In May 1988, 3 newly-introduced Baikal seals (Pusa sibirica) died one after another from their acute symptoms. Besides, the disease infected 3 other seals that were originally kept in the same tank as well, and one of those three seals finally died, too. The others survived and recovered about one month later. All of the dead seals showed severe congestion of lungs and remarkable hindrance of circulation, and clinical symptoms of respiratory embarrassment and convulsion. Severe diarrhea and liver disorder were observed in otherwise, one seal showed green colored eyes. These symptoms and the invalidity of antibiotics suggested that the cause of the seals, death was virus infection.

To identify the type of virus, we carried out virus neutralization tests using Canine Distemper Virus (CDV). 5 serum samples of Baikal seals had high antibody titres of CDV, but samples of Harbour seals showed low titres in contrast.

As a result of the epidemiological survey, we concluded the CDV or closely related morbillivirus have been brought into the Aquarium from Lake Baikal by new comer seals.