An 11-year-old boy with relapsed Ewing sarcoma complicated by pulmonary metastases was hospitalized in early September with fever, neutropenia, and worsening posterior thigh pain. He completed the most recent round of chemotherapy with topotecan, cyclophosphamide, and vincristine within 1 week before admission. Two days before admission, he began complaining of pain in his right upper posterior thigh. His father noted a small, less than 1 cm diameter, black, circular lesion in the area, thought to be a bruise. The patient denied recent trauma to his leg. Four days before hospitalization, he removed a small sliver from the sole of his left foot while visiting a local lake on the Iowa-Nebraska state borders. He waded, but did not swim in the lake. He also incurred mosquito bites on the lower extremities that day.
On the day of admission, the patient had a fever of 38.6°C, pulse of 128/minute, respiratory rate of 20/minute, and blood pressure of 117/64 mm Hg. He was a well-appearing male with alopecia. Physical examination was notable for a loose primary lower left canine (#22) with a purple hue to the underlying gum, normal heart and lung examinations, an accessed port in his central chest that was clean and intact, a liver edge palpable just below the costal margin, and normal extremities. The skin examination revealed a mosquito bite on each of his anterior shins, a small puncture site at the location of the sliver he incurred in the sole of his left foot without any surrounding erythema, and a 1.5-cm gray-black macular lesion on his posterior superior right thigh. Laboratory findings revealed a white blood cell count of 270/μL with 20% neutrophils (absolute neutrophil count 50/μL), 72% lymphocytes, and 8% monocytes, hemoglobin 10.2 g/dL, hematocrit 29.5%, and platelets 45,000/μL. He was treated with cefepime and vancomycin empirically for fever and neutropenia. During the next 24 hours in the hospital, the posterior, medial, upper thigh lesion grew to 3 cm in diameter. It remained macular and developed central, target-like clearing surrounded by a gray inner ring and black outer ring. An additional 1.5-cm ring of erythema encircled the lesion. It was not ulcerated, necrotic, or fluctuant, and had no associated drainage or visible disruption of the superficial skin. The antimicrobial regimen was changed to liposomal amphotericin B, gentamicin, piperacillin-tazobactam, and vancomycin was continued. He underwent wide surgical excision of the lesion late on the second hospital day. Biopsy of the lesion and cultures were diagnostic (Fig., Supplemental Digital Content 1, https://links.lww.com/INF/A892).
Permanent tissue sections stained with hematoxylin and eosin and Gomori's methenamine silver revealed the presence of sparsely septate, hyaline, irregularly branching hyphae with thin, nonparallel walls (Fig., Supplemental Digital Content 1, https://links.lww.com/INF/A892). The hyphae were 5 to 8 μm in width and occluded vessel lumens, with resulting thrombosis, invasion of the surrounding vessel walls, and extension into the surrounding soft tissue. There was a marked lack of inflammatory response to this process presumptively secondary to the patient's neutropenic state. The histologic appearance was not that of Aspergillus or a zygomycete. It involved the majority of the superficial and deep vessels of the skin and subcutaneous tissue but spared the epidermis. The presence of hyphae at one of the lateral surgical margins was seen on the silver stain, suggesting that diseased tissue remained in situ. The direct calcofluor white fungal stain showed the presence of fungal hyphae. Fungal cultures were initially set up on inhibitory mold agar and brain heart infusion agar. Growth of flat, spreading, cream-colored colonies with few aerial hyphae occurred at day 3 on these media as well as on the chocolate agar plate. Culture on additional agar plates including potato flake and Mycosel agars showed no sporulation and the preliminary culture report was “nonsporulating hyaline mold; unable to identify by routine methods.” The isolate was referred to Dr. Michael Rinaldi at the Fungus Testing Laboratory, University of Texas Health Science Center in San Antonio, TX, where identification of Pythium insidiosum was confirmed by molecular sequencing (D1/D2). Although there are no defined breakpoints for the MICs for P. insidiosum, susceptibility testing yielded 24-hour MICs reading of 1 μg/mL for amphotericin B, 0.06 μg/mL for voriconazole, 4 μg/mL for posaconazole, and 2 μg/mL for terbinafine. The in vitro combinations of posaconazole and terbinafine, and voriconazole and terbinafine both appeared synergistic.
The patient became afebrile on the first postoperative day. Neutropenia resolved with human granulocyte colony-stimulating factor injections, achieving an absolute neutrophil count greater than 1000/μL on postoperative day 3. Antibacterial therapy was subsequently discontinued. Computed tomography of the sinuses, chest, abdomen, and pelvis showed no signs of disseminated fungal infection and an echocardiogram was normal. He received 7 days of intravenous liposomal amphotericin B at 5 mg/kg daily. Antifungal therapy was changed to oral voriconazole, 200 mg twice daily following loading dosage, on postoperative day 6 in preparation for hospital discharge. The patient received a total of 9 months of oral voriconazole, discontinuing treatment 2 months after his final round of chemotherapy for relapsed Ewing sarcoma.
Pythium insidiosum is an aquatic oomycete in the Kingdom Straminipila. Of the 127 species currently classified in the genus Pythium, it is the only one known to infect animals and humans in addition to plants.1 It is phylogenetically more closely related to algae than to true fungi, though it can be confused with fungal pathogens from the class zygomycetes that have morphologically similar hyaline, but much larger diameter hyphae-like structures.2Pythium insidiosum exists in 2 forms: as biflagellate motile zoospores that swim to and encyst on the host, and as irregularly branching, thin, nonparallel walled hyaline hyphae-like elements with rare septations which germinate in the tissue. The asexual zoospores have an affinity for lily and grass plants as well as human and animal hair and tissue. Encystment and formation of a germ tube occurs at the follicular end of a hair or at the edge of injured tissue leading to invasion and infection in the host. The hyphae-like elements are strongly angiotropic and invade and grow within blood vessels.1,3
Pythiosis insidiosi, infection caused by Pythium insidiosum, commonly manifests as cutaneous, pulmonary, intestinal, or osseous infections in horses, cattle, cats, and dogs. Pythium insidiosum was first reported as a human pathogen in 1985.3 It causes 4 major clinical syndromes in humans: cutaneous or subcutaneous painful, ulcerating lesions, ocular infection, vascular disease leading to arterial occlusion, and disseminated disease.4 The majority of pythiosis cases in human beings reported are from Thailand, and patients typically have a history of an agricultural career with significant exposure to swampy areas. Epidemiologic analyses highlight that 85% of patients with cutaneous, vascular, and disseminated disease had underlying thalassemia-hemoglobinopathy syndrome, but the majority of patients with ocular pythiosis had no underlying disease.4,5 Human pythiosis has also been reported in Australia, Brazil, Haiti, Malaysia, New Zealand, and the United States.6 Disease in the United States has mainly manifested in the ocular form with the exception of a case report describing a neutropenic 12-year-old child with AML who developed P. insidiosum pleuropericarditis complicating pneumonia.7 Two definitive cases and 5 additional putative cases of orbital pythiosis have been reported in the United States.2,8 These occurred in otherwise healthy children with exposures in states where pythiosis is endemic or has been diagnosed in animals (Oklahoma, Tennessee, Florida, Texas, and Arizona).2 Our patient is very unusual in that he had no history of recent travel to areas with endemic pythiosis. Review of the literature reports no cases from Nebraska.
Diagnostic methods for identifying Pythium insidiosum include serologic tests, PCR, and DNA sequencing once the organism has been isolated from the patient. Successful treatment of human pythiosis generally requires surgical excision or amputation of the infected site. Employing antimicrobial therapy for pythiosis is challenging because Pythium species may lack ergosterol, a typical target for antifungal therapy. However, the cell walls of Pythium species do contain β-glucan, the target of many newer antifungal drugs.4 There are case reports demonstrating successful use of traditional antifungals with and without surgical excision, including amphotericin B, itraconazole, and terbinafine.8,9 Susceptibility testing has been performed to guide treatment in past cases and was helpful in guiding our patient's postsurgical treatment regimen. Immunotherapy with a P. insidiosum vaccine with and without surgery has been used successfully in animals and in a few human beings on a compassionate basis.4
We suspect that our patient was infected with Pythium insidiosum via integumentary defects while wading in a local lake, and that his immunocompromised state secondary to chemotherapy contributed to the pathogenicity of the organism. Treatment involving wide surgical excision combined with susceptibility-guided antifungal therapy with oral voriconazole has proven successful thus far in our patient. Now, more than 1 year after initial infection, he continues to do well off chemotherapy and remains symptom-free.
This is a particularly unique case in both the geographic location of presentation and in the severe immunocompromised condition of the patient, differing from the well-described association of pythiosis with thalassemia and Southeast Asia. Clinician awareness of this emerging, potentially life-threatening infectious oomycete is imperative to improving patient outcomes worldwide.
The authors thank Dr. Michael Rinaldi at the Fungus Testing Laboratory, University of Texas Health Science Center, San Antonio, TX, for his expert consultation and editorial advice.
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