Etiology

Pythium insidiosum³¹ is an aquatic pathogen belonging to the class Oomycetes in the kingdom Stramenopila (Chromista). Pythium species and other oomycetes differ from true fungi in producing motile, flagellate zoospores and having cell walls that contain cellulose and β-glucan but not chitin. In addition, ergosterol is not an important component of the oomycete cell membrane. Furthermore, members of the genera Pythium and Lagenidium are sterol auxotrophs—they incorporate sterols from their environment rather than producing them. Other members of the genus Pythium are plant pathogens of some economic importance. Based on molecular studies, the pathogens most closely related to oomycetes are the Prototheca spp.⁹³ The infective stage of P. insidiosum is thought to be the biflagellate aquatic zoospore, which is released into warm water environments and likely causes infection by encysting in damaged skin or GI mucosa.⁵⁸ Although the specific factors that favor sporulation of P. insidiosum in the environment are unknown, some evidence suggests that certain types of plant material may be involved in the life cycle as a natural host.¹²¹

Epidemiology

In the United States, pythiosis is encountered most often in the Gulf Coast states but has been recognized with some frequency in animals living as far north as New Jersey, Virginia, Kentucky,¹¹³ southern Illinois, and southern Indiana and as far west as Oklahoma, Missouri, and Kansas. Surprisingly, GI pythiosis has also been confirmed in a number of dogs living in Arizona⁵⁹ and California.¹² Globally, pythiosis is most often encountered in Southeast Asia (especially Thailand and Indonesia), eastern coastal Australia, New Zealand, and South America but has also been recognized in Korea, Japan, and the Caribbean.

Pythiosis is most often identified in young, large-breed male dogs and is especially common in outdoor working breeds such as Labrador retrievers.⁴⁴ Affected dogs have been identified more often in the fall, winter, and early spring than in the late spring and summer months.¹²² In cats, specific breed and sex predilections have not been apparent in the few described infections. However, of 11 cats with cutaneous pythiosis diagnosed through the authors’ laboratory, six were younger than 10 months old, with an age range of 4 months to 9 years.⁶⁰

Infected dogs often have a history of recurrent exposure to warm, freshwater habitats. However, some infections develop in suburban house dogs with no known access to a standing body of water. Because P. insidiosum zoospores have been shown to have an affinity for damaged skin,¹¹⁶ it is likely that animals with cutaneous wounds or injury to GI mucosa are more likely to become infected. However, solid epidemiologic evidence to support this assumption is lacking. Other risk factors for the development of pythiosis have not been identified. Affected animals are typically immunocompetent and otherwise clinically healthy.

Clinical Findings

Clinical signs caused by P. insidiosum infection are the result of cutaneous or GI lesions, or of disease extension into adjacent tissues or regional lymph nodes. Cutaneous and GI lesions are rarely found together in the same patient. Systemic dissemination of pythiosis has only been described once.⁴⁵

Cutaneous pythiosis in dogs and cats typically causes nonhealing wounds and invasive masses that contain ulcerated nodules and draining tracts (Figs. 65-1 and 65-2).^45,174 Affected dogs are usually taken to the veterinarian because of solitary or multiple cutaneous or subcutaneous lesions involving the extremities, tailhead, ventral neck, perineum, or medial thigh. When present, regional lymphadenomegaly often reflects extension of infection rather than just reactive inflammation. Lesions in 11 cats with cutaneous pythiosis evaluated through the authors’ laboratory have included subcutaneous masses (some of which were highly invasive) in the inguinal, tailhead, or periorbital regions, and draining nodular lesions or ulcerated plaquelike lesions on the extremities (see Fig. 65-1), occasionally centered on the digits or footpad. Two cats with pythiosis had large subcutaneous masses affecting the extremities and cranioventral thorax but no cutaneous involvement.¹⁷⁴ In a third case, P. insidiosum infection resulted in nasal cavity and nasopharyngeal lesions and bilateral retrobulbar masses.¹³

GI pythiosis in dogs is characterized by severe segmental transmural thickening of the stomach, small intestine, colon, rectum, or (although rarely) the esophagus or pharyngeal region (Fig. 65-3).^{44,75,122,136} Mesenteric lymphadenomegaly is common and occasionally observed without accompanying GI tract lesions. The gastric outflow area, duodenum, and ileocolic junction are the most frequently affected portions of the GI tract, and it is not uncommon to find two or more segmental lesions in the same patient. Inflammation in affected regions is typically centered on the submucosa, with variable mucosal ulceration and occasional extension of disease through serosal surfaces, resulting in adhesion formation and peritonitis. Involvement of the mesenteric root may cause severe enlargement of mesenteric lymph nodes, which are often 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.¹³⁷ In addition, infection may extend from the GI tract into contiguous tissues such as the pancreas and uterus.^44,122 In one report, a prostatic abscess caused by P. insidiosum was observed in a dog with an adjacent colonic lesion.⁸⁵ Clinical signs associated with GI pythiosis include weight loss, vomiting, diarrhea, hematochezia, or all of these symptoms. Physical examination abnormalities are a very thin body and a palpable abdominal mass. Signs of systemic illness such as lethargy or depression are not typically present unless intestinal obstruction, infarction, or perforation occurs. Similar to clinical signs observed in dogs, those of GI pythiosis in cats have been attributed to partial intestinal obstruction, palpable abdominal masses, and mesenteric lymphadenomegaly.¹⁴⁵

Diagnosis

The definitive diagnosis of pythiosis has traditionally been challenging because histopathologic features are not specific and the organisms are not often successfully cultured. However, numerous highly specific serologic, immunohistochemical, and molecular-based tools for the diagnosis of pythiosis have been developed. These assays have made the definitive diagnosis of pythiosis possible in the majority of affected animals, even when culture is unsuccessful. In addition, they provide clinicians with an opportunity to make a diagnosis earlier in the course of disease, when lesions may be more amenable to surgical or medical therapy.

Pathologic Findings

A presumptive diagnosis of pythiosis, lagenidiosis, or zygomycosis can occasionally be established cytologically if lesions are accessible (Fig. 65-4). Cytologic examination of exudate from draining tracts, impression smears made from ulcerated skin lesions, and fine-needle aspirates of enlarged lymph nodes often reveal pyogranulomatous, suppurative, or eosinophilic inflammation (or all of these). In addition, macerated tissue fixed in 10% potassium hydroxide (KOH) may be examined microscopically for the presence of typical wide, poorly septate, branching hyphal elements.

Histologic findings associated with pythiosis are characterized by eosinophilic granulomatous to pyogranulomatous inflammation with fibrosis. Affected tissues typically contain multiple foci of necrosis surrounded and infiltrated by neutrophils, eosinophils, and macrophages. In addition, discrete granulomas composed of epithelioid macrophages, plasma cells, multinucleate giant cells, and fewer neutrophils and eosinophils may be observed.44,45,45 Vasculitis is occasionally present. Organisms are usually found within areas of necrosis or at the center of discrete granulomas.¹²² Although P. insidiosum hyphae are difficult to visualize on sections stained with hematoxylin-eosin (H&E) stain, they may be identified as clear spaces (hyphal ghosts) surrounded by a narrow band of eosinophilic material.¹²² Hyphae are easily visualized in sections stained with Gomori’s methenamine silver (GMS) but not with periodic acid–Schiff. They are broad (mean, 4 µm; range, 2 to 7 µm), rarely septate, and occasionally branching.^45,122

Cutaneous pythiosis typically causes severe nodular to diffuse ulcerative dermatitis and panniculitis. Because areas of inflammation are most often found in the deep dermis and subcutis, wedge biopsies are preferred to punch biopsies when pythiosis is suspected. Similarly, because inflammation in GI pythiosis centers on the submucosal and muscular layers rather than mucosa and lamina propria,¹²² the diagnosis of pythiosis may be missed on endoscopic biopsies that fail to reach deeper tissues. Therefore, pythiosis should be considered as a diagnostic possibility when endoscopic biopsies reveal eosinophilic or pyogranulomatous inflammation without identification of an etiologic agent.

Therapy

Aggressive surgical resection is the treatment of choice for pythiosis. Because it provides the best opportunity for cure, complete excision of infected tissue should be pursued whenever possible. When cutaneous lesions are limited to a single distal extremity, amputation is recommended. In animals with GI pythiosis, segmental lesions should be resected with 3- to 4-cm margins whenever possible. Despite the fact that mesenteric lymphadenomegaly is almost always present, P. insidiosum hyphae are often absent in enlarged mesenteric nodes. Therefore, the presence of nonresectable mesenteric lymphadenomegaly should not dissuade the surgeon from pursuing complete resection of a segmental bowel lesion. In this situation, enlarged lymph nodes should be biopsied and cultured for prognostic information. Unfortunately, most dogs with GI pythiosis are not taken to the veterinarian until late in the course of disease, when complete excision is not possible. In addition, the anatomic location of the lesion may prevent complete surgical excision when the esophagus, gastric outflow tract, rectum, or mesenteric root is involved.

Local postoperative recurrence of pythiosis is common and can occur either at the site of resection or in regional lymph nodes. For this reason, combination medical therapy with itraconazole (ITZ) and terbinafine is recommended for at least 2 to 3 months after surgery (Table 65-2). To monitor for recurrence, ELISA serology should be performed before and 2 to 3 months after surgery. In animals that have had a complete surgical resection and do not have a recurrence of disease, serum antibody levels drop significantly within 3 months of surgery.⁶⁵ If this occurs, medical therapy can be discontinued. If antibody levels remain elevated 2 to 3 months after surgery, medical therapy should be continued, with periodic reevaluation of ELISA serology.

Medical therapy for pythiosis has traditionally been unrewarding, likely because ergosterol (the target for most currently available antifungal drugs) is generally lacking in the oomycete cell membrane. Despite this fact, clinical and serologic cures have been obtained using medical therapy with ergosterol-targeting drugs in a small number of P. insidiosum–infected dogs, as well as with an infection in a 2-year-old child.¹⁶⁰ In the authors’ experience, a small number of dogs with nonresectable or partially resected GI pythiosis have responded to ITZ, AMB lipid complex, or a combination of ITZ and terbinafine (see Table 65-2). Although the overall percentage of animals responding is low (less than 20%), based on our subjective observations, the combination protocol seems superior to ITZ or AMB alone.

Caspofungin, the first antifungal in the newly developed echinocandin class of β-glucan synthase inhibitors to gain United States Food and Drug Administration approval, has the potential to be a more effective drug for the treatment of oomycosis because of the large amount of β-glucan present in the oomycete cell wall. However, the susceptibility of the form of β-glucan synthase present in P. insidiosum to inhibition by caspofungin is unknown. Results of two studies were moderate caspofungin-induced inhibition of in vitro growth of P. insidiosum, even at high concentrations.18,139 In rabbits, experimentally infected with P. insidiosum and treated with caspofungin intraperitoneally for 20 days, measurable disease progression slowed during treatment, but lesions quickly resumed growth when the drug was discontinued.¹³⁹ Considered together with the extremely high cost of the drug itself, these results are not encouraging regarding the potential clinical usefulness of caspofungin for the treatment of pythiosis. See the Drug Formulary in the Appendix and Chapter 55 for more information on caspofungin.

A vaccine derived from soluble mycelial antigens and secreted exoantigens of P. insidiosum has been used successfully to treat cutaneous granulomas in horses and vasculitis in people.82,173 Unfortunately, although controlled trials have not been completed, the efficacy of Pythium vaccines in dogs appears to be poor, and clinical improvement has not been observed in any of the authors’ patients. The limited published information available regarding the efficacy of Pythium vaccines in dogs is anecdotal.¹¹⁷ In one canine case report that suggested a vaccine-related therapeutic effect,⁷⁷ tissues were submitted to Louisiana State University that were obtained after the initial diagnostic wedge biopsies but before vaccine administration. The tissue culture was negative for oomycetes and failed to show any hyphae in multiple GMS-stained sections, suggesting that the disease may have been resolving before the vaccine administration.⁵⁷ Interestingly, the authors of this chapter are aware of one additional dog in which lesions associated with cutaneous pythiosis resolved completely without additional therapy after incomplete surgical resection.