Equine and Avian Respiratory System Diseases: Anatomy and Clinical Signs

Laryngeal Hemiplegia (Roaring)

Laryngeal hemiplegia, commonly known as "roaring," is caused by the paresis (weakness) or paralysis of the arytenoid cartilage and vocal fold. This is typically due to the progressive loss of large myelinated fibers in the distal portion of the recurrent laryngeal nerves, most commonly on the left side. While it affects all breeds, it is more prevalent in males and larger, long-necked breeds. The primary clinical sign is a noise heard during inspiration (inhalation) during exercise.

Dorsal Displacement of the Soft Palate (DDSP)

DDSP occurs when the caudal free margin of the soft palate moves dorsal to (above) the epiglottis, creating a functional obstruction to airflow. This can be caused by inflammation, congenital hypoplasia of the epiglottis, or as a complication of previous laryngeal surgery. Horses with DDSP may "choke down" during high-speed exercise.

Lower Airway Inflammatory and Vascular Diseases

Equine Asthma

Equine asthma is an allergen-mediated inflammatory disease of the lower airways. It encompasses conditions such as heaves, recurrent airway obstruction, and chronic obstructive pulmonary disease (COPD). Common triggers include being stabled in poorly ventilated areas with high exposure to straw or hay dust.

The pathology involves small-airway inflammation, mucus production, and bronchoconstriction. Chronic cases can lead to permanent airway narrowing and secondary bacterial pneumonia.

Exercise-Induced Pulmonary Hemorrhage (EIPH)

EIPH is most frequently seen in sporting horses, particularly racehorses. It is caused by extremely high pulmonary vascular pressure during maximal exercise, which leads to the rupture of capillaries and bleeding into the lungs. While many horses show no outward signs, some may present with coughing, frequent swallowing, or even epistaxis (nosebleed) in about 5% of cases.

Avian Respiratory Anatomy and Physiology

The avian respiratory system is significantly different from the mammalian model. Birds do not possess a diaphragm; instead, they have a single celomic cavity.

Key Components

• Air Sacs: These are thin-walled structures connected to the lungs and some bones. Their primary function is to increase the volume of air that moves through the lungs to facilitate efficient gas exchange.
• Syrinx: Located where the trachea divides into the primary bronchi, the syrinx is composed of cartilages, membranes, and muscles. It is the organ responsible for vocalizations and song by changing airflow to produce vibrations.
• Choana: This is a cleft in the roof of the mouth that serves as an opening between the oral and nasal cavities, allowing the bird to breathe with its mouth closed.

Clinical Signs of Respiratory Compromise

Recognizing respiratory distress requires observing both rate and quality of breathing. In birds, a normal respiratory rate for a chicken is approximately 35 breaths per minute.

Common Clinical Signs

• Increased Rate/Effort: Observed in both species, often accompanied by nostril flaring or neck extension.
• Abnormal Sounds: Persistent sneezing, coughing, or respiratory noise.
• Avian-Specific Signs: Open-beak breathing (which can also indicate stress or overheating), discharge from the eyes or nares, swelling of the infraorbital sinus, and a cyanotic (blue) comb or wattle.

Exam Note: Differential Diagnoses When a patient presents with respiratory signs, remember that the cause is not always within the respiratory system itself. Consider environmental irritants, trauma, cardiovascular disease, anemia, or even non-respiratory diseases (like obesity or reproductive issues) that may be compressing the air sacs.