Visna-Maedi

VISNA-MAEDI

Synonyms

  • Ovine progressive pneumonia,lymphoid interstitial pneumonia,Chronic viral encephalomyelitis of sheep

Definition 

  • The visna – maedi are different clinical manifestations of the same viral infection. The “visna syndrome” is a slow progressive demyelination, whereas maedi-syndrome is a slowly progressive interstitial pneumonia

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

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

Last modified: Monday, 19 March 2012, 6:21 AM