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Eight groups of 1-day-old chickens were inoculated with 0, 250, 5000, or 100,000 white blood cells of chickens infected with Marek's disease virus strain K (MDV-WBC). Four of these groups were additionally infected with 10(5) TCID50 chicken anaemia virus (CAV). At day 14 after inoculation, chickens infected with CAV had reduced haematocrit levels, reduced body weights, and depletion of the thymic cortex and bone marrow. Semi-quantitative immunohistochemical examination of nerves and visceral organs was performed at day 28 by immunoperoxidase staining in which a monoclonal antibody specific for leucocytes was used. CAV significantly enhanced the number of lymphoproliferative lesions induced by 5000 MDV-WBC. In contrast, CAV significantly reduced the number of lymphoproliferative lesions induced by 100,000 MDV-WBC. Comparable results were found at day 61 after macroscopic examination of nerves and visceral organs. These findings show that the pathogenesis of MD in experimental infections appears to be enhanced or inhibited by CAV, depending on the dose of MDV.

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.

Summary The prevalence of antibodies to various viruses was investigated in a series of serum samples collected from horses in the Netherlands between 1963 and 1966 and from 1972 onwards. Neutralizing antibodies to equine rhinopneumonitis virus, equine arteritis virus and to equine rhinovirus types 1 and 2 were detected in respectively 76%, 14%, 66% and 59% of the equine serum samples tested. The observed incidence of serum samples positive to equine adenovirus in the complement fixation test was 39%. Precipitating antibodies to equine infectious anaemia virus were detected only in serum samples from two horses imported from abroad. Haemagglutination inhibiting antibodies to Myxovirus influenzae A / equi-1, M. Influenzae A / equi-2, and Reovirus types 1, 2, and 3 were present in respectively 82%, 50%, 10%, 33% and 3.6% of the serum samples tested. The most frequently observed incidence of antibodies to the various equine respiratory viruses occurred in the groups of horses having repeatedly contact with other horses.