The significance of the "lung phase" in host-helminth relations

Many helminth infections have a "lung phase" whose significance considering the host-parasite relations has not yet been clarified. In some nematode infections larvae must pass through the lungs, in others adult worms live in the respiratory tract, and some others cause tropical pulmonary eosinophilia. Some examples of these infections (Nippostrongylus brasiliensis, Schistosoma mansoni, Trichinella spiralis and Toxocara canis) are considered to clarify whether this phase of infection is a "pro-host" or a "pro-parasite" mechanism. Whereas in primary infections--in some cases--transit through the lungs may render the parasite more resistant to host defence mechanisms (e.g., Schistosoma and perhaps Trichinella infections) or enables completion of the cycle (Nippostrongylus and Toxocara infections), in secondary infections the lungs might be the site of parasite killing.