Types of Pneumonia

One of the most common methods of describing pneumonias is based on their morphologic pattern, that is, the location where the pneumonia is located. Knowing what anatomic structures are affected can actually help you determine the most likely etiology, how the animal became exposed to that agent, and the subsequent pathogenesis. The most common patterns of pneumonia are bronchopneumonia, interstitial pneumonia, granulomatous pneumonia and embolic pneumonia.

Bronchopneumonia

Fundamentally, the inflammation in bronchopneumonias is found at the junction between the bronchioles and the alveoli. These pneumonias are most commonly caused by bacteria or aspiration of feed or stomach contents. Bacteria most commonly gain entry to the lungs via aerosols or inhaled nasal flora.

Bronchopneumonias are almost exclusively found in the cranioventral lung lobes, likely from inhaled particles settling into these lung lobes first due to gravity. The inflammatory response to the bacteria or foreign material results in progressive filling of the alveoli and bronchioles with an inflammatory exudate, eventually causing consolidation. Consolidation can be identified grossly as a lung lobe that is firmer than normal and may sink in formalin. Typically consolidation includes some component of atelectasis, as well as the accumulation of exudate.

There are two main forms of bronchopneumonia: suppurative and fibrinous.

Suppurative Bronchopneumonia

As the name suggests, suppurative bronchopneumonia is characterized by suppurative inflammation, otherwise known as pus. This pus can be seen grossly within the bronchi, and can be extruded by squeezing the lung (gross). The inflammation in this type of pneumonia often appears lobular, because the inflammation stays within its lobule, and doesn’t spread across the lobular septae. In species with very prominent lung lobules, like cattle and pigs, this type of pneumonia can produce a checkboard pattern of affected and unaffected lobules.

Suppurative bronchopneumonias are most commonly caused by bacteria like Pasteurella, Bordetella, Trueperella, Streptococcus, Mycoplasmas and E. coli. They can also be caused by aspiration pneumonia.

Fibrinous Bronchopneumonia

Fibrinous bronchopneumonia has an exudate that is rich in fibrin, the major clotting protein. Typically, fibrinous bronchopneumonias are not restricted by the lobular septae, causing entire lung lobes to become affected as the exudate spreads through the lobules. Often the pleural surface is involved in these pneumonias, producing a fibrinous pleuritis.

Fibrinous bronchopneumonia has a classic “bread and butter” appearance… meaning the appearance of a piece of bread with butter on it after you dropped it on the ground. These pneumonias are often associated with yellow thoracic fluid, because in order for fibrin to leave the vessels, they must be quite leaky!

Like their suppurative counterparts, fibrinous bronchopneumonias are also primarily caused by bacteria. Some examples include Mannhemia, Histophilus, Actinobacillus and Mycoplasma. These pneumonias rarely resolve completely, often leaving behind significant scarring and adhesions to pleural surfaces. In some cases, aspiration of irritating substances like stomach acid can also lead to fibrinous bronchopneumonia.

Interstitial Pneumonia

With interstitial pneumonias, the inflammation is within the interstitium (duh), otherwise known as the alveolar walls. Unlike bronchopneumonia, the location of the exudate in this type of pneumonia is not grossly visible. Typically, these pneumonias have a “rubbery” or “meaty” texture, due to the accumulation of inflammatory cells in the interstitial tissue. You might see impressions of the ribs indenting the lungs. The lungs will also fail to collapse when the thorax is opened.

Interstitial pneumonias tend to have a diffuse distribution, affecting all lobes. This is because these pneumonias are typically caused by things that spread hematogenously, or cause diffuse damage to the alveoli lining. For example, epithelial damage can be caused by inhalation of toxic gases or certain cytotoxic viruses. Hematogenous causes include sepsis and septicemia, blood-borne toxins like endotoxins, or blood-borne cytotoxic viruses.

A brief note on bronchointerstitial pneumonia:

Sometimes this terminology is used when there are components of both bronchopneumonia and interstitial pneumonia… for example the last photo. This is a common finding with viral infections that cause damage to both the alveolar epithelium and the bronchiolar epithelium.

Embolic Pneumonia

Embolic pneumonias are characterized by multiple nodules throughout all of the lung lobes. This distribution makes sense, because this type of pneumonia is caused almost exclusively by hematogenous bacteria showering the lungs and forming abscesses. Grossly, these pneumonias have a very classic appearance of multiple, pus-filled abscesses. Easy to diagnose!

However, it’s important to remember that those showering bacteria came from somewhere. The most common locations are hepatic abscesses rupturing into the caudal vena cava (especially in feedlot cattle), umbilicus infections, chronic bacterial infections or contaminated catheters. Some of the most commonly isolated bacteria are Trueperella, Erysipelothrix, Streptococcus and Staphylococcus.

Granulomatous Pneumonia

Granulomatous pneumonia doesn’t refer to a specific location of inflammation, however it’s still an important type of pneumonia to think about. This type of pneumonia forms granulomas, large nodules of macrophage infiltration. These macrophages accumulate in large numbers because there is something very difficult for them to eat! Typically, these inedible things are fungi, parasites or some robust bacterial species like Mycobacterium tuberculosis.

These lungs appear similar to embolic pneumonia, with a multifocal distribution of firm nodules. However, these nodules typically contain firm, caseous material that may even be mineralized.

Comparison Table

Check Your Understanding

Photos© Metro, Mosier, Saunders, Jennings, Wright, Sagartz licensed under CC BY-SA 4.0.

Zachary JF. Pathologic Basis of Veterinary Disease, Sixth Edition.