Nine cases of photosensitization in horses that affected the skin and eyes have been reported by researchers, who put it down to eating hay containing wild parsnip.
Primary photosensitization rarely occurs in horses and can easily be misinterpreted.
Researchers with the Free University of Berlin, writing in the journal BMC Veterinary Research, described the photodermatitis and eye problems in the nine horses in a just-published case series.
The animals were affected by skin erosion and inflammation around their muzzles and on the skin over their penis.
Primary photosensitization occurs in animals and humans when phototoxic compounds accumulate in the skin, cornea or mucous membranes after eating, or having direct contact with, plants or chemicals that can trigger the condition.
Photosensitization outbreaks have been reported in scientific journals from 20 countries, with the highest number of reports from Australia and Brazil. Only 12 reports related to Europe.
Sheep were the most frequently reported species, followed by cattle and horses.
Different plants have been identified as causing photosensitization, depending on location. St John’s wort (Hypericum perforatum) is common in the United States, South America, Europe, New Zealand, and Australia, while Brachiaria decumbens has been identified as a cause in Brazil, Colombia and Nigeria.
Panicum grasses, Lantana species and the fungus Pithomyces chartarum are the most commonly reported causes in Australia.
The nine cases in Germany, from three different stables in Berlin and Brandenburg, most likely involved the wild parsnip (Pastinaca sativa).
Wils parsnips contain furocoumarins, a family of photodynamic pigments. When exposed to ultraviolet radiation from sunlight, furocoumarins interact with oxygen and produce reactive substances that can damage cell membranes in the skin and eyes.
Unfortunately, furocoumarin concentrations were not measured in the case series because of a lack of analytic equipment and the invasive nature of sample collection.
The affected horses, of different breeds, had variable degrees of skin redness, scaling, crusting and death of unpigmented skin at the head and prepuce (the fold of skin that covers the head of the penis).
Turning to the eyes, conjunctivitis was seen in seven of the nine horses. Blepharitis (inflammation along the edges of the eyelids) was identified in three of the nine horses, corneal edema without additional signs of keratitis and/or uveitis in two of the horses, and corneal edema with signs of uveitis in one horse.
The eyes of four of the horses were sensitive to light, the authors reported.
One horse developed corneal erosion.
Analyzed liver enzymes were within the normal reference ranges in all horses.
All three stables housing the horses had fed hay from the same distributor. Analyzed samples showed high contents of wild parsnip – plants, seeds and roots.
The horses were treated according to severity of their clinical symptoms with flunixine meglumine or prednisolone. Topically, either gentamicin, dexamethasone and/or atropine (1x/day) were applied. Skin care was provided with almond oil or dexpanthenol.
All horses were kept in a dark environment or were treated with sunscreen and facemasks, with treatment varying from 6 to 30 days.
The researchers concluded that ingestion of wild parsnip can trigger primary photosensitization with dermatitis and eye injury in horses.
They noted that wild parsnip is widespread in Europe and the hay distributor reported that it was growing in the field where the relevant hay was harvested, but usually in very low amounts.
In 2018, the relationship of grasses and wild parsnip changed in favor of parsnip. In Germany and other European countries, the years 2017 and 2018 were particularly difficult for hay production because of heavy rain in the summer of 2017 and severe drought in 2018.
“As a result, hay quality was low, prices increased, and hay producers were pressured to use all available grassland for hay production.
“Hence, weather-resistant plants may have gained an advantage in growth, leading to higher concentrations of uncharacteristic plants in hay and silage.”
They cautioned that, in times of extreme weather, hay may alter in botanical composition, resulting in high amounts of uncharacteristic plants causing novel problems.
The case report team comprised Winter, Katharina Thieme, Corinna Eule, Eva-Maria Saliu, Olivia Kershaw and Heidrun Gehlen, all with the Free University of Berlin.
Winter, J.C., Thieme, K., Eule, J.C. et al. Photodermatitis and ocular changes in nine horses after ingestion of wild parsnip (pastinaca sativa). BMC Vet Res 18, 80 (2022). https://doi.org/10.1186/s12917-022-03162-2