Date Published: February 27, 2019
Publisher: Public Library of Science
Author(s): Yuchi Chen, Jane C. Quinn, Leslie A. Weston, Panayiotis Loukopoulos, Juan J. Loor.
Photosensitisation is a clinical condition occurring in both humans and animals that causes significant injury to affected individuals. In livestock, outbreaks of photosensitisation caused by ingestion of toxic plants are relatively common and can be associated with significant economic loss.
The agents that are most commonly implicated in outbreaks of photosensitisation have not been formally investigated on a global scale. To address this question, a systematic review of the literature was undertaken to determine the most common causative agents implicated in outbreaks of photosensitisation in livestock in Australia and globally, as well as the prevalence and morbidity of such outbreaks.
A systematic database search was conducted to identify peer-reviewed case reports of photosensitisation in livestock published worldwide between 1900 and April 2018. Only case reports with a full abstract in English were included. Non peer-reviewed reports from Australia were also investigated. Case reports were then sorted by plant and animal species, type of photosensitisation by diagnosis, location, morbidity and mortality rate and tabulated for further analysis.
One hundred and sixty-six reports qualified for inclusion in this study. Outbreaks were reported in 20 countries. Australia (20), Brazil (20) and the United States (11) showed the highest number of peer-reviewed photosensitisation case reports from this analysis. Hepatogenous (Type III) photosensitisation was the most frequently reported diagnosis (68.5%) and resulted in higher morbidity. Panicum spp., Brachiaria spp. and Tribulus terrestris were identified as the most common causes of hepatogenous photosensitisation globally.
Hepatogenous photosensitisation in livestock represents a significant risk to livestock production, particularly in Australia, Brazil, and the United States. Management of toxic pastures and common pasture weeds may reduce the economic impact of photosensitisation both at a national and global level.
Photosensitisation is a global health issue affecting domestic livestock production with numerous underlying aetiological causes . Clinical photosensitisation occurs when photodynamic compounds accumulate in the skin, cornea, and/or mucoid membranes . Any portion of the animal exposed to sunlight and lacking protective fleece, hair or pigmentation can develop lesions within minutes to hours of exposure . Severity can range from mild erythema and oedema to severe necrosis and skin sloughing .
Following review of the peer-reviewed scientific literature, 78 reports presenting with a full text or detailed abstract were analysed (Fig 1). Data on causal plant species or organisms; geographical location; type of photosensitisation (primary, hepatogenous, congenital, unknown); outbreak years; animal species; size of flock or herd, percentage of morbidity and mortality were extracted and tabulated. A summary of information presented in the peer-reviewed literature is shown in Table 1.
Photosensitisation is a common, but likely underreported, entity in the literature. Hepatogenous photosensitisation is by far the most common presentation. Some species, the Panicum genus of grasses and Pithomyces chartarum in particular, consistently were reported in photosensitisation cases. Novel species are also implicated in outbreaks of photosensitisation, including pasture legumes Biserrula pelecinus, but primary photosensitisation is a rare occurrence in general. Significant variation in both morbidity, mortality and severity was observed in both peer-reviewed and non-peer reviewed reports. Variations in reported outbreaks may reflect true differences in morbidity rates between aetiological agents, but may also be partly due to the fact that mild presentations are overlooked, and lesions are not consistently graded by an unified standard. Together, our findings help identify the aetiology and geographical patterns, the plant and animal species implicated, and morbidity and mortality patterns of photosensitisation in livestock globally. This suggests that control of pasture species or weeds known to cause toxic outbreaks would have a significant impact on the prevalence of the condition in livestock globally, but also particularly in Australia, Brazil, and the United States where these outbreaks appear to be more common.