Age‐related resistance to Avian Leukosis virus I. Influence of age at exposure on mortality and congenital transmission

Abstract

Groups of White Leghorn chickens were inoculated at 1 day and at 2, 4, 6 and 8 weeks of age respectively with a mixture of leukosis viruses of subgroups A and B. The five infected groups were kept in a filtered air positive pressure house. A sixth group was accommodated separately in a similar house as a control. All birds which died or were removed were subjected to pathohistological examination; diagnosis of lymphoid leukosis was made upon either gross lesions plus microscopical lesions or microscopical lesions only. The incidence of lymphoid leukosis in the infected groups appeared inversely proportional to age of infection, i.e. the mortality due to lymphoid leukosis decreased from 54.3% in the group infected at 1-day-old to 7.4% in the group infected at 8 weeks of age. Prevalence of leukosis in the latter group may be attributed to a small number of chicks already infected vertically with the virus. Congenital transmission of leukosis virus was demonstrated in embryos in the groups infected at 1-day-old, 2, 4 and 6 weeks of age. In the latter group congenital transmission was extremely low; from 214 pooled embryo extracts (1007 embryos) only 2 (0.9%) contained leukosis virus. In the group infected at 8 weeks of age no virus was detected in the embryos. Congenital transmission of leukosis virus appeared to be related to age of infection, i.e. early infection went parallel with a high rate of transmission. The pattern of congenital transmission was erratic and the number of hens shedding leukosis virus continually was small. From the results in this trial it is concluded that both lymphoid leukosis and congenital transmission occur rarely if chickens (born free of leukosis virus) are kept free from infection during the first 6-8 weeks of life and subsequently are injected with a relatively high dose of leukosis virus.

Horizontal transmission of lymphoid leukosis virus. influence of age, maternal antibodies and degree of contact exposure

“…The chicks of groups 1, 3, 4, 5 and 6 were derived from a spf White Leghorn strain A parent flock of the Central Veterinary Institute (Maas et al, 1980). The chickens of groups 2, 7 and 8 originated from a flock of the same strain of chickens present in the flocks participating in our experimental LL control programme.…”

“…The procedures and criteria for pathological examination have been described previously (Maas et al, 1980).…”

Theoretical considerations on the relative importance of vertical and horizontal transmission for the maintenance of infection by exogenous avian lymphoid leukosis virus

“…There are several ways in which this might be brought about. Maas et al (1980) have reported that the occurrence of vertical transmission following horizontal infection is agerelated: infection by LLV before 4-8 weeks of age (by injection) results in congenital transmission, whereas infection at 8 weeks does not. Differences in response to contact infection are apparently not so clear although it seems likely that there would be a similar trend (Weyl and Dougherty, 1977).…”

The development of two white Plymouth Rock lines resistant to Marek's disease by breeding from survivors1

“…Burmester (Regional Poultry Research Laboratory, East Lansing, U.S.A.) in 1967. The second strain, the WL strain A, developed as a leukosis virus-free flock (Maas et al, 1980), was at that time of unknown susceptibility to MD. From the WL line 7 2 ,50 out of 100 one-dayold chicks were placed in an isolation unit, and inoculated intra-abdominally with strain K MDV and the remainder served as contact controls (penmates).…”