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This article was published in 2005
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Unusual presentations of Cheilanthes sieberi and Marsilea drummondii poisonings in the Narrabri RLPB district

Shaun Slattery, District Veterinarian, Narrabri RLPB

Cheilanthes sieberi (Rock or Mulga Fern) and Marsilea drummondii (Nardoo) are ferns with well known poisonings of stock.

Both can grow abundantly in different parts of the Narrabri RLPB district. This article reports on poisoning syndromes with these plants that are outside those commonly reported in the literature.

Rock Fern (Cheilanthes sieberi)

C. sieberi is commonly associated in the literature with a bone marrow depressing condition in cattle, polioencephalomalacia in sheep and neoplasia of the urinary bladder in cattle. The first is common in certain winters in the Narrabri district.

In the winter of 1999 a blindness syndrome in cattle, suspected to have been caused by the ingestion of C. sieberi was investigated. Such a syndrome has not be previously recorded, confirmed or suspected with C. sieberi.

The syndrome affected three from thirty-six mixed British breed four-seven month old weaners.

The pasture they were grazing was typical of those associated with the bone marrow depressing form of C. sieberi poisoning. It consisted predominantly of dry standing summer growing Arista sp (Wire Grass). As the poisoning occurred in May, the only green feed in such a pasture was on the banks of a semi-permanent creek that ran through the paddock. This green feed consisted mostly of C. sieberi. The extent of C. sieberi growth was on the owner's account far greater than normal. The weaners were observed to be primarily grazing this watercourse area.

The syndrome presented as apparent sudden blindness in the affected weaners, indicated by behaviour such as aimless wandering and constantly walking large circles. Observation and clinical examination could find no nervous signs (eg ataxia) besides the blindness. The only other clinical sign observed was mild diarrhoea.

The blindness first became apparent three weeks after introduction to the paddock. The first weaner died a week later. The two other weaners did not recover and deteriorated in condition, presumably due to an inability to feed, and were euthanased four and six weeks after signs were first observed.

All weaners were autopsied. No gross pathology was observed. Haematology was normal. Histologically there was "severe bilateral retinal atrophy and optic neuropathy". Further it was found that "The optic neuropathy has resulted from optic nerve compression during an acute episode of swelling due to myelinic oedema".

This lesion, and also the clinical course and signs, are similar to that seen with Stypandra glauca poisoning and closantel toxicosis.

However careful inspection of the paddock could not find any other plant species in large numbers or any species associated with blindness. The weaners had not been treated with any anthelmintic.

A condition of retinal degeneration, called bright blindness is seen in sheep in the United Kingdom grazing Pteridium sp (Bracken Fern). However it is a gradual condition affecting sheep that have grazed Pteridium sp for a least a year. A preliminary examination of the literature suggests the retinal lesion is different and the optic nerve lesions are not present.

Also in the United Kingdom there is a report of blindness and histological lesions similar to those seen in this case in calves grazing a paddock containing rhizomes of Dryopteris felix-mas (Male Fern).

A more thorough examination of the literature reporting these syndromes may be beneficial.

Nardoo (Marsilea drummondii)

Marsilea drummondii (Nardoo) poisoning causes polioencephalomalacia. It is commonly associated in the literature with the poisoning of sheep and occasionally horses. The usual circumstance cited is during drought conditions when Marsilea drummondii growing in flooded depressions is the only available feed. It usually exhibits as a syndrome with proportionally large mortalities.

However experience with Marsilea drummondii in the Narrabri district since 1990 is that it causes sporadic cases in sheep grazing abundant lush pastures that include Marsilea drummondii as a minority of the pasture.

For example in one flock of 2,500 ewes grazing excellent waist high summer grasses after the wet spring of 1998. There was also was isolated patches of the Marsilea drummondii. In such pastures there was no need for the sheep to eat Marsilea drummondii. The first signs of problems were the finding of occasional carcasses. Eventually a ewe with clinical signs was found. Its rumen contained only Marsilea drummondii. Histology confirmed polioencephalomalacia. Other were found and responded to thiamine. At mustering thirty from 2,500 ewes were found to be missing.

In neighbouring flock with similar feed conditions, six from 3,000 ewes were observed with signs. On a third property with gilgais containing Marsilea drummondii but with abundant summer pasture, five from 800 ewes died over three weeks.

This presentation suggests that some sheep are attracted to Marsilea drummondii and will eat it out of preference.

A case of poisoning in calves was confirmed in conditions more typical of thoses described with Marsilea drummondii in the literature.

In this case a mob of fifteen, eight week old poddy calves were being held a small paddock in drought conditions and being fed calf pellets. The paddock contained two gilgais containing water pools from a recent storm. A ring of Marsilea drummondii was growing around the water's edge and was the only available feed. The calves were actively grazing this. The first two cases were seen two days after introduction to the paddock with a further case two days later. Clinical signs were consistent with polioencephalomalacia and it was confirmed by histology.


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