Physical examination shows that Champ is cyanotic and depressed. He has a fever and is tachypneic. Upon auscultation, you note that his heart and respiratory sounds are muffled: the lungs sound wet with a good deal of crackling, and Champ seems to find it painful to breathe deeply. Radiographs of the thorax indicate consolidated regions in the cranial ventral portions of the lungs and some pleural effusion.
Champ has pneumonia: specifically he has an "aspiration pneumonia," pulmonary inflammation and congestion resulting from non-infectious foreign material in the lungs. The aspirated material is almost always ingesta that has been vomited or regurgitated. If the material can't be coughed out of the trachea, it becomes trapped in the lower respiratory system where it has to be cleared out by macrophages. This is a long and inefficient process that results in ongoing inflammation, pulmonary granulomas, and secondary bacterial infection. Aspiration pneumonia can be very difficult to treat since the cause of the damage is hard to remove.
In this case, the aspiration pneumonia is secondary to Champ's megaesophagus. Megaesophagus is dilation of the esophagus with loss of controlled peristalsis: animals that have it can't easily pass ingesta through the esophagus into the stomach. They tend to regurgitate their meals and aspiration of the undigested food is a common consequence. Megaesophagus patients can be managed by feeding liquified food (for easier passage through the gastroesophageal sphincter) and by feeding from an elevated position (to let gravity help with esophageal transit), but aspiration pneumonia is a constant worry.
Aspirated food causes immediate physical lesions by occluding airways and filling bronchi with ingesta. If it came from the stomach, the gastric acid can cause further damage to the delicate lung tissue. The secondary bacterial infection associated with the persistent inflammation is most dangerous factor for the patient, because it can fulminate or persist at a low grade for extended periods. Persistent infections take a toll on the immune system and open the lungs to superinfection with other organisms.
Another possible source of aspiration pneumonia is improper intubation during surgery. The reason why animals are denied food before surgery is because they may vomit while under anaesthetic: if this happens and the inflatable cuff on the tracheal tube isn't properly seated, the vomit may be inhaled, with potentially fatal consequences. Food denial minimizes the volume of vomit and the cuff keeps it out of the airways. Iatrogenic aspiration pneumonia is avoidable.
This low magnification image of a lung demonstrates the "hemorrhagic" and "purulent" aspects of this condition nicely. The alveoli and air spaces are diffusely occluded by inflammatory exudate consisting of neutrophils, fibrin, and edema. Multiple foci of intraseptal hemorrhage are present and some hemorrhage is evident within the airways.
Below are two more magnified views of the lung parenchyma. The airways can be seen to be filled with neutrophils, and there's evident interstitial edema in the form of clear spaces within the section. Pink-staining fibrin is disseminated throughout the tissue. Small foci of hemorrhage are visible around the airways and there's also a lot of interstitial hemorrhage.
The alveolar septae are discernible as light blue-staining tissue. The septae are thicker than normal (a result of the edema) and don't appear continuous with one another. This is partly due to the effects of the edema but it's also a side effect of the collateral damage caused by the invading neutrophils. Not only is there hemorrhage, but even where the vessels have remained intact, there's hyperremic vasculature and small numbers of erythrocytes visible within airways.
At right above is an airway occluded by neutrophils and necrotic debris: its walls are edematous and swollen and a there's a pronounced hyperremia of the local vasculature. The presence of inflammatory exudate within the larger airways puts this inflammation into the category of bronchopneumonia.
Here's the cause of the trouble: aspirated foreign material. One of the surest ways to provoke a strong immune response is to put something that doesn't belong there into a sensitive organ like the lung. This particle of food was inhaled after being regurgitated. The infiltrate around the material is primarily neutrophilic but some lymphocytes are visible as are occasional macrophages.
As expected, hemorrhage and necrotic debris are also present in this area of intense inflammation.
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