Special Veterinary Pathology

HISTOPLASMOSIS

Definition

• It is an infectious granulomatous disease of man, dogs, cattle, horses, sheep of which dogs are most often affected.

Aetiology: Histoplasma capsulatum

The organisms is whitish brown in colour having spherical spores of two sizes

• Microconidia 3 to 4 µ in daimeter
• Macroconidia 8 to 12 µ in diameter

In the soil and at room temperature it grows as a myclium, while in cultures at 37°C and in the body, it occurs as a yeast cell form

Transmission

• Fungus present in soil and infection is by inhalation

Clinical signs

Two forms

1. Benign inapparent form
2. Fatal disseminated form

Emaciation, diarrhoea, vomition, ascites, chronic cough, anorexia, pyrexia, anaemia, swelling of lymph node, splenomegaly and hepatomegaly.

Gross lesions

• Discrete nodules and enlargement of organs
• Enlargement of spleen, lymph nodes, liver and adrenals
• Nodules in the lungs.

Microscopic lesions

• Marked proliferation of cells of rerticulo endothelial cells with formation of macrophages and & epithelioid cells.
• These cells swell and undergo degeneration and karyolysis.
• Infiltration of lymptocytes and plasma cells
• No giant cells formation
• Benign variety, fibrous capsule is present
• Malignant variety, capsulation is not a feature

Diagnosis

• Histological examination of fungus in the reticuo endothelial cells
• Cultural examination of the organisms
• Skin test with 'histoplasmin'
• Complement fixation test(CFT), agar gel precipitation test(AGPT)

EPIZOOTIC LYMPHANGITIS

Definition

• Epizootic lymphangitis is a disease of the skin and superficial lymphatics of horses, mules and donkeys and rarely humans.

Aetiology

• Histoplasma farciminosum. The organism is yeast shaped in tissue and cannot be differentiated from Histoplasma capsulatum

Incidence

• It is remains endemic in India

Transmission

• Unknown. Direct contact does not appear unless infective material is transmitted to previously injured skin

Pathogenesis, clinical signs and gross lesions

• The clinical features are chronic indurative ulceration of the skin especially in the limbs, with thickening of the superficial lymphatics, enlargement of regional lymph nodes, formation of abscesses and discharge of purulent material.
• This is followed by the development of the new indolent ulcers.
• Sometimes infection may occur as conjunctivitis, or naso-lachrymal infection
• Only rarely it becomes generalized, involving internal organs.
• The disease runs a chronic course over a period of months, but eventually resolves.

Microscopic lesions

• Cutaneous lesions reveal granulomatous tissue reaction with a large number of macrophages and sometimes multinucleated giant cells.
• Their cytoplasm is distended with oval organisms, each about 2.0 by 3.0 µm and enveloped by a thin, clear capsule
• The central mass of fungus in sections can be demonstrated with haematoxylin and eosin(H&E)
• The capsule remains unstained
• Lymphocytes, plasma cells and some neutrophils are present within the granulomatous tissue reaction.
• Secondary bacterial infection leads to the formation of abscesses which may drain to the surface.

Diagnosis

• Demonstration of typical fungi in tissue sections, cultures and stained smears of exudates