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This article was published in 1951
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.

"Blue Couch" Poisoning in Sheep

G. CHARLES. B.V.Sc., Inspector or Stock, Forbes.

There are a number of plant species in New South Wales which are cyanogenetic, and in most of these the prussic acid content is present as a glucoside, being liberated during the digestion by an enzyme which may be contained in the same plant or in some other plant eaten.

One of the commonest of these cyanogenetic plants in the Central West is "Blue Couch"—Cynodon incompletus—and during the months of December-February this grass makes very rapid growth following thunderstorms; and mortalities occur when it is ingested by sheep or cattle. Edgar (1938) remarks: "It is extremely paradoxical that nature should provide adequate conditions for the growth of a grass so attractive to stock, and yet when consumed by the animals in sufficient quantity will cause rapid death." Madden (1950) refers to the many mortalities which have occurred in the North-west of N.S.W.; and describes one smash in particular.

The susceptibility to the poison is conditioned by the state of the animal consuming the grass. Hungry stock such as travelling mobs, or sheep which have been yarded for 24 hours or longer, will eat greedily if given access to the grass, which is often the only greenstuff available, and they then develop toxic symptoms very rapidly. Stock normally grazing in a paddock where the grass occurs are seldom affected; probably because their intake is less, and slower, so that excretion may balance absorption of the HCN.

During the week prior to 11th January, 1951, there had been a heavy thunderstorm in the Bundaburra Bridge area, about 7 miles from Forbes on the Marsden Road. This storm was followed by warm to hot weather, and the Blue Couch made rapid growth. On the afternoon of the 10th January, an owner yarded a mob of sheep near the bridge and left them there until about 8 a.m. on 11th January. When they were let out they rushed the green growth and ate ravenously; with the result that 150 of them died within the short space of no more than 15 minutes.

Examination of the carcases showed all visible mucous membranes to be cyanotic. On opening the carcase there was a marked engorgement of all blood vessels, but no other abnormality. The blood after exposure to the air for about an hour, was a bright cherry-red colour. This is regarded by Udall (1943) as diagnostic in cases of cyanide poisoning. On opening the rumen there was a faint but distinct odour of bitter almonds. Six samples of paunch contents were taken from each of two sheep, and tested by the sodium picrate test for the presence of cyanides.

Only one of each of these two sets of samples gave a positive reaction, despite the pervading odour noticed when opening the paunch, and this result, especially as fresh grass gave strongly positive reactions, seemed a little difficult to follow; until the comments of Finnemore and Jaffray (1935) were read. These workers found specimens of air-dried Blue Couch to contain as much as 0.198 per cent. HCN, and using this figure with the formula evolved by Seddon and King (1930) they calculated that only 80 gms. of grass containing this percentage of HCN would be a minimum lethal dose for a sheep weighing 160 lbs. Allowing for a sheep to ingest three times this quantity, or about 8 ozs., it still follows that the amount ingested is only a small proportion of the total ruminal contents; and death is so rapid that peristaltic action would not have time to effect a complete distribution throughout the rumen.

As a result of this it is suggested that where ruminal contents are being tested for the presence of HCN, at least six samples should be taken from various parts of the rumen, and one of these should be from an area close to the entrance of the oesophagus into the rumen.

Treatment of this condition is frequently impossible owing to the rapidity with which death takes place, but both sheep and cattle which are affected and down do show a remarkable response to the intravenous inject on of sodium nitrite / sodium thiosulphate solutions. For cattle the dose is 10 mls. of 20% sodium nitrite followed immediately by 30 mls of 20% sodium thiosulphate; also intravenously. For sheep the dose is 10 mls. of 10% sodium nitrite, and 20 mls of 10% sodium thiosulphate.

References

Edgar, G. (1938) — Year Book, Institute of Inspectors of Stock. N.S.W. page 79
Udall, D.H. (1943) — The Practice of Veterinary Medicine. page 695
Finnemore, H. &: Jaffray, A.B. (1935) — Commonwealth of Australia Journal C.S.I.R.: viii: 136
Seddon, H.R., & King, R.O.C. (1930) - ibid iii: 14-24
Madden, F.J. (1950) — Year Book, Institute of Inspectors of Stock. N.S.W., page 53