Resistance to Marek's disease at hatching in chickens vaccinated as embryos with the turkey herpesvirus.

Chickens vaccinated with herpesvirus of turkey (HVT) as 18-day embryos or at hatching were challenged as neonates with pathogenic Marek's disease (MD) virus (MDV). Embryonally vaccinated chickens had much greater resistant to challenge than chickens vaccinated post-hatch. Embryos became readily infected with HVT regardless of whether the vaccine was deposited into the body of the embryo or extraembryonally, such as in the amniotic sac. Embryonally vaccinated chickens were viremic with HVT at hatching and remained persistently viremic through the duration of the experiment. The titer of recoverable virus was higher in the embryonally vaccinated chickens than in the chickens vaccinated post-hatch. Embryonal vaccination did not affect hatchability. Vaccination at any stage of embryonation tested protected better against neonatal challenge than did vaccination at hatching. Protection against an early challenge was greatest when the embryos were 17 or 18 days old at the time of vaccination. Lower protection in chickens vaccinated as 11-day embryos was not due to humoral immunologic tolerance. Chickens vaccinated at the 11th day of embryonation were poorly protected against MDV challenge at three or eight days of age but were well protected if the challenge was delayed until the 14th day of age.

Shedding of lymphoid leukosis virus in chickens following contact exposure and vaccination.

Chickens contact-exposed to lymphoid leukosis virus at various ages up to 32 weeks responded with relatively high rates of infection as determined by the presence of neutralizing antibody. Virus shedding as determined by cloacal swab and albumen testing occurred in 7 of 8 groups of such chickens, but the incidence was 10% or less and sporadic. Vaccination of chickens immediately before exposure with a low pathogenicity virus of subgroup A at 8 weeks of age did not eliminate subsequent shedding.

An evaluation of methods for eradication of avian leukosis virus from a commercial breeder flock.

Two trials were conducted to determine whether lymphoid leukosis virus (LLV) could be eradicated from chicken breeder stocks in one generation. Dams were selected as potentially virus-free parents on the basis of negative tests for virus in progeny embryos (trial 1) or in vaginal-cloacal swabs (VCS) (trial 2) of the dams. In trial 1, 8 of 12 groups of chickens hatched from selected breeders remained free of LLV infection through 36 weeks of life. In trial 2, VCS appeared to be more efficient in detecting possible shedder dams, and only 1 of 72 groups of chickens showed evidence of infection at 14 weeks after hatching. Within that positive group, a single chicken was shown to be the possible cause of the infection. The results show that eradication of LLV can be accomplished in one generation by: 1) virological examination of dams for shedding; 2) elimination of the shedder dams; and 3) small-group rearing of the progeny chicks.

Differential effect of maternal antibodies on efficacy of cellular and cell-free Marek's disease vaccines.

The protective efficacy of cell-free and cell-associated turkey herpesvirus (HVT) vaccines against Marek's disease (MD) was determined by a quantitative protective dose 50% (PD(50)) assay in susceptible chicks from HVT-vaccinated, MD-exposed dams or from genetically comparable, isolation-reared, specific pathogen free dams. Comparison within trials showed that cell-associated and cell-free vaccines were equally efficacious in chicks lacking maternal antibodies (median PD(50)s were about 1-4 plaque-forming units (PFU) ). However, in chicks with maternal HVT/MD antibodies, PD(50)values were increased by 2- to 8-fold for cell-associated vaccine and by 15-to 80-fold for cell-free vaccine. The apparent requirement in antibody-positive chicks for greater doses of cell-free than of cell-associated vaccine to give 50% protection may be of practical importance in establishing the optimum number of PFU in a field dose.

Lymphoid leukosis: interrelations among virus infections in hens, eggs, embryos, and chicks.

Hens from a commercial source were selected because they were infected with lymphoid leukosis virus (LLV). LLV was detected in vaginal swabs from 17 viremic hens and from 27 of 44 hens that were not viremic. All hens that were positive on the vaginal swab test (VST) produced one or more eggs with virus in albumen or in embryos, whereas in comparable tests, virus was detected only in eggs from 5 of 17 hens that were negative on VST. Congenital transmission of LLV was erratic and neither the VST nor tests for virus in egg albumen prior to incubating eggs identified all hens that transmitted infection. For example, 14 hens negative on VST produced 50 eggs negative for virus in albumen and yet one of the embryos from these eggs was infected. Eggs from other hens had infectious virus in albumen and about half of the embryos from these were infected. Tests for virus in cloacal swabs from one-day-old chicks were as sensitive as tests on embryos for detecting congenital transmission. Titers of LLV in the meconium of congenitally infected chicks were as high as 10(7) infectious units per ml. The cloacal swab test should be a valuable adjunct to the VST and tests on egg albumen in programs designed to eradicate lymphoid leukosis from chickens.

Immune responses of chickens fed the androgen analog mibolerone.

Mibolerone, an androgen analog (17 beta-hydroxy-7-alpha, 17-dimethylestr-4-en-3-one), induces a slow but progressive involution of the bursa of Fabricius when fed to chickens at microng levels during the first 7 weeks of life. Chickens receiving mibolerone remained immunologically competent, evidenced by: 1) their antibody response to nonreplicating antigens and infectious antigens; 2) the number of antibody-producing cells in their spleens; 3) the stimulation of their peripheral leukocytes with the plant mitogen phytohemagglutinin; and 4) their capacity to resist challenge with Marek's disease virus and Newcastle disease virus after vaccinations with turkey herpes-virus and the B-1 LaSota strain. This, coupled with the fact that it prevents experimental lymphoid leukosis, makes mibolerone a potential agent to be used under field conditions for the control of lymphoid leukosis.

Oncogenicity of avian leukosis viruses of different subgroups and of mutants of sarcoma viruses.

Leukosis viruses of seven subgroups were tested for oncogenicity in chickens susceptible to virus infection and to development of lymphoid leukosis (LL) tumors. All subgroup A viruses and the subgroup B virus tested produced a high incidence of LL and other related neoplasms. Viruses of subgroup C and RAV-61 of subgroup F produced a low level of LL. The RAV-50 of subgroup D produced osteopetrosis. In these tests, the viruses of subgroup E and G and one virus of subgroup F were not pathogenic, possibly because infection was not established in the chickens, the chickens were not susceptible to tumor development by these viruses, or the viruses lacked oncogenicity. All temperature-sensitive mutants of Rous sarcoma virus produced sarcomas, but the level varied. One nontransforming mutant produced sarcomas, and the other three tested produced LL. All three mutants that cause cells to grow as colonies in agar produced a high incidence of sarcomas. Thus, sarcoma viruses, by back-mutation, may lose the ability to transform cells in vitro, to make cells grow in agar colonies, or to induce sarcomas in vivo, yet they retain the ability to produce LL. Conversely, it was previously shown that leukosis viruses may be changed into viruses that transform cells in vitro and produce sarcomas in vivo by suitable passage in chicks.

Lymphoid leukosis viruses and GS antigen in unincubated chicken eggs.

Lymphoid leukosis viruses and viral group-specific antigen were found in albumen of unincubated chicken eggs stored at 8 degrees C. Infectious virus was detected for up to 22 days and antigen was stable for 63 days. Tests for virus were conducted on albumen withdrawn from eggs prior to incubation and on extracts of embryos from the same eggs. When albumen was from eggs stored no longer than 6 days, virus was isolated from 20% more albumen samples than embryo extracts. The techniques described should be useful in programmes to eradicate lymphoid leukosis viruses from commercial poultry.

Oncogenesis by Marek's disease herpesvirus in chickens lacking expression of endogenous (gs, chick helper factor, Rous-associated virus-O) and exogenous avian RNA tumor viruses.

Chickens free of exogenous avian leukosis virus (ALV) infection, replicating endogenous ALV (Rous-associated virus-O), gs antigen, and chick helper factor were fully susceptible to induction of Marek's disease (MD) by ALV-free MD viruses. Dual infection with Rous-associated virus-2 and MD virus did not significantly alter the character of the MD lesions. Thus exogenous ALV infection was not requisite for MD virus-induced oncogenesis. Although participation of endogenous RNA tumor virus genes in MD lesion induction could not be excluded, expression of such genes in MD tumors as gs antigen was not established.

Phenotypic mixing test to detect and assay avian leukosis viruses.

A phenotypic mixing (PM) test for detecting and assaying avian leukosis viruses (ALV) of the A, B, C, and D subgroups is described. An ALV and Rous sarcoma virus RSV-0) are phenotypically mixed by co-cultivating on C/O (cells susceptible to all subgroups of ALV) cells for a certain period. Then the RSV with the new virus property is assayed on C/E cells (cells resistant to infection with subgroup E leukosis/sarcoma viruses). The test is relatively simple and rapid, and its results are unequivocal. It is as sensitive as the more lengthy complement-fixation test (COFAL). The system is suitable for detecting avian leukosis viruses in samples such as heparinized blood, plasma, and embryo extracts.

Pathogenesis of Marek's disease in old chickens: lesion regression as the basis for age-related resistance.

Chickens of various age levels, free from prior infection, were simultaneously exposed to Marek's disease virus, and the response of each age group was recorded. Four- and 20-week-old chickens of lines 15x7 and CM (commercial source) had substantial resistance to mortality and gross lesions. In contrast, in line 7, which was tested at 1-day, 2-, 4-, 8-, 12- and 16-week age levels, 4-week-old chickens were fully susceptible to clinical Marek's disease (MD), although resistance was demonstrated at 8-week and older age levels. Genetically resistant chickens of line 6 maintained their resistance at all age levels tested. Pathogenesis of MD was compared in 12-week-old and 1-day-old chickens of line 15x7. Within the 1-day-old group, 23% of the chickens died because of MD, whereas there were no deaths in the 12-week-old group. Both groups developed viremia although duration, incidence, and levels of virus in the 1-day-old group were higher than in the 12-week-old group. Although initially the 12-week-old group responded by producing higher levels of antibody, the long term incidence of agar gel precipitin, immunofluorescent, and virus neutralization antibody in the two groups was similar. Gross and microscopic lesions of MD developed in both groups, but lesions regressed in the 12-week-old group and persisted in the 1-day-old group. It was concluded that age resistance to MD was expressed through lesion regression.