A5 Q2 Torch Summer Magazine 2024 - Flipbook - Page 18
Case history... Photosensi琀椀sa琀椀on
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Lambs are on good grazing, not yet weaned.
Have been treated for nematodirus with a white drench 4 weeks ago
Had 昀椀rst Ovivac P Plus 4 weeks ago at same 琀椀me as BZ drench.
One lamb a昀昀ected in a group of 600. Rest appear to be 昀椀ne, this has “only
just happened”.
Figure 1 - picture credit Jona-
Figure 2 - picture credit Jonathan Hobbs
Signs start within 4-5 days of exposure to the inci琀椀ng cause.
There can be two di昀昀erent underlying mechanisms.
Primary: sheep directly grazing plants that contain a photodynamic agent
Secondary: underlying liver damage. This prevents the liver from adequately
breaking down the chlorophyll in grass. This produces phylloerythrin, which is a
photodynamic agent. The a昀昀ected sheep may also be jaundiced.
In both cases, the photodynamic agent enters the general circula琀椀on. There
are areas where the blood vessels are close to the skin and not protected by
either pigmenta琀椀on (black skin) or wool, such as the face and ears. When UV
light strikes the photodynamic agent in these areas, it produces a very localised
chemical burn. This is similar to sunburn, except that it takes only a few minutes
of exposure to sunlight to produce the burn.
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Plants associated with photosensitisation include
Bog Asphodel
Ragwort
Grazing brassicas when still growing (especially rape and kale)
Buckwheat
Mycotoxins - e.g. blue green algae
Giant hogweed (direct contact, not grazing)
It is said that photosensi琀椀sa琀椀on issues o昀琀en occur when sheep are newly moved
onto a pasture that contains the toxic plants in a decent percentage of the sward.
This is less true on modern leys. We 昀椀nd sheep can be sporadically a昀昀ected when
grass is less plen琀椀ful and sheep are more likely to be 昀椀nding green pick in the
hedgerows. Bog asphodel may be a risk on higher or we琀琀er ground, e.g. moorland
grazing.
In this par琀椀cular scenario, only one lamb appeared to be a昀昀ected. It could be that
this was the only lamb to 昀椀nd and consume the toxic plant. Or it could be that
there are more lambs that are either in the early stages or more subtly a昀昀ected
than this single lamb.
