Issue Description Disseminated protothecosis is most commonly
seen in dogs. The algae enters the body through the mouth or nose and
causes infection in the intestines. From there it can spread to the
eye, brain, and kidneys.
The two species of Protetheca
that are known to cause disease in animals are P. zopfii and P.
wickerhamii. Protothecosis is an opportunistic infection that occurs
when the organism comes into contact with injured skin or mucosa. It
is believed that those animals that have suppressed immune systems or
are overwhelmed by a pre-existing disease are at greater risk of
infection. It is postulated that weak cell mediated immunity is
responsible for allowing the organism to gain entrance into the body.
However, very few canine cases have undergone significant immune
system assessment due to the rarity of this disease. It has also been
theorized that the organism can become highly virulent and is able to
evade the host's immune response. It is believed that in the systemic
form of the disease the organism is ingested, enters the body via the
intestinal mucosa and spreads through out the body hematogenously or
through the lymph system. In contrast, the cutaneous form of disease
originates with punctures, cuts, or abrasions in the skin.
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
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
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.
Treatment Protothecosis is a difficult disease to treat.
Amphotericin B, tetracycline, Ketoconazole, Itraconazole, fluconazole
and clotrimazole have been used to attempt to treat this disease.
Unfortunately, these drugs have shown little success in treating
systemic protothecosis. Surgery is used to treat both systemic and
skin forms of the disease.
Canine protothecosis is a very
rare disease that should only be considered after other more common
diseases have been excluded. Protothecosis is very difficult to treat
and is often fatal.