A challenging case: An unusual cause of blindness in a Siberian husky - Veterinary Medicine
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A challenging case: An unusual cause of blindness in a Siberian husky
Despite a lack of other obvious physical abnormalities or signs, systemic disease was suspected of causing this dog's eye problems.


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.


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