Investigation of Panicum grasses as causative agents of hepatogenous photosensitisation in livestock

The purpose of this thesis is to gain a better understanding of the prevalence and aetiopathogenesis of hepatogenous photosensitisation related to Panicum grasses globally and in Australia.
A systematic review of the literature revealed that photosensitisation is a common but underestimated disease that affects livestock worldwide. Australia showed the highest number of photosensitisation case reports compared to 19 other countries. Hepatogenous photosensitisation was the most frequently reported diagnosis and resulted in higher morbidity in livestock than other types. Panicum spp. were the most common causes of hepatogenous photosensitisation globally.
To better understand the impact of photosensitisation in Australian livestock industry, an online survey was circulated to livestock veterinarians, producers, and traders to evaluate their awareness and perception of this issue. Responses confirmed that veterinarians and livestock producers commonly encounter photosensitisation in livestock but that the economic significance depends on the stakeholder’s position in the value chain.
To determine the species most commonly related to Panicum outbreaks in the Riverina locality of NSW Australia, and area with, anecdotally, a high number of reported outbreaks of hepatogenous photosentisation on an annual basis, a plant survey was conducted. To identify Panicum species with a high level of accuracy, traditional taxonomic identification methods were used alongside a novel DNA barcoding method to confirm species identity. This barcoding method used one nuclear (ITS) and two chloroplast regions (matK and trnL intron-trnF) to distinguish Panicum species regardless of phenological growth stage. This multi-locus DNA barcoding method proved to be an accurate and cost-effective adjunctive tool for further distinguishing Panicum spp. at the species level. This plant survey also found that one previously uncommon species, Panicum hillmanii, was found to be highly prevalence across the entire collection region, a surprising result with interesting implications.
To determine the importance of Panicum species involvement in outbreaks of hepatogenous photosensitisation, targeted and non-targeted metabolomic profiling via UHPLC/Q-TOF-MS confirmed the existence of steroidal saponins in five examined Panicum species (P. capillare, P. hillmanii, P. gilvum, P. decompositum, and P. effusum). The work demonstrated that each species has distinct chemical profile that is strongly associated with their genetic divergence, and that P. hillmanii has a higher abundance of saponins, especially protodioscin, compared to the other four species examined. This novel finding suggests that P. hillmanii may has the potential of causing more photosensitisation outbreaks than others if this saponin fingerprint can be correlated with the hepatotoxicity.
Together, these findings demonstrate that photosensitisation outbreak patterns exist in different livestock species associated with specific plants, plant distribution and climate conditions worldwide. Multiple stakeholders across the livestock chain perceive photosensitisation as a common issue with a significant economic impact on the livestock industry in Australia. Panicum spp., and particularly P. hillmanii, possess specific saponins/sapogenins profiles with higher abundances are likely the leading putative cause of hepatogenous photosensitisation in livestock. Accurate species identification with the assistance of DNA barcoding would promote a better understanding of the distribution of Panicum species in Australia and their potential hepatotoxic potential.