THE MERCK VETERINARY MANUAL
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Oomycosis

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Oomycosis is caused by pathogens in the class Oomycetes. These organisms are not true fungi but are aquatic pathogens in the kingdom Stramenopila. They are more closely related to algae than fungi but cause disease that closely resembles zygomycosis (see Zygomycosis). Organisms of significance in veterinary medicine include various species of Saprolegnia and Achyla (eg, S diclina), which are the common agents of cutaneous disease in fishes; Pythium insidiosum, the cause of a cutaneous and subcutaneous mycosis in horses (bursatti, swamp cancer, leeches), a cutaneous, subcutaneous, and GI disease in dogs, and a cutaneous and paranasal disease of cats; and Lagenidium spp, the cause of cutaneous and systemic lesions and large-vessel aneurysms in dogs. Pythiosis is a common disease of domestic animals in some tropical and subtropical areas of the world. In dogs, pythiosis is most often encountered in southeast Asia, eastern coastal Australia, South America, and in the USA, especially along the Gulf coast. In the USA, the disease most often is seen in fall and winter months.

In horses, lesions are large, roughly circular, granulomatous, ulcerated, fistulated nodules, or subcutaneous swellings with yellow-gray necrotic masses or cores. The lesions are most common on the legs (especially the lower limbs), abdomen, chest, and genitalia. Distribution of lesions is attributable to the aquatic nature of the organism. The lesions are pruritic, discharge a mucosanguineous exudate, and often are self-traumatized. The granulomas contain firm, yellowish, coralliform masses of necrotic tissue known as “kunkers,” which may be removed intact. Kunkers are foci of coagulative necrosis in vessels that have become sequestered from the surrounding tissue; they contain broad, branching aseptate hyphae and are 1–10 μm in diameter. Bone involvement may be a feature of chronic pythiosis. Enteric pythiosis in horses is characterized by fibrosing and stenotic GI lesions containing intralesional foci of caseous material and fungal hyphae.

Specimens removed at surgery or necropsy consist of fibrous tissue with irregularly spaced, firm, focal areas of necrosis that vary in size and color. Microscopically, alterations vary from foci of acute exudative inflammation with numerous eosinophils to a granulomatous reaction with sequestered areas of necrosis and a framework of hyphae that are thick-walled, branching, and slightly irregular in width.

GI and cutaneous forms of pythiosis are seen and are characterized by severe granulomatous and eosinophilic inflammation. P insidiosum infection is seen most often in the GI tract of young adult dogs, especially Labrador Retrievers. The stomach, proximal small intestine, and ileocolic junction are affected most commonly, but any part of the intestine, esophagus, and colon can be diseased. Clinical signs include vomiting, weight loss, and anorexia. The weight loss can be severe, but affected dogs usually do not appear systemically ill until late in the disease. The lesions are typically characterized by severe transmural thickening of the gastric or intestinal wall, with mesenteric lymphadenopathy in which the lymph nodes are embedded in a large, firm granulomatous mass involving the surrounding mesentery. Bowel ischemia, infarction, or acute hemoabdomen may develop due to extension of disease into mesenteric vessels. Enteric pyogranulomas typically consist of necrotic foci infiltrated and surrounded by neutrophils, eosinophils, epithelioid macrophages, plasma cells, and multinucleated giant cells. Etiologic agents may not be apparent on sections stained with H&E. Sections stained with Gomori methenamine silver show branching, rarely septate hyphae.

Cutaneous pythiosis is typified by nonhealing wounds, invasive masses, and ulcerated nodules with draining tracts. The extremities, tail head, ventral neck, or perineum are affected most commonly. Pythiosis in cats is rare and typified by either cutaneous or nasopharyngeal lesions.

Lagenidiosis is an oomycotic infection of dogs characterized by progressive, multifocal cutaneous and subcutaneous lesions, most often affecting the extremities, mammary region, perineum, or trunk. Regional lymphadenopathy is common. At least two species of Lagenidium have been shown to affect dogs. One of the Lagenidium spp causes more aggressive cutaneous infection with systemic involvement, whereas a less aggressive Lagenidium spp tends to cause more slowly progressive cutaneous disease. Cutaneous lesions are characterized as firm dermal or subcutaneous nodules, or as ulcerated, thickened, edematous areas of deep cellulitis with regions of necrosis and numerous draining tracts. In contrast to the clinical course of cutaneous pythiosis, in dogs with aggressive Lagenidium spp, involvement at distant sites is often seen. Thoracic and abdominal lymph nodes, lungs, and especially great vessels may be affected. Animals with great vessel or sublumbar lymph node involvement typically have cutaneous or subcutaneous lesions on the hindlimbs and often develop hindlimb edema. Great vessel aneurysms may acutely rupture, resulting in hemoabdomen and sudden death.

In horses, lesions of pythiosis are similar to those of zygomycosis (see Zygomycosis) and may be confused with cutaneous habronemiasis (see Cutaneous Habronemiasis), excessive granulation tissue, and certain equine neoplasms. In pythiosis, the necrotic cores are distinct from the surrounding tissue, and a seropurulent discharge from the sinus tracts is prominent. The lesions contain irregular, branching (at right angles), rarely septate hyphae, 4–8 μm in diameter.

In dogs, diagnosis can be made by isolation of P insidiosum from infected tissues. Culture identification or PCR has been used. Immunoblot serology for detection of anti-P insidiosum antibodies is available and appears to be both sensitive and specific. Immunoblot serology for detection of anti-Lagenidium antibodies in canine serum can provide a presumptive diagnosis of lagenidiosis but must be interpreted in conjunction with results of serologic testing for P insidiosum infection because of the potential for cross-reactivity in serum from dogs with pythiosis. The histologic features of lagenidiosis are similar to those of pythiosis and zygomycosis. However, Lagenidium hyphae are usually much larger and visible on H&E-stained tissues. Definitive diagnosis of lagenidiosis and pythiosis is best made by culture and PCR identification.

The prognosis for pythiosis or lagenidiosis is poor if surgical excision cannot be done. Complete surgical excision is the treatment of choice, but the disease is often too extensive at the time of diagnosis to allow complete resection. In animals with lesions limited to a single distal extremity, amputation is recommended. Medical therapy for pythiosis should include itraconazole (10 mg/kg/day) and terbinafine (5–10 mg/kg/day). Steroids may decrease inflammation and improve clinical signs in the short term. Treatment with amphotericin B lipid complex can also be attempted. Approximately 20% of dogs with pythiosis respond to longterm antifungal therapy. Lagenidiosis appears to be poorly responsive to medical therapy. In horses, the prognosis is guarded, and timely recognition and treatment are essential for successful management. Factors that influence prognosis include size and site of lesion and duration of infection. Small lesions of short duration that have not invaded critical structures usually respond best to treatment. Surgical excision, immunotherapy, or a combination of both may be effective. Immunotherapy consists of a series of intradermal or SC injections of killed, sonicated, whole-cell hyphal antigens or precipitated soluble antigens of the causative fungus. Subcutaneous abscesses at the sites of injection, osteitis, or deep-seated laminitis may be a complication of such therapy. Surgical removal plus systemic or local administration of amphotericin B may be a satisfactory treatment if the disease is localized.

Last full review/revision May 2014 by Joseph Taboada, DVM, DACVIM

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