Equis ISSN 2398-2977

Histoplasma spp

Contributor(s): Susan Dawson, Richard Walker




  • Genus:Histoplasma.
  • Sexual stage:Ajellomyces spp.


  • Gk:histos- tissue;plasma- mold, image, formation.

Active Forms

This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login

Clinical Effects



  • Free-living dimorphic fungus.
  • Prefers neutral to alkaline soil with neutrogen enrichment in areas with annual rainfall of 35-50 in (85-125 cm).
  • Found in topsoil, especially in the presence of bird and bat feces.
  • Birds are passive carriers, whereas bats undergo intestinal infections.


  • Free-living form consists of septate hyphae producing microconidia and macroconidia (asexual reproductive units).
  • Becomes a yeast in animal hosts or suitable culture   →   reproduces by budding.
  • A sexual state,Ajellomyces capsultatus, has been described.


H. capsulatum
  • Inhalation, possibly ingestion and occasionally wound infection.
    H. farciminosum
  • Arthropods may be vectors.
  • Infection via skin wounds.
  • Occasionally respiratory, gastrointestinal or conjunctival routes.

Pathological effects

H. capsulatum
  • Disseminated disease in humans and dogs is found in association with immunosuppression.
  • Recovery and resistance require cell-mediated immunity.
  • Recovered animals are immune.
    H. farciminosum
  • Skin sensitivity follows exposure.
  • Circulating antibodies can be demonstrated following infection.
H. capsulatum
  • Thoracic lymph nodes become enlarged and lungs may contain nodules.
  • May disseminate to skin, mucous membranes, abdomen, central nervous system and bone marrow.
  • Dogs present with chronic intractable cough, diarrhea, emaciation + pyrexia unresponsive to antibiotics.
  • Cats: emaciation, pyrexia, dyspnea, ocular lesions.
  • Occasionally infects horses, usually subclinically.
    H. farciminosum
  • The cause of equine epizootic lymphangitis (pseudoglanders) .
  • Local skin nodule   →   ulcerated arrows may recur.
  • Local lymphatic nodules   →   discharging sinus tracts.
  • Some cases show hematogenous spread   →   visceral organs.
  • Lesions progress from suppurative   →   granulomatous.

Other Host Effects

H. capsulatum
  • Found in the top soil of endemic areas, particularly in the presence of bird and bat guano.
  • Birds are mainly passive carriers; bats undergo intestinal infection.
  • Subclinical infections are common in dogs, cats and humans; occasionally seen in horses.
    H. farciminosum
  • May colonize the gastrointestinal tract.


Control via animal

  • Treatment of equine histoplasmosis rarely attempted.
  • Euthanasia   Euthanasia  advised in non-endemic areas.

Control via chemotherapies

Canine and feline histoplasmosis


  • Not available.


This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Johnstone P F, Reams R, Jakovljevic S et al (1995) Disseminated histoplasmosis in a horse. Can Vet J 36 (11), 707-709 PubMed.
  • Goetz T E & Coffman J R (1984) Ulcerative colitis and protein-losing enteropathy associated with intestinal salmonellosis and histoplasmosis in a horse. Equine Vet J 16 (5), 439-441 PubMed.