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cases and one unpublished case.8 Of the 26 cases, 20 had ocular lesions.8 Subsequently, a case was described involving a German shepherd with severe gastrointestinal signs but no ocular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.