Photosensitisation occurs in grazing animals due to accumulation of photodynamic molecules in the circulatory system and dermal tissues. The most severe reaction is in non-pigmented skin, which has the least protection from UV or visible light exposure (Quinn et al 2014). Sheep are affected in areas not protected by wool such as the ears, face and occasionally the legs. Primary photosensitisation is the result of accumulation of phototoxic plant molecules in the skin of herbivores. Secondary photosensitisation occurs when the photodynamic compound phytoporphyrin, a microbially produced metabolite of chlorophyll, is not cleared by the diseased liver of a herbivore and accumulates in the circulatory system and then in the skin (Quinn et al 2014).
Panicum spp. grasses are known to cause toxicity in sheep through accumulation and blockage of biliary ducts and renal tubules with saponin crystals (Radostits et al 2007). Several species occur in the Albury region including P. gilvum (sweet panic), P. miliaceum (common millet), P. capillare (witchgrass) and P. effusum (hairy panic) (Australia’s Virtual Herbarium, 2015). Signs of photosensitisation occur secondary to hepatic insufficiency. A definitive diagnosis of crystal associated cholangiohepatopathy is made when acicular negative outlines are found in bile ducts of the liver and cortical tubules in the kidneys (Button et al 1987).
Common Heliotrope (Heliotropium europaeum) is a well-known and widespread poisonous plant throughout the winter dominant rainfall area of south-eastern Australia (Cunningham et al 1981). Deaths in sheep most commonly occur due to chronic liver damage from multiple seasons of exposure to pyrrolizidine alkaloid toxins in the plant (Seaman 1987). This report describes a case where an initial diagnosis of acute common heliotrope toxicosis in lambs was made. As this is not a common diagnosis, the histopathology was later reviewed and the diagnosis was revised to crystal associated cholangiohepatopathy.
Initial contact was made on 9 December 2014 when the owner contacted the Murray Local Land Services office, reporting two crossbred lambs with apparent blindness and weeping eyes. He also described swollen, droopy ears and a presumptive diagnosis of photosensitisation was made over the phone.
A visit was made to the property in the Albury Shire to examine the lambs and inspect the pasture for potential causative plant species. The affected mob was comprised of 450, ten-month-old White Suffolk x Merino lambs. They were three weeks off shears and had been mostly grazing an unimproved pasture that included a small flood plain. Since shearing, they intermittently grazed a new lucerne stand, which was heavily infested with common heliotrope. The lambs crash grazed the paddock three times for 3-4 days at a time in an attempt by the owner to minimize possible risks associated with grazing lucerne and heliotrope.
The two most severely affected lambs had swollen, droopy, sunburnt ears, swollen faces including blepharitis obstructing vision, mucoid discharge from the eyes, nose and mouth and some distal limb swelling. Blood samples were taken for liver biochemistry. The remainder of the mob was inspected in the paddock and it was estimated that 20 were mildly affected. The farmer was advised to inspect the mob in the yards and draft any affected lambs out and monitor their recovery in the shed, giving access to fresh water and medium quality hay. A total of 23 affected lambs with swollen ears were removed from the mob to recover in a shed. The remainder of the mob was moved to a new paddock close to the yards and shed.
After two days most of the affected sheep had recovered and there were no new cases. Blood results showed marked increases in liver enzymes, bilirubin and copper (Table 1). The two worst affected lambs had not appeared to recover and they were euthanased as their prognosis was poor. Post-mortem examinations were performed and samples were taken for histopathological examination. The lighter lamb was extremely icteric with a slightly enlarged liver, dark kidneys and thick, dark bile (Figures 1 & 2). The heavier lamb had similar changes but less severe.
|Test||Normal||Heavy Lamb||Light Lamb|
|GGT||0-55 U/L||245 H||147 H|
|GLDH||0-30 U/L||23||67 H|
|AST||0-130 U/L||1427 H||1345 H|
|BIL||0.0-6.8 umol/L||168.6 H||276.8 H|
|CK||0-300 U/L||497 H||1062 H|
|Copper||7.5-20.0 umol/L||35.6 H||31.4 H|
The two paddocks were closely examined for hepatotoxic plants. Panicum sp. along with common heliotrope were identified. While pyrrolizidine alkaloids from heliotrope were initially suspected as the causative agent, the severity of signs from the short exposure time cast some doubt. Cholangiohepatopathy caused by Panicum sp. such as witchgrass and hairy panic and resulting in photosensitisation is well described during this season in this region. The unimproved pasture was inspected for Panicum spp plants, and although present, it was initially considered that they were in low enough proportion not to cause disease. Lucerne has been suspected as a cause of primary photosensitisation and was therefore a less likely agent in this case given the clinical signs and hepatic pathology.
Initial histopathological examination of the liver revealed mild hepatic megalocytosis and other changes associated with cholangiohepatopathy including bile retention, mild round cell hyperplasia around portal areas, moderate anisocytosis and anisokaryosis. There were dilated tubules with degenerate epithelium in the kidneys, with proteinaceous material in tubules and Bowman’s spaces. This supported the presumptive diagnosis of pyrrolizidine alkaloid toxicity from heliotrope.
Based on this diagnosis, the lambs were not grazed on the lucerne paddock again until the common heliotrope had died off. They were moved back to the original natural pasture paddock eight days after they had been removed as this was not considered a risk. No more cases of photosensitisation were observed.
As acute pyrrolizidine alkaloid poisoning in lambs has not been reported before in the searched literature, the case was discussed with senior pathologists with experience in the region. The histopathology slides for the liver and kidney were re-examined and acicular negative outlines resembling saponin crystals were found in each (Figure 5). This supported a revised diagnosis of crystal associated cholangiohepatopathy due to Panicum sp. toxicity.
The few seedling grass plants suspected to be a Panicum sp. could not be identified at the time and were no longer present in the pasture when the re-diagnosis was made (Figure 4). Identification would require growing the plant to the senescent stage before examination by a botanist, as Watt et al (2013) were able to do. The owner of the property, a trained agronomist, did not observe any plants resembling Panicum sp. during the remainder of the growing season, though he was not specifically looking for their presence.
While this case is a seemingly typical Panicum sp. toxicity, diagnosis was made difficult by several factors. Anecdotally, toxicity occurs in young lambs grazing pastures where Panicum sp. make up more than 50% of the available pasture (Morton 2010). Morbidity rates of 20% are expected in affected mobs. The low morbidity rate in this outbreak (23/450 or ≈5%) may be attributed to the low presence of the implicated plants.
The grazing pattern that caused the disease also included crash grazing of a lucerne stand that was heavily infested with common heliotrope. Lucerne has been associated with photosensitisation of unknown pathogenesis (Robson 2007) while hepatopathy due to pyrrolizidine alkaloid poisoning has been thoroughly studied (Lanigan et al 1978).
It is plausible, in this author’s view, that although the pathology in the liver due to crystal associated hepatopathy was significant, it may have been compounded by the addition of common heliotrope or lucerne in the diet. This hypothesis is supported by the absence of clinical cases in the mob while they were grazing the naturalised pasture before they were grazing the lucerne paddock, and once they were reintroduced to the paddock without access to the lucerne paddock. The cumulative effects of grazing these plant species has not been studied and further work in the area is required to test the hypothesis.