Clinical history:

A 7-year-old, male neutered, Cavalier King Charles Spaniel presented with a four-week history of progressively worsening tachypnoea, developing into dyspnoea. CT imaging under sedation revealed generalised ground glass appearance throughout the lung fields, deemed to be consistent with non-cardiogenic oedema. The dog was treated with trimethoprim-potentiated sulfonamides and oxygen supplementation, but deteriorated and sadly died from cardiopulmonary arrest. A post-mortem sample of lung was submitted to VPG for histopathology.

Histopathology:

The lung tissue was diffusely consolidated. The alveoli were lined by hyperplastic type II pneumocytes and the interstitium was fibrotic. Alveolar spaces were flooded with oedema, fibrin or surfactant-type material, and large numbers of inflammatory cells, mainly macrophages and neutrophils. There were accumulations of foamy eosinophilic material visible in some areas within the alveolar spaces. When Grocott-Gomori methenamine silver (GMS) stain was applied, the foamy eosinophilic material was shown to be composed of numerous 3-4 micron diameter fungal organisms. This appearance was diagnostic for Pneumocystis sp. infection (pneumocystosis).

Figure 1.

This low-power image (H&E) shows lung consolidation. Alveolar spaces are no longer filled with air (should show as clear space). The bronchioles are still visible containing clear space though.

Figure 2.

This high-power view (H&E) shows the alveoli infiltrated with inflammatory cells (arrow head), multifocally filled with foamy eosinophilic material (horizontal arrows), and lined by type II pneumocytes (vertical arrows).

Figure 3.

With GMS silver stain, in the areas of foamy material, there are numerous round organisms (arrow heads), typical of Pneumocystis sp. Note that most of the foamy material is non-staining, which corresponds to the trophozoites. The silver stain only stains the cyst walls.

Histological diagnosis:

Interstitial pneumonia with intra-alveolar Pneumocystis sp

More information on Pneumocystis:

Clinical cases of canine pneumocystosis are rare. The clinical history of gradual onset respiratory disease, usually longer than 4 weeks, is common. The other most common clinical signs are exercise intolerance, cough, and weight loss despite normal feed intake. It is thought that underlying immune impairment is required for the causative agent, Pneumocystis carinii, to cause clinical disease, and this may be inherited or induced by immunosuppressive therapy. The Cavalier King Charles Spaniel is a predisposed breed for this infection owing to a suspected underlying immune deficiency. There is a known common variable immunodeficiency inherited in the Miniature Dachshund, making this the other most commonly affected dog breed. Foals with severe combined immunodeficiency (SCID) are also predisposed. At 7 years of age, this spaniel was older than most affected dogs (median 1 yr).

Pneumocystis carinii is a yeast-like fungus that is highly adapted to infect type I pneumocytes that line the alveoli in health. The typical histological picture is the presence of foamy material filling alveoli. This material is composed of numerous fungal cysts and trophozoites. The individual organisms are difficult to appreciate without a special silver stain (GMS) and can be overlooked if they are present in low numbers and only standard H&E stain is used.

This patient unfortunately succumbed to pneumocystosis. Dogs can recover but early treatment is generally needed to achieve this. Obtaining an antemortem diagnosis can be problematic, but bronchio-alveolar lavage fluid cytology is an effective and noninvasive method.

Alex Civello DipACVP FRCPath MRCVS

Board Certified Anatomic Pathologist

References:

1. Weissenbacher-Lang C, Fuchs-Baumgartinger A, Guija-De-Arespacochaga A, Klang A, Weissenböck H, Künzel F. Pneumocystosis in dogs: meta-analysis of 43 published cases including clinical signs, diagnostic procedures, and treatment. J Vet Diagn Invest. 2018 Jan;30(1):26-35. doi: 10.1177/1040638717742429.

2. Lobetti R. Common variable immunodeficiency in miniature dachshunds affected with Pneumonocystis carinii pneumonia. J Vet Diagn Invest. 2000 Jan;12(1):39-45. doi: 10.1177/104063870001200107.