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The role of immunity in controlling Parastrongyloides trichosuri infections in possums needs to be known before the parasite can be used as a biological control agent or vector for DNA. Australian brushtail possums (Trichosurus vulpecula) were captured from the wild in New Zealand from a region where the specific nematode parasite P. trichosuri was common. Existing worm burdens were removed with anthelmintics. Three infections with P. trichosuri filariform larvae (5,000; 10,000; 20,000) were each truncated with anthelmintics after 7 days in an attempt to stimulate an immune response to infection. Subsequently, these possums resisted a challenge infection more effectively than untreated possums, developing a mean of 29 worms (5/10 fully resistant) compared with 127 (1/10 fully resistant) in untreated possums. Treated possums all had mucosal-like mast cells in the small intestine lamina propria and circulating antibody to a 65 85 kDa fraction of filariform larvae. In a second experiment, possums treated with the immunosuppressant dexamethasone trimethylacetate twice weekly were not significantly more susceptible to challenge with filariform larvae than were untreated possums. These results indicate that immunity was not highly-developed in the wild-caught possums, but could be stimulated artificially.