Rhinosporidiosis

Definition/organism

Rhinosporidium seeberi — classically taught as fungus; molecular data suggests Mesomycetozoea (aquatic protistan). Endemic India + Sri Lanka; sporadic elsewhere. Water/soil contact.

Sites

Nasal polyp (typical) · nasopharynx · larynx · conjunctiva.

Morphology

Polyp structure (oedematous stroma, mixed inflammatory infiltrate, respiratory epithelium) PLUS:

• Sporangia (thick chitinous wall) containing RBC-sized spores — thousands per sporangium
• Rupture → spores into submucosa/surface → new sporangia (local spread mechanism)
• Surrounding: granulation tissue (plasma cells, lymphocytes, histiocytes, neutrophils)
• Overlying epithelium: hyperplastic, focal thinning, occasional ulceration

Clinical relevance

Geographic clustering + bleeding nasal polyp + sporangia-with-spores histology = distinctive triad in endemic-region patient. Surgical excision needs WIDE excision + base cauterisation (not simple polypectomy) — ruptured sporangium spillage → local recurrence risk.

Definition and Organism

Rhinosporidiosis is a chronic granulomatous infection caused by Rhinosporidium seeberi, classically taught as a fungus though its precise taxonomic classification has been debated (molecular studies have suggested it may actually belong to a distinct group of aquatic protistan parasites, the Mesomycetozoea, rather than true Fungi — a point worth knowing even though it is conventionally taught alongside the mycoses). The disease is endemic in India and Sri Lanka, occurring sporadically elsewhere in the world, and is thought to be acquired from contact with stagnant water or soil contaminated by the organism.

Sites of Involvement

Typically presents as a nasal polyp, but lesions may also occur in the nasopharynx, larynx, and conjunctiva.

Morphology

Beyond the general structure of an inflammatory or allergic nasal polyp (oedematous stroma, mixed inflammatory infiltrate, overlying respiratory epithelium), the diagnostic feature is the presence of large numbers of the organism within thick-walled sporangia:

• Each sporangium contains numerous spores, each roughly the size of a red blood cell, enclosed within a chitinous wall.
• A single mature sporangium may contain several thousand spores.
• On rupture of a sporangium, spores are released into the submucosa or onto the mucosal surface, from where new sporangia develop — the presumed mechanism of local spread within the tissue.
• The intervening tissue is inflammatory granulation tissue, containing plasma cells, lymphocytes, histiocytes and neutrophils.
• The overlying epithelium shows hyperplasia, with areas of focal thinning and occasional ulceration.

Clinical Relevance

• The combination of geographic clustering (India/Sri Lanka), presentation as a bleeding nasal polyp, and the histologic finding of thick-walled sporangia packed with erythrocyte-sized spores is distinctive enough that rhinosporidiosis should be specifically considered — rather than assumed to be a routine allergic or inflammatory nasal polyp — in a patient from or with travel history to an endemic region.
• Because a ruptured sporangium releases thousands of spores capable of seeding new sporangia locally, surgical excision of a rhinosporidiosis polyp carries a recognised risk of local recurrence if spillage of spores into adjacent tissue occurs during removal, which is why wide local excision with cauterisation of the base is the preferred surgical approach over simple polypectomy.

Diagram 1: Sporangium Structure in a Nasal Polyp

Draw a low-power cross-section of the polyp stroma, with several rounded, thick-walled sporangia embedded within an inflammatory granulation tissue background, and the mucosal surface at the top.

• Sporangia: draw as large, thick-walled round structures, each packed with numerous small round spores (dozens shown to represent “thousands”), varying in maturity — some sporangia shown intact, one shown ruptured with spores spilling into the surrounding tissue and toward the surface.
• Background stroma: fine stippling to represent granulation tissue with scattered plasma cells, lymphocytes, histiocytes and neutrophils.
• Overlying epithelium: shown thickened (hyperplastic) in most areas, with one small area of thinning/ulceration directly over a ruptured sporangium.

Labels required

• The chitinous wall of the sporangium and the individual spores within it must both be labelled, since the two-part structure (wall + spore-packed interior) is the diagnostic feature.
• The ruptured sporangium must be labelled as the mechanism of local spread, connecting histology to the clinical point about recurrence risk on incomplete excision.

Errors commonly made

• Drawing sporangia as solid or homogeneous structures rather than showing them clearly packed with individual spores, which is what distinguishes this organism from other fungal forms on histology.
• Omitting the ruptured sporangium and its spilled spores, losing the visual link between organism biology and the clinical recurrence risk after incomplete surgical removal.