Pyrrolizidine alkaloidosis commonly affects grazing livestock in Southern Australia. Usually mature sheep show signs of hepatopathy after one or more seasons of exposure to plants high in pyrrolizidine alkaloids (PAs) such as Paterson's Curse (Echium plantagineum) and common heliotrope (Heliotropium europaeum) (Seaman JT 1987, Salmon D 2012). However, PAs are toxic to many species including humans and it has been documented that babies have died of PA exposure in utero and that they are more sensitive to the effects than their mothers (Rasenack et al. 2003).
This case report describes three cases of severe chronic liver disease seen in autumn-drop crossbred lambs from mixed farming properties in Central Western NSW from October to December 2018. The lambs had little if any exposure to PA-containing plants but their pregnant dams had grazed stubbles containing common heliotrope over the summer of 2017-2018. It is hypothesised that the lambs suffered chronic hepatopathy as a result of in utero exposure to PAs.
In October 2018, the death of 10/600 second-cross autumn-drop lambs over the previous few weeks was investigated. These lambs had been early weaned and had been in a feedlot for eight weeks. The ration consisted of barley, cereal hay and a concentrate additive. They had been vaccinated with 5-in-1 clostridial vaccine within the previous eight weeks. The owner noticed that the lambs were 'stargazing' before they died. The lamb examined was noted to be depressed and appeared blind but died before examination. The lamb was in light body condition.
On post-mortem there was abundant peritoneal and pleural fluid that did not clot on exposure to air. There was pericardial fluid with a fibrin 'chicken fat' clot. Grossly the liver was shrunken and irregular. It was very firm and pale. The rumen had a lot of fibre in it with minimal grain. The abomasal wall was oedematous. There were splash haemorrhages on the epicardium.
In November 2018, sudden death in stud Poll Dorset ewes was investigated. They had been losing ewes for a number of months and presumed that it was copper toxicity. However, they continued to die despite been drenched with sodium molybdate twice in the last two months, as prescribed by the private veterinarian. This property was next door to a property recently diagnosed with anthrax. Two immunochromatographic tests (ICT) were performed on two recently-dead ewes and both were negative.
During the investigation the owner commented that they had found a dead weaner lamb that morning and had another one sick. These were first-cross autumn-drop lambs born to Merino ewes. The ewes had grazed heliotrope over summer, however the farmer had not reported losses in these ewes.
Both ewes were golden yellow in the skin so a necropsy was performed on the fresher one. The carcass was very jaundiced and the liver friable. The kidneys were melting. The liver from this case was too autolysed to get a diagnosis, however liver sent from a later case confirmed PA toxicity with secondary copper toxicity.
The second lamb died prior to necropsy. It had been early weaned and the mob was on a feeder of barley. On post-mortem the liver was hard and pale and the kidneys pale.
There were ongoing losses in the mob with approximately 30/250 dying.
In December 2018, weight loss in a mob of 700 homebred second-cross lambs born in February of the same year was investigated. About 1% were losing weight and two had died. Seven had been drafted off for examination. All were listless and thin although well grown. Their mothers had grazed stubbles with common heliotrope and they were born onto similar paddocks.
One lamb was humanely destroyed. On post-mortem the liver was pale and firm with 1mm white spots through it.
Histopathological hepatic lesions were similar for the lambs in all three cases. There was abundant fibroplasia (more severe in cases 1 and 2), with concomitant loss of hepatocytes. The fibrous tissue, admixed with proliferating bile ducts (ductular reaction), expanded portal areas and bridged between portal and centrilobular areas.It dissected between individual hepatocytes and groups of hepatocytes, disrupting the normal hepatic architecture.Multifocally there was mild centrilobular fibrosis. Mild to moderate infiltrates of scattered lymphocytes and plasma cells were present.Many hepatocytes were enlarged (megalocytosis), up to about twice normal size. Hepatocyte nuclei were also enlarged (karyomegaly), up to almost twice normal size. In cases 1 and 3 the liver contained scattered foci of smaller, more normal-sized, hyperplastic hepatocytes (nodular hyperplasia).
The histological lesions were consistent with poisoning due to PA exposure, though the degree of fibroplasia is more severe than is usually seen in adult sheep.The megalocytosis, though present, was milder than is often seen with PA poisoning.Other than PAs, there was no evidence of exposure to any other hepatotoxins. Chronic aflatoxin poisoning can cause similar lesions, however there was no evidence that the sheep had access to mouldy feed.The mycotoxin, phomopsin, can cause a hepatopathy but could be ruled out since the sheep had not grazed lupin stubbles.
A testing methodology that detects sulphur-bound pyrroles, which are derived from PAs, in fixed liver tissue, has been used to confirm PA exposure (Small 1993). A positive result on this test would have provided further confirmatory evidence for PA intoxication, however unfortunately this test was not available.
On these three mixed-farming properties pregnant ewes grazed stubbles containing common heliotrope (Heliotropium europaeum). The affected lambs had no or limited access to heliotrope themselves. The liver changes were severe and chronic with much of the architecture lost and replaced by fibrous tissue. The clinical signs were consistent with a liver that was not functioning adequately. There were no clinical signs or losses due to secondary copper toxicity as is commonly seen in ewes with PA toxicity.
There is evidence in the literature of in utero PA intoxication in humans and some laboratory animals. In 2003, a 2-day-old baby died of liver disease caused by PAs. The mother's liver showed no toxic effects from PAs consumed during pregnancy from drinking herbal tea containing comfrey (Rasenack et al. 2003). They concluded that foetuses are more susceptible to PA poisoning than adults. Wiedenfeld et al., (2011), also state that foetuses and children show the highest sensitivity for PA poisoning.
If foetuses were generally more susceptible to PA poisoning than adults, one might expect to find multiple reports of congenital intoxication in farm animals. However, suspected congenital intoxication in farm animals is very rarely reported. There is one report of probable in utero PA intoxication in a foal (Small et al. 1993). We, the authors of the current case report, could find no published reports of either experimental or field congenital PA intoxication in cattle, goats or sheep. It seems that there is currently insufficient evidence to confirm whether or not ruminant foetuses, like human foetuses, are more susceptible to PA poisoning than adults, or indeed whether congenital intoxication occurs in these species. Individual PAs may vary in their ability to cross the placenta and cause foetal liver damage.
There is considerable variation in species susceptibility to PA intoxication. Pigs are more susceptible than ruminants, horses and cattle have similar, intermediate susceptibilities, while sheep and goats are least susceptible (Jubb, Kennedy and Palmer 2016). Sheep and goats appear to be the most resistant ruminants because their rumen flora contains more active PA-degrading bacteria and their hepatic drug-metabolising enzymes detoxify PAs more efficiently than cattle and horses (McKenzie 2002).
Young animals are regarded as much more susceptible than adults (Jubb, Kennedy and Palmer 2016). For neonatal ruminants this may be at least partly due to not yet having a functioning rumen. A case of PA poisoning was reported in calves that ingested bedding straw contaminated with Heliotropium europaeum (Harper et al. 1985). Neonatal animals can also become poisoned by PAs contained within maternal milk (Wiedenfeld et al. 2010). In our report some limited exposure to PAs through milk or direct ingestion of Heliotropium perhaps can't be entirely excluded. Besides the species variation in susceptibility to PA intoxication, there is also variation in the concentrations of alkaloids within different plant species and variation in relative toxicity between individual PAs. It is perhaps worth noting that H. europaeum has a higher concentration of alkaloids (range 0.5 to 3.1% dry weight), than Echium plantagineum (0.07 to 0.5% dry weight) (Anon 1978), another common pasture weed in the central west of NSW.
Though it cannot be proven, the hepatopathy in these lambs may represent a case of in utero intoxication due to H. europaeum. The potential for in utero exposure to PAs needs to be considered when advising producers on grazing pregnant stock on pastures containing plants high in PAs.