Endemic pneumonia in five- to eight-week-old pigs induced microscopic lesions of proliferative interstitial pneumonia which were compatible with a viral aetiology. The disease was transmitted experimentally to conventional and gnotobiotic pigs by means of a lung homogenate filtered through a 0.22 micron filter. No common viral respiratory pathogens of pigs were isolated. Two types of virus particles were observed in cell culture by electron microscopy; one was about 70 nm in diameter and had an envelope and short surface spicules, the other also had an envelope, was elongated, pleomorphic, measured 80 x 320 nm and was coated by antibodies.
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