In 1984, 229 (208 male and 21 female) California sea lions were admitted to the California Marine Mammal Center with clinical signs strongly suggestive of leptospirosis. Many were diagnosed serologically and by wetmount darkfield microscopic examination, however the organism was never successfully cultured. Of the 229 sea lions brought to the Center, 68 (30%) survived, and 161 (70%) died.

In 1988, 123 (130 male and 5 female) live California sea lions were admitted to the California Marine Mammal Center with signs of leptospirosis. Twelve animals were dead on arrival, but were suggestive of leptospirosis on histopathology. Of the live animals, 36 (30%) survived and 87 (70%) died. The smaller sea lions, estimated to be between one and two years, that were admitted at weights under 50 kg (66%), had a better survival rate than the older, larger animals. Eighty nine per cent of the animals that were estimated to be sub-adults to adults, and weighed over 50 kg on admission, died of complications due to the leptospirosis, primarily renal disease.

Treatment consisted of injections of long acting tetracycline 2.3 mg/kg intramuscularly once a day or if the animal was eating fish, 22mg/kg tetracycline orally three times a day. Animals were give fluids orally or subcutaneously.

As in 1984, the source of the 1988 leptospirosis outbreak is unknown. Outbreaks of leptospirosis have been documented in California sea lions in 1947, 1970, 1984 and 1988. Other isolated cases have been reported in sea lions in other years, and a few cases have been seen in 1985 and 1986 at the Center. Interestingly there were no cases of leptospirosis diagnosed at the California Marine Mammal Center in 1987.

It is difficult to say what the cause of the periodic outbreak of leptospirosis might be. Animals that survived the 1984 outbreak would likely have immunity to the disease in 1988. This would explain the large number of younger animals with the disease.