Characterization of Otostrongylus circumlitus, a lungworm from seals, and host-parasite interaction in harbor and northern elephant seals

The adult strongylid nematode, Otostrongylus circumlitus, parasitizes juvenile Pacific harbor seals, Phoca vitulina richardsi (PHS) and northern elephant seals, Mirounga angustirostris (NES) along the central California coast. In these two hosts there are striking differences in the nematode distribution and severity of infection. In juvenile NES, signs and symptoms are more severe but nonspecific, and occur during the pre-patent period of infection. Thus, it is difficult to diagnose O. circumlitus infection in this host species. Morphological and molecular methods were combined to determine whether O. circumlitus is the same species in NES and PHS. The nematodes from the two hosts were indistinguishable. A PCR-RFLP assay was developed to distinguish O. circumlitus from Parafilaroides sp., a strongylid parasite of the same hosts. Immunoblotting was used to examine the IgG responses of NES and PHS to O. circumlitus tissues. Each seal positive at necropsy for O. circumlitus responded to all O. circumlitus tissues. However, yearling NES and most PHS had a stronger response to the excretory-secretory (ES) glands than juvenile NES of 2–9 months. All positive juvenile NES had a strong response to a 28 kDa band, immunodominant in the female reproductive tract. A faint band of 28 kDa was also present in Parafilaroides sp. However, O. circumlitus could be distinguished from whole Parafilaroides sp., Anisakis sp., and Pseudoterranova sp., nematodes infecting seals, by immunoblotting. The morphology of the ES system of O. circumlitus was examined by light and transmission electron microscopy. The system was the rhabditoid type, with two long and metabolically active associated gland cells. These data suggest that O. circumlitus is the same species in NES and PHS, and can be distinguished from Parafilaroides sp. by PCR-RFLP. Otostrongylus circumlitus probably die and disintegrate in PHS and NES and there are marked differences in IgG responses to O. circumlitus among NES of different age classes and between seal species. A 28 kDa band has potential as a diagnostic marker for O. circumlitus in juvenile NES. The ES system of O. circumlitus is similar to that of other strongylids, with the ES glands most likely specialized for parasitism.