What is the primary sign of pasteurellosis in its upper respiratory form?

The primary sign of pasteurellosis in its upper respiratory form is rhinitis, commonly known as "snuffles." This condition is characterized by a thick, sometimes purulent nasal discharge, sneezing, and nasal congestion. It occurs due to the infection caused by the bacteria Pasteurella multocida, which affects the respiratory tract, leading to inflammation of the nasal passages.

In rabbits, "snuffles" is particularly notable because it can escalate if not managed properly, potentially leading to more severe respiratory issues. Understanding this primary symptom is crucial for identifying and addressing pasteurellosis early to prevent it from worsening or spreading.

Other signs listed in the options relate to different conditions or forms of illness. Abdominal distention and skin discoloration are not typical manifestations of upper respiratory pasteurellosis, and bronchopneumonia, while a serious condition, refers to infection of the lung tissue, which is more indicative of lower respiratory involvement rather than the upper respiratory symptoms associated with "snuffles."