Adult, female, warmblood, equine (Equus caballus).This mare presented to the referring veterinarian for increased respiratory noise. On physical examination a mass was noted in the area of the left arytenoid cartilage. This mass was surgically removed and sent for histopathologic examination. This mare had been imported to Canada from Argentina in apparently good health several years prior to examination for this problem.

Gross Description:  

The received surgical biopsy is a 2.5 cm x 1.5 cm wide, white with brown-black areas, cerebriform mass with a small elliptical base.

Histopathologic Description:

Larynx (per contributor): From a cartilaginous base numerous papillary projections of markedly hyperplastic epithelium extend with dramatic down growths into a fibrovascular stalk. Variably sized clusters of glandular tissue are noted within focal regions at the periphery of the biopsy, occasionally exhibiting dilation with small amounts of basophilic granular material within the lumen. Multifocally throughout the submucosa are numerous variably sized, often exceeding 100 μm in diameter, spherical structures (sporangia) with a variably prominent cuticle-like lining and filled with abundant variably sized and colored endospores. Few of the sporangia are ruptured and associated with a marked neutrophilic inflammatory response. Within the fibrous tissue and dissecting between sporangia there is a marked inflammatory infiltrate composed of plasma cells, lymphocytes and fewer macrophages. Although few, sporangia are noted within 1 mm of the lateral margins. No sporangia are within the cartilage of the deep margin.

Morphologic Diagnosis:  

Larynx: Laryngitis, lymphoplasmacytic, histiocytic, severe, multifocal to coalescing, chronic, with marked epithelial hyperplasia, and intralesional sporangia consistent with Rhinosporidium seeberi.

Lab Results:  

Polymerase chain reaction (PCR) for the 18S rRNA gene sequence of Rhinosporidium seeberi was performed using the formalin fixed, paraffin embedded biopsy material. The resulting amplified DNA was sequenced and the nucleotide sequence was confirmed to be identical to samples of Rhinosporidium seeberi present within GENBANK.


Rhinosporidium seeberi

Contributor Comment:  

Once thought to be a fungus, Rhinosporidium seeberi is an unusual organism now included in the class Mesomycetozoea, where it is the only member known to cause disease in mammals and birds.(7) Other members of this class are known to cause disease in aquatic animals, primarily fish.(7) Most commonly reported as a pathogen in tropical regions of the world, particularly India, Sri Lanka and Argentina, it has been sporadically reported in numerous locations, including Europe and North America.(6) Typically, these cases are thought to have been acquired in endemic areas and the animals then brought to non-endemic countries, as exemplified in a group of four polo ponies in the United Kingdom thought to have been infected in their native Argentina and not diagnosed until sometime after arrival into the United Kingdom.(5) However, both human and canine cases apparently acquired within Canada have been reported.(2,5)

Most lesions appear as pale to white, single or multiple, polypoid masses; these are most typically found in the nasal cavity.(2) Cases of laryngeal rhinosporidiosis are seemingly extremely rare, although one case has been reported in a Belgian Warmblood.(8) In horses, affected animals may present with epistaxis due to bleeding of traumatized nasal polyps, increased respiratory noise, or they may be asymptomatic.

It is presumed that the infection is most commonly acquired through exposure to contaminated water. The organism is thought to enter the body through preexisting damage to the normal mucosal barriers. Once in the body, endospores are released from the sporangia where they enlarge and mature to become juvenile, intermediate and finally mature sporangia filled with new endospores which are then available to repeat the cycle.(7)

JPC Diagnosis:  

Squamous mucosa with supporting cartilage, larynx (per contributor): Laryngitis, lymphoplasmacytic and neutrophilic, proliferative and polypoid, multifocal, moderate, with fibrosis and numerous intralesional sporangia and endospores, etiology consistent with Rhinsporidium seeberi.

Conference Comment:  

As noted by the contributor, conference participants identified neutrophilic inflammation primarily in association with ruptured sporangia. The aspect of this case that makes it somewhat challenging is tissue identification, and most participants identified the specimen as nasal mucosa. Given the histomorphologic features of the tissue and lesion, and combined with the fact that nasal rhinosporidiosis is much more common and reports of laryngeal infection are exceedingly rare, nasal cavity is a reasonable conclusion as the source of the affected tissue.

Based on the histomorphology of the organism, the etiologic differential diagnosis includes Coccidioides immitis and Emmonsia parva or E.crescens (formerly Chrysosporium spp.). The organisms of C. immitis are typically smaller, with spherules ranging from 20-200 μm in diameter, and endospores measuring 2-5 μm and more uniform in size and shape throughout the spherule. Emmonsia spp. form adiaspores in tissue that are characterized as large, spherical, uninucleate conidia that measure 10-20 μm in diameter for E. parva and up to 300 μm for E. crescens. Emmonsia spp. have a thick wall which is PAS-positive and helps distinguish them from C. immitis.(1)


1. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy and Palmers Pathology of Domestic Animals. Vol. 2, 5th ed. Philadelphia, PA: Elsevier Ltd; 2007:641, 644-645.

2. Harissi-Dagher M, Robillard N, Corriveau C, Mabon M, Allaire GS. Histopathologically confirmed ocular rhinosporidiosis in two Canadians. Can J Ophthalmol. 2006;41:226-229.

3. Hill SA, Sharkey LC, Hardy RM, Wilke VL, Smith MA, Anderson GM. Nasal rhinosporidiosis in two dogs native to the upper Mississippi river valley region. J Am Anim Hosp Assoc. 2010;46:127-131.

4. Hoff B, Hall DA. Rhinosporidiosis in a dog. Can Vet J. 1986;27:231-232.

5. Leeming G, Smith KC, Bestbier ME, Barrelet A, Kipar A. Equine rhinosporidiosis in United Kingdom. Emerg Infect Dis. 2007;13:1377-1379.

6. Lupi O, Tyring SK, McGinnis MR. Tropical dermatology: fungal tropical diseases. J Am Acad Dermatol. 2005;53:931-951, quiz.

7. Mendoza L, Taylor JW, Ajello L. The class mesomycetozoea: a heterogeneous group of microorganisms at the animal-fungal boundary. Annu Rev Microbiol. 2002;56:315-344.

8. Nollet H, Vercauteren G, Martens A, et al. Laryngeal rhinosporidiosis in a Belgian warmblood horse. Zoonoses Public Health. 2008;55:274-278.

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