Vet Voice: Summer and photosensitivity

VET VOICE PHOTOSENSITIVITY

Summer and photosensitivity

Words by: Lisa Whitfield

Production Animal Veterinarian

If you have been lucky enough to have a break over summer, your gumboot tan may have had too much time in the sun, resulting in quite uncomfortable sunburn. The pain from severe sunburn is mild compared with what cows experience when they suffer skin burns from photosensitivity.

The long hot days will mean summer crop feeding is under way for many, and facial eczema spore counting has likely started in most northern regions.

Facial eczema and brassica photosensitivity are two commonly encountered photosensitivity diseases, and they both have overlap in the time of year that they occur. January through to the end of the warm season is the high-risk period for these diseases.

BRASSICA PHOTOSENSITIVITY Rape and leafy turnip are two species of brassica that are more often associated with signs of photosensitivity in cattle, although any species of brassica can cause it. With summer crop feeding usually starting in January, it is the first suspect for photosensitivity in the New Year.

An unknown compound in the plants results in liver damage, and this damage leads to a buildup of photoreactive compounds in the blood. Sunlight exposure triggers a chemical reaction and leads to the development of skin burns. The liver damage in brassica photosensitivity is not usually permanent, however.

A brassica crop is more likely to cause photosensitivity under the following conditions: 1. Feeding a crop that is still immature 2. A crop that has been droughtstressed.

Crop feeding should be done in a way that allows the cows to get used to the new diet, with a slow build up over at least a week. Cows should all have equal access to brassica crops, so rather than letting

Facial Eczema lesions on the udder – red, painful burnt teats and thickened udder skin. The initial liver damage that resulted in the development of photosensitivity would have occurred a few weeks before.

them go on following milking, give them a grass break first, then move them onto the crop together. Make sure you measure your crop yield so you are not under or overestimating how much the herd is being offered.

FACIAL ECZEMA Facial eczema is a widespread disease in warmer parts of the country. This fungal disease of ryegrass also leads to liver damage, however in this case the damage is permanent. Liver damage leads to skin burns in much the same way as brassica photosensitivity.

Spore counting is the method used to assess the level of risk in pasture. Pasture spore counts of more than 60,000s/g are considered high risk for the development of clinical signs of photosensitivity. However, unlike brassica photosensitivity which occurs soon after grazing an affected crop, the display of clinical signs with facial eczema is usually delayed by a number of days or weeks, following grazing of affected pasture.

Farms should perform spore counting from their own pastures rather than relying on regional spore counts due to the wide variability in where hot spots occur. Spore counting should be performed on a weekly basis, and zinc should be provided once spore counts are trending above 20,000s/g. ACUTE SIGNS OF PHOTOSENSITIVITY Cattle may show subtle signs when photosensitivity is first developing. Look out for signs such as: • Restlessness • Agitation • Tail-swishing • Stamping legs • Head-shaking • Shade-seeking • Redness and inflammation of teat skin.

As signs progress, the cow may become very agitated. She may develop a fever from the skin inflammation. If no shade is provided, she may be lying down and standing up constantly. Closer examination may reveal swelling of white areas of skin, which you can detect by running your fingers over the edge from black skin to white, revealing a raised edge.

THERAPY Affected animals should be provided with complete shade shelter to prevent exposure to the sun. Due to inflammation and severe pain associated with photosensitivity, anti-inflammatories should be given, particularly during the first few days. Antihistamine injections, especially for cattle may also make the cow more comfortable.

Cattle suffering from brassica photosensitivity should be removed from the crop and provided with alternative feeds. Often these cows will respond within days of being taken off the crop.

Clinical signs of facial eczema will often take weeks to show outward improvement, but the liver damage is permanent. Consideration should be given to the welfare of severely affected animals.

• Lisa Whitfield is a Manawatu-based production animal veterinarian.

Scan Me! This QR code takes you to a video of a cow showing signs of photosensitivity after grazing a droughtstressed rape crop.