Final answer:
In grazing calves after weaning, a type II infection or hyperinfection of Ostertagia ostertagi occurs, which involves the ingestion of infective larvae from pasture and can lead to severe disease with symptoms such as profuse diarrhea and weight loss.
Step-by-step explanation:
The type of Ostertagia ostertagi infection that occurs in grazing calves after weaning is known as a type II infection or hyperinfection. This stage of the parasite's lifecycle corresponds to the ingestion of infective larvae that have survived on pasture through the colder months and are ingested during spring or early summer.
When calves, which are typically weaned in the fall, begin to graze in the following spring, they are at high risk of ingesting these overwintered larvae, leading to the development of type II ostertagiosis. In this condition, large numbers of larvae can emerge from the gut lining all at once, causing severe disease characterized by profuse diarrhea, weight loss, and poor growth. These symptoms result from the damage to the stomach lining where the larvae emerge, impairing digestion and nutrient absorption.