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This article was published in 1985
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Rock Fern Poisoning in the Coonabarabran District
F.K. Nottle, B.V.Sc., Veterinary Inspector, COONABARABRAN
Rock fern poisoning is an interesting condition in the variety of the clinical picture presented. Whilst many cases exhibit typical and observable signs of haemorrhagic disease there have been a number of cases where a non-specific syndrome was apparent. With experience with the disease it is possible to establish a provisional diagnosis from owner's descriptions. However, there remain outbreaks of rock fern poisoning in which a necropsy is required to reach a diagnosis.
THE PLANT
Rock fern is a name interchangeable between the species Cheilanthes sieberi (also known as mulga fern) and Cheilanthes tenuifolia (also known as rock lip fern). In practical terms it may be difficult to distinguish between the two. Everest notes that there is no definite evidence linking C. tenuifolia with cases of stock poisoning. The plants involved in my cases best fit the descriptions of C. sieberi and the term rock fern used in this paper refers to that species.
Rock fern is a perennial fern growing to 50cm high. It has thin dark brown to black stalks with small dark green, stiff triangular fronds. A feature of the fern is its ability to respond rapidly to rainfall even though it has dried to the extent where the fronds are brown and brittle. This rapid renaissance is important in the epidemiology of rock fern poisoning.
Rock fern is killed by cultivation so survives in areas never ploughed or infrequently so. I have seen it on all types of soil. It is most commonly found amongst rocks, in timber and along fence lines. However, in 3 of the cases reported here it was abundant within native grass tussocks on cleared paddocks.
THE TOXIN
Rock fern contains thiaminase enzyme activity and a casual role is proposed for thiaminase enzyme in rock fern poisoning (Beckett 1985).
THE DISEASE
As mentioned during the introduction the disease may present in a variety of ways. In the period 1980 to 1985, I have diagnosed it 11 times in the area. An acceptable diagnosis of rock fern poisoning could have been made on 5 cases seen by private practitioners over the same period. Each of the 11 cases is briefly described to illustrate the variance of the outbreaks in the signs observed, the necropsy picture and the grazing history.
CASE 1
(June 1980)
Second calf hereford cows recently moved from a scrub paddock onto grazing oats and being fed hay. Some deaths had occurred in the scrub and continued on the oats. The cows showed marked respiratory embarrassment, were in very poor condition but a temperature was not always evident. The necropsy picture varied between animals. The most common features were ulceration of the ruminal and intestinal mucosa, marked consolidation of the lungs with gelatinous material in the interlobular septa, small necrotic areas in the liver and a mild jaundice. 17 cows died out of 89. Rock fern was prolific amongst rock knolls in the scrub. Very little pasture was available.
CASE 2 (June 1980) 240 mixed sex yearlings were purchased at various sales and held on a small paddock prior to introduction onto a grazing oat crop. 1 animal died prior to, and 5 subsequent to, release onto the oats. Affected animals showed an unsteady gait, general depression and obvious fresh blood on the faeces. Haemorrhages were evident in all areas of the body being particularly marked in the gall bladder. The liver was yellow-brown in colour with necrotic areas and haemorrhages. There was marked jaundice. There was ample feed in the small paddock but within almost every tuft of grass there was a rock fern plant. As the feed became shorter the stock had to ingest the fern nearly every time they ate.
CASE 3
(January 1981)
70 cows and calves grazing a forest reserve. 3 weeks after summer rain, calves began to die. The calves were depressed and passing blood. The usual haemorrhagic changes were present together with approximately 2 litres of free blood in abdominal cavity. 6/70 3 to 6 month old calves died but the cows were not obviously affected. The fern was the only green feed available following the rain.
CASE 4
(August 1981)
4 cows out of 40 grazing a paddock with short feed died over 3 days. Typical haemorrhagic changes. The fern was present in timbered areas along fence lines.
CASE 5
(August 1982)
Aged hereford cows grazing a short feed paddock with access to bush country. 1 died and 1 sick out of 50. Presenting sign was depression with no obvious blood on the faeces. Changes seen were extreme jaundice, air filled septa in lungs with consolidation of tissue, haemorrhages in rumen, intestines, heart etc. and orange coloured liver and kidney. Severe fatty degeneration in the liver and kidney were probably associated with a pregnancy toxaemia/starvation ketosis state as the cow had only calved 2 days before.
CASE 6
(November 1982)
Sudden deaths were reported in 2 out of 30, 2 to 4 week old calves. Post-mortem showed very watery blood and massive haemorrhaging into intestinal lumen over large portion of its length. Histopathology showed 'changes in keeping with but not pathognomonic for rock fern poisoning.' As the calves had no rumen contents indicative of grazing, any toxin must have been ingested via the milk. The cows were picking amongst rocks on a hill in search of more palatable feed and bitten off rock fern was found in this area.
CASE 7
(May 1983)
1 weaner steer in 4 score condition was seen to be depressed, have an unsteady gait and a stringy nasal discharge. The typical changes were evident. The 60 head were grazing a roughly cleared paddock with a lot of tussock patches containing large amounts of fern.
CASE 8
(June 1983)
2 yearling steers died and 3 others were visibly affected out of 16. The steers were depressed, passing blood and had an unsteady gait. Typical changes were present. The paddock contained plenty of tall dry native pasture with short improved species. In seeking the shorter succulent feed the steers ingested significant amounts of rock fern within the native pasture tussocks.
CASE 9
(August 1984)
This is the only case of rock fern poisoning in sheep I have diagnosed in the area. Merino ewes with 4-10 week old lambs at foot were yarded for lamb marking and then were moved to a new paddock. 4 days later 15/330 were dead with about 6 more obviously affected. The ewes were in sternal recumbency, unable to rise with black faeces. Post-mortem showed ulceration of the abomasal surface with blood filled ileum and colon. Old rock fern stems were present in the rumen contents. The owner claimed that despite an excellent lambing, he expected only a 30% weaning rate. Possibly the lamb losses were due to the rock fern via the ewes milk. The previously grazed paddock had a large area of rocky ridge country with abundant fern.
CASE 10
(October 1984)
This case is recorded although it is questionable given the histopathology of the liver. For the past 3 years the owner reported 1-2 deaths per year in mature cows with marked abdominal distension and fluid accumulation. No necropsies were performed. One cow died in the winter of '84 with similar signs. Another cow was observed in poor condition, mild abdominal distension and a profuse dark fluid scour with obvious blood. The necropsy changes were identical with those of rock fern poisoning, viz haemorrhaging throughout the body, jaundice, consolidated lungs with gelatinous interlobular septa, numerous gastrointestinal ulcers, blood in intestinal lumen etc, but the liver was firm with a mottled yellowing appearance. The histopathology report on the liver (the only tissue submitted) read 'severe hepatopathy. There is extensive loss of hepatocytes with replacement fibrosis and bile duct proliferation involving 80% of the liver. The remaining hepatocytes showed marked megalocytosis. Throughout the sections there were isolated nodules of more normal hepatocytes (regenerative nodules) many of which contained areas of necrosis and associated haemorrhage. Comment - The liver pathology is in keeping with severe chronic pyrrolizidine alkaloid toxicity.' The cows would have had access to rock fern, Paterson's curse, fireweed and heliotrope at various times. They had come down from the hills, where rock fern was plentiful, 2 weeks previously. The pathological changes match those of rock fern poisoning. Equally they are equivocal for pyrrolizidine alkaloid toxicity.
CASE 11
(December 1984)
3 out of 15 3 year old cows died around the time of calving. One died prior to calving, one calved normally with a dead calf and the other died 5 days after a dead calf was pulled. The cows were in 3 score condition and showed general depression. Typical haemorrhaging into the intestine was not present although there was some capillary rupture in the intestines. Generalised small (1-3mm) haemorrhages were present on the serosal surface particularly in the thorax. There was yellowing and swelling of the liver and infarcts in the kidneys. The most dominant feature was the lungs. At best there was only 5% of normal tissue with spectacular emphysema to 100% with plugs of degenerating tissue and areas with liver red bloody consolidation. The cows had been consuming rock fern along the edges of a creek.
SEASONAL OCCURRENCE
with the ability of the plant to respond rapidly to rainfall, poisonings are likely to occur when rain follows a dry spell. Cases have occurred throughout the year. However, there has been a winter dominance with 10 out of 16 cases in the May to August period.
TREATMENT
Treatment was not attempted on the majority of cases. A period free of any stress together with prevention of access to the fern seems to have prevented further losses in most cases.
References
Beckett, R.J. (1985) Ph.D. thesis. University of Sydney
Everest, S.L. (1974) Poisonous Plant of Australia