Visna-Maedi Virus

Maedi-Visna

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

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)