Etiology

• Iatrogenic; drenching, medicating or tube feeding.
  • Aspiration pneumonia is commonly diagnosed in hypocalcemic cows. Often this is as a result of farmers attempting to drench these cows whist they are too weak to swallow or because they were drenched in lateral recumbency.
• Any disease leading to pharyngeal or esophageal dysphagia may lead to the aspiration of rumen contents, food, water and/or saliva into the trachea and the rest of the respiratory tract. The most common of these conditions is esophageal obstruction or choke Esophageal obstruction: probang technique.
• Other esophageal disorders that could also lead to regurgitation include strictures, esophagitis Esophagus: diseases and disorders and congenital disorders such as persistent right aortic arch Vascular ring anomalies. 
• Neurological disorders may affect the control of swallowing and lead to pharyngeal paralysis or paresis, such as tetanus Tetanus, rabies, botulism Botulism, etc.
• Down cows, in lateral recumbency may also be at risk. Down cow: overview
• Cleft palate can lead to aspiration of milk into the trachea:
  • Common observations, by the farmer, include a calf that starts suckling vigorously but then starts coughing and struggles to drink properly .
  • Foreign bodies or a mass in the pharynx (subepiglottic or pharyngeal cysts or tumors of esophagus can cause dysphagia.
• Actinobacillosis Actinobacillosis in the throat/ esophageal area is a risk factor for aspiration pneumonia.
• Various poisonings may lead to dysphagia, including plants, lead Lead poisoning, arsenic Arsenic poisoning, etc.

Predisposing factors

General

• Milk fever: especially if animal becomes cast -> aspiration of rumen contents -> toxemia. Milk fever
• Iatrogenic: drenching/ medicating/ stomach tubing.
• Holding out the tongue when drenching/ medicating.
• If the animal is bellowing at the time of drenching/ medicating.
• If the head is thrown back/  neck overstretched at the time of drenching/ medicating.
• Food, saliva, blood and exudates can be aspirated during general anesthesia.
• Animals with aspiration pneumonia should not be anesthetized if at all possible.

Pathophysiology

• The severity of the injury to the pulmonary tissue following aspiration depends on the amount and composition of the aspirated material.
  • Evidence appears to be lacking but it is suspected that water or saliva aspiration is unlikely to be as critical as aspirated rumen contents (high foreign body and microbe load) or drugs/ drenches/ milk.
  • Practically it may very hard to establish what has been aspirated on farm and economics may hamper detailed investigation.
• If large quantities of fluids are aspirated the animal may die acutely.
• If rumen contents are aspirated then the damage can be severe with pulmonary edema and hemorrhagic pneumonia. Toxemia is a common sequelae and can prove fatal in 1-2 days.
• In general, aspiration of material into the lungs leads to bacterial contamination of the lower respiratory tract which overwhelms the normal respiratory defence mechanisms resulting in pulmonary infection, lung consolidation and in some cases pleuropneumonia and/or lung abscess formation.
• The bacteria involved are mixed and anerobic infections are quite common.
• Often normal inhabitants of the nasopharynx; such as Mannheimia haemolytica Mannheimia haemolytica, Pasteurella multocida Pasteurella multocida and Histophilus somni Histophilus somni infection are isolated pathogens.
• The cranioventral parts of the lung are at most risk because of the structure and orientation of the bronchi and other conducting airways.
• Lateral recumbency may lead to unilateral pathology.

Timecourse

• In early cases, clinical signs may not be obvious, being limited to fever and depression.
• As the pneumonia progresses, the clinical signs become more apparent.
• Following milk fever and aspiration of rumen contents, resulting toxemia can prove fatal in 1-2 days.