WSAVA 2002 Congress
Carlos Eduardo Larsson
Full professor of Internal Medicine-FMVz/USP, Av. Prof. Dr. Orlando Marques de Paiva
São Paulo, Brasil

Pythiosis is a py granulomatous, cutaneous or gastroenteric, disease caused by a protist agent, currently known as Pythium insidiosum. Since its first report in horses, by Kerr in 1829, has received different names (Hyphomyces destruens, Pythium destruens, Pythium gracile) until the actual taxonomic designation, and systematically included in the kingdom Protista, class Oomycetes, order Peronosporales and family Pythiaceae.

Pythiosis has been reported in many countries, mainly in moist, tropical or subtropical ones, such as Indonesia, Japan, Nova Guine, Thailand, Australia, USA (States of Missouri, Georgia, South and North Carolina, Tennessee, and Kentucky), Costa Rica and Brazil5 (States of São Paulo, Mato Grosso, Piauí, and Rio Grande do Sul). Besides horses are more commonly diagnosed with the disease, it may affect dogs, cats, bovine and man. It is not considered a classic zoonosis, although it is classified as a saprozoonosis by some authors.

During a long period of time pithyosis was erroneously considered a fungal disease, from the Phycomycosis complex that would include not only the infection by this protist, but also the infections caused by true fungus of class Zygomycetes. However, it is known that Phytium insidiosum is the only specie of the genera able to infect mammals, through flagellated, motile, fresh water surviving zoospores. Its cellular wall contains cellulose and ß-glucan, but not chitin; in the plasmatic membrane there are no estherols, like ergostherol, that is susceptible to the action of the majority of the antifungal drugs.

There are several major differences in comparison with the kingdom Fungi mainly regarding cellular division, mitochondrial structures and Golgi apparatus.


When motile zoospores are placed in contact with various leaf and animal tissues they migrate towards the tissue. Once in the vicinity of the attractants, the zoospores became sluggish, lose their flagella, assume a rounded form and become encysted on the tissue. They have an evident tropism by horse cutaneous tissue and hair, human hair and for vegetals (water-lily and grass leaves). After encystment, the zoospores develop germ tubes in the direction of the affected tissue, and these structures allowed for penetration and invasion tissues6. In spite of the unclear pathogenesis, it is almost certain that Pithyum insidiosum is a true pathogen, not just an opportunistic organism, as experimentally showed in health and immunosuppressed rabbits10.

Experimental studies6,7,8 about zoospores motility, in different temperatures, have explained the major incidence of pithyosis in warm months, that is, last summer and rise fall.

Many cases of pythiosis are caused by previous exposition of animals to wet and swampy environments. Apparently zoospores do not penetrate in whole skin. There is no evidence that immunosuppression (iatrogenic or by virus) can predispose to infection


Cutaneous and gastroenteric forms of pythiosis are reported in dogs, while in the rare feline cases ophthalmologic, bone, cutaneous and subcutaneous pictures are referred 3,10

Large-breed, young (36 month or less) animals either male or female, mainly those utilized for hunting, herding or that remain standing in stagnant water are most commonly affected. In Brazil, the disease is more frequent in mongrel dogs5, while in the USA, German shepherds appear to be particularly susceptible2,4.

The gastroenteric form, not reported in Brazil6, but frequent in the USA, has been diagnosed in young animals presenting chronic anorexia, emesis, and diarrhea.

Dysphagia, ptialism, regurgitation, hematochezia and lymphadenopathy are related symptoms. Emesis is due to obstructive gastrointestinal disorders, and the lesions generally are situated in stomach and small intestine as well as in esophagus and large intestine10.

Cutaneous pythiosis is the most common observed clinical form, involving either epiderm or hypoderm, being able to metastasize to secondary lymph nodes and many other tissues, especially when glucocorticoids are used. In general, the lesions are observed in previously injured and excoriated areas on legs, tail head, perineum, face and flank. Early lesions are solitary, resembling those of acral-lick dermatitis or multiple that progress to eroded or ulcerated lesions, accompanied by intense pruritus leading to self-mutilation. They rapidly develop into large, firm to boggy masses with ulceration and draining traits. The lesions have well-delimited edges, necrotic, centrifugal growth sometimes with a honeycomb appearance.

Unlikely to horses, the called kunkers are seldom evidenciated in dogs and cats. However, sanguinolent or purulent material may be drained when lesions are pressed. Lesions generally are covered by hemorrhagic crusts5. Depending on the evolutive phase, lesions may resemble those of foreign body induced, deep pyoderma, subcutaneous or systemic fungal disease or neoplasia.


Anamnestic data regarding breeding type, animal use, possible travel to endemic regions, evolution time, early lesions, general symptoms etc. Besides the general physical and dermatologic exams, complementary exams may contribute to the diagnosis1,3,4,7,9,10.

Thus, exudate material cytology, histopathology of wedge biopsy, culture of wedge biopsy, and immunohistochemical assays may be required. The best stain for exudate cytology is Gomori methenamine silver (GMS) that allows hyphal structures evidenciation, but this is a limited diagnostic method. Histopathology of wedge biopsy from injured skin fragments, obtained by excisional biopsy, and submitted to GMS really demonstrate hyphal structures, as wide occasionally septate and irregular branching. Such structures may be seen in granulomatous lesions center. Histologically it seems a nodular or diffuse granulomatous or py granulomatous dermatitis and panniculitis, with many eosinophils. The process circumscription by Splendore-Hoelpli phenomenon is not uncommon. Lesions fragments, not maintained at low temperatures, and washed with sterile saline with or without antibiotics (like ampicillin), may be spread in Sabourand dextrose agar, BHI, corn meal agar and vegetable extract agar. Once samples are applied to the culture media they may be incubated at 35-37°C. Immunohistochemical assays have been developed to be used in formalin prefixed infected equine tissues. According to THOMAS & LEWIS10 (1998), it has been successfully used in dogs, cats, equines and human, with promising results. Enzyme-linked immunoabsorbent assay (ELISA) has been recognized since 1997 by MENDOZA et al7 in the serodiagnosis of human and animal pythiosis. ELISA test sensibility is much better than that of agar gel immunodiffusion.


1.  Brown, C.C.; Mcclure J.J.; Triche. P; Crowder, C. Use of immunohistochemical methods for diagnosis of equine pythiosis. Am. J. Vet. Res., v. 49, p. 1866-68, 1988

2.  Fischer, J.R.; Pace, L.W.; Turk Jr - Gastrointestinal pythiosis in Missouri dogs: eleven cases. J. Vet Diagn Invest, v. 6, p. 380-2, 1994.

3.  Foil, C.S.O. In: Greene, Ce (ed). Infectious diseases of the dog and cat. Philadelphia, Saunders, 1998. p. 420 - 423, 1998.

4.  Foil, C.S.O.; Short, B.G; Fadok, V.A.; Kunkle, G.A.. A report of subcutaneons pythiosis in five dogs and a review of the etiologic agent Pythium spp. JAAHA, v. 20, p. 959-966, 1984.

5.  Larsson, C.E.; Meng, M.C.; Nahas, C.R.; Michalany, N.S.Newton, J.; Rosa, P.S; Bonavito, D.; Gambale, W. Pitiose canina - aspectos clínicos e epidemiológicos de casoS em São Paulo. In: 15° Congresso Brasileiro De Clínicos Veterinários De Pequenos Animais, Rio de Janeiro, 1993. Proceedings.

6.  Mendonza, L.; Hernandez, F.; Ajello, L.; Life cycle of the human and animal oomycete pathogen Pythium insidiosum. J. Clin Microbiol. v.31, n. 11, p. 2967-73, 1993.

7.  Mendonza, L.; Kaufman, L.; Mandy, W.; Glass, R. Serodiagnosis of human and animal pythiosis using an enzyme-linked immunoabsorbent assay. Clín Diagn Lab Immunol, 1997.

8.  Miller, R.I Investigations into the biology of three "phycomycotic" agents pathogenic for horses in Austrália. Mycopathologica, v. 81, p. 23-28, 1983.

9.  Scott, D.W.; Miller Jr., W.H.; Griffin, C.E. In: Muller & Kirk´S Small Animal Dermatology. 6 ed., Philadelphia, Saunders, P. 381-84, 2001.

10. Thomas, R.C.; Lewis, A.T - Pythiosis in dogs and cats. Compend. Cont. Educ. v. 20, n.1, p.63-73, 1998

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Carlos Eduardo Larsson
Full professor of Internal Medicine-FMVz/USP, Av. Prof. Dr. Orlando Marques de Paiva
São Paulo, Brasil

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