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This article was published in 1962
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Some Observations on Addiction to Swainsona spp. in Sheep
PETER T. DIPLOCK, B.V.Sc., Veterinary Inspector, Warialda
The term "addiction" is defined by Webster as "apply habitually" or being "wholly given over to", and on this basis this term has been applied to the continuing ingestion of Swainsona spp.Such continued ingestion leads to the condition usually known as "Pea Struck" in Australia and "Loco Disease" in the U.S., according to Whittet (1958) in his book on Weeds; published by the N.S.W. Department of Agriculture. He has grouped both conditions under the heading of plant poisons of the "Delriant" type, affecting the nervous system of animals.
The precise pathogenesis of "Loco Disease" in the U.S. is now well on the way to elucidation and is concerned largely with the "Selenium Converter" action of such plant species as Astragalus, Oxytropis and Stum spp. (Underwood, 1956). The true position as regards Swainsona spp. is still obscure, however.
Hurst (1942) has stated that even the classification of this plant species requires considerable study; the present position being little less than chaotic and such that little reliance can be placed on reports of a particular genus from any given area, as the generic name is in some cases known to vary from place to place.
Although numerous genera of Swainsona spp. have been described from wide areas of this continent, it would appear from the records that most trouble in this state in the past has been due mainly to the five genera listed below; the main differences in appearance between them being based on flower colour. These are as follows:—
| Spp. | Flower Colour |
|---|---|
| S. coronillifolia ("Darling Pea") | Purple to Pink |
| S. galegifolia ("Red Darling Pea") | Deep Red to Brick Red |
| S. Greyana ("Darling Pea") | Pinkish |
| S. luteola ("Horse Doping Pea") | Purple, Blue or Pinkish |
| S. procumbens ("Broughton Pea") | Purple to Blue |
SYMPTOMS
The symptoms of "Pea Struck" in sheep are variable, but in typical cases most of the following signs are seen:—
(1) Apprehensive appearance when roused, with the head held high.
(2) Variable degree of emaciation.
(3) Stiffness of the hindquarters.
(4) Disturbances of vision(?) and prominent, staring eyes.
(5) Looseness of incisor teeth and dental irregularities, largely confined to the incisors.
(6) Hypersensitive reflexes (in varying degrees).
(7) Deaths appear to be largely due to misadventure.
LINES OF INVESTIGATION
Three main lines of investigation have been followed to date. These comprise:—
(a) Aetiology.
(b) Histopathology.
(c) Treatment; which so far has been largely empirical.
(a) AETIOLOGY
(i) Possible Alkaloid
In the majority of poison plants, with the exception of cyanogenetic and nitrate-containing species, the poisonous principle is an alkaloid, but a search of the literature has failed to reveal any record of the isolation of any specific toxic substance from Swainsona spp. In the Agricultural Gazette of N.S.W. in 1893 an analysis of Darling Pea is given which states that the analysts were unable to find any conclusive evidence of the presence of an alkaloid in the plant. They go on to say that if such is present it must be in exceedingly minute quantities.
Unfortunately, there appears to have been no later analysis performed on this plant utilising the more advanced techniques now available, but in view of these findings such lines of investigation do not appear to hold out much promise.
(ii) Selenium
During a discussion with the District Agronomist, Mudgee, in 1958, the possibility of selenium poisoning being a factor in "Pea Struck" was raised, in view of the rather similar syndrome occurring in the North Central States of the U.S. (Filan, 1958).
with this in mind, samples of Swainsona spp. were collected at Mudgee and forwarded to Glenfield for selenium assay. Early in 1960 further samples were collected in the Inverell P.P. District and also submitted.
Due to technical difficulties, however, these assays have not yet been performed.
Further information in this connection has come to hand from another quarter, however, and the C.D.A.I. advised early in 1961 that workers at the C.S.I.R.O. Field Station at Armidale considered from work they had performed on Swainsona galegifolia that there was no evidence that this plant was a selenium accumulator (Watts 1961).
(iii) Calcium
Following conversation with a landholder in the Inverell District whose W.S.L.S. block is badly infested with Swainsona spp., it was ascertained that he has to use fairly heavy dressings of lime to be able to grow legumes of a comparable quality to those grown by his neighbour on the opposite side of the road, who never uses lime and who has virtually no Swainsona problem although the country appears to be essentially the same. He also stated that his sheep "don't do as well" as those of his neighbours without a limestone-salt supplement, and that he thinks he has more cases of "Pea Struck" sheep when he does not provide such a supplement
The possibility of a "Calcium Converter" action on the part of the leguminous pea and the known effects of calcium imbalance on the nervous system led to the collection of serum samples for calcium estimation. Six samples have been submitted to Glenfield—three from "Pea Struck" sheep, one from a normal lamb, one from a normal adult and one from a "Pea Struck" sheep on a Warialda property for purposes of comparison.
The results were as follows:—
| mg.Ca/ 100ml | mg.Mg 100ml | |
|---|---|---|
| (a) "Pea Struck" Sheep, Inverell | 9.5 | 2.5 |
| (b) "Pea Struck" Sheep, Inverell | 9.6 | 2.4 |
| (c) "Pea Struck" Sheep, Inverell | 9.2 | 2.6 |
| (d) Normal Lamb, Inverell | 10.7 | 2.5 |
| (e) Normal Adult, Inverell | 9.5 | 2.2 |
| (f) "Pea Struck" Sheep, Warialda | 9.9 | 2.7 |
| (Normal Range (Dukes, 1947)) | 9-12 mg.ca/100ml |
As these serum calcium values are within the normal limits, although somewhat on the low side, further investigations along these lines do not appear promising, although it is intended to submit further samples to check on these findings.
(b) HISTOPATHOLOGY
During the course of investigation into this problem at Mudgee in 1968, two samples of brain and upper spinal cord tissue from "Pea Struck" sheep which were almost moribund were submitted to Glenfield for histopathological examination.
This examination failed to show any focal lesions of demyelination and the then D.V.R. commented that although "Pea Struck" has been alleged in the past to be due to demyelination, he personally had never been satisfied on this point and had in fact seen a lot of material from "Pea Struck" sheep which did not show demyelination (Edgar, 1958).
There appears to be little record of much histopathological work on this condition, and it is hoped to submit further tissue samples for examination at the conclusion of the current Swine Fever outbreak, when the time should be more opportune.
It should be most significant if investigation shows the condition to be one of a true addiction rather than a permanent irreversible organic change.
(c) TREATMENT
To date a number of small trials have been carried out using two types of drugs:—
(i) Bromides (Hypnotics).
(ii) Chlorpromazine and Serpasil (Tranquillisers of the Phenothiazine and Rawolfia types respectively).
The first type appear to exert their main action on certain areas of the cerebral cortex; while the second act mainly on the hypothalamus and reticular areas.
Trial 1 (Nov., 1957, at Mudgee)
Trial group consisted of eight affected Merinos divided up as follows:—
(a) Four treated with seven consecutive daily doses of 4ml. Triple Bromide mixture per head.
(b) Four untreated controls.
Both groups were running on a Pea-free paddock.
Results
Eight days after the last administration of Triple Bromide the results were most noticeable. The treated group had improved considerably to forward store condition and had lost their obvious appearance of apprehension; while their reflexes, which had been hypersensitive, had returned to normal.
When seen again some four months later, all four treated sheep continued to show a marked improvement with no suggestion of being "Pea Struck", while the untreated controls were in a much poorer condition than those that had received treatment, regardless of the fact that they had been grazing on Pea-free paddocks for the previous two months.
Trial 2 (March, 1958, at Mudgee)
This group comprised a total of 21 sheep affected with Swainsona poisoning and were treated as follows:—
(A) Five treated with Triple Bromide at 4ml. daily for 10 days.
(B) Five treated with Serpasil (Ciba) 1mg. by intramuscular injection daily for 5 days; reduced to 0.5 mg. daily after the first 2 days due to severe reaction.
(C) Eleven untreated controls. Half of each group were allowed free grazing on Pea, while the other half were kept off it for 6 weeks.
Results
Those on Serpasil showed a rapid improvement within 4 days of the cessation of treatment.
Two months after the concluding treatment, however, no significant differences could be observed between the sheep treated with Triple Bromide, those treated with Serpasil and the control group.
TRIAL 3 (Sept. 1961 at Inverell)
In this trial eleven "Pea Struck" sheep were used as follows:—
(A) Two treated with Chlorpromazine (1 mg. per kg. bodyweight daily for 17 days) and kept on Pea-free country during this time.
(B) Two untreated controls on Pea-free country for the same period.
(C) Three similarly treated with Chlorpromazine but still allowed to graze on Swainsona.
(D) Four untreated controls with unrestricted grazing on Swainsona.
Results
At the conclusion of this Trial the treated sheep in both groups showed a definite improvement in condition. The gait had improved and the overall condition and weight of the treated animals had improved considerably. The apprehensive appearance had decreased and the condition of the teeth was close to normal.
The untreated sheep in both groups, however, also had improved a little due to the seasonal conditions on this property, which had been favourable for clover growth with a tendency to reduce the amount of Swainsona available.
This improvement immediately after treatment with Chlorpromazine is similar to that observed in the group treated with Serpasil and may be due to a non-specific growth stimulating effect of the tranquilliser, rather than to a specific effect on the addiction to Swainsona spp. (Soule, 1958).
However, the owner reported, about one month after this trial concluded, that the treated sheep appeared to be holding their improvement and did not seem to seek out Swainsona plants as they had before, although grazing in a paddock in which the plant was prolific, but they did eat the plant if they came across it when grazing.
Further trials are now being organised in an endeavour to throw some more light on these problems.
CONCLUSION
Despite the limited number of investigations in the trials reported, it does appear that the condition of Swainsona intoxication may well be one of addiction involving portion at least of the cerebral cortex rather than a condition associated with irreversible organic changes, but its precise aetiology, pathogenesis and histopathology still await elucidation. Until this is complete, rationally based therapy will not be possible.
ACKNOWLEDGEMENTS
The advice and encouragement of all the Departmental Officers who have assisted in this work to date is greatly appreciated; particularly that of Mr. D. G. Christie, B.V.Sc., District Veterinary Officer, Orange, in whose area the early trials were performed; and Mr. A. Harris, B.V.Sc., D.V.O., Armidale, who has assisted with the later work.
REFERENCES
- Dukes, H. H. (1947) — "The Physiology of Domestic Animals". (6th ed.) Comstock, Ithaca, NY
- Edgar, Grahame, (1958) — Personal Communication
- Filan, F. (1958) — Personal Communication
- Hurst, Evelyn (1942) — "The Poison Plants of N.S.W." N.S.W. Poison Plants Cmtee, Sydney, N.S.W.
- Soule, R. P. (1958) — "Nutritional Effects of Tranquillisers on Ruminants". Chas. Pfizer and Co. Inc., Nevada Nutrition Conference, Reno, Nevada
- Underwood, E. J. (1958) — "Trace Elements in Human & Animal Nutrition". Academic Press Inc., New York, N.Y.
- Watts, R. M. (1961) — Personal Communication
- Whittet, J. N. (1958) — "Weeds", N.S.W. Dept. of Agric., Govt. Printer, Sydney, N.S.W.