Incidence of Photosensitization in Husbandry Animals: A Meta-Study on the Effects of Feed Diversity and Feed Choice

Abstract

As this is a meta-study, we examined whether plant species diversity in the feed and the possibility of feed choice would influence the number of cases of photosensitization in farm animals. We evaluated 110 scientific references which described 172 cases of photosensitization worldwide, mainly in husbandry animals between 1926 and 2022. More than 50% of the cases occurred in South America and Australia. Among the animal species, sheep and cattle were statistically overrepresented. A total of 35 organisms were revealed to be phototoxic: 24 herbs, 2 grasses, 7 woody species, and 2 kinds of fungi. Animals developed mainly secondary photosensitization due to fresh feed (71.8%) of normal quality (88.1%), indicating that the phototoxic agents are from liver-toxic plants such as the grass Brachiaria and the herb Froelichia. Horses fell ill chiefly with primary photosensitization due to directly acting phototoxic agents of plant species such as the herbs Medicago and Pastinaca, both in fresh and conserved feed. Goats manage to avoid phototoxic plants under both high and low feed diversity if they still have free choice between plant species. High feed diversity reduced the incidence 2.4-fold, while enabled selection possibility even reduced it 7.5-fold. Since the lack of choice between forage plants was revealed to be the main cause of photosensitization, this knowledge could be used to prevent the disease in livestock.