Chapter 86: Pythiosis and Lagenidiosis

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Pythiosis and Lagenidiosis

Pythium insidiosum and Lagenidium spp., the causative agents of pythiosis and lagenidiosis, are pathogenic “water molds” in the class Oomycetes. Although they often are grouped with fungi because they grow on mycologic media and produce hyphal structures in tissue, they differ from true fungi in producing motile, flagellate zoospores; having cell walls that lack chitin; and having cell membranes that generally lack ergosterol. Both pythiosis and lagenidiosis cause lesions characterized by eosinophilic and pyogranulomatous inflammation associated with broad, irregularly branching, sparsely septate hyphae; as a result they often are confused with zygomycoses, which are infections caused by true fungi in the class Zygomycetes that produce similar lesions. Although the clinical syndromes associated with P. insidiosum infection have been described for many decades, Lagenidium spp. have been recognized as mammalian pathogens only since 1999.


P. insidiosum infection is a devastating and often fatal cause of gastrointestinal or cutaneous lesions in dogs and cats. In small animal patients, pythiosis is encountered most often in the southeastern United States but also has been identified in animals living in New Jersey, Virginia, Kentucky, southern Illinois, Indiana, Oklahoma, Missouri, Kansas, Arizona, California, and the Cayman Islands. Young large-breed dogs (especially outdoor working breeds such as Labrador retrievers) are infected most often. In cats, specific breed and sex predilections have not been observed in the few cases that have been reported to date. However, the development of cutaneous pythiosis in very young animals appears to occur more often in cats than in dogs. Of 27 cats with cutaneous pythiosis diagnosed through my laboratory in the past 8 years, 11 were younger than 1 year of age, and the age range was 4 months to 9 years.

The infective form of P. insidiosum is thought to be the motile biflagellate zoospore, which is released into aquatic environments and likely causes infection by encysting in damaged skin or gastrointestinal mucosa. Many dogs with pythiosis have a history of recurrent exposure to warm freshwater habitats. However, disease also is identified regularly in suburban house dogs with no history of access to lakes or ponds. Affected animals typically are immunocompetent and otherwise healthy.

Clinical Findings

Gastrointestinal Pythiosis

In dogs, gastrointestinal pythiosis typically results in severe segmental transmural thickening of the stomach, small intestine, colon, rectum, or rarely the esophagus. The gastric outflow area, proximal duodenum, and ileocolic junction are the most frequently affected locations, and it is not unusual to find two or more segmental lesions in the same patient. Mesenteric lymphadenopathy is common but most often represents reactive hyperplasia rather than infection. Involvement of the mesenteric root may cause severe enlargement of mesenteric lymph nodes, which typically are embedded in a single large, firm granulomatous mass that is palpable in the midabdomen. Extension of disease into mesenteric vessels may result in bowel ischemia, infarction, perforation, or acute hemoabdomen. In addition, infection in gastrointestinal tissues may extend into contiguous organs such as pancreas and uterus. Gastrointestinal pythiosis is rare in cats but was described recently in two young adult male cats with focal intestinal lesions that were amenable to surgical resection (Rakich et al, 2005).

Clinical signs associated with gastrointestinal pythiosis include weight loss, vomiting, diarrhea, and hematochezia. Physical examination often reveals a very thin body condition and a palpable abdominal mass. Signs of systemic illness such as lethargy or depression typically are not present unless intestinal obstruction, infarction, or perforation occurs. Laboratory abnormalities that may be associated with pythiosis include eosinophilia, anemia, hyperglobulinemia, hypoalbuminemia, and rarely hypercalcemia. Abdominal radiography and sonography usually reveal severe segmental thickening of the gastrointestinal tract, an abdominal mass, or mesenteric lymphadenopathy.

Cutaneous Pythiosis

Cutaneous pythiosis in dogs typically causes nonhealing wounds and invasive masses that contain ulcerated nodules and draining tracts, most often involving the extremities, tail head, ventral neck, or perineum. Unlike in dogs with gastrointestinal pythiosis, regional lymphadenopathy in dogs with cutaneous pythiosis usually reflects extension or postsurgical recurrence of infection rather than just reactive inflammation. Cutaneous and gastrointestinal lesions rarely are encountered together in the same patient.

Cats with pythiosis most often present with nasopharyngeal lesions; invasive subcutaneous masses in the periorbital, tail head, or inguinal regions; or draining nodular lesions or ulcerated plaquelike lesions on the extremities, sometimes centered on the digits or footpad. In contrast to dogs, cats with pythiosis often have firm, nodular, subcutaneous lesions without overlying cutaneous lesions or alopecia.


Cytologic and Histopathologic Analysis

In dogs with pythiosis, pyogranulomatous and eosinophilic inflammation often is apparent on cytologic evaluation of exudate from draining tracts, impression smears made from ulcerated skin lesions, or fine-needle aspirates of enlarged lymph nodes or thickened gastrointestinal tissues. Hyphae are observed occasionally, and their morphologic appearance (broad, rarely septate with tapered, rounded ends) in conjunction with a typical inflammatory response can provide a tentative diagnosis of pythiosis, lagenidiosis, or zygomycosis. Microscopic examination of macerated tissue that has been digested in 10% potassium hydroxide may be more likely to reveal hyphal elements than other cytologic specimens.

Histologically, pythiosis is characterized by eosinophilic pyogranulomatous inflammation. Affected tissues contain multiple foci of necrosis surrounded and infiltrated by neutrophils, eosinophils, and macrophages. In addition, discrete granulomas composed of epithelioid macrophages, plasma cells, multinucleated giant cells, and fewer neutrophils and eosinophils often are observed. Organisms typically are found within areas of necrosis or at the center of granulomas. Vasculitis is present occasionally. In gastrointestinal pythiosis inflammation centers on the submucosal and muscular layers rather than the mucosa and lamina propria. Therefore the diagnosis of pythiosis may be missed on endoscopic biopsy specimens that fail to reach deeper tissues. Similarly, disease in animals with cutaneous pythiosis typically is found in the deep dermis and subcutis, so that deep wedge biopsy rather than punch biopsy is required for optimal evaluation. P. insidiosum hyphae are not visualized routinely on hematoxylin and eosin–stained sections but may be identified as clear spaces surrounded by a narrow band of eosinophilic material. Hyphae are visualized readily in sections stained with Gomori’s methenamine silver but usually do not stain well with periodic acid–Schiff stain. Pythium hyphae are broad (mean, 4 µm; range, 2 to 7 µm), infrequently septate, and occasionally branching (usually at right angles).


Isolation of P. insidiosum from infected tissues is not difficult when appropriate sample handling and culture techniques are used. For best results, room temperature (i.e., not refrigerated) tissue samples should be wrapped in a saline-moistened gauze sponge and shipped at ambient temperature to arrive at the laboratory within 24 hours. Small pieces of fresh, nonmacerated tissue should be placed directly on the surface of vegetable extract agar supplemented with streptomycin and ampicillin (or an alternative selective medium) and incubated at 37° C (98.6° F). Growth typically is observed within 12 to 24 hours. Isolation of P. insidiosum from swabs of exudate collected from draining skin lesions generally is unsuccessful. Identification of P. insidiosum should be based on morphologic features; growth at 37° C; production of motile, biflagellate zoospores; and, if possible, species-specific polymerase chain reaction amplification (Grooters and Gee, 2002) or ribosomal RNA gene sequencing. Although production of zoospores is an important supporting feature for the identification of pathogenic Oomycetes, it is not specific for P. insidiosum. Species-specific PCR amplification also can be used to identify P. insidiosum DNA in fresh, frozen, or paraffin-embedded tissues.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Chapter 86: Pythiosis and Lagenidiosis

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