What are the Clinical Signs of Maedi Visna?

Although maedi visna virus can infect any age of sheep including newborns, because of the slow nature of the virus most signs we see in sheep are in adults - usually greater than 2 years of age. Within an infected flock, the older the sheep the more likely they are to be infected, and the more likely they are to show signs of disease and lost productivity.

Chronic Pneumonia (Maedi) 

The most dramatic sign of MV is chronic pneumonia, in which an adult sheep will develop a chronic productive cough without a fever.  Sheep are usually > 3 years of age and more often 4 to 5 years when signs are evident. As the disease progresses, the animal will develop dyspnea and exercise intolerance, although it will still come up to the feeder or be observed grazing.  But inevitably, the sheep continues to deteriorate – losing weight and becoming more depressed, until it is found dead or is culled.  From 2 to 24% of sero-positive animals develop maedi.  These differences may be due to differences in strain pathogenicity, breed and management.

Video of sheep with respiratory form of MV

Mastitis

A much higher proportion of infected ewes develop a chronic lymphocytic mastitis either affecting one or both glands.  This differs from bacterial mastitis with which inflammation is due to neutrophils (polymorphonuclear leucocytes or PMN's). Inflammation is diffuse through the parenchyma in lymphoid follicles.  As time progresses, this is replaced with scar tissue.  As a result much less milk is produced and the udder feels uniformly hard to the touch – particularly at lambing.  The ewe is not ill and the milk appears normal.  The major effect though, is reduced milk production that negatively affects lamb growth. MV Udder

Neurological Disease (Visna) 

The neurological component is most often accompanied by cachexia (no appetite) and chronic wasting.   It is less common than maedi.  Affected animals can be younger, usually < 2 years of age. The sheep may be ataxic, paretic and drag a leg.  Hind legs are often affected more than the front but all 4 can be involved. They may also have depression, head tilt and fine tremor of the lips.  Occasionally they may also appear to be blind.  Differential diagnosis includes diseases such as rabies, listeriosis, polioencephalomalacia, aberrant larval migration, spinal abscess and scrapie, and so a diagnostic necropsy should be performed.

MV Thin Ewe

Wasting 

Sheep with either chronic pneumonia or the neurological form of the disease will lose condition, despite adequate nutrition and despite the rest of the flock appearing to be healthy.   Other causes of chronic wasting include dental disease, Johne’s disease, scrapie, parasitic abomasitis, abomasal emptying syndrome and caseous lymphadenitis.   Occasionally, sheep with MV will lose condition without other signs of disease.

Arthritis 

Much less dramatically than in goats, adult sheep may develop arthritis of the carpus and tarsus.  There is excess synovial fluid and soft tissue thickening of the periarticular regions.  Bursitis may also occur.  The articular cartilage is damaged and scarring of the periarticular cartilages occurs as the disease progresses.

 

Why Don't All Sheep Show Signs of MV When They are Infected With the Virus?

Antigenic variation of MVV is very common, and so there are many different strains of MVV and some clinical signs may predominate in a flock and be absent in another.  Additionally, there is a genetic link to severity of disease with some breeds being more affected and some animals within a flock.  There are differences in genetic susceptibility to infection and disease.  While it appears that the sheep can be infected, some may be better able to control viral expression. Good nutrition and management can delay some of the clinical signs of disease – or sheep may be culled for another reason before they succumb to MV.