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Waxing and waning lethargy and ataxia. Left head tilt, positional vertical nystagmus, generalized ataxia.

T2w sagittal

T1w sagittal

T2w transverse

T2-FLAIR transverse

T1w +C sagittal

T1w +C transverse

There is marked thickening and intense contrast enhancement of the choroid plexus of the fourth ventricle and its extensions into the lateral apertures. The mesnecepathlic aqueduct and 4th ventricle are distended due to the presence of poorly defined material or tissue which is hyperintense on T2w, T2-FLAIR and pre-contrast T1w images, without signs of susceptibility artifact on T2* GRE images. This material is not significantly enhancing, most suggestive of exudate or necrotic debris.
Meninges are diffusely mildly enhancing. Within the pons there is minimal enhancement. The 3rd and lateral ventricles are mildly distended, likely from mild obstructive hydrocephalus and/or decreased resorption by the choroid plexus. In the ventral aspect of the frontal olfactory lobes there is patchy signal hyperintensity and contrast enhancement, likely a combination of pulsatile/motion artifact as well as mildly increased meningeal enhancement.


Findings are consistent with an exudative form of infectious or inflammatory choroid plexitis and meningoencephalitis. Atypical choroid plexus neoplasia (such as infiltrative carcinoma or lymphoma) is considered highly unlikely due to the symmetry of the changes.

The CSF analysis indicated a marked purulent eosinophilic pleocytosis, and numerous green algal cells identified as Prototheca spp. were identified in the cerebrospinal fluid, leading to a diagnosis of protothecal meningoencephalitis. Further treatment was declined and the patient was humanely euthanized.

Prototheca algae are ubiquitous environmental organisms, which are endemic in some areas of the southeastern US. The algae are associated with the soil, seeping tree wounds and sewage which can subsequently contaminate ingested water or food. Canine protothecosis is rare, but affected dogs most often present with hemorrhagic large bowel diarrhea, ocular disease and severe, often fatal, systemic infection (sometimes including the CNS). Antifungal agents have been used for treatment with very limited success in dogs with systemic infection and CNS infection has a very poor prognosis. In people and cats, Protothecosis tends to be more limited, leading to granulomatous skin lesions. Affected individuals can be immunosuppressed or have concurrent disease.

This patient had an atypical presentation because of its primarily neurological signs without other overt signs of systemic illness. The MRI findings shared some similarities with two other reported MRIs from dogs with protothecal meningoencephalitis. Multifocal contrast enhancing lesions were seen on MRI of one previous canine patient throughout the brain and meninges, including poorly defined, asymmetric mildly enhancing mass effect in the pons (1). T2 hyperintensity of the left caudate nucleus, cerebellar mass effect and borderline herniation, dilation of the mesencephalic aqueduct, fourth ventricle, increased meningeal enhancement and hydromelia were seen in another previously reported canine patient’s MRI (2).

Although rare, prototheca meningoencephalitis is a possible differential for patients with CNS disease that come from endemic areas such as the southeast US, especially those with a history of persistent diarrhea and multifocal CNS signs.

In people, non-hemorrhagic and hemorrhagic choroid plexitis alone, or in combination with meningitis, meningoencephalitis and brain abscesses have been identified on MRI or CT from bacterial, fungal and parasitic infections (3). Metabolic and toxic disorders and DIC  have also been documented to cause choroid plexus disorders, especially hemorrhagic choroid plexitis (3). In this patient, however, hemorrhage was not evident and there were the additional signs of meningoencephalitis, making infectious or inflammatory disease likely.


  1. Le Roux A, Gumber S, Bauer RW et al. Algal meningoencephalitis due to Prototheca spp. in a dog. Case Rep Vet Med 2013; Article ID 474731.
  2. Young M, Bush W, Sanchez M et al. Serial MRI and CSF analysis in a dog treated with intrathecal Amphotericin B for Protothecosis. J Am Anim Hosp Assoc 2012; 48: 125-131.
  3. Mathews VLP and Smith RR. Choroid plexus infections: Neuroimaging appearance of four cases. Am J Neuroradiol. 1992; 13: 374-378.