Life Cycle of the Human and Animal Oomycete Pathogen Pythium insidiosum

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 of P. 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 walls. 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 of P. insidiosum is proposed.