When investigating sudden death, several aetiological categories need to be considered. These include toxicities (plant and otherwise), infectious diseases, nutritional accidents, misadventure and anaphylaxis (Vermunt, Malmo & Parkinson, 2010). In the case presented, two heifers were found dead well after it was feasible to conduct an autopsy. Is a history and property examination sufficient for reasonable speculation on the cause of death despite the limitation of not having supportive necropsy and laboratory findings. The authors will present what they consider the evidence for and against a diagnosis of hemlock poisoning.
The death of 2 of 35, homebred 12-month-old Angus heifers was investigated on 23 September 2011. The cattle were running in an 18 ha paddock. The first heifer was found dead in the creek running through the paddock two and a half weeks prior to the veterinarian's visit, while the second heifer was found dead in the same location and position one week later. Both heifers were judged to have been in good body condition (body condition score approximately 3/5), and neither showed any indications of illness prior to their deaths although there was a two-day time lapse between the last sighting of the cattle alive and the discovery of them dead. No obvious abnormalities were noted in the carcasses, with no signs of bloat in these or the remainder of the herd. As the cattle had died well prior to the veterinarian's visit, a necropsy was not considered.
The heifers were born in spring 2010 and weaned in April-May 2011, with no new introductions into the group. The heifers were vaccinated at marking in early March with a 5-in-1 and again at weaning. They had not been recently drenched (exact worming history unknown). The remainder of the herd was moved from the paddock immediately following the discovery of the second death, and there were no subsequent mortalities. There is no history of significant infectious or nutritional herd diseases affecting the property.
The heifers had been run in the paddock since weaning in April-May. The paddock was inspected on foot. A small, slow-flowing creek ran through the paddock, with no evidence of blue-green algae. The pasture consisted of lucerne (Medicago sativa), phalaris (Phalaris aquatica), cocksfoot (Dactylus glomerata), prairie grass (Bromus catharticus), ryegrass (Lolium perenne) and occasional recently sprayed serrated tussock (Nasella trichotoma). The lucerne was short and semi-dormant at the time of the heifer deaths. Common hemlock (Conium maculatum) grew in abundance under trees by the creek (refer to Figure 1), with shorter clusters interspersed throughout the remainder of the pasture. It would have been difficult for the cattle to avoid the hemlock and some plants had been trampled although there was no sign that it had been grazed. There had been no recent rain, thunderstorms or application of fertiliser on the pasture.
Silage bales wrapped in plastic were kept in the paddock and bits of this plastic were scattered around the property. An old shelter standing in one corner of the paddock contained scrap metal but no batteries or other lead sources. A large red-bellied black snake (Pseudechis porphyriacus) was sighted.
A sample of the hemlock was submitted to agronomist Belinda Hackney from the Department of Primary Industries to confirm its identification as hemlock (Conium maculatum, refer to Figures 2 and 3). It was differentiated from the rarer water hemlock (Cicuta spp.) based on its characteristic hollow, stout stem with purple colourations and distinct taproot (Everist 1981; Panter, Keeler and Baker 1988). The agronomist relayed reports of suspected cattle deaths from hemlock poisoning several years earlier near Rockley, NSW.
Hemlock is a well-known toxic plant containing eight pyridine alkaloids, including coniine. All of the plant is toxic although the alkaloids are particularly concentrated in the seeds and flowers (Hurst, 1942; Everist, 1981; Panter, Keeler and Baker, 1988; Galey, Holstege and Fisher, 1992; Pokorny and Sheley, 2000; Williamson et al., 2007; Davis et al., 2009).
Hemlock toxicity has been reported in cattle, pigs, horses, poultry, sheep, goats and humans (Hurst, 1942; Everist, 1981; McBarron, 1983; Panter, Keeler and Barker, 1988; Lopez, Cid & Bianchini, 1999). Cattle are reported to be 10 times more susceptible than sheep to the effects of acute intoxication and goats appear particularly resistant. An intramuscular dose of 16mg/kg caused death in a cow compared to a dose of 240mg/kg in a sheep (McBarron, 1983; Panter, Keeler and Baker, 1988; Lopez, Cid and Bianchin,i 1999). Fresh plant given at 5.3 g/kg bodyweight is the reported lethal oral dose in cattle (Lopez, Cid and Bianchini, 1999).
Hurst (1942) provided a review of livestock losses associated with suspected hemlock ingestion in Australia, including the death of cattle in Warrnambool Victoria in 1909, the death of four cattle and sheep in Queanbeyan and Cooma in 1922, the death of 11 cows within 24 hours of demonstrating signs of paralysis in Cooma in 1932, and the death of sheep and cattle in the Bathurst district in 1931. Cattle and sheep have been known to consume the plant without signs of intoxication in Australia (Hurst, 1942). In the United States, green-chopped hay contaminated with hemlock was the suspected source of poisoning of 226 dairy cows, including 50 deaths (Panter, Keeler & Baker, 1988). Another outbreak in US involved 20 of 30 Angus cows that demonstrated clinical signs of toxicity following consumption of contaminated alfalfa hay; 2 subsequently died (Galey, Holstege and Fisher, 1992).
Onset of clinical signs of hemlock poisoning in livestock can occur within 15 minutes of ingestion and death within 2-3 hours, although times may vary according to species (Lopez et al., 1999; Williamson et al., 2007; Davis et al., 2009). When death does not result, clinical signs generally cease after 6 to 7 hours (Vetter, 2004). Tremors, teeth grinding, nervousness, depression, inappetance, muscular weakness and spasms, knuckling at the fetlocks, lateral rotation of limbs, incoordination, mydriasis, bloat, excessive salivation, lacrimation and urination (may be coffee-coloured), diarrhoea, hypothermia, rapid, weak pulses, collapse, respiratory distress and death from respiratory paralysis are sequelae described (Hurst, 1942; Everist, 1981; McBarron, 1983; Galey, Holstege & Fisher, 1992; Pokorny & Sheley, 2000; Vetter, 2004; Williamson et al., 2007; Davis et al., 2009). Ingestion of smaller doses of hemlock at approximately 40 -75 days gestation may also produce congenitally deformed calves, and abortion has been reported (Hurst, 1942; Everist, 1981; Panter, Keeler & Baker, 1988; Pokorny & Sheley, 2000; Davis et al., 2009).
There are no reports of specific gross or microscopic pathology associated with the hemlock poisoning (Lopez, Cid and Bianchini, 1999). The diagnosis of hemlock intoxication is aided by identification of the fresh plant source or of contaminated hay (an alkaloid analysis may be appropriate), evidence of the plant in the rumen contents, a mousy odour to rumen contents or urine, blood in the faeces, gastrointestinal irritation and evidence of respiratory arrest (Everist, 1981; Galey, Holstege and Fisher, 1992; Vetter, 2004).
Hemlock is a common weed with legendary status as a poisonous plant. Arguments in favour of hemlock as the cause of death include:
1. alternate diagnoses were ruled out or considered less likely.
Based on history and a paddock inspection the following causes of sudden death were either ruled out or considered unlikely: acute phalaris poisoning, nitrate, cyanide, lead or blue green algae poisoning, acute fascioliasis, anthrax and hypomagnesaemia. Some lucerne was present in the paddock but at the time of the deaths was not growing actively and none of the remaining cattle showed any evidence of bloat. Phalaris was also present but acute phalaris poisoning occurs when hungry unaccustomed stock first gain access to the plant. These cattle had been in this paddock for several months. Enterotoxaemia is also a possibility. These cattle had been vaccinated at marking and weaning but immunity to the enterotoxaemia fraction of clostridial vaccines is imperfect.
Death due to accident or misadventure is possible but seems unlikely. Snakebites, lightning strikes (although lack of recent storms eliminates this differential) and gastrointestinal foreign bodies due to ingestion of plastic wrappers observed scattered around the property (Vermunt, Malmo & Parkinson, 2010) are also possibilities. Death of the two heifers due to unrelated causes cannot be excluded.
2. the toxic plant, hemlock was abundant in the paddock.
Hemlock was abundant and accessible in the paddock. While the dense clumps of hemlock appeared not to have been grazed, small individual hemlock plants were found growing in the pasture beside the creek. It would have been difficult for grazing cattle to avoid ingesting these plants.
3. deaths ceased when source of toxin removed.
No further deaths were reported following removal from this pasture. The cattle received no other treatment and did not receive a clostridial booster vaccine.
4. the literature provides some support for the diagnosis.
While reports of hemlock intoxication of grazing cattle are scant, this incident is consistent with these. Furthermore, cattle are known to be relatively susceptible to hemlock intoxication and death can occur soon after ingestion.
5. the affected cattle are at a susceptible age.
Adolescent cattle, free from maternal guidance and lacking the wisdom of experience are most prone to gustatory experimentation.
However, there are arguments against a diagnosis of hemlock poisoning.
1. cattle frequently graze hemlock infested paddocks without incident.
Hemlock is a common weed along the waterways of the central tablelands and elsewhere. It is often abundant in pastures grazed by cattle and yet, presumably because it is not palatable, it appears to be rarely eaten and therefore rarely implicated in livestock mortalities.
2. There is no autopsy or laboratory support for the diagnosis.
We conclude that there is some support for a diagnosis of hemlock poisoning. However, the lack of fresh evidence and the fact that hemlock poisoning is rarely diagnosed despite the weed being widespread means that the cause of death remains uncertain.