JPC SYSTEMIC PATHOLOGY
Signalment (JPC #2416085): Adult female African clawed frog (Xenopus laevis)
HISTORY: This frog was submitted for necropsy. The frog had become thin and lethargic and had an area of brown discoloration on its dorsum.
MICROSCOPIC DESCRIPTION: Skin: The epidermis is diffusely thickened up to four times normal with acanthosis and spongiosis. Multifocally within the epidermis there are numerous adult aphasmid nematodes, which measure up to 100 um in diameter with a thin cuticle, a stichosome, inapparent musculature and bacillary bands, a uterus containing many eggs, and an intestine lined by multiple cuboidal cells. Frequently, the epidermis is expanded by a cystic space (tunnel) that contains degenerating nematodes and cellular debris. Occasionally basal keratinocytes are swollen with a clear vacuole (hydropic degeneration) and rarely cells within the stratum spinosum are rounded with intensely eosinophilic cytoplasm (dyskeratosis). Occasionally, granulocytes transmigrate the epidermis, and there are a few lymphocytes and plasma cells in the dermis. There is orthokeratotic hyperkeratosis with few degenerateing nematode fragments within the stratum corneum. There is a focal area of sub-basilar erosion where the epidermis has lifted off of the basement membrane, and the remaining cleft contains a moderate amount of homogenous eosinophilic to fibrillar fluid (serum, fibrin).
MORPHOLOGIC DIAGNOSIS: Skin: Hyperplasia, epidermal, diffuse, moderate, with viable and degenerating intraepithelial aphasmid nematodes, and orthokeratotic hyperkeratosis, African clawed frog (Xenopus laevis), amphibian.
ETIOLOGIC DIAGNOSIS: Cutaneous capillariasis
ETIOLOGY: Pseudocapillaroides xenopi
ETIOLOGY SYNONYMS: Capillaria xenopodis
CONDITION SYNONYMS: Flaky skin disease, gray skin disease
- Common parasite of wild caught and captive bred clawed frogs, including those in research colonies
- Causes a wasting syndrome characterized by anorexia, changes to skin color and excessive epidermal desquamation which often results in death
- May be asymptomatic until stressed
- Parasites typically found in dorsal skin; ventral lesions usually associated with bacterial infection from secondary invaders
- Not completely understood but thought to be direct, and completed within the epidermis
- Transmission via ingestion of eggs in desquamated skin or through autoinfection
- Epithelial damage predisposes to secondary bacterial infection, which may progress to septicemia
- In amphibians, the skin is important in maintaining hydration, osmoregulation, thermoregulation, and respiration
- Dependent on thymus-mediated immune response
TYPICAL CLINICAL AND GROSS FINDINGS:
- Dry, rough, flaky, dull gray skin usually over the dorsum with varying degrees of ventral erythema
- Excess mucus, ulceration, and sloughing of the skin
- Small (2-4mm x 1mm) white, sigmoid shaped nematodes in mucus overlying epidermis
- Anorexia and emaciation
- Gradual onset, may take up to 12-18 months after infection for clinical signs to develop
TYPICAL MICROSCOPIC FINDINGS:
- Nematode adults and larvated eggs within epidermal tunnels and subepidermal clefts
- Adults: 100-200u diameter, thin cuticle, inapparant musculature, esophagus with stichosome glands, hypodermal/bacillary bands, lacks sensory papillaeFemales are twice as large as males, and contain embryonated and larvated eggs within uteri
- Males have a caudal spiculeEggs are thin-walled with bipolar plugs and contain embryonated larvae
- Acanthosis, parakeratosis and dyskeratosis
- Variable granulocytic and monocytic inflammation surrounding parasites and within the superficial dermis
- Epidermal ulcerations and erosions
- Degenerate parasites within the superficial epidermis, viable adults within the deeper layers
Granular degeneration of dermal glands in severe cases
ADDITIONAL DIAGNOSTIC TESTS:
- Direct examination of the skin of a live frog under dissecting microscope
- Microscopic examination of skin scrape or wet smear of desquamated skin
- Aeromonas hydrophila (Red-leg)—causes acute septicemia, cutaneous hemorrhages and death; common secondary invader
- Basidiobolus ranarum—broad, irregular, rarely septate Zygomycete
- Batrachochytrium dendrobatidis (chytridiomycosis)—causes cutaneous erythema and orthokeratotic hyperkeratosis; fungal thalli have distinct discharge papillae
- Chromomycosis—dematiaceous fungi of the genera Fonsecaea, Phialophora, Cladosproium, Rhinocladiella and Scolecosbasidium cause disseminated disease in amphibians and have dark-brown hyphae or thick walled muriform cells (sclerotic bodies)
None (Pseudocapillaroides xenopi is the only epidermal nematode of South African clawed frogs)
Other nematodes that localize in the epithelium include:
- Trichosomoides crassicauda in urothelium of rats
- Gongylonema in the esophagus of ruminants, does not elicit an inflammatory response; in the tongue and infrequently the esophagus of nonhuman primates and small mammals where it often elicits considerable inflammation
- Anatrichosoma in the nasal mucosa of nonhuman primates
- Eucoleus boehmi in the nasal mucosa of dogs
- Cunningham AA, Sainsbury AW, Cooper JE. Diagnosis and treatment of a parasitic dermatitis in a laboratory colony of African clawed frogs (Xenopus laevis). Vet Rec.138:640-642.
- Feldman SH, Ramirez MP. Molecular phylogeny of Pseudocapillaroides xenopi (Moravec et Cosgrove 1982) and development of a quantitative PCR assay for its detection in aquarium sediment. Jour Am Assoc Lab Animal Science 2014; 53(6): 668-674.
- Wright KM, Whitaker BR. Amphibian Medicine and Captive Husbandry. 1st ed. Malabar, FL: Krieger Publishing Company; 2001:210, 436-437.