Final answer:
Dyspnea in rabbits is most frequently caused by pneumonia with Pasteurella multocida infection, which leads to fluid-filled alveoli and respiratory distress.
Step-by-step explanation:
The most frequent etiology of dyspnea in rabbits is pneumonia with Pasteurella multocida infection. Pneumonia is a condition where the alveoli of the lungs fill with fluid, impairing gas exchange and causing significant respiratory distress. This condition, in rabbits, is often caused by the bacterium Pasteurella multocida, which can lead to severe respiratory illness. On the other hand, hypersensitivity pneumonitis (HP) is caused by an allergic reaction to inhaled substances like dust, bird feathers, or molds and is considered an occupational disease because it is primarily associated with specific environmental exposures at work.