Compare the histologic sections. Describe the pathology that you see.

Does the injury primarily involve the alveolar spaces, the interstitial space, or both?

Is there normal lung in any of the images of diseased lungs?

Can you identify any borders? Does that help you determine the etiology?

Which of these cases seems to have the most tissue destruction? What does that tell you about the virulence of the organisms?

The low power image of the bacterial pneumonia shows a classic lobar pattern with normal lung across the fissure in the adjacent lobe. Note the prominent alveolar infiltrate of neutrophils. Despite the extensive infiltrate, the lung parenchyma is relatively spared, suggesting that the causative organism is less virulent.

The viral pneumonia demonstrates thickening of the interstitial spaces and prominent hyaline membranes. If the patient survives, viral pneumonias tend to resolve completely.

The fungal pneumonia shows almost complete obliteration of the lung parenchyma. The propensity of aspergillus to invade blood vessels makes the specimen quite hemorrhagic. Invasive fungal pneumonias are very serious and generally occur in very ill patients; however, if the patient survives the destruction here is so significant it would likely lead to the development of a cavitary lesion.

Aspergillus can infect the lung in three ways. An allergic hypersensitivity reaction in the bronchi can occur as a result of inhalation of a large number of spores. Alternatively, Aspergillus can colonize an abscess cavity, creating a “mass” lesion often called an aspergilloma. This form typically does not invade. The most devastating is invasive Aspergillosis, in which the organism invades the pulmonary parenchyma. Aspergillus has a particular propensity to invade vessels, resulting in hemorrhage, thrombosis, and infarction. The infarcted tissue also acts as “culture media” for growth of Aspergillus as well as a number of other infectious agents. More on this next year!