Most herbaceous perennial pastures and shrubs used for livestock production provide a valuable and good quality food source.
However, occasionally some of these plants can poison livestock. Even though you may never encounter such a situation, it is worthwhile to be aware of the possibility.
This will enable prompt action and minimise losses should poisoning occur. For many of these plants there are reasonably well known predisposing factors involved in the development of toxicity. These will vary with the particular plant and type of toxin involved.
However, with most plants, the risk of poisoning is increased if the plant is the dominant pasture species available. Several clinical syndromes are involved in the poisoning of livestock following consumption of herbaceous perennial pastures and shrubs.
Photosensitisation:
Photosensitisation occurs after exposure to sunlight and is essentially an inflammation of the skin (dermatitis) and sometimes an inflammation of the conjunctiva and cornea of the eye (conjunctivitis/keratitis).
Photosensitisation is caused when light sensitive molecules, usually derived from plants, are deposited in the skin and cornea. These are activated by exposure to sunlight to cause considerable local damage. Access of sunlight to these molecules is blocked or diminished by pigmentation in the skin or long hair or wool.
For this reason, the physical changes of photosensitisation are most apparent in areas with white hair or wool and those least covered by hair or wool, such as eyes, eyelids, ears, lips, face and coronets. Photosensitisation is not the same as sunburn.
It is caused by exposure to visible light and does not require prolonged exposure. Sunburn is caused by ultraviolet light and requires prolonged exposure. Animals showing clinical signs of photosensitisation might:
- appear restless
- seek out shade
- shake their heads, or
- rub against other objects.
Affected parts become swollen due to fluid accumulation under the skin (oedema) and this might cause the ears to droop. This will progress to weeping of the oedema fluid though the skin, death of the skin, formation of hard scabs and sloughing of the dead skin to leave raw wounds.
It is likely there will also be discharges from the eyes and the cornea may become opaque. In many cases animals may appear lame because the coronets become inflamed. If animals are removed from the source of the problem and into shade as soon as the early behavioural changes or signs of oedema are noticed, they will recover well.
However if removal is delayed, extensive damage may occur and some animals may die. There are two types of photosensitisation:
- Primary photosensitisation occurs when plants contain light sensitive molecules and these are absorbed unaltered by the animal eating the plant. This is very rare in perennial pastures but has been reported in lucerne and birdsfoot trefoil.
- Secondary photosensitisation occurs when ruminants and occasionally horses suffer damage to their liver and then graze green pasture. A plant toxin is the most common cause of the liver damage. Chlorophyll from green pasture is metabolised by bacteria in the alimentary tract to phylloerythrin, a very potent light sensitive molecule.
When this is absorbed it is normally extracted from the blood by the healthy liver and excreted, however a damaged liver is unable to remove all the phylloerythrin from the blood and its concentration increases, causing photosensitisation. A number of sub-tropical grasses can cause secondary photosensitisation including signal grass, bambatsi panic and panic grass.
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