Life cycle of the human and animal oomycete pathogen Pythium insidiosum

Abstract

Pythium insidiosum, the etiologic agent of pythiosis insidiosii, causes life-threatening infections in humans and animals. Previous studies of the epidemiology of this disease hypothesized about the possible life cycle of this oomycete. Details, however, were not provided on the steps required to cause infection. We investigated the life cycle ofP. insidiosum by inoculating pieces of equine skin and plant leaves and then studying the ensuing events with a scanning electron microscope. Our observations revealed that zoospores had a strong tropism for skin tissue, horse and human hair, and water lily and grass leaves and a weak attraction to a variety of other leaves. Encysted zoospores were observed on the favored leaves and skin. There they produced germ tubes and later abundant hyphal filaments that penetrated leaf tissues. Young sporangia had compact, thick wails. The sporangial wall was reduced to a fragile membrane when the sporangia had produced well-differentiated biflagellate zoospores. The encysted zoospores secreted an amorphous material that permitted the zoospores to adhere to skin and plant tissues. On the basis of these findings, a model to explain the life cycle ofP. insidiosum is proposed.

Gastrointestinal Pythiosis in 10 Dogs from California

“…Previous investigators have hypothesized that P. insidiosum uses a plant substrate to maintain the asexual phase of its life cycle in aquatic environments, a concept supported by its ability to infect pieces of water lily and multiple grass species in the laboratory. [12][13][14] Frequent exposure to outdoor water sources previously has been reported as a potential risk factor for pythiosis. 1,[15][16][17] Of the 8 dogs in this report for whom environmental information was available, 2 had access to flooded rice fields, 2 to ponds, 1 to a lake, 1 to a creek, and 1 to the Sacramento River.…”

Development of a Nested Polymerase Chain Reaction Assay for the Detection and Identification of Pythium insidiosum

“…1,15 Furthermore, few veterinary microbiologists are familiar enough with the morphologic characteristics of oomycetes to definitively distinguish P insidiosum from other oomycetes that have similar features. Although the production of motile biflagellate zoospores has been considered diagnostic for P insidiosum by some authors, 3,36 these characteristics are shared by a number of other oomycetes, 18,21,37 and thus should not be used alone to make a definitive identification. An exoantigen test (utilizing polyclonal serum raised against secreted antigens of P insidiosum) has been available in the past through the CDC j for the identification of P insidiosum isolates, 14 but its availability has been limited in recent years.…”

Clinicopathologic Findings Associated with Lagenidium sp. Infection in 6 Dogs: Initial Description of an Emerging Oomycosis

“…Examination of 1 biopsy specimen that was macerated and then digested in potassium hydroxide (KOH) for 30 minutes disclosed numerous broad, poorly septate, right angle-branching hyphae. Histologic evaluation of the biopsy samples identified severe pyogranulomatous dermatitis with numerous broad (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25), infrequently septate hyphae with irregular nonparallel walls. Zygomycosis was suspected because of the large hyphal diameter.…”

“…The infective form of P insidiosum is thought to be the asexual reproductive structure, which is a motile biflagellate zoospore produced and released into water environments (probably in association with plant material). 9,10 The definitive diagnosis of pythiosis historically has been challenging. Although a presumptive diagnosis often is made on the basis of typical histologic findings (pyogranulomatous and eosinophilic inflammation surrounding broad, sparsely septate hyphae with nonparallel walls), these characteristics are shared by infections caused by zygomycete fungi in the order Entomophthorales, such as Conidiobolus coronatus and Basidiobolus ranarum.…”