B161 - MAEDI-VISNA

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B161 - MAEDI-VISNA

Nature of the disease
Maedi-Visna is a contagious disease of goat due to a lentivirus of the Retrovirirdae family. It is very similar to the caprine arthritis/ encephalitis agent. The same agent causes two different chronic diseases: Maedi (respiratory form) and Visna (nervous form). 
Classification
OIE, List B disease.
Susceptible species
Ovine and in a less extent caprine.
Distribution
Maedi-Visna is present in Europe, North America and a few countries in Africa and South America. It has never been reported in the Pacific Region.
Clinical signs 
  • In the Maedi form:
    • Incubation period of 2-3 years,
    • Loss of condition progressing to emaciation,
    • Intolerance to exercise,
    • Increasing dyspnoea,
    • Coughing
    • Death within 3-10 months.
    • Sometimes chronic mastitis
  • In the Visna form:
    • Incubation period of 2 years,
    • Progressive loss of condition,
    • Progressive ataxia, starting with hind limbs,
    • Hypermetria, 
    • Tremor of facial muscles, trembling,
    • Death within a year
Post-mortem findings 
  • In the Maedi form lesions are restricted to the thoracic cavity and include:
    • Enlarged and heavy lungs with grey coloration,
    • Solid parts raised above normal lungs, not collapsible,
    • Excess of frothy fluid in the bronchi,
    • Enlarged mediastinal lymph nodes.
  • In the Visna form lesions there is no gross lesions part from muscle depletion.
Differential diagnosis 
Specimens required for diagnosis 
Full blood on EDTA or heparin or specimens of lungs, mediastinal lymph nodes and brain in neutered buffered formalin can be submitted for virus isolation or PCR.

More commonly diagnosis is made from serums of suspected animals using Agar gel immunodiffusion test (recommended by OIE) or ELISA test. 

Transmission   
Transmission can be vertical through colostrum and milk.

Long and intense direct contacts can lead to horizontal transmission, indirect transmission is considered anecdotic.

Risk of introduction   
Maedi-Visna could be introduced by importation of incubating animals.
Control / vaccines  
There is no treatment nor vaccine. 

If introduced in a previously free country the eradication of the disease can be attempted by elimination of the contaminated flock and research of in-contact herds.

References
  • GEERING WA, FORMAN AJ, NUNN MJ, Maedi-Visna In Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, p. 163-168
  • Maedi, In n Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p. 1070-1075
  • Office International des Epizooties, 2002
  • Office Vétérinaire Fédéral Suisse
  • Visna, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 1120-1121