Otostrongylus circumlitus (Railliet, 1899), the large pulmonary worm of seals, is commonly found in ringed seals, Phoca hispida and Atlantic harbor seals, Phoca vitulina vitulina, and has been reported in Pacific harbor seals, Phoca vitulina richardsi, from northern elephant seals, Mirounga angustirostris and a grey seal, Halichoerus grypus.

In 1992, The Marine Mammal Center (TMMC) handled an unprecedented increase in stranding calls with a total of 794 patients admitted. Among phocid pups, 0. circumlitis, showed an increased incidence. 0. circumlitis was infrequently detected in northern elephant seals, M. angustirostris, at necropsy at TMMC prior to 1990. A total of 5 cases (13.5%) were documented in 1991. In 1992, it was the leading cause of mortality among M. angustirostris weaner pups admitted (45.3% of necropsies which represents 39 cases). Unweaned pups, as young as 8 weeks old, died from fulminating disease with worms up to 14 cm in length in the respiratory tree and vasculature. Weaned pups were significantly more affected than other age classes. Among P. vitulina examined post mortem, the infection rate increased significantly from an historical incidence of 2.4% (from 1984 to 1989), 0.4% in 1991, to 16.5% in 1992. TMMC has documented the parasite in the hearts of two otariids, juvenile Zalophus californianus in 1992.

A program to detect infection antemortern and to develop an effective treatment protocol was hastily instituted mid-pupping season 1992. Fresh feces were collected and examined for the presence of larvae via a sedimentation concentration technique. Positive animals were treated with 50 mg/kg body weight with fenbendazole paste (PanacurR) PO SID for 10 to 30 days. Dexamethasone, antibiotics and an oral expectorant (guaifenesin) were also administered. Animals were nebulized with acetylcysteine and given coupage therapy SID to QID depending on the severity of clinical signs. Typical clinical signs consisted of: dyspnea with rales and wheezes, emaciation, hemoptysis, epistaxis, anorexia, pyrexia, depression and coagulation. Radiographic signs were similar to canine heartworm disease. Gross pathology revealed severe consolidating pneumonia and lesions consistent with cardiopulmonary collapse and septicemia.

A severe vasculitis and lesions secondary to verminous pneumonia and were seen histopathologically. Treatment was successful in a number of Miruounga and two Phoca. Research is currently being undertaken to determine the life cycle of this parasite and to further document the pathobiology of the disease.