Pyrrolizidine alkaloids (PA) cause liver damage in pigs, horse, cattle and sheep, and result in a chronic fatal hepatoxicity. In Australia, PAs are amongst the most important natural toxins affecting livestock. The main sources of PAs are plant families including Asteracae (Senecio spp., Ageratum spp.), Boraginaceae (Heliotropium spp., Echium plantagineum) and Fabaceae (Crotalaria spp.) (Parkinson, Vermunt & Malmo 2010).
This report details an ongoing case of pyrrolizidine alkaloid toxicity in a group of dairy heifers over a 10 month period.
A mob of 60 autumn calving Holstein dairy heifers, from a commercial dairy in the Finley district, were first investigated by a private veterinarian in June 2013 for sudden death, drop in production, ill thrift and weight loss/poor body condition. The heifers had calved six weeks prior to the initial investigation.
The cattle had been fed lucerne hay in racks since February 2013. The lucerne was grown on the property by the owner, cut by a contractor, and baled by the owner. It was later noticed by the owner that the lucerne hay which had been fed to the cattle was contaminated with common heliotrope (Heliotropium europaeum).
The investigation has continued over the past 10 months with the cattle showing poor body condition and reduced milk production. Deaths have continued at the rate of one or two per week. A total of 40 deaths have occurred over the past 10 months since the initial deaths. Three or four of the remaining heifers are ill.
Affected cattle presented dull and depressed, in very poor body condition, and have had reduced milk production. As the investigation progressed affected cattle developed photosensitisation and neurological signs including reduced awareness, wandering and blindness.
Three necropsies have been performed on affected cattle that died over the past 10 months.
The first post-mortem showed a very inflamed abomasum and an oedematous heart with increased pericardial fluid. The liver was slightly rounded and had a nutmeg appearance.
The second post-mortem showed grossly oedematous mesentery and a rounded liver. There were petechial haemorrhages in the omentum over the gall bladder and reticulum.
The third post-mortem showed an enlarged liver with a thickened liver capsule and fibrous bile ducts. The gall bladder and hepatic lymph nodes were significantly enlarged. Slight jaundice was observed through the GIT.
Virology and serology testing from the initial deaths in June ruled out bovine ephemeral fever, liver fluke and persistent BVDV infection. However, a strong positive was identified for the pestivirus antibody AGID test.
Throughout the investigation fixed samples of abomasum, liver, and heart were collected from the post-mortems for histopathology.
The pathologist consistently reported chronic liver damage suggestive of pyrrolizidine alkaloid toxicity or aflatoxin exposure. There was biliary hyperplasia with bridging portal fibrosis, as well as megalocytosis and mild karyomegaly of hepatocytes.
Histopathology of sections of the abomasum revealed occasional multifocal areas of lymphoid aggregates in the lamina propria, prominent diffuse lymphangiectasia, and severe diffuse submucosal non inflammatory oedema.
Histopathology of the heart showed a non-suppurative, oedematous epicarditis with a moderate vasculitis.
Biochemistry results from the initial investigation showed elevated liver enzymes, globulin, and pepsinogen, as well as a decreased albumin/globulin ratio. The results were as follows (please note the following table only shows abnormal levels. All other parameters were within normal reference ranges):
Blood samples were collected from 11 of the 20 remaining affected heifers during December 2013. The biochemistry revealed increases in GGT, GLDH or AST in eight of the samples. Three of the sampled cows also had an increased CK.
The differential diagnosis was either pyrrolizidine alkaloid toxicity or aflatoxin toxicity
A final diagnosis of pyrrolizidine alkaloid toxicity was made based on the clinical signs, histopathology results of the liver, increased liver enzymes and history of feeding heliotrope contaminated lucerne hay.
Common heliotrope is often toxic to livestock in Australia. Although it is abundant in predominantly winter rainfall areas, common heliotrope also depends on summer rainfall for development. Such conditions occur regularly in the Riverina, where heliotrope covers several thousand square kilometres (Parsons and Cuthbertson 2001).
Heliotrope, like Patterson's Curse (Echium plantagineum), contains PAs which produce toxic pyrrolic metabolites when ingested, that inhibit mitosis of hepatocytes resulting in liver damage and a chronic fatal hepatotoxicity (Parkinson et al. 2010; Radostits, Gay, Hinchcliff, Constable 2007).
Cattle are reported to be 30 to 40 times more susceptible to PA poisoning than sheep and goats (Radostits et al. 2007), and clinical signs may appear within a few weeks (Parsons & Cuthbertson 2001). Calves and younger cattle are more susceptible, while older cattle are more resistant to clinical disease (Parkinson et al. 2010).
Pyrrolizidine alkaloids are most toxic when heliotrope is consumed while green. However heliotrope is still toxic and is more palatable, following ensilage or drying in hay (Parkinson et al. 2010). This may have been a contributing factor to the development of PA toxicity in the heifers in this case, as the heliotrope was mixed in the lucerne hay.
Clinical signs of PA toxicity in cattle may initially be indistinct and variable, but may include depression, anorexia, diarrhoea and tenesmus, ill-thrift and wasting. Specific clinical signs develop when liver function is lost in affected animals, and may include jaundice and mild photosensitisation (Parkinson et al. 2010; Radostits et al. 2007).
Neurological signs will also develop with chronic PA toxicity as a consequence of liver insufficiency leading to hepatic encephalopathy. A compromised liver is unable to process nitrogen from protein which results in high blood ammonia levels. The accumulation of ammonia has been shown to be associated with cerebral oedema which results in nervous signs such as depression, aimless walking, head pressing, increased awareness, and blindness (Radostits et al. 2007).
Post-mortem findings of PA toxicity usually include a small, hard and sometimes nodular liver. The liver capsule is usually thick and grey and the cut surface is generally tougher than normal. Although atrophy of the liver was not seen in the post-mortems performed in this case, it is generally caused by a combination of malnutrition and the anti-mitotic effect of the toxin. Atrophy and fibrosis of the liver will impede blood flow through the portal vein, resulting in portal hypertension which may sometimes produce ascites and (as in this case) oedema of the abomasum, omentum and mesentery (Parkinson et al. 2010).
Megalocytes (hepatocytes with enlarged nuclei and increased cytoplasmic volume) are a characteristic histological lesion of PA toxicity, as seen with this case. PAs prevent cell division but not DNA synthesis as the hepatocytes try to divide to replace the cells that have undergone necrosis. Although megalocytosis is indicative of PA intoxication, it is not pathognomonic as it occurs with other toxins such as aflatoxins and nitrosamines. Chronic PA intoxication is typically also associated with hepatic fibrosis, biliary proliferation, and in some cases nodular regeneration (McGavin and Zachary 2007).
Once severe liver fibrosis occurs the changes are irreversible and are usually fatal, as in this case. Even with supportive therapy, the disrupted hepatic blood flow and the inability of the remaining hepatocytes to divide prevents hepatic regeneration, therefore prognosis is poor and slaughter should be considered (Parkinson et al. 2010).
We would like to acknowledge the Finley Veterinary Clinic for their work on this case, and the EMAI for performing the pathology on samples collected from this case.