Bracken fern (Pteridium esculentum) is a native warm-season perennial fern that has stiff erect fronds. It is adapted to most soil types and is often found in open forest and paddocks in the Mid North Coast of NSW. In cattle two distinct syndromes may be seen. The first an acute haemorrhagic syndrome where bone marrow suppression leads to leukopaenia, thrombocytopaenia and anaemia and the second a chronic toxicity known as Bovine Enzootic Haematuria associated with urinary bladder tumours.
This report describes a case of the acute haemorrhagic syndrome associated with bracken fern toxicity in beef cattle.
In March 2012 I was called to a property near Kempsey on the Mid North Coast of NSW to investigate cattle deaths. The 600ha property comprises open grazing land of predominantly native grasses and significant areas of bush. The property runs about 200 head of cattle. The owner reported losing two heifers at about the same time of year last year and had just found another heifer dead in an advanced state of decomposition and one sick heifer. The sick heifer was described as being depressed, separated from the mob and salivating excessively. The three deaths and sick animal were all found in the same area of the same paddock, with two out of the three dead animals dying in or around a water source. All affected animals were yearlings.
The sick heifer was in good body condition and when yarded appeared slightly weak. Rectal temperature was 40.5° celcius. There were petechial haemorrhages evident on vulval and oral mucous membranes and haemorrhage was also evident in the sclera of the eyes (See Figures 1 and 2). Mucous membranes appeared slightly pale. Respiration rate and heart rate were elevated though the chest sounded clear on auscultation. Faeces were soft, formed, a brown-red colour and malodorous. Blood was collected in plain and EDTA tubes for haematology and biochemistry.
A tentative diagnosis of bracken fern toxicity was made at this stage given the history, clinical signs and presence of bracken fern on the property. A poor prognosis was given, however the owners felt the animal appeared a little brighter than earlier that day and wanted to attempt treatment. A recommendation to treat with Oxytetracycline was made given the likelihood of secondary infection.
The heifer died overnight and a post mortem was conducted the following morning.
The carcase was generally pale and blood clotted poorly. There was widespread evidence of haemorrhage throughout the carcase (see Figures 3-5). There were some areas of ulceration in the intestinal tract (see Figure 4). These findings were consistent with the earlier diagnosis of bracken toxicity. Given that blood samples had been submitted to the laboratory the previous day no samples were collected at post mortem, however bone marrow is the recommended post mortem sample from suspected bracken toxicity cases.
Haematology findings included anaemia (packed cell volume 17%), leukopaenia (1.2 x 109/L) with low neutrophils (0.07 x 109/L), lymphocytes (1.1 x 109/L) and monocytes (0.02 x 109/L). Fibrinogen was mildly elevated. Platelets were reduced. These results are consistent with bracken fern toxicity.
The toxic principle in bracken is Ptaquiloside, which is a norsequiterpene glycoside of the illudane type (Radostits et al. 2000). Cattle generally need to have grazed paddocks containing bracken for two to four weeks before signs will be seen and clinical signs of toxicity may be seen for several weeks after being taken off the bracken fern.
Bracken fern toxicity is the second most common plant poisoning seen in this area, behind lantana toxicity. The majority of cases seen are of the acute type, with cases of enzootic haematuria being much less common. Cases of toxicity are most commonly seen when feed is scarce, though this was not the case on this property with other available feed being abundant at the time. The acute haemorrhagic syndrome is most frequently seen in young cattle, as in this case, or in introduced stock, whereas enzootic haematuria is most commonly seen in older animals. Often there is a history of recent slashing or burning, with regrowth of bracken being five to ten times more poisonous than mature fronds. (Radostits et al. 2000; Rose et al. 2011). Rhizomes are also said to be some five times more toxic than mature fronds (Radostits et al. 2000) and cattle may actively eat rhizomes when they are bought to the surface in ploughed paddocks.