With the arrival of summer when animals become more exposed to ultraviolet light, cases of photosensitization can occur.
In ruminants (cattle, sheep and goats), many causes can be caused by ingestion of certain plant species. As well, infectious causes such as liver flukes or leptospirosis can spark cases.
The bottom line is that liver damage results in an accumulation of a byproduct in the skin leading to the susceptibility to sunburn.
It will first show up in the thin-haired or lightly pigmented area of the animals.
Affected animals usually don’t die but anyone who has been badly sunburned knows the road to recovery is long. There is a definite economic toll on weight loss and failure to thrive. These animals can be in pain, so appropriate care is required, and they must be removed from the sun.
This applies to horses as well, and because horses often can have white socks, or other white areas, it can be very harmful to them as well.
Veterinarians may be confronted with only one or several cases at the same time. Depending on which areas are affected, there could be lameness if it involves the feet or weight loss if the mouth/muzzle and udder/scrotum or vulval area is involved.
On close examination, the skin may be swollen and sloughing in white areas. With dark-pigmented cattle (red especially) we see clinical signs more often around the mouth or lightly coloured udders, scrotums or the vulval areas.
It may start as being uncomfortable and result in a strange gait or being itchy. Areas exposed to lots of sunshine and that are thin-haired is where to focus attention for diagnosis. Even the whites of the eyes can be involved. Some pastures are open and lack access to good shade trees, sheds or any place offering substantial shade.
In severe cases, animals must get in the shade and stay there for long periods.
Severely affected cattle may need to be pulled and treated with supportive treatment, which could involve fluids if dehydrated and NSAIDs because open wounds can be painful.
If you discover one case, take a close look at other cattle, focusing in on the white areas.
A common plant that affects horses is white clover. I have seen numerous horses affected in the white areas. They often must be pulled off pasture and treated. Other species of plants should be introduced to the pasture, or it might be hard to use in summer for horses.
With photosensitization comes complications arising from open wounds and skin sloughing due to fly strike and bacterial infections.
Similar to a human burn patient, there is loss of fluids, so you and your veterinarian must react to the severity of the case.
Hydrotherapy, ointments, covering the wound and treating with antibiotics for secondary bacterial infections have been used in certain cases.
Predominantly white-haired breeds such as Charolais, all-white Herefords (RWF) and Holsteins being predominantly white will generally show the effects earlier than some other cattle.
The goal from a herd-health perspective is to determine what is causing the liver issue and then try and prevent it for the next sunny season. Even in overcast days, the ultraviolet light is strong, so burning is still possible.
As well, more cases may be seen following a wetter spring with lush growth during May and June.
When looking for the cause, we must consider various plants and a few infectious processes. Keep in mind anything that causes liver damage could cause this sunburn susceptibility.
For plants, white or alsike clover, St John’s wort and hogweed have been linked to this. Any plants that produce the pyrrolizidine alkaloids can cause liver damage, among other things.
Grasses such as perennial ryegrass, buckwheat or trefoil have also been identified as culprits. Mycotoxins produced in certain feed or blue-green algae are also potential causes.
When it comes to infectious causes, leptospirosis and liver flukes, in cases where enough liver damage has been caused, can be to blame.
If you encounter photosensitization, treat the clinical cases by removing those animals from the pasture, and determine the cause.
Plant control or controlled grazing at the times of year when the UV light is highest may help.
Cases may also have to do with an individual animal’s grazing patterns or individual susceptibility. Most severely affected animals are usually culled.
If we can determine that a plant species is the cause, eliminating it or avoiding grazing it at susceptible times of May and June are possibilities.
Roy Lewis works as a veterinarian in Alberta.