TOXIC CAUSES

Photosensitisation

This is a condition seen in grazing animals, and it is equally as common in adult cows as in young stock. It is caused by an accumulation of light-reactive pigment in the skin.

• Primary This is caused by the animal actually eating the photosensitising compound. Exam­ples include the chemicals contained in such plants as St John’s wort and buckweed and also lantana, which is common in southern Africa.

Plate 10.18. Skin tuberculosis (skin TB) is typically seen as a row of nodules under the skin of the

• Secondary In this instance there is a neck. dysfunction in the liver and chemicals which

Plate 10.19. Photosensitisation. The initial insult probably occurred 3-4 weeks previously and by this stage the damaged skin is virtually painless and flaking off, with new (pink) skin forming underneath.

are normally detoxified in the liver accumulate in the body. The best example of this is facial eczema, which is caused by ingestion of toxins produced by Pithomyces, a fungus which grows on ryegrass in New Zealand.

An obstructed bile duct can also lead to the accumulation of toxic products. The most common of these is phyllo- erythrin, a breakdown product of chlorophyll, the green pigment found in plants.

Clinical signs

In the very early stages of the acute disease the animal may simply be showing signs of liver failure, e.g. depression, off food and incoordi­nation.

Treatment

In the early stages the aim is to minimise the effects of the photoreactive chemical. If you suspect the condition, take the animal away from direct sunlight as soon as possible and shut it in a loose-box. Your vet will probably prescribe antihistamines and/or anti-inflammatory drugs to counteract the effects of the histamine and reduce any further skin damage. Antibiotic cover may be given to prevent skin infection and vitamins (especially A and D) to promote healing. While the skin is very raw, that is immediately after the initial ‘peeling’, fly repellents are useful to reduce irritation and to prevent fly strike and subsequent maggot infestation. Keeping the skin supple with a bland emollient cream also helps healing.

It can easily take a whole summer for the skin to completely heal and during this time the animal should be allowed out to graze at night only. There is no reason why photosensitisation should recur the following year, although the skin may be permanently damaged, similar to the scarring left after severe burns.

Urticaria (Blaine)

This is an allergic or hypersensitivity condition with a sudden onset. The chemical which causes the allergy is often not identified. Animals of all ages can be affected, but most cases are seen in cows and heifers over 12 months old. The face, eyelids, lips and sometimes the vulva become swollen, with oedema or dropsy fluid accumulating under the skin. A good test for this is to squeeze the tissue gently between your forefinger and thumb. You can make quite a significant depression and when you remove your hand the finger marks remain. Sometimes the allergy is so severe that the whole head and neck are affected and this may interfere with breathing. In addition to swollen eyes and face, the cow in Plate 10.20 also had raised lumps on her skin. She recovered rapidly with treatment.

Mild cases disperse without treatment, but if severe, antihistamines and anti-inflammatories can be used to alleviate the symptoms, and diuretics, drugs which remove fluid from the body, may help to decrease the swelling. The syndrome is also known colloquially as ting.

Septicaemia

Calves which have been severely ill due to septicaemia may lose their hair, especially around the head and face.

Plate 10.20. Blaine. Note the swollen eyes and face and the lumps on the skin over her shoulder.

Scouring

Calves which scour badly often lose the hair over their hind legs, as seen in Plate 2.22. This is thought to be due to undigested fat and other substances in the diarrhoeic faeces reacting with the skin. Multivita­mins may assist recovery, but most calves eventually recover naturally.

Poorly Mixed Milk Substitute

Poorly mixed milk substitute, in which the fat rises to the top of the milk, can produce hair loss around the muzzle. A typical example is shown in Plate 2.18. Details of milk substitute problems are given in Chapter 2.

Alopecia

Alopecia simply means hair loss, and a few calves seem to lose their hair for no obvious reason. They have neither scoured nor been ill and yet they may suffer from total hair loss over the whole body. Apart from giving an injection of vitamins A, D and E to improve skin condition, there is little that can be done for treatment. All the cases I have seen have recovered, although it may take two to three months.