How does MVV affect productivity?

Since only a proportion of animals that become infected, actually develop disease and because only animals at least 2 years of age show signs of disease, it becomes necessary to ask if MV infection is worth worrying about.  The answer is yes and there are several studies to back this up (Table 1). 

Table 1.  Summary of research into effects of maedi-visna viral infection on productivity of sheep.

Findings

Author

5 flocks in Nova Scotia. Sero-positive ewes were only 2/3 as likely to lamb; Birth weights were 3 to 6% lower; 50 day weights were lower (AGID test)

Dohoo IR et al. 1987. Canada

1 range flock – did not find differences in productivity
11% of yearlings infected vs 93% of 7+ ewes (AGID test)

Snowder GD et al. 1990. western USA

66 flocks – sero-positive ewes greater than 5 years of age raised lighter lambs at 100 days of age. (AGID test)

Campbell et al. Ontario 1991.

66% of sero-positive ewes in one flock had damage to udder.  Damage was directly negatively correlated with lamb weights at 80 days of age with lambs averaging 1.7 kg less at weaning.

Pekelder JJ et al. 1994.
Netherlands

 

9 range flocks.  Found many indicators that sero-negative ewes had much higher productivity, e.g. 4.95 more kg lamb weaned per ewe exposed to ram; 0.11 more lambs weaned per ewe lambing; 0.09 more lambs weaned per ewe exposed; More likely to lamb (odds ratio 1.9); 0.15 kg higher birth weight; Pre-weaning mortality lower (OR= 0.8) (non-significant trend) (rTM ELISA)

Keen JE et al. 1997.  Western USA

No effect on SCC, or milk production in a flock of dairy sheep (AGID)

Legrottaglie et al. 1999.  Italy

Decrease of 0.94 kg/lamb at weaning for sero-positive ewes ≥ 4 years of age.
Increase of risk of lamb mortality (OR: 1.65)
No effect on litter size or lamb birth weight (CFIA ELISA)

Arsenault J et al.  2003. Quebec

Much of the loss of productivity can likely be traced back to reduced milk production due to mastitis.  Other losses may be connected to chronic wasting and early culling and death.  More research is required on the effect of MVV infection in dairy ewes.