In Japan, over a period from 1971 to 1973, a sero-epizootiological survey was performed by using the hemagglutination-inhibition test in order to investigate the infection of swine in the field with a porcine parvovirus (PPV) and the relationship between stillbirth among gilts and the PPV infection.
It was presumed that the epizootic season of PPV might extend from July to September. It was almost the same as the epidemic season of Japanese encephalitis virus (JEV). Few sows were infected with PPV in winter. A fast spread of PPV among pigs reared in a contaminated district was observed in the epizootic season.
There was a tendency that pigs advanced in age in months showed a higher possession rate of PPV antibody than younger pigs; that is, pigs 5 to 6 months old exhibited a possession rate of 7.7-28.6%, pigs 7 to 10 months old 48.9-66.7%, and pigs 11 to 16 months old 83.8-100
In Fukushima Prefecture, the pre-breeding possession rate of PPV antibody was lower in gilts bred in February to July (5.9-33.3%, or 20.6% on the average) than in those bred in the other months (57.1-85.7%, or 75.6% on the average). The average annual possession rate was 37.4%.
In the same prefecture, the infection rate during the period of pregnancy was higher in gilts in bred in March to July (38.5-81.3%, or 68.5% on the average) than in those bred in the other months (0-25.0%, or 12.0% on the average). The average annual infection rate was 48.2%. The same results were obtained in Ibaraki Prefecture.
In 7 prefectures, 808 gilts bred in April to August in 1971 exhibited a pre-breeding possession rate of PPV antibody of 30.8% and an infection rate of 57.8% during the period of pregnancy.
In Fukushima Prefecture, the pre-breeding possession rate of PPV antibody was higher in sows which had delivered once or twice before than in gilts. Sows bred in June, July, and the other months exhibited a possession rate of 44.4%, 83.3%, and 100%, respectively. The average annual possession rate was 94.3%. Sows bred in June, July, and the other months exhibited an infection rate of 55.6%, 20.0%, and 0%, respectively, during the period of pregnancy. The average annual infection rate was 6.2%.
Over a period from 1971 to 1973, 880 gilts bred in April to August were immunized by two inoculations of Japanese encephalitis live virus vaccine (the S-strain) for the prevention of still-birth caused by JEV. They were divided into two groups after the enzootic period of PPV. The first group consisted of gifts which had been infected with PPV during the period of pregnancy. The second group consisted of gilts which had not been infected with PPV during the period of pregnancy. The results of delivery young were obviously better in the second group than in the first group. The average number of normal young delivered was by 1.1 per litter larger, and the average number of stillborn piglets by 0.8 per litter smaller in the second group than in the first group. In the first and the second group, a stillbirth rate of farrowed young was 19.0% and 9.3%, respectively.
From the results mentioned above, it was presumed that PPV infection might induce stillbirth mostly in gilts which had been bred in spring and summer. The stillbirth rate was about 10% in these gilts.
