| Popis: |
A study was made of the pattern of congenital transmission of RIF, a naturally occurring avian leukosis virus, in a flock of chickens selected for a high incidence of neoplasms. It was found that about 1 out of every 6 hens had a persistent RIF-viremia, and all such hens infected their embryos regularly. One out of every 7 non-viremic hens was also a congenital transmitter of RIF, but the transmission was somewhat more erratic than in the case of the viremic birds. There was no indication that roosters, viremic or otherwise, could transmit RIF to their progeny, and it was concluded that the congenital transmission of RIF was strictly maternal. Congenitally infected chickens produced relatively high concentrations of virus for the entire 7-month duration of the experiment. Since these birds had no detectable antibody to RIF it was concluded that they had developed immunological tolerance to the virus by virtue of congenital infection. Birds which had not been infected through the egg became infected after contact with congenitally infected birds. The contact-infected chickens exhibited transient low-level viremia. The viremia was followed by the formation of antibodies to RIF, which cleared the blood of detectable virus. The frequency of congenital transmission of RIF of commercial flocks was found to be only about one-tenth that of the selected flock. The virus must have spread efficiently in the commercial flocks, however, since almost all adult birds had antibodies. A flock of chickens reared in strict isolation and free of leukosis virus was found to have no antibodies to RIF. Tumors diagnosed as visceral lymphomatosis occurred about 6 times more frequently in congenitally infected birds than in those infected by contact. Osteopetrosis, another neoplasm of the leukosis complex, occurred only in congenitally infected birds. Cases diagnosed as neurolymphomatosis on gross postmortem examination occurred in both congenitally infected and contact-infected birds, but the discrepancy between results of gross and microscopic examinations of affected tissues leaves doubt about the class of bird most frequently affected by this manifestation of infection. |
