A challenging case: An unusual cause of blindness in a Siberian husky


A 12-year-old 57-lb (26-kg) castrated male Siberian husky was presented to the Colorado State University Veterinary Teaching Hospital for evaluation of blindness of three weeks' duration, depression, and polydipsia.

A 12-year-old 57-lb (26-kg) castrated male Siberian husky was presented to the Colorado State University Veterinary Teaching Hospital for evaluation of blindness of three weeks' duration, depression, and polydipsia.

Vital Stats


The dog had undergone a laparotomy at Rowley Memorial Animal Hospital in Springfield, Mass., five months before presentation. During the surgery, an adrenal and a liver mass were excised. Histologic examination revealed that the masses were an adrenocortical adenoma and a well-differentiated hepatocellular carcinoma and that hepatocellular swelling with lymphohistiocytic and eosinophilic inflammation was present. The prognosis for the liver mass was considered good because hepatocellular carcinomas rarely metastasize and the histology report stated that complete removal is often curative. At that time, there was no serologic evidence of hyperadrenocorticism, thus the adrenocortical adenoma was considered nonfunctional.

The dog had recovered well from the surgery, but three months later had been presented to Rowley Memorial Animal Hospital because of a cloudy right eye. Uveitis was diagnosed, and diagnostic tests were done to attempt to identify the underlying cause. The results of a CBC, serum chemistry profile, and urinalysis were normal. An abdominal ultrasonographic examination revealed a mass where the adrenal tumor had been removed, which was thought to be either a recurrence of the tumor or a suture reaction. A thoracic radiographic examination revealed a mild bronchointerstitial pattern in the caudal dorsal lung field that was suggestive of thickening of the lungs due to an infection or an infiltrative neoplasia but was not considered consistent with metastasis. The dog was sent home with prednisolone acetate 1% topical eye drops to be administered three times a day and deracoxib (2 mg/kg) to be given once a day for five days.

Several weeks later, the owner had moved to Colorado. After the move, the dog became blind and was presented to a veterinarian who started treatment with oral prednisolone (5 mg every day) and continued the topical prednisolone drops. The dog was referred to the Colorado State University Veterinary Teaching Hospital for further evaluation.


On presentation, the dog was alert and active, although it bumped into things in the examination room. The dog's temperature, pulse, and respiratory rate were normal, as were findings of the remainder of the physical examination.

Figure 1

An ocular examination revealed a severe, bilateral panuveitis. Aqueous flare (2+ on a scale of 1 to 4) was observed in both eyes. The right eye had 360-degree posterior synechiae and iris bombé. In addition, there was a dense, immature cataract that precluded fundic examination. The left lens had dense nuclear sclerosis through which a massive retinal detachment and retinal hemorrhages could be discerned (Figures 1 & 2). The intraocular pressures measured by applanation tonometry (TonoPen Vet—Medtronic) were low in both eyes (5 mm Hg in the right eye and 6 mm Hg in the left eye; normal = 10 to 20 mm Hg) despite the posterior synechiae in the right eye. Since bilateral panuveitis is often indicative of a serious systemic disease, a complete diagnostic workup was recommended.

Figure 2


Blood was drawn for a cbc, a serum chemistry profile, fungal titers (to rule out aspergillosis, blastomycosis, coccidioidomycosis, cryptococcosis, and histoplasmosis), and Leptospira species titers. A fecal sample and urine sample (collected by cystocentesis) were submitted for analysis. The history of neoplastic mass removal along with the previous thoracic radiographic findings suggested a metastatic neoplasm could be contributing to the dog's panuveitis, so abdominal ultrasonographic and thoracic radiographic examinations were performed. In addition, ocular ultrasonography was performed to allow better visualization of the posterior segment of the right eye.

The results of the CBC were normal. The serum chemistry profile revealed mild azotemia (blood urea nitrogen = 35 mg/dl, normal = 7 to 32 mg/dl; creatinine = 2.3 mg/dl, normal = 0.4 to 1.5 mg/dl). The dog was seronegative for all fungal and Leptospira species, and the results of the fecal examination were negative. Urinalysis revealed a low urine specific gravity (1.006), a few red blood cells, and moderate numbers of bacteria (later identified on culture as Serratia marcescens, a relatively common opportunistic pathogen). No other organisms were seen in the urine sediment.

No evidence of metastatic neoplasia nor other abnormalities were identified on the thoracic radiographs, and the abdominal ultrasonographic examination revealed hypoechoic nodules in the spleen and liver and a small heterogeneous mass in the area of the previously removed left adrenal gland. An ultrasound-guided fine-needle aspirate of this mass was obtained after the patient was sedated. Interpretation of the abdominal ultrasonographic examination was that the liver and splenic nodules likely represented metastases from the previously diagnosed hepatic adenocarcinoma and that the mass in the adrenal region was a hematoma or granuloma.

Figure 3

Ocular ultrasonographic examinations showed bilateral retinal detachments (Figures 3 & 4). A pocket of fluid was seen in the nasal and ventral retrobulbar areas of both eyes, and the right globe appeared distorted. A sample of fluid from beneath the detached retina of the right eye was obtained with the aid of ultrasonography and submitted for cytologic evaluation.

Figure 4


Cytologic examination of the subretinal aspirate revealed a high cellularity and a finely granular eosinophilic background. The cellular content consisted of 10-to-20-µm, ovoid-to-reniform, pale basophilic organisms (Figure 5). Each had 1-to-2-µm, round-to-globular basophilic structures within its cell wall. Rare degenerate neutrophils were phagocytizing the organisms, which were tentatively identified as Prototheca species. Protothecosis was confirmed by culture (on Sabouraud's dextrose agar), and the organism was further identified as Prototheca wickerhamii.

Figure 5

The results of cytologic examination of the fine-needle aspirate from the left adrenal area showed that there were cells that appeared to be adrenal in origin but did not possess criteria of malignancy. In addition, a few oddly appearing cells were identified that were probably Prototheca organisms.

The dog was discharged from the hospital, and the owner was informed that the disease carried a grave prognosis. The owner was given the option of administering antifungal therapy with amphotericin B, with or without itraconazole, but because of the prognosis, specific antiprotozoal therapy was declined. However, the urinary tract infection was treated with enrofloxacin (4 mg/kg orally once daily for 14 days), and palliative therapy for the uveitis (prednisolone acetate 1% solution, one drop four times daily in both eyes) was instituted.

The owner was traveling to Texas in a week, so the dog was scheduled for a recheck appointment with a veterinarian there. No changes were noted at the time of that recheck. Two weeks later, the dog developed severe diarrhea and was euthanized. Necropsy was not performed.


Protothecosis is an uncommon disease in mammals. It is caused by ingesting a saprophytic algae of the genus Prototheca, which is found worldwide in organic waste such as sewage, soil, and even tree slime (slime flux).1-4 The organism lacks chlorophyll, but its life cycle is similar to that of green algae of the genus Chlorella.5-8 Two species have been documented as causing most clinical cases of protothecosis in domestic animals: P. wickerhamii and Prototheca zopfii.

Protothecosis has been diagnosed in many species, including cattle,9 deer,10 dogs,2-4,6,8,11-34 cats,35 and people.36,37 In the United States, most cases have occurred in the Southeast, although cases from most geographic areas have been reported. In people, protothecosis usually occurs as a cutaneous form,36,37 which arises from traumatic, cutaneous lesions. The cutaneous form is also most common in cats.35 As early as 1952, the organism was isolated from a cow that had mastitis, which appears to be the most common manifestation in cattle.9,10 A more severe, systemic form of the disease is usually manifested in other mammals.5,6,28,29 In addition to the eyes, the organism has been isolated from the heart, brain, liver,2,6 intestines,32 lymph nodes,16,32 lungs, and kidneys.2,4,6,16

Dogs with protothecosis often present for evaluation of clinical signs indicative of gastrointestinal disease, such as severe bloody diarrhea.2,17,19,26,32-34 These signs are frequently either preceded or followed by ocular signs. Alternatively, ocular manifestations may be the only initial clinical signs of the disease.22,34 Neurologic signs such as ataxia, progressive limb paresis, and head tilt were also reported in dogs with protothecosis.6

Ocular involvement is frequently seen in dogs with protothecosis. A literature review published in 2000 found 25 published cases and one unpublished case.8 Of the 26 cases, 20 had ocular lesions.8 Subsequently, a case was described involving a German shepherd with severe gastrointestinal signs but no ocular lesions.31 More recent reports include a case of protothecosis in a 10-year-old mixed-breed dog that had developed unilateral blindness but had no diarrhea32 and a series of 13 cases, at least two of which involved ocular lesions.4 In that retrospective study, six of the 13 dogs had renal failure attributable to Prototheca species infection.4 Most reported cases involved chronic gastrointestinal signs of diarrhea, weight loss, and debilitation. Several dogs had signs of neurologic dysfunction such as ataxia, paresis, head tilt, and circling.8 Interestingly, many of the earlier reported cases involved collies (six of 26).11,13,17,21,24,28 In the retrospective case series involving 13 dogs, affected dog breeds were not listed.4 No specific breed predisposition can be inferred because the percentage of collies in the dog populations of the represented areas was not given.

The ocular lesions associated with protothecosis are usually severe and vision-threatening. They consist primarily of granulomatous choroiditis, with occasional anterior uveitis.15 Hemorrhage may occur intraretinally or subretinally. Aqueous flare is less commonly seen. The clinical manifestations include red eyes, blepharospasm, corneal cloudiness due to edema, and often unilateral or bilateral blindness. The blindness is usually due to retinal separation from the choroid by the presence of inflammatory cells, fluid, and Prototheca species organisms.11,13,17-20,27,30,34 The retinal separation may be seen ophthalmoscopically and appears as a thin veil in the posterior segment.

Diagnosing protothecosis may be difficult initially. The results of CBCs and serum chemistry profiles of affected animals are usually normal until the patients are severely debilitated.8,24,32 Fungal cultures from any location are generally negative. Urinalysis results may be normal, although the organism has been identified in the urine sediment of many dogs with renal disease caused by Prototheca species.4 In our patient, a bacterial urinary tract infection was identified, but no protothecal organisms were seen in the urine sediment. Whether the cystitis was due to the protothecosis is unknown. The findings from fecal flotation are usually negative for protothecal organisms; however, colonic scraping appears to have some value in isolating these organisms (based on cytologic examination).8,24 In one case, protothecal organisms were observed in biopsy samples in the mucosal layer of the colon and in the abdominal lymph nodes.32

Historically, thoracic radiography has not been helpful in diagnosing protothecosis.6,13,20,21,30 Although in our patient abnormalities were identified in the initial thoracic radiographs taken at Rowley Memorial Animal Hospital, they were not seen in subsequent radiographs taken at Colorado State University. In addition, the possibility that the dog had a metastatic neoplasm confounded the radiographic interpretation. Plain abdominal radiographs are normal in dogs with protothecosis, but using a contrast medium may help identify a thickened intestinal wall or decreased lumen size.14,16

One of the most useful and least invasive tools for diagnosing protothecosis is to identify the organism from an aspirate from the posterior segment of the eye.13 The procedure may require referral to an ophthalmologist. In many case reports, the procedure was called a vitreous tap but was more likely an aspirate of the fluid from the subretinal space.8,34 The organism appears to have an affinity for the subretinal space, and in cases involving a retinal separation, numerous organisms were obtained from the fluid aspirated from this area.4,13,20,22,30,32,34 On the other hand, aqueous aspirates do not appear to be useful in diagnosing the disease because organisms are rarely present in the fluid.4,20

Treatment of systemic protothecosis is usually unrewarding.4,8,30 In people, a few reports of successful antifungal therapy have been reported, but none of the treatments was successful long-term in dogs. A 39-year-old man with hepatic and biliary protothecal infections treated with amphotericin B and oral ketoconazole was completely cleared of the disease.38 Although some Prototheca species are sensitive to antifungal drugs in vitro,4 antifungals only provide temporary remission in vivo. Intravenous or per rectum amphotericin B, with or without oral itraconazole or ketoconazole and with or without immunostimulatory therapy, has been tried in people and dogs.4,22,25,30,31 Amphotericin B in liposomes has been used in people and dogs and has proved to be less nephrotoxic than amphotericin B without liposomes, but it is expensive. A collie with cutaneous protothecosis responded to six months of oral ketoconazole therapy; all clinical signs resolved except for a scrotal lesion, which was removed surgically.28 In a dog exhibiting only ocular signs, treatment with oral itraconazole apparently did not slow the progression of the ocular lesions but allowed the dog to survive for 11 months.30 In our patient, treatment with amphotericin B (regular and in liposomal form) and itraconazole was offered, but because of the grave prognosis, the owner declined.

Although Prototheca species are ubiquitous in nature, they rarely cause systemic disease in animals or people.39 People with systemic protothecal infections are almost always immunocompromised.4,39 Suggested predisposing causes in people include neoplasia, human immunodeficiency virus infection, immunosuppressive drug use, systemic fungal infections, and deficiencies in cell-mediated immunity.4,39-41 Immunosuppression may not be apparent clinically, and protothecosis has been reported in a person who had no concurrent disease and exhibited immunocompetence based on all test results.42 This patient had elevated antibodies to the organism and responded to antifungal therapy; an underlying cause for the patient's infection was not established.

In dogs, the systemic disease is more common than it is in people, but the reason for this is unknown. In most reported cases in dogs, underlying immunosuppression was not found,2,6,11,13,19,24,26,31,32 but few of these patients have undergone immune status assessment.8 A case has been reported in which a dog with protothecosis had deficits in both T-lymphocyte and polymorphonuclear activity.21 Although our patient's immune system status was not specifically assessed, the dog may have been immunocompromised. The presence of neoplasia in people has been documented as a predisposing cause for protothecosis,41 and this dog had confirmed adrenal and hepatic tumors. The dog's previous major abdominal surgery could also have led to immunosuppression. Because the dog began receiving glucocorticoids after its ocular signs developed, it appears that the dog did not acquire the infection because it was pharmacologically immunosuppressed. But it is possible that this therapy may have allowed the infection to further disseminate.


Consider systemic protothecosis in your list of differential diagnoses for ocular inflammatory diseases, especially when severe chorioretinitis is present. Affected patients may present with ocular signs (especially blindness) alone or in conjunction with signs of gastrointestinal, neurologic, or renal disease. Diagnosing protothecosis may be difficult, but aspirating the subretinal fluid of dogs with retinal detachments will often yield Prototheca organisms.

At this time, protothecosis is minimally treatable and the prognosis is grave because the organism has usually disseminated throughout the body before it is detected. In the future, treatments directed at strengthening a patient's immune response and earlier detection methods may improve the outcome in patients with protothecosis.

Juliet R. Gionfriddo, DVM, MS, DACVO

Department of Clinical Sciences

College of Veterinary Medicine

Colorado State University

Fort Collins, CO 80523


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