Interstitial Lung Diseases

Brendan M. Corcoran, Edinburgh, Scotland

The lung interstitium is that part of the lung that does not include the airspaces, the capillary endothelial cells, and the alveolar lining epithelium. However, in respiratory disease it would be very unusual for these structures alone to be affected by a pathologic process, and inflammatory and infiltrative diseases of the true interstitium are likely to involve the adjacent alveolar walls. When infiltrative processes extend to include the alveolar spaces, the disease process is more properly categorized as pneumonia.

Diseases of the Lung Interstitium

Compared with humans, there appears to be a limited number of true interstitial lung diseases (ILDs) in dogs and cats, and those that are recognized are poorly defined or have been described only partially. By inference these diseases involve pathologic processes that are restricted to the interstitial structures, but in reality they tend also to involve adjacent structures, so that the pathologic profile is more complex. In addition, these diseases compromise respiratory function and can secondarily initiate other types of respiratory abnormalities.

The most readily recognized disease entities are pulmonary infiltration with eosinophilia (PIE) (see Chapter 163) and idiopathic pulmonary fibrosis (IPF). Parasitism in dogs and cats can be a contributory factor in the eosinophilic airway and lung diseases. The type of parasites involved depend on geographic location but can include Filaroides spp. (Oslerus osleri, Filaroides hirthi), Crenosoma vulpis, Aelurostrongylus abstrusus, Capillaria aerophilia, and the heartworms Angiostrongylus vasorum and Dirofilaria immitis (see Web Chapter 57).

Connective tissue and vasculitic disorders also are a significant problem in human medicine and can involve the lung, but identifying similar disease entities in dogs and cats is difficult, or it is difficult to show that such entities consistently affect the lungs of dogs and cats. Several ILD conditions reported in humans are pathologic descriptions, such as lymphocytic infiltrative disorders, and are unlikely to be identified in dogs and cats without more widespread availability of pathologic data. It is possible for infectious agents such as viruses and neoplastic processes such as lymphoma to cause ILD. In addition, groups of clinical conditions can involve the interstitium, such as pulmonary edema (cardiogenic and noncardiogenic) and syndrome entities like acute respiratory distress syndrome; but invariably these involve other parts of the lung and strictly speaking are not ILDs. Bacterial bronchopneumonia and the mycotic pneumonias (in specific geographic locations worldwide) also can be included in this group. Interstitial lung changes detected radiographically can be the consequence of a disease process elsewhere in the body, such as metastatic mineralization with hyperadrenocorticalism, or even the result of natural changes with aging (Web Box 56-1).

Web Box 56-1 Diseases Involving the Pulmonary Interstitium

Pulmonary Fibrosis

Idiopathic pulmonary fibrosis

Fibrosis secondary to other disease processes (e.g., chronic bronchitis)

Normal aging changes?

Eosinophilic Pneumonias

Pulmonary infiltration with eosinophilia

Eosinophilic bronchopneumopathy

Parasitic infections (pulmonary parasites, cardiac parasites)

Feline asthma (airways)

Pulmonary Mineralization

Hyperadrenocorticalism

Pulmonary neoplasia

Idiopathic mineralization

Pulmonary edema (cardiogenic, noncardiogenic)

Pulmonary neoplasia

Pneumonia (viral, bacterial, mycotic, protozoa)

Acute respiratory distress syndrome

Note that the diseases listed here can present with radiographic evidence of interstitial lung involvement, but many are not classified as interstitial lung diseases.

In humans there is a large group of occupational and toxin- or drug-induced ILDs, but of the the only one that can be identified readily in dogs is paraquat poisoning (Web Box 56-2). Many of the ILDs reported in human medicine are extremely rare, and it is possible that some exist in dogs and cats but have not yet been characterized. Most of the occupational category of ILDs can be excluded, unless a dog or cat is in the same environment and inadvertently exposed to the same agents as the owner. This chapter concentrates on IPF, a disease recognized predominantly in the West Highland white terrier and to a lesser extent in other terrier breeds (Corcoran et al, 1999, 2011; Heikkila et al, 2011; Johnson et al, 2005) and cats (Williams et al, 2004).

Web Box 56-2 Classification of Interstitial Lung Diseases in Humans

Examples (not exhaustive) of the types of interstitial lung diseases recognized in human patients. Some of these diseases are recognized to have analogous conditions in the dog and cat, but the majority of human interstitial lung diseases have not been described in veterinary species.

Drug or Toxin Induced