Causes, incidence, treatment of pulmonary and systemic fungal infections in horses
Systemic fungal infections have an insidious progression, often presenting with non-specific clinical signs. A thorough clinical evaluation usually is required to confirm diagnosis.
Fungi are eukaryotic organisms with a definitive cell wall made up of chitins, glucans and mannans. Within the cell wall, the plasma membrane contains ergosterol, a cell membrane constituent frequently targeted by anti-fungal agents.
Pathogenic fungi can be divided into three groups: multinucleate septate filamentous fungi; non-septate filamentous fungi; and yeasts.
Dimorphic fungi can change between forms, depending on environmental conditions.
In soil and decaying matter, the mycelial form usually is present and is composed of a collection of hyphea. The mycelia produce infective spores that can inoculate vertebrate tissue.
Pulmonary fungal infections in horses can cause pulmonary granulomas, diffuse pneumonia or pleuropneumonia. Mycotic granulomas of the upper respiratory tract have been found in the nasal passages, paranasal sinuses, nasopharynx, guttural pouch and trachea of infected horses.
The most common clinical signs of upper respiratory fungal infection include unilateral or bilateral serosanguineous or mucopurulent nasal discharge as well as inspiratory and expiratory noise.
Other clinical signs include facial deformation and dyspnea caused by partial blockage of nasal passages by granulomatous masses.
Fungal pneumonia sometimes affects horses that are immunocompromised or neutropenic or that have enteritis/colitis, bacterial pneumonia or neoplasia. Systemic infections can have variable clinical signs depending on the location and extent of the infection. Fungal infections can affect multiple organs and body cavities.
Weight loss, colic and diarrhea often occur with infection within the abdominal cavity. Immunodeficiency, congenital or acquired, or glucocorticoid therapy may predispose a horse to fungal infection.