Acute-onset high-morbidity primary photosensitisation in sheep associated with consumption of the Casbah and Mauro cultivars of the pasture legume Biserrula.

Background: Primary photosensitisation (PS) subsequent to ingestion of the pasture legume Biserrula pelecinus L. (biserrula) has recently been confirmed in grazing livestock. Currently, the stage of plant growth resulting in acute primary PS has not been determined, nor whether both commercial varieties were able to cause this disorder. Given the potential utility of this pasture species in challenging climates, a grazing trial was undertaken to examine if both varieties 'Casbah' and 'Mauro' were able to cause photosensitisation in livestock, and if this could be mitigated by grazing in winter, or in combination with other common pasture species. Results: A controlled grazing trial was undertaken in winter in Australia. Plots were established in a randomised block design containing either a dominant pasture of Biserrula pelecinus L. cv. 'Cashbah' or 'Mauro', or mixed biserrula / ryegrass or mixed lucerne / perennial ryegrass / subclover control populations. A photosensitisation scoring system was established to quantitate mild / moderate / severe PS. 167 prime meat ewe lambs were introduced to the plots and monitored twice daily for clinical signs of PS. Mild clinical signs were observed at 72 hours on pasture. All animals were removed from biserrula dominant stands at this point (72 hours). Four of the animals grazing B. pelecinus L. var. 'Casbah' dominant pasture rapidly proceeded to severe clinical photosensitisation in the following 12 hour period. Animals remaining on mixed stands of biserrula / ryegrass did not exhibit severe PS (PS score <11/20) but showed an 89% (71/80) incidence of mild to moderate photosensitisation over the following 14 days. No animal showed signs of icterus. Where PS was identified, those animals were withdrawn from pasture at that point. Animals on 'Casbah' dominant pastures had significantly higher PS score compared to the animals on 'Mauro' at day 6. Animals on mixed lucerne showed significantly lower PS score than animals grazing biserrula varieties of any composition. The trial was halted at 14 days as only plots with a low proportion of biserrula still contained unaffected animals. As acute, severe PS has not been investigated on biserrula previously, the most severely affected lamb was subjected to complete diagnostic investigation. Necropsy revealed bilateral severe multifocal to coalescing erythematous ulcerations and alopecia on both aspects of the ear pinnae, severe bilateral periorbital and conjunctival oedema and variably severe subcutaneous facial oedema. No evidence of hepatopathy was present. A diagnosis of acute unseasonal primary photosensitisation caused by biserrula ingestion with no other underlying pathology was confirmed, the first documented report of this condition. This is also the first formal report of primary PS in animals grazing B. pelecinus L. var. 'Mauro'. Conclusions and clinical importance: We report an unseasonal outbreak of acute photosensitisation, established in <72 hours, in sheep grazing Biserrula pelecinus L cv.'Casbah' and 'Mauro' with exceedingly high morbidity. This study is the first to identify that both commercial varieties of biserrula can cause primary photosensitisation in sheep, and expands our definition of seasonal and temporal risk periods for biserrula photosensitisation. The speed of onset of clinical photosensitisation subsequent to biserrula ingestion is defined (<72 hours). The current study identifies the earliest phenological presentation of clinical photosensitisation to occur well prior to the reproductive phase of the plant's life cycle, indicating that the currently unidentified photodynamic compound that causes photosensitisation may exist across all life stages of the plant. Mixed pastures are not a guaranteed protection against PS in animals grazing biserulla. A low composition pasture of B. pelecinus L cv. 'Mauro' might be the least photosensitising option available to producers. A grading system is also proposed as a tool for objective and consistent clinical appraisal of future PS outbreaks.