To analyze pathogenicity changes of new isolate LL09 from chick embryos, 120 1-day-old specific pathogen free (SPF) chickens were intranasally inoculated with broth cultures of isolate LL09. Its colonization pattern in different chicken tissues was studied by bacterial isolation and indirect immunoenzyme histochemistry. Results showed that the bacteria were isolated from tracheas and lungs at 1 h post-infection. Afterwards, they colonized livers, hearts and spleens at 120 h and then infected kidneys at 168 h. The peak infection appeared on 21 d post-infection. They persisted in these organs and caused injuries up to 42 d. With growth of chickens, B. avium began to be gradually cleared away from livers, hearts and spleens, except that it could still be detected in tracheas, lungs and kidneys until 56 d post-infection. It demonstrated that lungs and kidneys would possible be colonized for a long time by B. avium in addition to the tracheas.

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