White Leghorn chickens raised from one day old in an environment contaminated by the infectious bursal agent (IBA) had lower geometric mean titers (GMT) as measured by the hemagglutination-inhibition (HI) test to the Newcastle disease virus (NDV), than control Leghorns reared in an uncontaminated environment. Immunosuppression, defined as a reduction in GMT, was most pronounced at 35-56 days old for Leghorns vaccinated with NDV at 1 and 28 days or at 28 days. In a separate trial with broilers, immunosuppression was similar at 42-56 days old. This study also demonstrated that IBA infection in chickens increased susceptibility to Marek's disease (MD). The unvaccinated control chickens infected with IBA averaged 56.3% MD lesions, whereas unvaccinated controls not exposed to IBA averaged only 18.1% macroscopic lesions. It was also found that 20.7% of the HVT-vaccinated chickens exposed to IBA had gross MD lesions, whereas those HVT-vaccinated chickens reared in an environment free of IBA had 2.99% gross MD lesions.

One-day-old chicks used in this study were from breeder flocks vaccinated with live (B1 and LaSota) or inactivated oil emulsion Newcastle disease (ND) vaccine. Chicks were vaccinated against ND by various procedures. The vaccination response was evaluated by hemagglutination-inhibition antibody titers and by challenge. Chicks from breeder flocks vaccinated with live virus vaccine had a geometric mean hemagglutination-inhibition antibody titer (GMT) for Newcastle of 7 (low maternal antibody titer) at 1 day of age, whereas those chickens derived from breeder flocks vaccinated with inactivated oil emulsion ND vaccine had a GMT of 84 (high maternal antibody titer). One-day-old chicks injected with live B1 vaccine were not immunized against ND regardless of breeder flock source. However, chickens with low maternal antibody titers were effectively immunized against ND when injected at 1 day of age with an inactivated or inactivated plus live ND vaccine. Chicks with high maternal antibody titers were not effectively protected when vaccinated with inactivated vaccine at 1 day of age; however, these chicks were protected when injected with a combined live and inactivated ND vaccine. Chicks from both breeder flocks were effectively immunized against ND when injected at 1 day of age with a live or inactivated ND vaccine and revaccinated by aerosol at 21 days of age with live B1 ND vaccine. Even though they were protected against ND, there appears to have been an interference phenomenon in chicks derived from breeder flocks vaccinated with the live ND vaccine. Beak-O-Vac vaccinated chickens were not effectively protected against ND when compared with chicks vaccinated by aerosol at 1 day of age. Water vaccination at 7, 14, or 21 days of age was as effective as aerosol vaccination when administered to chicks with low maternal antibody titers. However, water vaccination was not as effective as aerosol vaccination when administered to chicks with high maternal antibody titers.

Four isolates of Marek's disease herpesvirus (MDV) were obtained from 21-week-old broiler breeders with lesions and mortality from Marek's disease (MD). They had been properly vaccinated with turkey herpesvirus (HVT). The isolates were designated Ala-7, 8, 9, and 10. These 4 MDV strains were compared with the GA isolate of MDV in chickens vaccinated with graded doses of HVT. Challenge with the MDV strains was also with graded doses. All 5 isolates of MDV were similar in virulence. However, protection afforded against MD lesions by HVT was significantly lower in the Ala-7, 8, and 9 isolates than with the Ala-10 or GA isolates. The Ala-7, 8 and 9 isolates of MDV induced small-cell plaques in chick embryo fibroblasts, the Ala-10 isolate induced both large and small cell per plaque types, and the GA isolate produced predominantly small cell plaques with a few plaques of the large cell type.

Mycoplasma gallinarum was isolated from tracheas and air-sac lesions from broilers in flocks having higher than normal condemnations due to airsacculitis. A representative M. gallinarum isolant, given by aerosol or by air-sac inoculation, produced air-sac lesions in young chickens when given in combination with a vaccine combining Newcastle disease and infectious bronchitis or with a field strain of infectious bronchitis virus.

A cone-selected Lasota strain of Newcastle disease (ND) was found to be more immunogenic than the B1 strain but less immunogenic than the regular Lasota strain while having the same pathogenic index as the B1 strain. The geometric mean hemagglutination-inhibition (HI) titers induced in chickens vaccinated with the cloned Lasota strain were higher than those induced in chickens vaccinated with the B1 strain but were found to be slightly less than the titers obtained in chickens vaccinated with the regular Lasota strain. The clone-selected Lasota strain had essentially the same spreading potential as the regular Lasota strain, as indicated by geometric mean titers and challenge mortality of nonvaccinated chickens which were placed in contact with the vaccinated chickens. Oral, ocular, or aerosol vaccination of maternally immune chickens with the clone-selected Lasota strain gave essentially the same protection as those vaccinated with the regular Lasota strain.