Photosensitivity is caused by compounds in the blood that react to sunlight. Liver function is the key. Anything affecting the liver's ability to metabolise or excrete harmful compounds predisposes an animal to photosensitivity. If the liver is not functioning, toxic compounds build up in the blood. When toxins reach the skin and react with sunlight, they cause photosensitisation.
The source of these compounds can be primary, through the animal's diet or an inherited defect in the animal's metabolism of its red blood cells, or secondary due to liver damage from facial eczema (FE), leptospirosis, or anything that interferes with normal metabolism.
Signs of photosensitivity include:
- Twitching, flicking of ears and tail
- Irritability, stomping and kicking at self
- Swelling around the eyes, ears, udder, and feet
- Hair loss, reddening, thickening, and peeling of affected skin
Diagnosis is based on signs; a blood sample can be taken to assess liver function.
Treatment is primarily supportive, including the provision of shade, topical treatment with zinc oxide-based sunblock ointments and fly control. Depending on the cause, antibiotics for secondary skin infection and/or prevention of hepatitis may be useful. Early use of anti-inflammatory drugs reduces the severity of the disease and improves the welfare of affected animals.
Prevention depends on the cause - FE, spring eczema and brassica crops are the three most common causes of photosensitivity in New Zealand.
Facial eczema causes photosensitivity and/or death due to liver injury. Humid conditions trigger growth and formation of fungal spores Pithomyces chartarum. These produce the toxin sporidesmin, which when ingested, causes liver damage, especially to the bile system, and impairs the excretion of phylloerythrin (a breakdown product of chlorophyll). Symptoms of FE liver damage are seen 10 to 20 days after consumption of spores.
Strategies to minimise the risk of FE include the use of spore-counting to identify risk periods and areas. At high-risk times, graze the most exposed, poorly sheltered pastures (often west or south-facing); increase rotation rate so pastures maintain some leaf length (don't graze down to base levels where the spores are); feed fodder crop or other supplements if available; spray pasture with fungicide and use zinc salts, such as zinc sulphate in water troughs or zinc oxide for oral drenching, as these have a protective effect against the toxin.
The cause of spring eczema is uncertain. Unlike FE, the liver is not necessarily affected but previous liver damage doesn't help.
Brassica crop photosensitisation is most often seen within 10-14 days of going onto summer turnips, especially if under drought stress. Eating immature rape can also cause photosensitization (‘rape scald'). Prevent the risk of breakouts by maintaining adequate voltage and double-fencing of breaks; gradually introducing crops and grazing them at the optimal stage of maturity; and offering supplements or pasture before going onto new crop.
