Respiratory health plays a considerable role in the overall welfare and productivity of sheep flocks, impacting both meat and wool production. When the respiratory system is compromised, sheep may experience poor growth rates, reduced feed efficiency, and diminished reproductive success. Lung health issues are a major cause of illness and mortality in sheep populations, translating directly into significant economic losses for the industry. Understanding the anatomy, common diseases, and diagnostic methods related to the ovine respiratory tract is essential for effective flock management.
Structure and Function of the Ovine Respiratory System
The ovine respiratory system is structured to manage the high volume of air required for gas exchange in a grazing ruminant. Air enters through the nostrils and travels down the trachea, which is reinforced by C-shaped cartilage rings to keep the airway open. The trachea bifurcates into the left and right bronchi, which then branch into progressively smaller airways called bronchioles, eventually leading to the alveoli where gas exchange occurs.
The sheep’s lungs feature a distinct lobe arrangement: the left lung is typically divided into cranial and caudal lobes, while the right lung possesses cranial, middle, caudal, and an accessory lobe. A notable anatomical feature is the presence of connective tissue septa that separate the lung tissue into distinct lobules. This separation means that an infection often remains localized within a lobule, which limits the spread of disease but can also affect how ventilation is distributed.
Gas exchange is the primary function, occurring across the thin barrier between the alveoli and the surrounding pulmonary capillaries. Oxygen diffuses from the inspired air into the bloodstream, while carbon dioxide moves out of the blood and into the alveoli to be exhaled. The healthy lung also performs a defense function, filtering inhaled air through the mucociliary escalator—a system of mucus-producing cells and cilia that trap and propel foreign particles and pathogens out of the airways.
Common Respiratory Pathologies in Sheep
Respiratory diseases in sheep are frequently complex, often involving a combination of viral, bacterial, and parasitic agents. These conditions can manifest as acute, sudden illnesses or as chronic problems that lead to progressive wasting and poor performance. The economic impact of these diseases stems from both sudden death and the long-term effects of chronic ill-thrift.
Bacterial pneumonia, commonly referred to as pasteurellosis, is often caused by Mannheimia haemolytica and is a major cause of acute death, especially in young sheep. This bacterium normally resides in the nasopharynx but proliferates when the immune system is weakened by stress or preceding viral infection. M. haemolytica produces a potent leukotoxin that damages white blood cells, resulting in severe fibrinous bronchopneumonia, inflammation, and fibrin deposition in the lungs.
Ovine Pulmonary Adenomatosis (OPA), also known as Jaagsiekte, is a contagious and progressive lung cancer caused by the Jaagsiekte Sheep Retrovirus (JSRV). The JSRV retrovirus infects and transforms type II pneumocytes, causing them to proliferate and form tumorous masses that replace normal lung tissue. These tumor cells secrete a large volume of clear fluid, which accumulates in the airways and impairs breathing. Since there is no effective treatment or vaccine, OPA is invariably fatal and presents a serious control challenge.
Parasitic lungworm infections, particularly from Dictyocaulus filaria, contribute to respiratory problems, especially in wet or cool environments. The adult worms reside in the bronchi, causing irritation and inflammation of the airways. Worm eggs are coughed up, swallowed, and passed as larvae in the feces, which develop into infective stages on pasture. The resulting inflammation, known as verminous pneumonia, leads to chronic coughing, reduced weight gain, and increased susceptibility to secondary bacterial infections.
Diagnosing Ovine Lung Health Issues
The initial step in identifying respiratory problems relies on careful clinical observation of the flock, looking for changes in behavior or breathing patterns. Farmers may notice signs such as a persistent cough, increased respiratory rate (tachypnea), or the presence of nasal discharge. A veterinarian will perform a physical examination, which includes auscultation, or listening to the lungs, to detect abnormal sounds like crackles or wheezes that suggest fluid or inflammation in the airways.
For more definitive diagnosis, especially with chronic diseases, technical tools are necessary. Transthoracic ultrasound (TUS) has become an invaluable tool for on-farm screening, particularly for OPA. TUS allows visualization of the lung surface and reliably detects the characteristic fluid-filled or solid tumor lesions, often identifying them at a preclinical stage. TUS is effective because OPA lesions tend to be located in the ventral lung lobes near the chest wall.
Laboratory testing provides confirmation of the causative agent and is often necessary to distinguish between diseases with similar clinical signs. Samples, such as nasal swabs or bronchoalveolar lavage fluid, can be analyzed using Polymerase Chain Reaction (PCR) to detect the genetic material of specific viruses or bacteria. For parasitic lungworm infections, diagnosis is achieved by analyzing fecal samples for the presence of first-stage larvae using specialized techniques. Post-mortem examination (necropsy) remains the gold standard for definitive diagnosis, allowing for direct visual inspection of lesions and subsequent histopathology.