Protective efficacy of Marek's disease virus (MDV) CVI-988 CEF65 clone C against challenge infection with three very virulent MDV strains.

Comparative 50% protective dose (PD50) assays were performed using a plaque-purified preparation of Marek's disease virus (MDV) strain CVI-988 at the 65th chicken embryo fibroblast (CEF) passage level (MDV CVI-988 CEF65 clone C) and three commercial MD vaccines: herpesvirus of turkeys (HVT) FC126, MDV CVI-988 CEF35, and a bivalent vaccine composed of HVT FC126 and MDV SB-1. In addition, comparative PD50 assays were performed in groups of chickens with maternal antibody to each of the three vaccines. Three representatives of the newly emerged biovariant very virulent (vv) MDV strains-RB/1B, Tun, and Md5-were employed as challenge virus.

Age related resistance to avian leukosis virus. III. Infectious virus, neutralising antibody and tumours in chickens inoculated at various ages.

Viraemia and neutralising antibodies were determined in chickens of six age-groups following inoculation with leukosis virus of subgroups A and B at the age of 1 day, and 2, 4, 6, 8 and 10 weeks respectively. The birds were kept in a filtered air positive pressure (FAPP) house. A seventh age-group, accommodated in a separate FAPP-house, was used as an untreated control.

Evaluation of resistance to Marek's disease in three white Plymouth Rock lines and in two reciprocal crosses.

Two White Plymouth Rock lines (WPR), both developed into lines resistant for Marek's disease (MD), and one commercial WPR line, supposedly susceptible to the disease, were mated in a number of combinations to evaluate resistance to MD in their pure- and crossbred progeny. For this purpose the pedigreed 1-day-old progeny chicks were exposed to 3-week-old spreader chicks which had been inoculated at 1-day-old with virulent MD virus (MDV) and reared with them for the whole experimental period of 15 weeks. Response to the challenge showed that resistance to MD in the cross-breds was intermediate to that of resistant and susceptible parental lines.

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

Eight groups of 1-day-old or 8-week-old chickens were exposed by contact to lymphoid leukosis virus (LLV) infection. Five groups of about 60 spf chickens were used. Three groups of the same size were progeny from LLV vaccinated hens.

The development of two White Plymouth Rock lines resistant to Marek's disease by breeding from survivors.

A programme to develop resistance to Marek's disease (MD) in chickens in two non-inbred White Plymouth Rock (WPR) lines by breeding from survivors was initiated in 1968 and since then nine generations have been produced. In each generation only sires and dams which survived heavy exposure to virulent MD virus (MDV), Dutch strain K, either by inoculation (for the first two generations) or by contact-exposure (for the following seven generations) were used. In this long term trial a high level of resistance to MD was reached within five generations; thereafter no marked further increase in resistance could be obtained.