In the last week I have seen two cases with very similar clinical signs. The first was a bull that I examined a day previously for routine fertility exam and test. On that day we noticed a very mild rash on his scrotum which we said we would monitor.

The next morning I got a call to say he was completely off-form. He had a very red and inflamed muzzle and scrotum and his legs were swelled. He was also in huge discomfort continually swishing his tail. The second case was a cow that had gone off her milk and appeared in pain. On closer exam she also had a reddened inflamed udder, nose and around her vagina. Both were cases of a condition I see occasionally on my rounds at this time of the year that is called “photosensitisation”.

It is quite a complex biochemical reaction within the cow’s body that leads to the clinical signs.

There is a primary cause of the disease which involves the cow ingesting a plant like St Johns Wort which subsequently reacts in the liver. This breakdown product is then excreted in blood causing a reaction with UV light (sunlight) at skin level. This causes the severe inflammation in certain areas of the skin like around the muzzle, udder, scrotum and other parts of the cow’s skin.

The other form of the disease we see is secondary where there is liver damage due, for example, to fluke. There is a subsequent reaction, or problem with chlorophyll breakdown product in the skin leading to the clinical signs I described earlier.

So when I see these typical signs I will usually treat mild cases with anti-inflammatories and house them for two weeks.

Some breeds are more prone such as cows with whiter coats. I have seen it myself in all breeds. I think we see more of the secondary type of cases in the last number of years due to the increase of fluke in cattle.

So maybe it is another indicator for the importance of fluke control. It really is a condition that farmers need to spot early and if getting larger numbers of affected cases look for the underlying factors involved. So it’s worth thinking about this in cows with some of these symptoms as, hopefully, we will see more sunlight for the coming months.