Histoplasma capsulatum in Cats (Felis) | Vetlexicon
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Histoplasma capsulatum

ISSN 2398-2950

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Synonym(s): H. capsulatum

Introduction

Classification

Taxonomy

  • Dimorphic fungus.

Significance

  • Cause of histoplasmosis.

Active Forms

Active Form 1

Morphology

  • Smooth yeast-like colonies.
  • Microscopy shows oval budding yeast cells 3-4 um in diameter with narrow neck.
  • Budding occurs within phagocytes in tissues.

Color

  • White.

Tolerances

Temperature
  • 37°C.
  • Withstands freezing and thawing.
  • Tolerates heating for up to an hour at 45°C.
Humidity
  • Prefers a moist environment.
Ultraviolet
  • Relatively resistant to ultraviolet light.
Other
  • Prefers environment rich in nitrogen, such as bird droppings.

Development

Growth
  • Mould form in environment; produces infectious spores.
  • Cultured in brain heart broth, plus 5% blood agar.
  • May take more than a week for colonies to appear.
Reproduction
  • Yeast form reproduces by budding.
  • Mould form in environment.
Longevity
  • Survives in the environment for months to years.

Active Form 2

Morphology

  • Small colonies with cotton-like aerial hyphae after 2-4 weeks incubation.
  • Microscopy reveals 2 types of conidia: large 8-14 um macroconidia and small tear-shaped microconidia borne on septate hyphae.

Color

  • Cream.

Tolerances

Temperature
  • 25°C.
Other
  • Survives at ambient temperatures for months and at refrigerator temperatures for years.
  • The mycelial form is the more stable form.

Resting Forms

Clinical Effects

Epidemiology

Habitat

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

Lifecycle

  • 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.

Transmission

  • Inhalation, possibly ingestion and occasionally wound infection.

Pathological effects

  • Disseminated disease in humans and dogs is found in association with immunosuppression.
  • Thoracic lymph nodes become enlarged and lungs may contain nodules.
  • May disseminate to skin, mucous membranes, abdomen, central nervous system and bone marrow, also intestines.
  • Dogs present with chronic intractable cough, diarrhea, emaciation and pyrexia - unresponsive to antibiotics.
  • Cats: emaciation, pyrexia, dyspnea, ocular lesions.

Other host effects

  • 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.

Control

Control via chemotherapies

Vaccination

  • Not available.

Diagnosis

Useful samples

  • Exudate.
  • Tissue.
  • Blood for serology.

Specimen storage

  • Samples may be refrigerated if not processed immediately.

Transport of samples

Laboratory diagnosis

Adequate safety measures, including the use of a safety cabinet, must be taken.
  • Macroscopic and microscopic appearance.
  • Cytology for organism best from:
    • Lymph node aspirate.
    • Bone marrow aspirate.
    • Lung aspirate.
    • Intestinal biopsy.
  • Exoantigen test.
  • Fluorescent antibody technique to identify yeast form in tissue.
  • Skin test: remains positive indefinitely; unreliable.
  • Complement fixation test: antibodies disappear approximately 9 months after infection.
  • Immunodiffusion:
    • Detects antibody.
  • Latex agglutination test:
    • Detects antibodies early in disease.
  • Intraperitoneal inoculation of mice: the yeast form can be recovered from the liver and spleen in 2-4 weeks.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Dixon D M, Casadevall A, Klein B et al (1998) Development of vaccines and their use in the prevention of fungal infections. Med Mycol 36 (Suppl 1), 57-67 PubMed.
  • Vanden Bossche H, Dromer F, Improvisi I et al (1998) Antifungal drug resistance in pathogenic fungi. Med Mycol 36 (Suppl 1), 119-128 PubMed.