Symptoms and Diagnosis

The systemic form of protothecosis is most often caused by P. zopfii. Organs commonly infected include the eyes, kidney, liver, heart, large intestines, skeletal muscle, myocardium, lymph nodes, thyroid, pancreas, peritoneum, diaphragm, and brain. Clinical signs depend on the organ systems involved and the severity of the lesions. White to tan granulomatous lesions can be seen within affected organs (Fig. 1). These lesions range from 0.5 to 2.0 mm in diameter. The inflammatory cell types seen on histopathologic evaluation are highly variable and may include plasma cells, macrophages, lymphocytes, and neutrophils. Plasma cells often predominate.

Gastrointestinal Signs
The most commonly reported clinical sign of disseminated protothecosis is intermittent bloody diarrhea presenting as hematochezia or melena. The colon is most often affected; however, lesions can be found throughout the intestinal tract. Grossly, colonic mucosal lesions can range from large white nodules to diffuse hyperemia to ulcerations that may or may not be hemorrhagic. These lesions may extend into the submucosa.

Central Nervous System Signs
Clinical signs such as cervical pain, head tilt, depression, ataxia, circling, ataxia and paresis are seen in a large percentage of dogs with systemic protothecosis. At necropsy, the central nervous system may contain white to tan, granulamatous nodules. Microscopically, lesions in the brain and spinal cord can be highly disseminated and consist of regions of necrosis surrounded by mixed inflammatory cells. The number of organisms per lesion can vary from many to none.

Ophthalmic Signs
In a retrospective study, 20 of 26 dogs with systemic protothecosis presented with or developed ophthalmic signs. These dogs generally presented with red, painful eyes and blindness. On ophthalmic examination, the pupil or cornea appeared cloudy. The most common histologic finding in affected eyes is choroditis characterized by exudative granulomatous inflammation and retinal detachment.

Cutaneous Infection
Cutaneous infection is associated with infection by P. wickerhamii. This form of protothecosis is less commonly observed. Skin lesions consist of nodules and draining ulcers with crusty exudates on the extremities, trunk and mucosal surfaces. Hyperkeratosis may be present as well as secondary bacterial infections. Microscopically, masses of Prototheca cells may be found in the dermis, subcutis, and adjacent skeletal muscle. Occasionally, the organisms may spread to the regional lymph nodes. In some animals, the cutaneous form of disease may transform to systemic infection as organisms spread to other tissues and organ systems.