Histoplasmosis or Epizootic Lymphangitis

Histoplasmosis is also known as Epizootic Lymphangitis, Pseudo-glanders, Equine blastomycosis, and Equine histoplasmosis.

Histoplasmosis or Epizootic Lymphangitis is a chronic, fungal disease affecting different species of animals and persists for 3-12 months.

Histoplasmosis or Epizootic Lymphangitis is a zoonotic disease.

Etiology

Histoplasmosis or Epizootic Lymphangitis is caused by a dimorphic fungus, Histoplasma capsulatum.

It is a Gram positive, yeast like cells with a characteristic double walled capsule are easily found in discharges.

These organisms are located both intracellularly and extracellularly in giant cells and macrophages.

Epidemiology

Prevalence of infection

The course of the disease is prolonged with mortality rate of 10-15%. Disease occurs as outbreak during winter when large numbers of horse are gathered together for military or other purposes.

It is a rare systemic mycosis of farm animals such as horses, cattle and pigs occurs with a high prevalence in specific geographic localities.

Its occurrence has been noticed in parts of Iran, Asia, India, Northern Africa, and the Mediterranean littoral.

Source of infection

The fungus is able to survive for the period as long as 4 months in soil and water.

Bedding, grooming, utensils, horse blankets or harness acts as medium for spread of infection. Both animate and inanimate objects and environment gain access into abrasions in lower limbs.

Transmission

• Infection generally occurs by inhalation of contaminated dust.
• Direct contact with bedding, grooming, utensils, horse blankets or harness acts as conduit for spreading infection.
• Fungal spores carried in both animate and inanimate objects and environment gain access into abrasions in lower limbs.
• The isolation of organism from alimentary tract of biting flies indicates that the biting flies also contribute the spread of infection.

Host Affected

• Affects solipede animals such as horse, donkeys and mules.
• The disease rarely affects cattle and camels.

Pathogenesis

• Histoplasmosis is the most common cause of mediastinitis and this  remains as a relatively rare disease.
• Severe infections can cause hepatosplenomegaly and lymphadenopathy.
• Lesions have a tendency to calcify as they heal. Ocular histoplasmosis causes damage to the retina of the eyes.
• Scar tissue is left on the retina which can experience leakage, resulting in a loss of vision not unlike macular degeneration.
• This lung infection develops gradually over several weeks, causing a cough and increased difficulty breathing.
• Symptoms include weight loss, a mild fever, and a general feeling of illness.

Clinical Signs

Initially, lesions are ulcerative, suppurative, pyogranulomatous dermatitis and in many cases lymphangitis developed.

Ocular form

• Keratitis, conjunctivitis, sinusitis and pneumonia occur in other form of the disease.
• Lesions usually develop on the limbs, particularly about the hocks, and may be present on the back sides of neck, vulva and scrotum.
• Occasionally lesions appear on the nasal mucosa, inside the nostrils without involving nasal septum.
• An indolent ulcer develops at the portal of entry making its appearance several weeks to 3 months after infection occurs.
• A spreading dermatitis, lymphangitis is evident as corded lymphatics with intermittent nodules develops.
• Nodules rupture and discharge thick creamy pus.
• Local lymph nodes also enlarged and can rupture.
• Thickening of the skin in the areas and general swelling of the whole limb is common but with painless lesions.

Pulmonary signs

• Fever, pneumonia, chronic cough, nasal discharge, lethargy, dyspnoea, tachypnoea and pleural effusion.

Extrapulmonary signs

• Ocular discharge, ophthalmitis, wasting, abscess formation, icterus, diarrhoea, lymphnadenitis, hepatic insufficiency with jaundice and anasarca, placentitis, abortion, widespread lesions in neonates especially in foals.

Zoonosis

• Infection is transmissible to human beings.
• Upon entry in to wounds, the fungus invades subcutaneous tissues and develop a local granuloma / ulcers and from there spread along the lymphatic vessels.
• Ocular form of disease ensues due to injection of organism into eyes by biting flies.

Necropsy Findings

• Pulmonary consolidation and granulomatous pneumonia, enlargement of splanchnic lymph nodes.
• Gross enlargement of liver containing necrotic foci observed.
• Lesions are usually confined to the skin, subcutaneous tissues lymphatic vessels and lymphnodes.
• Granulomatous lesions may be found in lungs, liver and spleen.
• Histological lesions is quite characteristic and consists of pyogranulomatous inflammation with fibroplasia.

Diagnosis

• Based on clinical signs and necropsy findings.
• For screening the herd, histoplasmin skin test appears satisfactory.
• Mullein test is negative but a sterile filtrate of a culture of H. capsulatum var farciminosum, has been used in a cutaneous sensitivity test.
• Radiography: Miliary nodules.
• Immunology: CFT, Latex agglutination test.
• FAT
• Antibody to H. capsulatum var farciminosum are detectable in serum before or at the time of development of lesions.
• BUN level should be judged because BUN level should not be exceed 75 mg%.

Sample Collection

• Sample should be collected in a solution containing 500 units/ml penicillin.

Differential Diagnosis

• Glanders
• Ulcerative Lymphangitis
• Sporotrichosis

Treatment

• Parentral Iodides are as effective in some cases as amphotericin-B.
• Sodium Iodide is administered as a 10% solution at 1 ml per 5kg intravenously once weekly for 4 weeks.
• Amphotericin B is the drug of choice 0.125 to 0.25 ng/lb body weight slowly intravenously every 48 hours 3 treatment or 0.1% solution in 5% dextrose slow intravenously as the drug is nephrotoxic.
• Supportive treatment with sodium bicarbonate, B vitamins and analgesic.

Prevention

Formalized Aluminium Hydroxide, adsorbed and heat attenuated vaccines have been widely used apparently with success.

Control

Outbreak in uninfected areas are best controlled by slaughter of affected animals. In enzootic areas severe cases should be destroyed. Less severe cases should be kept in quarantine while undergoing treatment.

All infected bedding, harness and utensils should be destroyed or vigorously disinfected.