An extensive breakout of equine influenza occurred for the first time in Japan, involving the Tokyo, Nakayama, and Fukushima Race Courses and the Equestrian Park, Tokyo, belonging to the Japan Racing Association, in 1971. In the Equestrian Park, 168 (98.3%) horses of 171 in total were infected with equine influenza within only 6 days, or from December 15 to 20, 1971. Hematological examination was carried out on the infected horses in the Equestrian Park. Ten horses were selected at random and examined for the erythrocyte and leukocyte counts and blood pictures on the 2nd, 4th, 7th, 11th and 22nd days of sickness. All of them were proved to have a high hemagglutination inhibitor in the serum. Equine influenza viruses were isolated from two of them. Consequently, erythrocytes and leukocytes showed a tendency to decrease a little. In the blood picture, a very remarkable increase in the segment form of neutrophil leukocytes was recognized on the 2nd day of sickness, although no increase was seen in the rod form of neutrophil leukocytes, myelocytes, or metamyelocytes. An obvious increase in monocytes was found on the 4th day of sickness. On the 7th day of sickness, these hematological changes caused by the disease disappeared and normal conditions were regained. Therefore, the hematological changes by the disease could be found within a very short period, unless the disease was associated with any secondary infection. It was considered that a hematological finding characteristic of the disease at the beginning would be a very remarkable increase in the segment form of neutrophil leukocytes, in spite of a slight decrease in leukocyte count. Such finding would be a characteristic of the disease, as compared with any other equine viral disease (equine infectious anemia, Japanese B encephalitis, equine rhinopneumonitis, etc.). The disease shows a violent spread, so it should be quickly diagnosed on early time of the outbreak. Therefore the characteristic findings mentioned above would play an auxiliary role in the diagnosis of the early stage of the disease, if they are used at the same time with the results of clinical observation.
