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Human pythiosis was first described in Thailand in 1987, and thirty-two cases were published worldwide in 2002. Most cases are reported from tropical and subtropical regions; however, human infection has also been encountered in the United States, Israel, and Australia. The principal pathogen is identified as Pythium insidiosum, and at least one case of Pythium aphanidermatum infection has been reported.
Most case reports of pythiosis are published in Thailand, accounting for 78% of published reports in 2002 – over 90% of these in farmers. Pythium insidiosum is identified in irrigation water in endemic agricultural areas, and 64% of soil samples in endemic areas contain the organism. 102 cases of human infection were identified in Thailand through active case finding from 1985 to 2003 – 59% vascular, 33% ocular, 5% cutaneous/subcutaneous, and 3% disseminated.
Natural infection of dogs, cats, cattle, horses, and other mammals are described in South America (Argentina, Brazil, Colombia, and Venezuela), Central America, and the Caribbean (Costa Rica, Guatemala, Haiti, Jamaica, Panama, and Nicaragua), North America (the United States, notably in Florida, Louisiana, Mississippi, and Texas), Australia and Asia (India, Indonesia, Japan, New Guinea, New Zealand, North Korea, and Thailand). A single case (dog in Mali) was reported in Africa in 2005.
Infection is acquired through direct contact or trauma and presents as granulomatous cutaneous and subcutaneous lesions. Patients often have underlying thalassemia. Vascular pythiosis is characterized by ascending purulent infection and thrombosis of major arteries, usually of the lower extremities. If a major vessel is involved, the patient usually dies within weeks. Symptoms include intermittent claudication, resting calf pain, and other signs of arterial insufficiency. Fever and weight loss are not described.
Ocular disease is characterized by ulcerative keratitis, which may progress to endophthalmitis and require enucleation. Eleven cases of Pythium endophthalmitis were reported from a single Thai hospital in 2004. Legenidium, a related genus of oomycetes, has been associated with infections of the skin and eyes in Thailand.
Pythium insidiosum is not susceptible to antifungal agents, and excision is the only proven form of therapy. Saturated solutions of potassium iodide have been recommended for localized skin infections. Immune therapy consisting of injection of Pythium antigen into infected patients has been successful in animal models and used for human disease in Thailand.
A single outbreak (4 cases of keratitis) of pythiosis was reported in Thailand in 2009, and at least three instances of cross-border pythiosis have been published:
2005 (publication year) – Pythiosis was documented in a German Shepherd dog exported from Mali to France.
2011 (publication year) – An American soldier acquired traumatic wound infection by Pythium aphanidermatum in Afghanistan.
2015 (publication year) – A French traveler acquired Pythium insidiosum keratitis in Thailand.