History
A fully vaccinated 6-month-old 27-kg (59.4-lb) sexually intact male Staffordshire Terrier that was seemingly healthy suddenly developed ataxia and seizures. The dog was taken to a location with a beach and a river twice during the preceding week and became ill the day after the second visit. The dog also frequently came into contact with soil from a potted house plant.
Clinical and Gross Findings
On initial evaluation by the attending veterinarian, petechiae were present on the inner aspects of the dog's thighs and gums. The dog was panting excessively and salivating and had a dazed appearance. Two hours later, a seizure occurred, and the dog was subsequently unresponsive. It then had multiple successive seizures and was treated with pentobarbital (4 mg/kg [1.8 mg/lb], IV), dexamethasone (2.5 mg/kg [1.14 mg/lb], IV), and diazepam (1 mg/kg [0.45 mg/lb], IV) but never regained consciousness and was euthanized with an IV injection of pentobarbital. The dog underwent necropsy.
The dog was in moderate body condition (body condition score, 4/9), and the buccal and conjunctival mucous membranes were markedly pale with petechiae on the buccal mucosa. Multiple petechiae were also present in the inguinal and axillary areas. Multifocal, variably sized, 5 × 8 × 6-mm to 15 × 25 × 12-mm, irregular, poorly circumscribed, pale areas of necrosis were scattered throughout the cerebral white matter and cortex from the diencephalon to the midbrain. The kidneys were bilaterally, moderately swollen (right kidney, 9.2 × 4.1 × 3.5 cm; left kidney, 8.4 × 4.1 × 3.8 cm) with multifocal 1- to 2-mm × 3- to 5-mm × 2- to 5-mm, pale, slightly raised, poorly circumscribed, cream-colored, soft foci scattered throughout and occupying approximately 50% of the parenchyma of each kidney (Figure 1). The spleen was moderately enlarged (25 × 6 × 2 cm) with rounded edges; 10 to 15 small (3- to 5-mm × 3- to 5-mm × 2- to 4-mm) lesions, similar to those in the kidneys, were scattered throughout and occupied approximately 5% of the splenic parenchyma. Multiple irregular patches of hemorrhage occupied approximately 20% of the lung parenchyma. The liver was slightly enlarged and diffusely congested. Sections of brain, kidneys, spleen, lungs, and liver were placed in neutral-buffered 10% formalin, routinely processed, and embedded in paraffin, and 4-μm-thick sections were stained with H&E stain.
Photograph of the enlarged right kidney from a young dog that suddenly developed ataxia and seizures. The kidney has been longitudinally split and laid flat in this view. The dog was taken to a location with a beach and a river twice during the preceding week and became ill the day after the second visit. The dog also frequently came into contact with soil from a potted house plant. Notice the multifocal, pale, slightly raised, poorly circumscribed, cream-colored, soft foci scattered throughout and occupying approximately 50% of the parenchyma.
Citation: Journal of the American Veterinary Medical Association 256, 7; 10.2460/javma.256.7.775
Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →
Histopathologic Findings
On histologic examination of various tissue sections, there were multiple areas of necrosis in the brain, kidneys, and spleen, with a marked infiltrate of macrophages, multinucleated giant cells, neutrophils, and fewer lymphocytes and plasma cells (Figure 2). Mixed within the inflammation and necrotic debris were multiple, 10- to 15-μm-diameter, amoebic trophozoites with a prominent karyosome and occasional double-walled cysts with a wrinkled outer wall; the cyst wall reacted with Grocott methenamine silver stain (in the kidney and brain sections). In sections of the brain, the amoebas were especially prominent around blood vessels that had perivascular mononuclear cuffing and vasculitis. In the left eye, there was a small focal area extending from the caudal aspect of the ciliary process and caudally along the pars ciliaris retinae, where the inner and outer pigmented epithelial layers were separated by moderate numbers of similar inflammatory cells with rare amoebic trophozoites.
Photomicrographs of sections of the brain (A) and right kidney (B) of the dog in Figure 1. In each of these views, there is an area of necrosis with a marked infiltrate of macrophages and fewer lymphocytes and plasma cells. Multiple, 10- to 15-μm-diameter, amoebic trophozoites (arrows) with a prominent karyosome and occasional double-walled cysts are present. In both panels, H&E stain; bar = 50 μm.
Citation: Journal of the American Veterinary Medical Association 256, 7; 10.2460/javma.256.7.775
Paraffin-embedded sections of the brain and kidneys were sent to a laboratorya for further identification of the amoeba. Immunohistochemical analysis of the kidney and brain sections with an assay to detect Acanthamoeba spp and an assay that targeted infectious free-living amoeba (Naegleria fowleri, Acanthamoeba spp, and Balamuthia mandrillaris)1 was performed. Amoebic forms in the kidney and brain tissue reacted positively in both immunohistochemical assays (Figure 3). Acanthamoeba genotyping2,3,b of kidney tissue-derived DNA was performed; sequencing identified that this dog was infected with Acanthamoeba healyi, genotype 12.
Photomicrograph of a section of the right kidney after immunohistochemical analysis for Acanthamoeba spp. The trophozoites (arrows) appear red. Acanthamoeba-specific immunohistochemical stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 7; 10.2460/javma.256.7.775
Morphologic Diagnosis and Case Summary
Morphologic diagnosis: severe, multifocal to coalescing, necrotizing, granulomatous encephalitis and severe, bilateral, multifocal to coalescing, necrotizing, granulomatous nephritis with intralesional A healyi trophozoites and cysts; and multifocal, granulomatous splenitis and moderate granulomatous anterior uveitis of the left eye with A healyi trophozoites and cysts.
Case summary: systemic amoebic infection with granulomatous inflammation in a dog.
Comments
Acanthamoeba spp are ubiquitous, free-living amoeba that can be either opportunistic or nonopportunistic pathogens. They have a 2-stage life cycle, existing as a trophozoite or a dormant cyst.4 They can cause infections in humans and other animals, and individuals with compromised immune systems are particularly susceptible.5,6 In humans, Acanthamoeba spp can cause keratitis (nonopportunistic infection), granulomatous encephalitis, cutaneous lesions, and sinus infections. Most infections with free-living amoeba are fatal and are usually only diagnosed after death.7 Acanthamoeba spp can also harbor viruses, yeasts, protists, and pathogenic bacteria, which can result in development of concurrent infections in susceptible hosts.4
To the authors’ knowledge, this is the first reported case of systemic acanthamoebiasis in a dog in the Caribbean. Overall, there have only been a few reported cases of Acanthamoeba infection in dogs, with systemic involvement (as evident in the dog of the present report) described in almost every instance.8–13 In Acanthamoeba-infected dogs with neurologic signs that were examined at appropriate facilities, analysis of CSF samples has revealed trophozoites.14 For the dog of the present report, the organisms were notably detected in association with areas of anterior uveitis, which has not been previously reported but can be attributed to systemic involvement.
A rapid clinical course is a common feature of amoebic encephalitis in nonhuman species.8 The disease affects primarily young and immunosuppressed dogs.14 Systemic acanthamoebiasis has been detected in dogs infected with canine distemper virus,9,15 and it has been proposed that the viral infection increased those dogs’ susceptibility to infection with Acanthamoeba spp. The dog of the present report was only 6 months old and did not have any previous history of illness to indicate immunosuppression. Therefore, age-related susceptibility was a likely contributing factor to infection in this case.
In most cases of granulomatous amoebic encephalitis, organisms are inhaled through the nasal passages or are introduced through skin lesions. The dog of the present report had recent access to a river and a beach as well as frequent contact with plant soil, all of which are potential sources of infection.1 Spread of the organisms through the CNS is thought to occur hematogenously or directly through the olfactory neuroepithelium.1 The organism is likely to also have a respiratory route of transmission in humans, and immunocompromised people are also more likely to develop infection.4 This dog had visited a popular beach prior to becoming clinically ill; therefore, investigation to determine the source of infection was of public health concern. Although the disease is rare, systemic acanthamoebiasis should remain a differential diagnosis in animals with signs of encephalitis that are unresponsive to conventional antimicrobial treatments.
Acknowledgments
Supported by The University of the West Indies Campus Research and Publication Fund.
The authors thank Maggie Mootoosingh and Gerald Chandoo for histologic assessments.
Footnotes
Infectious Diseases Pathology Branch, Division of High-Consequence Pathogens and Pathology, National Center for Emerging and Zoonotic Infectious Diseases, CDC, Atlanta, Ga.
Free-Living and Intestinal Ameba Laboratory, Waterborne Disease Prevention Branch, Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC, Atlanta, Ga.
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