contamination, the larvae emerge in the host's intestine and begin the next phase of their lifecycle by taking up residence in the host's large intestine.
In the instance of Trichuris trichiura, a related species that infects humans and causes trichuriasis, the ingestion of infective eggs leads to the larvae emerging within the intestines and establishing infection.