Originally Bordetella bronchiseptica was thought to be the cause of canine distemper. Then, when a viral cause of distemper was found, it was thought to be only a secondary pathogen. Now in recent years it has been shown conclusively that it can cause respiratory disease in its own right.

The bacterium appears to attach by means of its fimbriae specifically to the cilia of the trachea and bronchi, and in in vitro studies it has been shown that ciliary cells and ciliostasis occurs. In contrast to other bacteria such as streptococci, Pasteurella multocida and staphylococci which are eliminated within 24 hours of experimental infection, B. bronchiseptica multiplies rapidly, and after a few days reaches a plateau. As the number of organisms reaches peak levels clinical signs start to appear. After 2-3 weeks the number of organisms starts to diminish and clinical signs resolve. However some bacteria still persist in the trachea and bronchial tree for several months after infection.
B. bronchiseptica stimulates an acute inflammatory reaction, with mucopurulent discharges present in the trachea and bronchial tree; in some cases there may be lung involvement, though this is uncommon. Immunity to B. bronchiseptica is slow to develop, and the organism is only fully cleared from the respiratory tract by 12-14 weeks after infection. Immunity lasts at least six months, but dogs become susceptible to challenge again by 14 months after infection.

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