Visna-Maedi Virus

Maedi-Visna

Pathogenesis

• Primary replication in lung macrophages
• These then carry infection to brain, lung, udder and/or joints
• T cells react: Type IV immune complex disease
• Target organs become chronically inflamed after 2-6 years
• Visna:
  • Inflammation results in demyelination with subacute meningitis around ventricles and choroid plexus
  • Posterior paresis progresses for up to a year until sheep can no longer stand
  • For more on joint pathology, see here
• Maedi:
  • Alveolar septa become infiltrated by lymphocytes and macrophages
  • Smooth muscle hypertrophy
  • PM: lungs are heavy, rubbery and do not collapse
• All result in loss of condition
• Infected animals remain antibody and virus positive

Epidemiology

• Transfer via aerosol, milk or colostrum
• Increased risk in winter housing

Diagnosis

• Clinical signs
• Serology: AGDT/ELISA for antibody

Control

• Seropositive and progeny must be removed from flock
• Farm can be re-accredited after 2 years of clear tests

• Caused by a retrovirus
• The respiratory from of the disease caused by maedi-visna virus (Maedi) is also called lymphoid interstitial pneumonia
• Transmitted by close contact and via milk
• The pulmonary lesions develop very slowly hence this disease is uncommon in sheep < 2 years old
• Increased respiratory rate upon exertion, loss of weight
• Remains in Monocytes and macrophages
• Gross findings
  • Severe interstitial pneumonia
  • Lungs fail to collapse properly on opening the chest and can weigh more than twice the normal weight
  • Impressions of the ribs remain on the visceral pleura
  • Lungs are a mottled grey/ tan colour - the lesions can vary from irregular grey speckling to homogeneous grey consolidation
  • Rubbery in consistence
  • Diaphragmatic lobes most affected
  • Associated bronchial and mediastinal lymph nodes are often enlarged
• Histologically
  • Major features are extensive lymphoid proliferation around perivascular, peribronchial and peribronchiolar sheaths associated with pulmonary lymphatics
  • Many of these areas contain germinal centres and smooth muscle hyperplasia (in walls of terminal bronchioles and alveoli)