Dogs are usually infected by aerosol.

Virus replicates in the tonsils and bronchial lymph nodes leading to viraemia (in macrophages) and further replication in widespread lymphoid tissues. Damage caused by this replication causes lymphopenia and immunosuppression. Dogs may eliminate the virus at this stage, but if it is a very pathogenic strain, or if the dog is not able to make a good immune response, then the virus spreads to the epithelial surfaces of the respiratory, enteric and urogenital tracts, skin and CNS. The relative involvement of these tissues determines the clinical signs which develop.

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