A clinical case of canine mycotic pneumonia

Aspergillus fumigatus Bronchopneumonia in a Hellenic Shepherd Dog

A 3 yr old intact female Hellenic shepherd dog was referred due to depression, partial anorexia, fever, and a mild productive cough of 2 mo duration. Thoracic radiographs showed increased opacity of all of the left lung lobes. Upon bronchoscopy, a sanguineous, purulent discharge was detected in the tracheal lumen with hyperplastic tissue narrowing the left main stem bronchus. Cultures were positive for bacteria (Bacillus spp. and Clostridium spp.) but negative for fungi. Due to the severity of the lesions, a complete left lung pneumonectomy was performed. Histopathological examination of the excised lung tissues revealed a severe granulomatous bronchopneumonia with numerous alveolar macrophages laden with structures stained positively by periodic acid-Schiff and Grocott stain that had morphology consistent with fungi. PCR and sequencing of internal transcribed spacer regions 1 and 2 from genetic material extracted from paraffin-embedded pulmonary tissue confirmed the presence of Aspergillus fumigatus. Itraconazole was administrated for 5.5 mo and the dog was clinically normal 26 mo after surgery.

Immune response to fungal infections

The immune mechanisms of defence against fungal infections are numerous, and range from protective mechanisms that were present early in evolution (innate immunity) to sophisticated adaptive mechanisms that are induced specifically during infection and disease (adaptive immunity). The first-line innate mechanism is the presence of physical barriers in the form of skin and mucous membranes, which is complemented by cell membranes, cellular receptors and humoral factors. There has been a debate about the relative contribution of humoral and cellular immunity to host defence against fungal infections. For a long time it was considered that cell-mediated immunity (CMI) was important, but humoral immunity had little or no role. However, it is accepted now that CMI is the main mechanism of defence, but that certain types of antibody response are protective. In general, Th1-type CMI is required for clearance of a fungal infection, while Th2 immunity usually results in susceptibility to infection. Aspergillosis, which is a disease caused by the fungus Aspergillus, has been the subject of many studies, including details of the immune response. Attempts to relate aspergillosis to some form of immunosuppression in animals, as is the case with humans, have not been successful to date. The defence against Aspergillus is based on recognition of the pathogen, a rapidly deployed and highly effective innate effector phase, and a delayed but robust adaptive effector phase. Candida albicans, part of the normal microbial flora associated with mucous surfaces, can be present as congenital candidiasis or as acquired defects of cell-mediated immunity. Resistance to this yeast is associated with Th1 CMI, whereas Th2 immunity is associated with susceptibility to systemic infection. Dermatophytes produce skin alterations in humans and other animals, and the essential role of the CMI response is to destroy the fungi and produce an immunoprotective status against re-infection. The resolution of the disease is associated with a delayed hypersensitive response. There are many effective veterinary vaccines against dermatophytoses. Malassezia pachydermatis is an opportunistic yeast that needs predisposing factors to cause disease, often related to an atopic status in the animal. Two species can be differentiated within the genus Cryptococcus with immunologic consequences: C. neoformans infects predominantly immunocompromised hosts, and C. gattii infects non-immunocompromised hosts. Pneumocystis is a fungus that infects only immunosupressed individuals, inducing a host defence mechanism similar to that induced by other fungal pathogens, such as Aspergillus.