NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Photosensitisation in Cattle

The NADIS records show that photosensitisation was first recorded in May last year

What is Photosensitisation?

Photosensitisation occurs when the presence of a chemical makes skin become sensitive to sunlight (particularly UV wavelengths). This leads to skin damage and loss. It is not very common in the UK, but it causes significant economic loss, particularly when it occurs in groups of animals.

There are three sorts of photosensitisation. Direct photosensitisation occurs when the chemical comes from a defect in the animals metabolism of its red blood cells, or, more commonly, from plants such as St. John’s wort. Secondary photosensitisation occurs in animals with liver damage. This damage interferes with the complete breakdown of chlorophyll, resulting in the accumulation of a photosensitive chemical. Local photosensitivity can also result as a reaction to the sap of some plants.

Most commonly affected sites are those exposed to direct sunlight, including the udder (which is exposed when the animal lies down)

Clinical Signs

· Non-pigmented skin affected

· Hair loss, reddening, peeling

· Ulceration of skin

· Crusting, bleeding

Diagnosis

· On the clinical signs described above

· A veterinary diagnosis is important in order to rule out liver failure

Treatment

· Removal to cool shaded housing

· Fly control

· Supportive therapy

· Treatment of liver failure (if present)

Prevention

Do not breed from animals with photosensitisation due to a genetic defect

Identify and remove possible plant sources of photosensitising chemicals

Richard Laven PhD BVetMed MRCVS