Pathogenicity

PATHOGENECITY

Clinical signs

  • The cases of epizootic lymphangitis can be grouped into four different forms, namely: cutaneous, respiratory, ocular and asymptomatic carriers.
  • The cutaneous form of the disease, after which the disease was named, is the most common.
  • The initial lesion is an open granulomatous wound along the course of a lymphatic vessel, which has a tendency to ulcerate, or to undergo alternating periods of discharge and closure for some weeks before healing with residual scar formation.
  • Lesions are most common in the forelimbs, the chest wall and the neck. In severe cases, skin over the entire body may be affected.
  • The lesions begin as indolent, chancre-like papules, becoming larger over the course of weeks and eventually form irregular pyogranulomatous nodules, which frequently ulcerate.
  • Mortality does not usually exceed 10% to 15% and the main loss results from the inability of animals to work for several weeks because of extremely painful lesions.
  • The ophthalmic form of the disease is less frequent. Infection may occur as conjunctivitis or a naso-lachrymal infection.
  • The infection rarely becomes generalised. Initial infection is characterised by a watery discharge from one or both eyes and some swelling of the eyelids, followed by the development of papules and ulcerating button-like growths on the conjunctiva and/or on the nictitating membrane .
  • The respiratory form of the disease is characterised by lesions which are mostly confined to the upper respiratory tract.
  • This form usually occurs as a late development in the cutaneous form of the disease.
  • On the nasal mucosa, the lesions begin as yellowish papules or nodules and these soon form crater-like granulating ulcers that bleed easily.
  • The lesions are usually found near the external nares. These lesions may also occur in the lungs.
  • Asymptomatic carriers can be identified clinically by the identification of fibrocalcific skin lesions at previous sites of infection. Such horses will give a positive result to an intradermal sensitivity test and positive reactions to serological tests.

Lesions

  • Gross lesions are manifested by pyogranulomas, purulent discharge of thickened superficial lymphatic vessels and enlargement of regional lymph nodes.
  • Histopathologically, a typical granulomatous tissue reaction occurs with a predominance of the large macrophages, many of which contain oval organisms in the cytoplasm.
  • Affected tissues stained by Gram stain revealed the presence of ovoid double-contoured yeast-like cells.
  • Periodic-acid Schiff or Gomori's methanamine silver stains are very useful to demonstrate the presence of the organisms.
  • Typical nodules of liquefied foci have also been recorded in the pleura, spleen, liver and bone marrow.
Last modified: Monday, 4 June 2012, 6:38 AM