Dogs

Sporotrichosis in dogs, as in humans, is an essentially a benign disease, despite rare osteoarticular and disseminated forms that can develop.35,94 The skin lesions may be solitary or multiple and more frequently located on the head, especially on the nose (Fig. 61-2), followed by the limbs and thorax. In a retrospective study involving 44 dogs, the duration of dermatologic lesions ranged from 2 to 48 weeks.⁸⁸ The cutaneous lymphatic form was only observed in 3 (6.8%) dogs and was usually associated with regional lymphadenomegaly. Nasal mucosal involvement was present in 9 (20.5%) dogs, with isolated mucosal lesions being detected in 3 (6.8%).

Cats

Cats are highly susceptible to S. schenckii infection, and the evolution of the disease is usually severe, unlike that which occurs in other species. Presumably because of their behavior, a higher prevalence of disease is found in adult male and sexually intact animals. The spectrum of signs ranges from subclinical infection, to a single skin lesion with spontaneous regression, to fatal systemic forms due to hematogenous dissemination. The most frequent clinical forms are multiple skin and mucosal (conjunctival, nasal, oral, or genital) lesions (Figs. 61-3 and 61-4). In addition to skin and mucosal nodules and ulcers, lymphangitis, regional lymphadenitis, and extensive zones of necrosis that expose muscle and bone can be observed.⁸⁷ The most affected body parts are the head, especially the nose and ears, tail, and hindlimbs. In a retrospective study involving 347 cats with sporotrichosis, the cutaneous lymphatic form was observed in only 19.3% of cats, whereas multiple skin lesions occurred in 39.5% of cases and involvement of mucosa of the upper respiratory and digestive tracts occurred in 34.9%.⁸⁷ Respiratory signs observed in 44.4% of cats (including three cats without skin lesions) were the most frequent extracutaneous signs.⁸⁷ Although no clinical signs suggestive of a septic syndrome were observed, S. schenckii organisms were commonly cultured both from the peripheral blood of cats with widespread cutaneous lesions and from cats in good general condition with localized lesions.^85,86

Some authors believe that the gravity of feline sporotrichosis may be related to immunodepression caused by co-infection with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV). However, there are few reports to support this theory.¹⁵ No significant differences in clinical signs or laboratory findings were found between cats that were co-infected with sporotrichosis and FIV/FeLV and cats that were not.⁸⁷ In cats with systemic sporotrichosis, dissemination via blood was not associated with the immunodeficiency caused by FIV/FeLV.⁸⁴ Additionally, in contrast to the disease in humans, the high frequency of sporotrichosis without apparent immunosuppression supports the assumption that this disease develops in a different manner in cats.⁸⁷

Sample Collection

Samples should be collected according to clinical conditions and features of the lesions and should include swab specimens of the nasal cavities, exudative lesions, and purulent or seropurulent content aspirated from nonulcerated abscesses, as well as incisional skin biopsy specimens.79,87 The swabs should be seeded on Sabouraud’s agar and Mycosel agar, where possible, and then sent to the laboratory, at room temperature for mycologic examination. The skin biopsies should be obtained from borders of active lesions with a 3- to 4-mm surgical punch. Two tissue samples can be collected, one fixed in 10% buffered formalin for histopathology and immunohistochemical tests and the second kept in a sterile glass flask with sterile saline and antibacterial agent and transported at refrigerated temperatures (4° C) to a diagnostic laboratory for fungal culture.⁴⁶ The best results for fungal isolation are obtained from skin biopsies from cats or dogs.^87,88 For cytologic examination, suitable samples can be obtained by impression smears on glass slides for microscopy of ulcerated skin or exudates, smears of swab specimens, tissue samples from biopsy, or skin scrapings, and also by aspiration of abscesses and nodules.^79,8,8 To diagnose dissemination, whole blood can be directly seeded in blood culture flasks.⁸⁶ Another sample useful for the diagnosis of systemic involvement in vivo is bronchoalveolar lavage.⁴⁶ At necropsy, tissue samples can be collected to mycology, cytology, histopathology, and immunohistochemical testing as previously described.

Cytologic Findings

This technique is very useful, and relatively accurate, for preliminary diagnosis of sporotrichosis.^65a Smears on glass slides stained with Wright’s or Romanowsky-type, or Gomori’s methenamine or Grocott’s silver stain (GSS) will permit good visualization of the yeast cells.^10,79 With Wright’s or Romanowsky-type staining, a positive cytologic exam suggestive of S. schenckii shows cigar-shaped to oval or round budding yeastlike organisms of 3 µm to 5 µm by 5 µm to 9 µm, with blue cytoplasm and a single, round, pink nucleus surrounded by a nonstaining cell wall (Fig. 61-5).^34,99 These organisms are seen mainly within cytoplasm of macrophages, and in neutrophils and also extracellularly.^6,99 Smears prepared from lesions from cats with sporotrichosis usually contain high numbers of yeastlike cells, although in some cases the fungi may be difficult to visualize. In dogs with sporotrichosis, organisms are often scarce.^6,79 Other organisms confused with S. schenckii include nonencapsulated forms of Cryptococcus neoformans and Histoplasma capsulatum.¹⁰

Fungal Isolation

The clinical material should be inoculated onto Sabouraud’s dextrose agar with chloramphenicol and Sabouraud’s dextrose agar with chloramphenicol plus cycloheximide (Mycosel agar), and incubated at 25° C to 30° C. Mycelial growth of S. schenckii can be observed in 3 to 5 days. Typical colonies are initially cream-colored, smooth, and moist, gradually becoming dark brown or black, and have a filamentous texture. However, much variation among isolates and even among subcultures of the same isolate can be observed. Various culture media can also influence the coloration and texture of the colonies. For blood culture, flasks with 3 mL of whole blood are incubated at room temperature in an inverted position. One milliliter of sediment is aspirated on the second and seventh days and inoculated into flasks containing agar brain–heart infusion (BHI) medium, incubated at 25° C, and observed for 6 weeks for fungal growth. Suspected isolates are subcultivated onto potato dextrose agar at 25° C. Confirmation of the identification depends on the morphologic characteristics of the mycelial form and its conversion to the yeast form on blood agar or BHI plus yeast extract agar when incubated at 37° C. The microscopic characteristics of the mycelial form are slender, hyaline, septate, and branched hyphae measuring 1.5 to 2 µm in width. The conidia rise isolated directly from the hyphae or in flowerlike groups in the apex of thin conidiophores, with sympodially arranged denticles that bear conidia in a flowerlike arrangement. Newly isolated cultures usually produce two types of conidia: either hyaline with thin walls and/or dark brown with thick walls. The intensity of the color of a colony is directly proportional to the number of dark brown thick-walled conidia. The yeast phase can be produced in vitro by culturing mycelia or conidia on rich media such as BHI agar at 37° C. The yeast parasitic phase is pleomorphic, showing oval or cigar-shaped cells and exhibiting single or multiple buds with elongated morphology.

Pathologic Findings

Apart from the cutaneous and mucosal lesions described in the clinical findings, the main gross lesions in cats are small whitish dots measuring approximately 1 mm in diameter on pleural surface and parenchyma of the lungs and enlargement of lymph nodes.⁸⁴ Histopathologic findings observed on hematoxylin and eosin stained tissue sections are not disease specific and can be related to other pathogenic fungi and protozoans.^58a Therefore, specific histochemical staining techniques such as GSS and periodic acid–Schiff (PAS) are needed to identify the structure of S. schenckii.^18,87 In addition, serial histologic sections of the same tissue sample may markedly improve the possibility to detect the fungus. The yeast cells suggestive of S. schenckii are oval or cigar-shaped, range in size from 4 µm to 6 µm, generally exhibit a single bud with a narrow base, and are dark staining in GSS and pink in PAS (Fig. 61-6). Pseudohyphae can be seen.¹⁰¹ Histologically, the cutaneous lesions of feline and canine sporotrichosis are characterized by ulcerative and variable but usually intense pyogranulomatous inflammatory reaction in dermis, which can reach the paniculus and subjacent skeletal muscles. Yeastlike cells can be seen inside macrophages and neutrophils and extracellularly in purulent exudate or inflamed areas. In cats with sporotrichosis, the number of Sporothrix organisms is usually high and can be detected in 62% of histopathologic examinations of skin biopsies.^19,87,87 However, in dogs with sporotrichosis, the fungus is visualized in tissue specimens in 17% to 42% of cases, and usually the yeastlike forms are rare, as in humans. The GSS-stained specimens showed a higher sensitivity compared to those stained with PAS in the histopathologic diagnosis of canine sporotrichosis, and 58.3% of positively diagnosed cases had a maximum of five fungal elements.⁵⁸ Asteroid bodies are not reported, and multinucleated giant cells are infrequent in dogs and cats compared to their occurrence in humans.^58,79,79 In addition, there is an inverse correlation between the presence of granuloma and the histopathologic finding of S. schenckii in cats and dogs.^58,87 The frequency of granulomas is low in cats, about 12% in skin biopsy specimens. However, in dogs, as in humans, well-organized granulomas are frequent and present in 38.2% to 45.2% of animals.^{5,58,79,87,88} Unlike dogs, systemic involvement is common in cats with mild to moderate mixed inflammatory infiltrate of polymorphonuclear and mononuclear cells and yeastlike structures observed histologically in lungs, liver, spleen, eye, kidney, adrenal glands, and lymph nodes.^18,84,84

Immunohistochemical Testing

Immunohistochemistry using fluorescein-conjugated antibodies can be used to identify the yeastlike cells of S. schenckii in the paraffin-embedded tissue from infected animals. Unstained histologic sections are deparaffinized and labeled with fluorescein isothiocyanate–conjugated polyclonal rabbit S. schenckii antiglobulin. This technique provides a quick and specific diagnosis of sporotrichosis and is very useful when multiple fungal cultures are negative and do not correlate with the cytologic and histopathologic findings.34,101

Genetic Detection

The polymerase chain reaction has been used to detect S. schenckii infection in biopsy specimens from infected cats, with good results.31,37 However, this technique has not been applied to a clinical test population and is not currently commercially available.

Serologic Testing

Testing for serum antibodies to the organism such as whole yeast agglutination, latex agglutination, enzyme-based immunoassay, and indirect fluorescent antibody testing can be used20a,100; however, these techniques do not distinguish exposure from active infection, and there may be cross-reactivity with other fungal organisms.