Photosensitivity Disorders in Grazing Ruminants
Summary
Photosensitivity disorders in grazing ruminants arise when light-sensitive compounds or their metabolites accumulate in the skin, leading to dermatitis, oedema and necrosis of non-pigmented areas. These conditions are broadly classified into primary photosensitisation, arising from ingestion of photodynamic agents; hepatogenous photosensitisation, resulting from liver damage and failure to excrete phylloerythrin; and congenital or iatrogenic forms. Clinically, affected sheep, cattle and goats exhibit restlessness, seek shade, develop erythema, crusting and sloughing of skin, and may suffer secondary infections. Outbreaks incur significant economic losses through reduced weight gain, veterinary costs and mortalities. Globally, toxic plants such as species of Brachiaria, Panicum and Tribulus, alongside fungi-derived pigments and anthropogenic chemicals, underpin most hepatogenous cases, whereas primary photosensitisation often follows ingestion of certain Amaranthaceae or Polygonaceae. Advances in biochemical analyses have clarified the role of steroidal saponins in grass-induced liver injury and illuminated the complex interactions between plant secondary metabolites, rumen microflora and host genetics. Integrated management—combining pasture monitoring, selective breeding for resistance and timely removal of toxic forage—remains central to controlling photosensitivity in extensive grazing systems.
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Photosensitivity Disorders in Grazing Ruminants publication trend
The graph below shows the total number of articles in photosensitivity disorders in grazing ruminants across all publications each year (not limited to Nature Index journals).
Technical terms
Photosensitisation: Skin damage caused by activation of light-sensitive compounds in exposed areas.
Primary photosensitisation: Direct skin injury following ingestion of photodynamic agents, without prior liver dysfunction.
Hepatogenous photosensitisation: Secondary photosensitivity resulting from impaired hepatic clearance of chlorophyll metabolites.
Saponins: Amphipathic plant glycosides that can induce bile duct obstruction and hepatotoxicity.
Phylloerythrin: A chlorophyll breakdown product normally excreted by the liver but phototoxic when accumulated.
References
- The aetiology, prevalence and morbidity of outbreaks of photosensitisation in livestock: A review. PLOS ONE (2019).
- Signal Grass (Brachiaria decumbens) Toxicity in Grazing Ruminants. Agriculture (2015).
- Content of the Saponin Protodioscin in Brachiaria spp. from the Eastern Plains of Colombia. Toxins (2017).
- Primary photosensitization in equidae and ruminants in the Brazilian semi-arid caused by Froelichia humboldtiana (Amaranthaceae). Pesquisa Veterinária Brasileira (2007).
- Identification of lamb flocks susceptible and resistant against Brachiaria poisoning. Pesquisa Veterinária Brasileira (2016).
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