Causes, incidence, treatment of pulmonary and systemic fungal infections in horses

Causes, incidence, treatment of pulmonary and systemic fungal infections in horses

Photo 1: Photomicrograph of an impression smear from a biopsy of a mass in the nasal passage of a horse with chronic serosanguinous nasal discharge and dyspnea.
Pulmonary or systemic fungal infections in horses historically have resulted in a high mortality rate. However, with earlier diagnosis and the increasing availability of affordable anti-fungals with known pharmacokinetic profiles, successful outcomes are becoming more common.

Systemic fungal infections have an insidious progression, often presenting with non-specific clinical signs. A thorough clinical evaluation usually is required to confirm diagnosis.

Photo 2: Radiograph of a horse with a history of chronic serosanguinous nasal discharge and dyspnea. Dorsoventral radiograph of the skull demonstrates significant increase in soft-tissue opacity within the nasal cavity and right maxillary sinus due to cryptococcal infection.
For pulmonary infections, thoracic radiography and collection of fluid samples by tracheal wash or broncho-alveolar lavage are useful. Cytologic or histologic examination of lesions may identify morphologic features of fungal organisms. The etiologic agent can be definitively identified by microbiologic culture, immunohistochemistry or polymerase chain reaction.

Treatment options depend on the site and extent of the infection, accessibility to surgical intervention, the etiological agent, the known antifungal susceptibilities, evidence-based study from human medicine and financial resources of the owner.

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.

Photo 3: A 12-year-old Quarter Horse mare presented with dyspnea and a three-week history of a serosanguinous nasal discharge. Conidiobolomycosis was diagnosed by culture and histology from a biopsy taken under endoscopic guidance of the mass.
Pulmonary-affected horses can present with signs similar to those of bacterial pneumonia and, if the condition is chronic, weight loss.

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.