An extensive outbreak of equine influenza occurred in Japan for the first time. It spread over many racecourses, farms and riding clubs in the eastern part of the country. Then, it involved the concentration center of imported horses which had passed quarantine in December, 1971, to be distributed to four places. The hemagglutination-inhibition (HI) test was performed on sera collected from horses in six places involved in the outbreak. Influenza A/equi 1/Prague /'56, Influenza A/equi 2/Miami/'63, and other viruses were isolated from the nasal secretions of infected horses in the allantoic fluid of incubated eggs and used as antigens for the HI test. The test was carried out by the standard tray method. Sera were inactivated at 60°C for 30 minutes after treatment with RDE in the case of the Prague strain. Sera were only inactivated at 60°C for 30 minutes in the case of the Miami and the isolated strain. In addition 5% suspension of chicken erythrocytes was used. The results obtained are as follows. 1) All the horses of riding club A into which some imported horses had been introduced were infected within 3 days. Their sera were proved to have HI titers ranging from 1:4 to 1:64 against the Miami strain and from 1:16 to 1:128< against the isolated strain between 10 and 16 days after the outbreak of the infection. 2) In the Equestrian Park, Tokyo, 168 horses (98.3%) out of 171 in total were infected within 6 days. Influenza viruses were isolated from among the infected horses. The sera of these horses showed an increase in HI antibody titer against the Miami and the isolated virus strain. The antibody titer maintained its peak for 40-50 days and then decreased. The sera of these horses, however, possessed no HI antibody against the prague strain. 3) In the Tokyo Race Course, 957 horses (99.4%) of 963 in total and in the Nakayama Race Course, 674 horses (93.3%) of 721 in total were infected. The sera of the infected horses of both racecourses exhibited an increase in HI titers against the Miami and the isolated strain. 4) In the Fukushima Race Course, 42 horses (39.6%) of 106 in total were considered to be infected, although the method of diagnosis used was not so reliable. The sera of the infected horses showed such increase in HI titer as similar to that mentioned above. 5) In the Shiroi Branch of the Nakayama Race Course, 120 horses of 209 in total were administered with killed vaccine (E1 150 CCA and E2 250 CCA) prepared by the Nippon Institute for Biological Science, Tokyo, but 135 horses out of these were infected with equine influenza. In the Shiroi Branch, many of the horses which had been free from the disease and administered with no vaccine were recognized to show lower HI titers than horses under similar conditions in the Tokyo and Nakayama Race Courses and the Equestrian Park. In the Shiroi Branch, horses which had been free from the disease and administered with vaccine were proved to show HI titers equal to or higher than those of horses recovered from natural infection. There was a large difference in HI titer between the horses under such conditions and horses free from the disease administered with no vaccine in any other racecourse. It was considered that the horses which had been free from the disease and had shown low HI titers might not have obviously been infected or not have been infected at all, and that the horses which had been free from the disease and had shown high HI titers might have been infected with equine influenza. 6) It was supposed that the present outbreak of equine influenza might have been caused by influenza A/equi 2 virus, but not by influenza A/equi 1 virus.
