A wide range of garden plants can cause sudden death to livestock that gain access to the garden or garden trimmings (Bourke, 2005). The perennial garden plant African Daisy or Cape Daisy (Dimorphotheca ecklonis, also known as Osteospermum ecklonis) is known to contain cyanogenetic glycosides, with the greatest amount in green leaves. In Australia there are scant reports of toxicity but D. ecklonis has on two occasions killed goats shortly after being given access to garden trimmings. D. jucunda is also recorded as having killed goats (Everist, 1974). Two alpacas suddenly died within 30 minutes after having eaten Osteospermum sp - probable Osteospermum ecklonis garden cultivar - plant material (McKenzie et al., 2009).
This report details a case in which three of 50 merino wethers died suddenly following access to a garden containing African daisies, identified as Osteospermum or Dimorphotheca.
On 23 February 2017 two of 50 rising 3-year-old merino wethers were found dead near the house and garden on a property in the Bathurst district. One wether had been found dead the week previously. The wethers, purchased in October 2016, were drenched with a combination drench and vaccinated with 6-in-1 on arrival and were vaccinated again six weeks later. They were drenched with levamisole and crutched and jetted with cyromazine at the end of January 2017.
The wethers were running in a dry grass paddock but also had access to the garden.
Two well grown wethers, fat score 3, were necropsied on 23 Feb 2017. Both carcasses were reasonably fresh. The mucous membranes were cyanosed and in both cases the lungs wet and red with white froth in the trachea. The livers were a normal dark red with sharp edges but were perhaps more autolysed than expected. Both wethers had clear pericardial effusions with protein clots.
Rumen contained fibrous yellow material but also moderate quantities of a broad leafed green plant matching a perennial daisy growing in the garden nearby. The owner identified this plant as "black eyed Susan." Samples were collected for identification both from the rumen and the garden.
The daisies Verbesina encelioides (native to America, naturalised in Australia) and Wedelia asperrima (native to Western Australia) are known to cause lethal pulmonary oedema and pericardial effusion. It was initially assumed that the garden daisy in this case may have a similar toxic principle. Enterotoxaemia was also considered as both wethers had a clear pericardial effusion with protein clots. However, it was considered unlikely as the wethers were on dry feed and had recently been vaccinated twice.
Heartwater was also considered because of the pericardial effusion and evidence of endothelial damage. However, it was considered a very low risk as there was no evidence of ticks or recent importation.
On 1 March 2018 the NSW Herbarium commented that the plant could be Osteospermum but that flowers were needed for confirmation. Once the plant was provisionally identified as Osteospermum an internet search showed that it was well known as a toxic plant containing a high concentration of cyanide.
Aqueous humor samples were negative for nitrate, ammonia levels were high (867 and 1296 umol/L) and calcium was within normal limits.
Epsilon toxin assay was negative.
The main finding was acute pulmonary congestion and oedema in both sheep. One sheep also had a minimal neutrophilic cholangiohepatitis of uncertain significance.
On 22 March 2017 a sample of the daisy was collected fresh and chilled for submission to the Regional Veterinary Laboratory, Benalla for HCN testing.
The sample was assayed as having 14,700 mg HCN/kg (on an 'as received' basis with dry weight determined gravimetrically at 20.4%). The laboratory comment was that feed concentrations greater than 200 mg cyanide per kg dry weight are considered to be potentially toxic to livestock. It was noted that potential toxicity is dependent on several additional factors. These include sulphur status and rumen function.
Deaths stopped after the wethers were moved. They were not re-vaccinated.
The plant was identified as Dimorphotheca pluvialis (Plant ID BIS 20465, sample 4006128 Det. L. Murray 20 Feb 2018).
Seanna McCune re-determined the sample 4006128 (Plant ID BIS 20465) as Dimorphotheca/Osteospermum - achenes required for a complete identification, in May 2019
Cyanide intoxication is difficult to diagnose with certainty due to the volatility of the toxin, unless sampling of animal tissues occurs promptly after death and samples are tested or frozen immediately. There are no specific histological changes associated with cyanide intoxication. Pulmonary congestion and oedema, as present in these two sheep, can be seen in cyanide intoxication, however, are not specific and accompany many other conditions.
The cause of the increased ammonia levels in the two sheep is uncertain. Ammonia levels increase with time following death.
The diagnosis of cyanide poisoning in these wethers is not conclusive. Cyanide was not demonstrated in the tissues of the sheep and the plant was tested one month after the deaths occurred. However, plant material consistent with Osteospermum / Dimorphotheca was found in the rumen and the plant when tested contained exceptionally high levels of cyanide, at over 70 times a potentially toxic dose.
Tissues including rumen contents were not tested for cyanide in part because cyanide was not suspected initially. However, tissue testing for cyanide is fraught because it is so labile (Paynter D, pers. comm.).
Cyanide poisoning of ruminants occurs when plants containing cyanogenetic glycosides are ingested. Rumen microbes metabolise these glycosides releasing free cyanide which becomes hydrocyanic (HCN or prussic acid) in water. HCN prevents cytochrome oxidase from releasing oxygen, leading to cellular anoxia and death (Parkinson et al., 2010). Many plants contain cyanogenetic glycosides including Sorghum spp.