Photosensitisation of livestock grazing Narthecium ossifragum: Current knowledge and future directions.

Photosensitisation diseases can cause production and animal welfare losses world-wide. In North-West Europe a photosensitisation disease complex known as 'plochteach', 'yellowses', 'saut' and 'alveld' occurs in lambs on extensive pastures containing bog asphodel (Narthecium ossifragum). Affected lambs develop lesions on the ears, face and sometimes the back, with erythema, oedema, ulceration and necrosis that can be followed by secondary infection and death. Adult sheep appear unaffected, the incidence in lambs varies from year to year and there are variations in susceptibility between- and within-breeds. The definitive cause remains uncertain although ingestion of N. ossifragum, which contains hepatotoxic saponins, has been implicated in the aetiology. However, problems replicating the disease complex by feeding N. ossifragum in a controlled environment have led to alternative hypotheses, including possible intake of toxins from fungal spores and cyanobacteria. Further research is required to assess the putative role of N. ossifragum, the scale of economic and animal welfare losses associated with the disease, how best to identify affected animals before external clinical signs appear and the treatment and management of clinical cases. Given the challenges involved in isolating the causative agent(s) of plochteach, an animal breeding route may be effective if heritability of resistance/susceptibility can be demonstrated.