Epizootic Lymphangitis is a highly contagious, chronic disease of horses, donkeys and mules, caused by Histoplasma capsulatum var. farciminosum, which is similar to H. capsulatum. The disease is widespread in Europe, Africa and Asia. It has been eradicated from the UK but is still under the notifiable Diseases of Animals Act.
Disease is spread by direct contact or indirectly, e.g. through infected grooming equipment and biting flies. Once established in a population it is very difficult to eradicate, which is not helped by the long incubation period. New cases can develop weeks or months after the infection appears to have been eradicated.
Ulceration and nodular lesions of the skin - mostly the neck and limbs as described above. Ulcers rupture, discharging blood-stained pus for several weeks. There will be swollen lymph nodes on physical examination and a typical lymphangitis appearance on the legs will be seen.
Pulmonary signs can also be seen in some cases.
Clinical signs and history would lead to a presumptive diagnosis.
Samples of lymph nodes and blood samples can be taken for diagnosis. The organism can be cultured and grows in the yeast phase at 37°C and in the mycelial phase at room temperature. Growth is slow, taking up to 8 weeks so is not ideal for primary diagnostic reasons. At room temperature, thick-walled chlamydospores can be seen. Organisms can also usually be seen in swollen leucocytes.
The organism can be stained using Gram, Giemsa, H&E and 10% Methylene Blue followed by Carbol Fuchsin and an enzyme-linked immunosorbant assay can be used to detect the fungi too.
Treatment and Control
In countries where this disease is notifiable, slaughter of the equid is required by law. In countries where treatment is used, options include potassium iodide, hamycin and Amphotericin B. Vaccination has also proven to be effective in endemic countries.
Injection of hyperimmune serum around skin lesions has been proven to be effective.
Life-long immunity follows infection, if the animal recovers.
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Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science
Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students University of Liverpool
Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) Equine Internal Medicine (Third Edition), Saunders.
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