Investigating unexplained lamb losses in Scottish hill flocks

Unexplained lamb losses are a major problem in many Scottish hill flocks, with average mortality recorded in a study averaging 18.6% and ranging from 8.4% to 25.8%.

These figures were presented by Fiona McAuliffe at last week’s Teagasc Hill Sheep Conference in Dungarvan, Co Waterford. Fiona carried out the research as part of her PhD investigating unexplained lamb losses, which she carried out with the support of the University of Edinburgh, SRUC and Nature Scot.

Fiona outlined that the Scottish sheep production system operated in many hill areas is extensive grazing on unenclosed upland areas. The main breeds of sheep are hardy native breeds which are not housed and lambed outdoors.

Flocks are typically handled at least five times a year – pre-mating (November), post-mating (January), marking (June), Shearing (July) and weaning (September), while lambs are typically sold as stores.

Blackloss

Lambs are not regularly supervised across extensive grazing systems, resulting in a lack of information on where and why lambs disappear. The term used for the unexplained loss of lambs is ‘blackloss’.

There are numerous suspected contributing factors including lambs succumbing to adverse weather, predation, parasitic infection, trace element deficiency, ewe effects such as mismothering etc, and a disease known as ‘plochteach’.

Plochteach is a disease which damages the liver and triggers photosensitisation on exposed skin. It is often first apparent on the ears of lambs and then is evident on their body when lambs lose wool through scratching, biting etc. Lambs become dull, cease eating and seek shade. Outbreaks typically occur in June and July.

Fiona explains that lambs may also die of shock or secondary infection. There is no specific treatment, but providing shade can prevent further photosensitisation. Not all lambs will succumb to the disease with wool growing back over the affected areas in lambs which recover. However, affected lambs are typically 4kg lighter than unaffected lambs.

Suspected cause

It is suspected that lambs ingesting bog asphodel is the cause of the disease with saponins which are found in the plant the root of liver damage.

This theory was reinforced through Fiona’s studies as areas where bog asphodel was dominant recording higher unexplained losses and also gave rise to the disease being identified at a much higher rate.

Extensive studies were carried out in SRUC’s high hill flocks across three large areas of mountain grazing, with records collected on over 3,000 lambs. The eight-year average blackloss rate was 8.3%, with the total lambs affected by plochteach recorded at 5.1%. The prevalence of plochteach in the blackloss population was recorded at 10.6%.

Further research

Fiona says that further investigation is required to improve our understanding of plochteach, to reduce the impact of the disease and identify appropriate treatment options.

Providing shade or shelter in upland areas through native scrub or woodlands is also viewed as a beneficial development.

Her studies also show that white-skinned (Scottish Blackface) single lambs raised in a hill environment were at higher risk of photosensitisation compared to black-fleeced sheep (Welsh Mountain).

Longer term, she said there is opportunities to possibly identify gene markers and investigate if sheep which are more tolerant to the disease can be identified.

On-farm study

An on-farm study carried out across four flocks including over 2,400 ewes found similar results, while tick-borne diseases were also found to possibly be a significant contributor to blackloss.

Gastrointestinal parasites (worms and fluke) were not seen as being as big of an issue as farmers suspected.

Other issues which were also not suspected, such as late castration and tail ringing were identified as contributing to lamb losses while killing with kindness – ie administering too many treatments on the one day were also identified by Fiona as leading to cases of mortality.