Visna-Maedi
Synonyms
Definition
Incidence
- Visna was reported in 1935 and Maedi was reported in 1939 in Iceland, in sheep
- In India, Maedi of sheep has been reported from different parts of the country
Transmisson
- Respiratory route
- Mononuclear cells in the colostrum and milk of infected ewes are infected with virus
- Passes through the intestinal mucosa and infects the lamb
Pathogenesis
- Entry of virus
- Replication in macrophages
- Cell associated viremia
- Dissemination of virus to brain and other organs
- Pulmonary secretion & milk containing infected macrophages transmit viruses
- Diseases like Jaagsiekte which increase the number of macrophages in lung secretions facilitate spread of virus
- Antigenic variation of virus occurs but neutralising antibodies are produced slowly
- Spread of virus is very fast than it can be neutralised
Clinical signs
- Incubation period – 2-3 years
- Only adult sheep are affected
- Clinical signs develop slowly
- Listlessness
- Loss of body condition
- Progressive emaciation
- Dyspnoea / coughing / nasal discharge
- Course of disease is 3-10 months
- In advanced cases, udder is enlarged and firm
Lesions of visna
- CNS – Demyelination with destruction of paraventricular white matter in the cerebellum and cerebrum
- Gliosis and lymphocytic infiltration
- Infiltration of lymphocytes and mononuclear cells in the meninges of brain and spinal cord
- Increased number of cells in the spinal fluid (pleocytosis)
LESIONS OF MAEDI / PROGRESSIVE PNEUMONIA
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Gross lesions
- Lungs do not collapse when thorax is opened
- Dense rubbery consistency
- Greyish color
- Distension of lungs 2-5 times as that of normal adult sheep lungs
- Cut surface is dry; no exudate
Microscopic lesions
- Thickening of the alveolar walls due to proliferation and infiltration of mononuclear cells,proliferation of fibroblast cells
- Lymphatic nodules in bronchi and bronchioles
- Intracytoplasmic inclusions in macrophages – appear blue color with Giemsa stain
- Follicular hyperplasia in spleen and lymphnodes, synovitis
Diagnosis
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Last modified: Monday, 19 March 2012, 6:21 AM