In the late summer of 2019, the Veekijker received numerous notifications of sheep displaying symptoms in keeping with photosensitivity. No clear cause could be identified at that time. Further investigation was necessary in order to further interpret the signals and to determine the cause. When yet another flock of sheep was discovered to have symptoms of photosensitivity at the end of August 2020, GD visited the farm in question.
Several adult animals had the characteristic symptoms of photosensitivity: swelling of the head and ears, in some cases accompanied by extreme itchiness and irritation of the skin on the head, particularly around the eyes and at the base of the ears. In severe cases, the skin became necrotic and fell off. Yellow mucus membranes were also found in a number of animals, and a blood test showed them to have serious liver damage. Even those animals not displaying any clinical symptoms had developed extremely abnormal liver values. The impact of the disease was greater than expected beforehand.
Liver damage and relationship to photosensitivity
The liver damage in a number of the animals was studied more closely during pathological examination. Severely thickened bile ducts could be seen under the microscope. Such typical abnormalities, in combination with the clinical symptoms, point very strongly in the direction of intoxication caused by sporidesmin. The sheep had very probably ingested mould spores of the fungus which forms sporidesmin, Pithomyces chartarum. Following ingestion of these spores, mycotoxin is released and is subsequently excreted with the bile at 100 times the concentration. This is extremely damaging for the bile duct tissue. In the rumen of ruminants, phytoporphyrin is produced from chlorophyll, which may cause photosensitivity. Normally, this phytoporphyrin would be converted in the liver and excreted with the bile. This excretion process is reduced or non-existent if there is hepatic impairment, resulting in photosensitivity of the skin.
Pithomyces fungus likes to stay warm
The fungus P. chartarum flourishes on dead plant material in the grass. It grows best under warm and moist conditions, as long as the minimum day temperature stays above 12 to 15 degrees Celsius for at least ten days. When grass samples were taken at this farm, large numbers of spores of this fungus were found.
When sheep display symptoms of photosensitivity, it is essential to screen the animals from daylight as quickly as possible. This prevents further irritation and damage to the skin. However, the damage to the liver has often already been done. There is no treatment for this disease, other than providing supportive therapy. A number of animals recover, but others will have permanent liver damage. At this farm, the farmer was forced to dispose of several animals. It is as yet unclear why this disease has reared its head over the past two years. Further research must clarify this pathology and the risk factors. GD is calling on farmers and veterinarians to contact the Veekijker if they recognise the symptoms in a flock.
Photo 1. Liver cell necrosis and thickening of a bile duct
Sheep farm manager
“The cooperation went smoothly”
“One day, I noticed that a couple of sheep had fluid under their jaws. They were also a bit lethargic. As this can be a sign of haemonchosis, I immediately rounded them all up. While moving the sheep indoors, I checked the flock together with my vet. One sheep had a swollen head, another had weak rear legs and yet another could only walk backwards. This had developed dramatically therefore within a short period of time. The vet initially suspected a cobalt deficiency, but the blood test proved otherwise. While we did see a reasonably high copper level, there was no possibility of copper poisoning because I don’t use copper anywhere.
We requested advice from a Veekijker vet, who visited the farm and checked the sheep and the land they had been grazing. He then suggested that it might be photosensitivity caused by mycotoxins. We took grass samples for examination, and did indeed find that fungus.
The fungus had originated in mown grass, which I had left lying among the sheep after mowing. We’ve done so for the past six years, so this was simply bad luck. The fungus will automatically disappear again, as it is only in the top five centimetres of the grass and theoretically cannot survive the winter. The young ewes are back in the same field by now. If I’ve mown the grass and left it to lie on the field, we now keep the sheep away from it for at least a month. That’s all you can do about it, it’s one of those things you simply need to know.
Due to the severe liver damage in my sheep and the re-emergence of the clinical symptoms as soon as the sheep were turned out again, I decided not to wait any longer but rather to dispose of my flock of older sheep. I’ve been able to make a fresh start with my ewe lambs and I’m pleased with that.
The cooperation between ourselves, our own vet and the Veekijker vet could not have been better! It all ran so smoothly, with short lines of communication. It was useful that the GD vet was already somewhat familiar with the farm situation. Our own vet was also very open to advice, and did not feel passed over. She regularly called to consult and wanted to learn from the situation.”
“Effective cooperation between GD, the farmer and their vet makes or breaks our insight into a new disease”
Liza Dekker, veterinarian at Dierenartsen Amstel, Vecht en Venen
“In the case of new diseases, you really need research and advice”
“My colleague was the first to visit the farm, where there was a suspicion of cobalt deficiency, because of swollen heads. I’m responsible for many of the small ruminants within our practice, but happened to have that Friday off. She thought it was somewhat strange, because older sheep were affected and there were also a number of sheep running a fever. I spoke to the farm manager on the following Monday, by which time more sheep had fallen ill. On Tuesday, we tapped some blood and tested the liver values and minerals.
The results certainly surprised us. While the cobalt level was fine, the blood test for the liver and copper levels showed strong hepatic impairment and a raised copper level. Yet the sheep showed no symptoms at all of copper poisoning, such as yellow mucus membranes or brown urine. I wasted no time in calling the Veekijker. Luckily we have short lines of communication, which is useful in such unique cases. Pretty soon, it was suspected that the fungus might be to blame. A GD Veekijker veterinarian visited the farm and took grass samples, after which the diagnosis was made very quickly.
Two or three weeks later, I visited another farm with the same symptoms. The GD Veekijker veterinarian wanted to come along, and the grass samples and blood tests showed this to be the same problem. These have been the only two farms affected in my practice. In the same period, another farmer did have a young animal with a swollen head, but that really was a cobalt deficiency. The main difference versus cobalt deficiency is that the latter is mainly found in young animals. Once you see swollen heads in older animals, that should set the alarm bells ringing. It’s then time to look into the liver values.
At the first farm, pathological examination of a number of animals showed that they had severely damaged livers. There were severe symptoms in the flock and the affected sheep showed signs of photosensitivity as soon as they were turned back out. The farmer therefore decided to dispose of his flock of older sheep and to continue with his young animals who had not been in contact with the problem. What struck me most about this case, was the moment that I received the blood test results. I really thought: ‘So how is that possible?’ It didn’t make any sense. With hindsight, the raised copper level was probably due to the scope of the liver damage. As the liver cells were destroyed, high levels of copper were released in the blood.
In situations like this, we practitioners really appreciate the short lines of communication with the Veekijker veterinarians. Of course we always try to keep up to date on any new diseases, but what you notice in practice is that something is different. It’s then very useful to be able to make a quick phone call and ask for advice. We really do need support and research to be able to diagnose such new diseases.”