Histoplasma spp

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Wright-Giemsa stained cytologic sample of a peritoneal fluid macrophage containing intracytoplasmic Histoplasma capsulatumorganisms.[1]

Histoplasma spp are a dimorphic, saprophytic fungus found worldwide and commonly reported in cats[2].

The mycelial form of Histoplasma spp thrive in organic, nitrogen-rich soils, such as those contaminated with bird or bat excrement. The most common route of infection is via inhalation of microconidia, which are small enough to reach the lower respiratory tract[3].

At body temperature, the organism transforms into the yeast phase, and can be phagocytosed by macrophages and systemically disseminated[4] via the bloodstream or lymphatic system to the reticuloendothelial and gastrointestinal systems and, sometimes, the bones, skin, eyes, or brain.

Co-infections with Leishmania spp are common in dogs in tropical regions[5].

Species which are pathogenic to dogs include:

  • Histoplasma capsulatum var capsulatum

Histoplasmosis in dogs is usually characterized by gastrointestinal, respiratory and/or systemic disease, although some dogs can be asymptomatically infected.

Clinically affected dogs usually present with chronic large-bowel diarrhea, generalized lymphadenopathy, inguinal or retroperitoneal masses[6] and pleural and peritoneal effusion. Weight loss, anemia and hypoalbuminemia are commonly observed[7]. In severe cases, disseminated intravascular coagulopathy has been reported due to granulomatous vasculitis[8].

A small number of dogs present with unusual symptoms such as osseous, ocular, dermatologic[9], or central nervous system involvement.

A weak association between systemic disease and carcinoma[10] and B-cell lymphoma[11] have been reported, suggesting a chronic inflammatory effect or production of carcinogenic metabolites by the fungus[12][13].

Diagnosis is based on isolation, Western blot[14], fungal culture or PCR assay identification[15].

Treatment usually involves itraconazole (5 mg/kg PO q 12h) as the drug of choice[16].

Amphotericin B has also be used combined with itraconazole for the first few days for improved effect. Terbinafine has also shown in vitro efficacy against this fungus[17].

Therapy is usually prolonged (4 - 6 months).

Beneficial effects of corticosteroid therapy have been documented in some cases of chronic respiratory disease[18].

References

  1. Tyre E et al (2007) Histoplasmosis in a dog from New Brunswick. Can Vet J 48(7):734-736
  2. Lin Blache J et al (2011) Histoplasmosis. Compend Contin Educ Vet 33(3):E1-E11
  3. Clinkenbeard KD et al (1989) Canine disseminated histoplasmosis. Compend Contin Educ Pract Vet 11:1347–1361
  4. Kerl ME (2003) Update on canine and feline fungal diseases. Vet Clin North Am Small Anim Pract 3:721–747
  5. Cordeiro RA et al (2011) Serological evidence of Histoplasma capsulatum infection among dogs with leishmaniasis in Brazil. Acta Trop 119(2-3):203-205
  6. Clemans JM et al (2011) Retroperitoneal pyogranulomatous and fibrosing inflammation secondary to fungal infections in two dogs. J Am Vet Med Assoc 238(2):213-219
  7. Wolf AM (2006) Histoplasmosis. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 3rd ed. vol 1. Philadelphia: WB Saunders. pp:577–584
  8. Gilor C et al(2011) DIC and granulomatous vasculitis in a dog with disseminated histoplasmosis. J Am Anim Hosp Assoc 47(3):e26-e30
  9. Nishifuji K et al (2005) Interdigital involvement in a case of primary cutaneous canine histoplasmosis in Japan. J Vet Med A Physiol Pathol Clin Med 52(9):478-480
  10. Murata Y et al (2007) Molecular epidemiology of canine histoplasmosis in Japan. Med Mycol 45(3):233-247
  11. Brunker JD & Hoover JP (2007) B-cell lymphoma in a dog with ehrlichiosis (Ehrlichia canis) and systemic histoplasmosis (Histoplasma capsulatum). Can Vet J 48(3):292-295
  12. Shacter E & Weitzman SA (2002) Chronic inflammation and cancer. Oncology 16:217–232
  13. Lane EP & Lobetti RG (2002) Renal T-cell lymphoma with cerebral metastases in a dog with chronic canine ehrlichiosis. J SA Vet Assoc 73:83–85
  14. Canteros CE et al (2010) Endemic fungal pathogens in a rural setting of Argentina: seroepidemiological study in dogs. Rev Iberoam Micol 27(1):14-19
  15. Raskin R & Meyer DJ (2001) Atlas of Canine and Feline Cytology. 1st ed. Philadelphia: WB Saunders. pp:55
  16. Brömel C & Sykes JE (2005) Histoplasmosis in dogs and cats. Clin Tech Small Anim Pract 20(4):227-232
  17. Sakai MR et al (2011) Terbinafine pharmacokinetics after single dose oral administration in the dog. Vet Dermatol 22(6):528-534
  18. Schulman RL et al (1999) Use of corticosteroids for treating dogs with airway obstruction secondary to hilar lymphadenopathy caused by chronic histoplasmosis: 16 cases (1979–1997). J Am Vet Med Assoc 214:1345–1348