Human infections with Pythium species sp., pythiosis, are increasing. Pythium is traditionally a plant pathogen but there are reports of increasing numbers of infections in humans. A risk factor for this infection is prior exposure to wet environments. These infections are difficult to treat, and in many cases to save the patient’s life, clinicians resort to surgical amputation for vascular pythiosis and penetrating keratoplasty for ocular pythiosis. Most ocular infections with Pythium lead to blindness or evisceration, so early recognition, diagnosis, and treatment are critical.
There are several laboratory techniques that can be used to help diagnose pythiosis. These include serology, microbial identification, molecular techniques, and proteomic assays. These all have their advantages and disadvantages including being time-consuming. Identification of Pythium species using zoospore production is a relatively easy and quick procedure. Once Pythium has been grown, these can be subcultured on non-nutrient agar containing plant (usually rice or carnation) leaves. After incubation at 37° for 24 hours, the infected leaves are transferred to an induction medium and incubated for five to six hours. During the incubation, the leaves are observing under a light microscope for the production of sporangia and motile zoospores. Once motile zoospores are visualized infection with Pythium insidiosum, or other species can be reported.
The Pythium cell wall is composed of an outer layer of cellulose and an inner layer of chitin. Due to its unusual cell wall, Pythium species are not susceptible to traditional antifungals. However, it can be susceptible to linezolid, azithromycin, and tetracyclines to test for sensitivity, zoospores are collected and spread on Muller-Hinton agar. Antimicrobial impregnated discs are placed on the surface of the agar, and plates incubated for 24 to 48hours. After that point, there should be growth inhibition around at least one of the antibiotics discs, and this can be reported as potentially useful for treatment.