ABSTRACT
A 5‐year‐old Basenji Mix achieved extended survival with treatment for disseminated protothecosis. The dog originally presented with iridal hyperemia and ocular ultrasonographic examination revealed a mass effect. Histopathology revealed ocular protothecosis and staging confirmed disseminated protothecosis. Treatment with itraconazole and amphotericin B (AmB) infusions resulted in remission. Four years later, restaging revealed recurrence of infection in colonic biopsy samples. An extended AmB protocol was restarted for a cumulative dose of 84 mg/kg and the dog received isavuconazole based on susceptibility testing. The dog was euthanized 4 years and 8 months after initial diagnosis due to decline of neurologic status. Necropsy confirmed the presence of disseminated protothecosis in multiple arteries, multiple lymph nodes, the pituitary gland, thyroid glands, kidneys, colon, cecum, right eyelid, right eye, heart, brain, and periosteum of one bone associated with the middle ear. This report documents the extended survival of a dog with widespread disseminated protothecosis after prolonged administration of AmB.
Keywords: algae, amphotericin, canine, colitis, isavuconazole, itraconazole, ocular protothecosis
Abbreviations
- BUN
blood urea nitrogen
- CSF
cerebrospinal fluid
- CT
computed tomography
- MRI
magnetic resonance imaging
- OS
left eye
- PAS
periodic acid‐Schiff
1. Introduction
Disseminated protothecosis is a devastating, frequently fatal disease in dogs. Caused by the unicellular, achlorophyllous algae of the same name, the infection most commonly establishes in the gastrointestinal tract, causing colitis and subsequent diarrhea [1]. If left untreated, the organisms replicate via endosporulation in the colonic submucosa and spread hematogenously or via the lymphatic system, resulting in disseminated infection. The eyes and brain are the most widely reported secondary sites of infection, though other tissues affected include the kidneys, heart, and lymph nodes. Cutaneous manifestations have also been reported and can be associated with systemic disease [2].
Treatment is complicated by the organism's resistance to conventional antibacterial and antimycotic drugs [3, 4]. Protothecosis is rare and requires cytology or histopathology as well as culture for definitive diagnosis, and the infection is therefore oftentimes not diagnosed until it has disseminated [5]. The largest case series to date describes the treatment and outcome of 17 dogs in Australia. Most died or were euthanized within 4 months of diagnosis. One dog lived to 17 months, and one was still alive at the time of writing (12 months documented survival) [6].
We report the case of a dog that survived with documented disseminated protothecosis at initial diagnosis for almost 5 years (1700 days), with a several‐year intervening period between initial treatment and suspected recurrence without evidence of detectable disease. The dog was treated with extended courses of AmB, as well as ancillary therapeutics including itraconazole, terbinafine, doxycycline, nystatin, and isavuconazole. This report documents long‐term survival in a dog with disseminated protothecosis.
2. Case
A 5‐year‐old spayed female Basenji Mix dog was evaluated at the Schwarzman Animal Medical Center in New York City in July 2019 for iridal hyperemia OS. Evaluation revealed that the dog was blind OS, with an absent pupillary light reflex, iris hyperemia, and suspected retinal detachment. Ocular ultrasonographic examination confirmed the presence of retinal detachment and revealed a mass effect. Diagnostic work‐up did not reveal other abnormalities.
A routine transpalpebral OS enucleation was performed 1 week later. Histopathology returned with evidence of pyogranulomatous inflammation with numerous intralesional round to oval algae with a refractile cell wall and displaying endosporulation. A diagnosis of ocular protothecosis was suspected.
In September 2019, staging for protothecosis was performed, including a full‐body CT scan and colonoscopy. The CT scan revealed mild bilateral retropharyngeal lymphadenopathy and was otherwise normal. Colonic biopsies revealed chronic, erosive, moderately lymphoplasmacytic, and mildly eosinophilic and histiocytic colitis.
Prototheca was isolated from urine and colonic tissue samples, confirming a diagnosis of disseminated protothecosis. The dog was administered itraconazole (generic, variable pharmaceutical manufacturers) at a dose of 5 mg/kg PO twice daily. Shortly after, liposomal amphotericin B (AmBisome, La Verne, CA, USA) infusions were initiated, which would consist of a cumulative dose of 24 mg/kg administered IV over the next 8 weeks. The AmB protocol involved administration of Lactated Ringer's solution, an isotonic crystalloid fluid, at an above maintenance rate between 70 and 75 mL/kg/day for 2 h before and after infusion of a 1 mg/kg dose of AmB. At each visit, renal analytes were monitored both before and after infusion for evidence of acute kidney injury (AKI), though none occurred.
Restaging performed in winter 2019–2020, including full‐body CT scan, colonic biopsies, rectal scraping, and colonic and urine cultures, returned with no detectable prototheca organisms. The dog continued to do well clinically on itraconazole PO, and restaging was again performed in March 2021. Colonic biopsies returned with no detectable prototheca organisms, and colonic and urine cultures returned negative, without evidence of Prototheca. The dog was monitored and restaged annually for evidence of recurrence and maintained on itraconazole at a dosage of 5 mg/kg PO twice daily throughout this time. Treatment efficacy was based on the dog's clinical status, as the bioavailability of itraconazole was not confirmed through measurements of blood concentrations.
In March 2023, almost 4 years after initial diagnosis, the dog developed ocular discharge, mydriasis, and lower eyelid swelling of the right eye. The dog had remained otherwise clinically well at home with no signs of gastrointestinal disease. A partial retinal detachment with subretinal cellular infiltrates was noted, suggestive of recurrence of prior protothecosis. The dog was administered terbinafine (10 mg/kg PO twice daily), and the itraconazole dose (previously 5 mg/kg PO twice daily) was decreased to once daily due to concerns regarding itraconazole resistance and antifungal drug interactions. Colonic samples returned positive for Prototheca and were considered resistant to all antifungals tested based on the elevated minimum inhibitory concentrations (MICs) obtained (Table 1).
TABLE 1.
Results of antifungal sensitivity panel performed at Cornell's AHDC and UT Health. a
| Drug name | MIC (μg/mL) |
|---|---|
| Amphotericin B | 4 |
| Anidulafungin | > 8 |
| Caspofungin | > 8 |
| Fluconazole | > 256 |
| 5‐Flucytosine | > 64 |
| Itraconazole | > 16 |
| Micafungin | > 8 |
| Posaconazole | > 8 |
| Voriconazole | > 8 |
| Nystatin a | 8 |
| Isavuconazole a | 1 |
This susceptbility testing was performed at UT Health. The other susceptibility testing was performed at Cornell's AHDC.
The dog's eyesight continued to deteriorate, and within a few weeks, the dog was blind in the right eye. Based on documented resistance, the itraconazole was discontinued, and the dog was treated empirically with doxycycline (10 mg/kg PO once daily), which has some efficacy against Prototheca [7]. In June 2023, the dog developed a new subcutaneous mass over the left hip, as well as daily regurgitation, polyuria/polydipsia, and urine leakage. Administration of doxycycline had been discontinued due to gastrointestinal adverse effects and hyporexia noted at home. The mass was excised, and histopathology revealed nodular granulomatous inflammation with intrahistiocytic algal organisms consistent with Prototheca. The dog then also developed an acute onset of lameness in the left pelvic limb. Radiographs revealed a smoothly marginated periosteal reaction with possible intramedullary sclerosis along the medial aspect of the right femoral diaphysis in addition to a multifocal, irregular periosteal reaction with suspected osteolysis along the left tibial diaphysis, suggestive of hematogenous osteomyelitis secondary to disseminated protothecosis.
At that time, the dog was administered the same AmB protocol as earlier despite the recently documented resistance, due to severe skeletal pain in the left pelvic limb. Once susceptibility results from UT Health in collaboration with Cornell's AHDC lab became available, the dog was administered nystatin (25 000 IU/kg PO three times daily), and terbinafine was discontinued. Lameness in the left hindlimb resolved shortly after restarting AmB infusions. By early September of the same year, repeat radiographs revealed an improving periosteal reaction of the left distal tibial diaphysis with resolved soft tissue swelling, consistent with a positive response to therapy. AmB was continued biweekly for another 2 months for an additional 30 mg/kg dosing due to clinical efficacy and lack of adverse gastrointestinal and renal effects.
In October 2023, PAS stain of colonic biopsy samples revealed algal organisms consistent with Prototheca, and the dog was administered AmB infusions. Between October 2023 and March 2024, the dog received an additional 30 mg/kg of AmB, utilizing the aforementioned protocol. The dog's renal analytes (creatinine and BUN) were closely monitored and remained normal before and after AmB treatment, with only mild transient increases during the 9‐month time frame that AmB was administered, allowing ongoing biweekly AmB infusions without any treatment delay. Throughout this time, additional susceptibility results returned from UT Health, and the dog was administered isavuconazole (initial starting dose of 3.4 mg/kg PO once daily, later increased to the recommended 6.8 mg/kg PO once daily in people). The dog's clinical signs initially improved with the addition of this medication, but ultimately plateaued.
In March 2024, the dog developed progressive severe signs of gastrointestinal disease and weight loss. Budesonide (1.5 mg PO once daily, later 2 mg PO once daily) was trialed empirically to reduce colonic inflammation. When no improvement was noted, an esophageal feeding tube was placed under general anesthesia to provide nutritional support and facilitate the administration of medications. The dog had a difficult recovery from anesthesia and presented less than 12 h later due to progressive signs of neurologic disease. The dog's neurologic status continued to deteriorate in the hospital, and the owners ultimately elected humane euthanasia, 56 months after initial diagnosis. In total, the dog received a cumulative dose of 84 mg/kg of AmB.
Further speciation of a colonic isolate from March 2024 was performed by Cornell's AHDC lab; previous isolates from this case were not available for speciation. Prototheca spp. was first identified based on morphological characteristics on wet mount after sample incubation on Sabouraud dextrose agar (SDA) incubated at 29°C for at least 48 h. Identification to the species level was then performed by sequencing cytb, as previously described [8]. The sequence resulted in 100% query coverage with 99.8% identity to Prototheca bovis [9]. Fluorescence in situ hybridization (FISH) testing was performed on endoscopic colonic biopsy samples from March 2024 and did not reveal evidence of adherent or invasive bacteria.
Necropsy confirmed the presence of disseminated protothecosis with involvement of multiple arteries, multiple lymph nodes, the pituitary gland, brain, heart, kidneys, cecum, colon, thyroid glands, right eye, and right palpebrae. The dog also had a concurrent mild to moderate acute bronchointerstitial pneumonia with extracellular mineral, pigmented acellular debris, bacteria, and yeast in air spaces. Bacterial and fungal cultures confirmed evidence of both pulmonary bacteria (both Escherichia coli and Streptococcus canis were isolated) and yeast (Candida spp.). Based on the distribution of lesions in the lung and the presence of foreign material in air spaces, aspiration was considered the primary differential for pulmonary inflammation.
3. Discussion
This report documents prolonged survival in a dog with disseminated protothecosis involving multiple organ systems. It is difficult to definitively determine whether the dog had a true relapse rather than a reinfection; though, given the rarity of this infection in a metropolitan area, as well as its resistance to previously prescribed drugs, relapse is considered more likely. The dog also received a large total dose of AmB, though the initial 24 mg/kg were administered 4 years before the subsequent 60 mg/kg. This case suggests that repeated AmB dosing in dogs with concurrent intravenous administration of fluid therapy can be considered for life‐threatening fungal infections. The dog's tolerance to high AmB dosing could be partially attributed to the lipid complex drug formulation used, as this formulation allows for a greater cumulative dose before inducing biochemical evidence of kidney injury [10].
Susceptibility testing performed throughout the dog's treatment protocol helped to inform clinical decisions, including the use of the newer antifungal isavuconazole, a novel treatment against disseminated protothecosis. Isavuconazole has fewer interactions with other drugs and fewer adverse effects when compared with other antifungals [11, 12]. It has a similar efficacy to voriconazole and has been used in human patients with renal insufficiency. It also crosses the blood–brain barrier and has demonstrated efficacy against infections that have spread to the central nervous system (CNS). No metabolic or clinical adverse effects were noted secondary to isavuconazole in this dog.
Disseminated protothecosis remains a rare but devastating disease in dogs, with a guarded long‐term prognosis. Based on the outcome in this single case, chronic treatment with an azole antifungal (itraconazole and isavuconazole) and possible repeat AmB infusions can be considered to extend survival.
Disclosure
The antimicrobials amphotericin B, nystatin, doxycycline, itraconazole, and isavuconazole are used off‐label.
Ethics Statement
The authors declare no institutional animal care and use committee or other approval was needed. The authors declare human ethics approval was not needed.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgments
The authors thank the Cornell University Animal Health Diagnostic Center Laboratory and the University of Texas Health Fungus Testing Laboratory for their contributions.
Landfield B. G., Appleman E. H., Daverio H., Guarino C., Moore M. E., and Gioia G., “Extended Survival in a Dog With Disseminated Protothecosis,” Journal of Veterinary Internal Medicine 39, no. 5 (2025): e70231, 10.1111/jvim.70231.
Funding: The authors received no specific funding for this work.
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