Farming Tips and Advice – Videos and Blogs from Tommy the Vet

Photosensitisation in Cattle


Sunlight and sore skin

While on a call last week I got a call to a very lame stock bull. On arrival I first examined him walking from the pen. I quickly observed that it was stiffness more than lameness and that the bull was unusually red around the hairless parts of his body. He was also swishing his tail continuously due to the pain. He was particularly red around the nose, testicles and anus. This skin inflammation had made him sore and reluctant to move. It was becoming quite obvious to me that he was suffering from a condition we see at this time of year called photosensitisation.

Photosensitisation  in cattle

Photosensitisation is a reaction that can occur in cattle and sheep when a chemical compound builds in the skin and then reacts with UV light from the sun. These chemical agents that build up in the skin are called photodynamic agents. When they react with sunlight they release chemicals which damage the cells in the skin leading to a lot of swelling redness and sometimes ulcerating wounds.

Typically like this bull, animals will appear sore and stiff before you actually see the visible signs of the disease I described. Very often these animals will be very twitchy uncomfortable and like the bull I saw doing a lot of tail swishing. In some animals with a severe reaction you will see large areas of skin peeling off and ulcerating. It will typically affect areas of skin with poor hair cover or white hair. Around the eyes, nose, teats, testicles and perineum are usually where the most severe reaction tends to occur.

So the combination of sunlight and these photodynamic agents in the skin is the reason this reaction occurs. There are two main causes for these photodynamic agents to build up in the cells of the skin.

They can be primary meaning the animal eats plants containing these agents or also when their skin comes in contact with plants containing them. There a number of plants that can cause this reaction like St John’s Wort, buckwheat and many more. From my experience looking for these offending plants can often be fruitless. Usually we only tend to see sporadic cases and where larger numbers of animals are affected we will look more closely at the pastures.

The second cause of this problem can be when ruminants liver function is compromised leading to a build-up of a photodynamic agent usually excreted in bile. For me this is becoming more common and my main suspicion would be because of liver damage due to liver fluke infestation. So when I see an older animal with this condition I often tend to prescribe a fluke dose as well. So anything that affects the liver or causes liver damage can lead to photosensitisation secondarily.

So what are my treatment options?

Due to the fact that it is quite a painful condition I usually recommend some pain relief or anti-inflammatory medication. We also recommend that the animal is housed for 3-4 weeks to keep them away from sunlight. Depending on how much infection is present some animals can require antibiotics. It can also be useful to apply fly pourons to prevent fly strike. You can also apply sun blocking agents to the sensitive areas but I’ve never seen this done.

Thankfully this condition is quite sporadic and doesn’t tend to affect large numbers. Apparently where large incidences due to occur Zinc supplementation may help, but again I have no personal experience of this.

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