Infection of Cattle with Parainfluenza 3 Virus with Special Reference to Udder Infection

In the previous study, parainfluenza 3 virus was isolated not only from nasal secretions but also from the milk of naturally infected cows. In the present study it was confirmed that the virus isolated by us is similar in its pathogenicity for cattle as the bovine strains of this virus previously reported. In view of the fact that the virus was isolated from milk, the virus was inoculated directly into the udders of lactating cows. The infected cows showed the same respiratory symptoms as well as the symptoms of fever, malaise, anorexia, diarrhea and leukopenia as observed in cattle infected by other routes of inoculation. The affected udders developed swelling and induration, and milk from these glands showed coagulation, color change, and increased pH and cell count. The milk contained increased numbers of glandular epithelial cells, mature and young neutrophils and monocytes. Virus was excreted in large amounts in milk from the quarters inoculated with virus. However, many of the uninoculated quarters also excreted virus in milk, but in smaller amounts. Virus was recovered from blood and nasal discharges of infected cows. Antibodies were produced in the infected cows and were excreted in milk. These findings, together with the clinical observations and the results of virus isolation from milk in natural cases, indicate that, in natural infection of cows with this virus, udder infection occurs frequently, but seldom results in overt clinical mastitis. However, the role and significance of the virus in the pathogenesis of mastitis should be further investigated.