Pathogenic properties of influenza A viruses introduced into embryonated chicken eggs via the allantoic cavity, the amniotic cavity or the yolk sac were studied using viruses with cleavable or non-cleavable haemagglutinin (HA), or reassortants derived from the highly pathogenic fowl plague virus (FPV) which has a cleavable HA. The various organs, membranes and fluids were analysed for virus yields, and by immunohistochemistry for production of viral nucleoprotein. Virus replication in primary tissue cultures derived from various embryonic organs was also studied. Some of the reassortants, which were previously found to be non-pathogenic for chickens and were temperature-sensitive () at 40 °C, multiplied at 37 °C to the same extent as the highly pathogenic FPV. The spread of other non-pathogenic reassortants in the embryo was restricted. For example, 81/Ho was able to multiply to a reasonable extent only in the chorioallantoic and the allantoamniotic membranes. After inoculation of the A/PR/8/34 strain containing a non-cleavable HA into the amniotic cavity, virus multipled only in the inner epithelial layer of the amnion, in superficial epidermal cells and in superficial epithelia of the oropharyngeal cavities, the nasal and paranasal sinuses and the oesophagus. Kidneys were free of virus antigen, although the virus multiplied to high titres in primary tissue cultures derived from embryonic kidneys. Thus influenza A viruses can be non-pathogenic for chickens because they are and unable to multiply at the body temperature of chickens (41 °C), or because their spread in the animal is impaired .


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