Fungal Pneumonia in Dogs and Cats: A Practical Clinical Overview for Veterinary Practice

Bacterial pneumonia remains the most commonly detected type of pneumonia in dogs and cats; however, fungal, parasitic and viral pneumonia must always be included in the differential diagnosis of patients presenting with respiratory distress¹. 

Mycotic pneumonia may be caused by Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, Cryptococcus neoformans and Aspergillus fumigatus. The respiratory tract acts as the primary portal of entry, and infection may remain confined to the lungs or disseminate systemically to various target organs¹. 

Clinical Presentation 

Clinical signs of fungal pneumonia are often non-specific and may mimic bacterial or chronic inflammatory airway disease. 

Common findings may include: 

• Persistent cough, often unresponsive to antibiotic therapy 

• Fever 

• Lymphadenopathy 

• Ocular involvement (e.g. hyphema) 

• Signs consistent with systemic mycosis¹ 

In cryptococcosis, pulmonary manifestation is considered less common than central nervous system or ocular involvement². However, pulmonary disease does occur and should not be overlooked. Cryptococcus spp. have also emerged as opportunistic pneumonia pathogens in immunocompromised humans². 

Opportunistic fungi such as Candida spp. may infect the respiratory tract, particularly in immunocompromised individuals. In humans, Candida spp. are among the most common opportunistic fungal lung infections. The true incidence of Candida pneumonia in dogs and cats remains unrecognised and warrants further investigation¹. 

Immunosuppressive therapy, including long-term corticosteroid administration, may predispose patients to opportunistic fungal infections³. A key clinical challenge is differentiating colonisation from true infection, particularly as delayed antifungal therapy in humans has been associated with high mortality¹. 

Thoracic Radiographic Findings 

Survey thoracic radiographs are typically the first diagnostic test performed when pneumonia is suspected. Radiographic findings suggestive of fungal pneumonia may include¹: 

• Diffuse interstitial pattern 

• Mixed interstitial and alveolar pattern 

• Miliary nodular pattern 

• Bronchointerstitial pattern 

• Pulmonary consolidation 

• Cavitary lesions 

• Hilar lymphadenopathy  

Although the miliary interstitial nodular pattern is considered characteristic of fungal pneumonia, radiographic changes can vary significantly¹. 

Radiographic patterns are not pathognomonic. For example, neutrophilic and eosinophilic inflammation and bronchial patterns may also be seen in chronic bronchitis. In humans, Candida pneumonia is described radiographically as alveolar or mixed alveolar–interstitial without a miliary nodular pattern¹. 

Therefore, thoracic radiography is an important screening tool but must be interpreted alongside additional diagnostics. 

Bronchoalveolar Lavage (BAL): Diagnostic Utility 

Bronchoalveolar lavage (BAL) plays a critical role in the diagnostic work-up of suspected fungal pneumonia. In human medicine, BAL culture is considered one of the main diagnostic methods for fungal identification within the respiratory system, with high sensitivity and specificity^1,4. 

In veterinary medicine, BAL is widely used to diagnose: 

• Bacterial pneumonia 

• Chronic bronchial disease 

• Neoplasia 

• Hypersensitivity disorders 

Cytological identification of pathogenic fungi has been reported in 71% of cases ¹. However, fungal organisms may not always be visualised cytologically, even when culture is positive. 

BAL cytology in fungal pneumonia may demonstrate: 

• Increased neutrophils 

• Increased eosinophils 

However, similar inflammatory profiles may also be present in chronic bronchitis. 

This overlap reinforces the importance of combining cytology with fungal culture and, where possible, additional diagnostic modalities such as PCR, serology or antigen testing. 

Practical Clinical Considerations 

• Fungal pneumonia should remain on the differential diagnosis list for dogs and cats presenting with respiratory clinical signs, particularly when there is poor response to antibiotic therapy. 

• Immunocompromised patients are at increased risk for opportunistic fungal infections. 

• Radiographic findings may be suggestive but are not definitive. 

• BAL culture is a valuable diagnostic tool, even when cytology is non-diagnostic. 

• Differentiating colonisation from infection can be challenging and may influence therapeutic decisions. 

Greater awareness and improved diagnostic pursuit in clinical practice may reduce underdiagnosis of opportunistic and pathogenic fungal respiratory infections. Continued veterinary research is needed to better define epidemiology and optimise diagnostic strategies for fungal pneumonia in companion animals. 

References   

1. Pavelski M, Seixas SV, Warth JF, Souza CD, Dittrich RL, Froes TR. Fungal pneumonia in dogs and cats with pulmonary clinical signs in southern Brazil. Pesquisa veterinária brasileira. 2018 Apr;38(04):696-702. https://www.scielo.br/j/pvb/a/J4DqWW6TnS9bNKkZLW6vCWJ/?format=pdf&lang=en  

2. Oliveira MD, Flores MM, Mazzanti A, Fighera RA, Kommers GD. Neurocryptococcosis in dogs and cats: Anatomopathological and fungal morphological aspects in a case series. Pesquisa Veterinária Brasileira. 2024;44:e07447. https://www.scielo.br/j/pvb/a/FmwrhWdtQDXD9GqYZXbwQxB/?format=html&lang=en  

3. McAtee BB, Cummings KJ, Cook AK, Lidbury JA, Heseltine JC, Willard MD. Opportunistic invasive cutaneous fungal infections associated with administration of cyclosporine to dogs with immune-mediated disease. Journal of veterinary internal medicine. 2017 Nov;31(6):1724-9. https://academic.oup.com/jvim/article-pdf/31/6/1724/66668207/jvim14824.pdf  
4. Kaur J. Diagnosis and therapeutic management of canine respiratory affections. MV Sc. Thesis, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana, India. 2019. https://krishikosh.egranth.ac.in/server/api/core/bitstreams/a1a37b47-79e3-42e6-a758-27abab472078/content