Bracken fern (Pteridium esculentum) is a robust Australian native fern that frequently invades pastures (McKenzie 2012). It contains ptaquiloside, a norsesquiterpene glycoside, which has been shown to cause both neoplasia and acute haemorrhagic syndrome in cattle (Vermunt et al., 2010). It is a common cause of cattle mortalities on the North Coast of NSW. The following case study describes Bracken fern toxicity in 3 heifers on a farm in the Pillar Valley, NSW.
In May 2013 disease was reported to the North Coast Livestock Health and Pest Authority (North Coast LHPA) in three Limousin Bazadais Angus cross heifers. The heifers were 13-14 months old and in a mob of 22.
The owner had noticed that the heifers were losing weight, salivating excessively and had various degrees of lameness. The demeanour of the heifers had changed dramatically and they had become listless and lethargic. There was variation in the length of clinical illness in the heifers. 1 heifer had been ill for only 1 day (heifer 1) while the other heifers (heifers 2 and 3) had been ill for 2 and 7 days respectively. Antibiotics (Oxytetracycline LA) and NSAIDs (Tolfedine) had been administered to heifers 2 and 3 with no improvement seen.
The herd was last treated for intestinal parasites in December 2012. The heifer's diet consisted of copra meal, mineral mix and poor quality native pasture. Recently the heifers were grazing a freshly slashed paddock that contained abundant young shoots of bracken fern approximately 10cm in height. There is also red lantana on the property.
The 3 heifers were examined by a North Coast LHPA district veterinarian and two Charles Sturt University veterinary interns.
Heifer 1: Dull and lethargic, enlarged sub-lingual and pre-femoral lymph nodes, hypersalivation, pyrexic (41.6°C), tachycardic and tachypnic and deceased gastro-intestinal activity. There were increased lung sounds on both sides.
Heifer 2: Losing body condition, salivating, bottle jaw, tachycardic and tachypnic. There were increased lung sounds on both sides.
Heifer 3: Losing body condition, pyrexic (41.2°C), tachycardic and tachypnic, pale mucous membranes. There were increased lung sounds on both sides.
Blood was taken from the 3 heifers for haematology and biochemistry.
Haematology revealed profound thrombocytopenia and leukopenia characterised by marked lymphocytopenia, neutropenia and monocytopenia in all 3 heifers. There was also mild anaemia. Biochemistry revealed mild alterations to varying parameters between individual animals.
Based on the history and these blood results a diagnosis of bracken fern toxicity was made. A poor prognosis was given to the owner and euthanasia was recommended.
A post-mortem was performed by the owner on heifer 3 and photographs were taken. The photographs revealed extensive internal haemorrhaging ranging in size from petechia to very large haemorrhages throughout all tissues. There were large volumes of sero-sanguinous fluid in all body cavities.
In this case ingestion of bracken fern caused a syndrome known as acute haemorrhagic disease. Bracken fern must be ingested in large volumes for a period of several weeks before haemorrhagic disease develops (Vermunt et al., 2010).
Pancytopenia occurs as a consequence of bone marrow suppression. The enlarged lymph nodes are presumed to be due to secondary infections due to the marked leukopenia. There was no external evidence of a clotting disorder, such as haematuria, petechial haemorrhage of mucous membranes or gross haemorrhage in these heifers at the time of the physical examination. However, the owner reported that heifer 3 began to bleed from the rectum shortly before death. The necropsy findings showed that the heifers were haemorrhaging internally.
The only way to prevent bracken fern toxicity is to restrict access of susceptible animals to bracken fern. Toxicity is more likely in young stock, hungry stock, those with uncontrolled access to ferny areas, stressed animals and animals newly introduced to the area.
The rhizomes are the most highly toxic part of the plant, closely followed by new young unfurled growth (Vermunt et al., 2010 and McKenzie 2012). For this reason, slashing or burning of bracken fern should not be followed by grazing. Pasture improvement incorporating the use of herbicides to reduce the level infestation is also a strategy that could be used to reduce stock exposure to bracken fern.
Removing all susceptible animals from pastures containing bracken fern is the most effective way to prevent further fatalities. Unfortunately deaths can occur for up to 6 weeks post removal from the bracken fern (Vermunt et al., 2010).