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. The experiments made feasible the differentiation between virulent MDV and vvMDV strains, within serotype 1. Vaccination with CVI-988 clone C vaccine resulted in PD50 estimates of about 5 plaque-forming units (PFUs) against challenge infection with each of the three vvMDV strains. The PD50 estimate of CVI-988 clone C vaccine was 12-fold below the PD50 of HVT FC126. The protective synergism of bivalent vaccine, composed of HVT and SB-1, was confirmed by groups given the lowest vaccine doses. The bivalent vaccine, however, resulted in incomplete protection in groups given the highest vaccine doses. Homologous maternal antibodies to serotype 1 caused a fivefold increase in the PD50 estimate of CVI-988 clone C. Heterologous maternal antibodies against HVT did not interfere with efficacy of CVI-988 clone C vaccination. However, the combination of maternal antibodies against both HVT and SB-1 (serotypes 2 and 3) showed a strong adverse effect on CVI-988 clone C vaccine.(ABSTRACT TRUNCATED AT 250 WORDS)

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. Serum samples, received at biweekly intervals between 1-17 weeks post-inoculation, from birds of the groups inoculated at 4, 6, 8 and 10 weeks of age, showed at 1 week post-inoculation a transient viraemia followed by neutralising antibodies at the later sampling times. Neutralising antibody to subgroup A virus was detected in nearly all birds tested; this was not so for antibody to subgroup B. In all four groups the average titre of the former antibody was higher than that of the latter. Midway through the laying period birds of each group inoculated with leukosis virus, and some of the uninoculated controls, were challenged by infection with either subgroup A or B virus. At termination of the experiment survivors from each group were tested for the presence of leukosis virus. The virus recovery was performed with plasma samples, white blood cell preparations and explant cultures of various organs. The plasma samples were all negative; the great majority of blood cell specimens received from birds inoculated early with leukosis virus were positive, whereas the majority of the preparations from the birds inoculated later remained negative. The organ explants from the two youngest age groups were mostly leukosis virus-positive, from the birds inoculated at 4 weeks of age the spleen and kidney explants contained leukosis virus whereas in the groups inoculated at 6, 8 and 10 weeks of age only the spleen explants of birds challenged with subgroup A virus In a subsidiary experiment, started 4 months after the challenge infection, four birds from each group (two challenged with leukosis virus of subgroup A and two with subgroup B) were accommodated in isolators. The birds were challenged again, this time with Rous sarcoma virus (RSV) of the homologous subgroup used for the previous challenge. The tests for virus just prior to the challenge showed leukosis virus only in the white blood cell preparations from the birds in the three youngest age groups; the birds from the older groups were virus-negative. The serological tests after challenge showed neutralising antibodies to both subgroups in birds of nearly all groups. Tumour formation at the site of injection was mainly observed in the chickens challenged with RSV of subgroup B. The virological and serological results as well as the tumour response show that the immune system of birds between 0-4 weeks of age is insufficiently developed to cope with a controlled exposure with leukosis virus, whereas in birds of 4-10 weeks of age an adequate immunological response has developed. The significance of the presence of leukosis virus in sera, plasma, white blood cell preparations and organ explant cultures is mentioned. In programmes for the control of lymphoid leukosis in reproductive stock the use of information on virus and neutralising antibodies is recommended.

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. Five groups were housed in one chicken house in close contact with a large number of immunologically tolerant chickens (virus "spreaders"). On two occasions infectious LLV was recovered from air/dust samples collected in this house. In the second house a small number of congenitally infected birds generated a mild degree of LLV exposure. It was demonstrated that infection by contact may lead to lymphoma formation and congenital virus transmission. The incidence of virus infection and LL mortality in the groups of birds exposed at 8 weeks of age were significantly lower than in chickens exposed at 1 day of age. In addition, about 100-fold differences in numbers of LLV-associated white blood cells were observed between both age groups. These results indicate that in addition to resistance to tumour formation, resistance to LLV infection develops in the chicken with increasing age. Maternal antibodies, present in three groups exposed at 1 day of age, reduced the rate of infection and the incidence of LL.

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. Susceptibility to MD, reflected by MD mortality in each generation, was expressed as % incidence of MD. Statistical evaluation of MD susceptibility in the nine generations of the two lines revealed a significant interaction between MD mortality and two sources of variation, sex and generation. No overall line effect was found, but there was a significant line x generation interaction. The remaining interactions were not significant. Results indicate that breeding from survivors for development of resistance to MD is feasible. Exposure of birds to either artificial (by injection) or contact challenge with virulent strain K MDV changed lines of birds highly susceptible to MD into highly resistant lines in a few generations. Complete resistance to MD was not attained.

Age-related resistance to avian leukosis virus. I. Influence of age at exposure on mortality and congenital transmission.

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.

The control of lymphoid leukosis in a flock White Plymouth Rock chickens.

Lymphoid leukosis (LL) was successfully controlled in a commercial basic breeding line of White Plymouth Rock chickens. The control method has been developed for breeder flocks and consists of three elements: --In the flock under study, homogenates of embryos from all eggs collected during a number of 14-day periods are tested for the presence of LL viruses. --Only eggs from hens that have been shown not to shed virus in their eggs are used for the production of progeny. The offspring are reared in isolation during the first two months of life, at which time the age-related resistance against tumour formation by LL viruses appears to be sufficiently developed. --The chickens are subsequently inoculated intramuscularly with LL viruses of subgroups A and B transferred to a conventional chicken house. The vaccination raises a solid immunity to horizontal LL virus exposure and, due to the age-related resistance, tumour formation does not follow. No excretion of LL viruses could be detected in three generations of White Plymouth Rock chickens to which the three elements of the control procedure were applied. Clinical disease was not observed in any of the chickens under notice.

The control of lymphoid leukosis in a flock White Plymouth Rock chickens.

Summary Lymphoid leukosis (LL) was successfully controlled in a commercial basic breeding line of White Plymouth Rock chickens. The control method has been developed for breeder flocks and consists of three elements: - In the flock under study, homogenates of embryos from all eggs collected during a number of I4-day periods are tested for the presence of LL viruses. - Only eggs from hens that have been shown not to shed virus in their eggs are used for the production of progeny. The offspring are reared in isolation during the first two months of life, at which time the age-related resistance against tumour formation by LL viruses appears to be sufficiently developed. - The chickens are subsequently inoculated intramuscularly with LL viruses of subgroups A and B transferred to a conventional chicken house. The vaccination raises a solid immunity to horizontal LL virus exposure and, due to the age-related resistance, tumour formation does not follow. No excretion of LL viruses could be detected in three generations of White Plymouth Rock chickens to which the three elements of the control procedure were applied. Clinical disease was not observed in any of the chickens under notice.