Kimberley horse disease ("Walkabout disease")

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Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 3 Western Australia, Series 3

Volume 7

Number 4 July-August, 1958 Article 4

7-1958

Kimberley horse disease ("Walkabout disease") Kimberley horse disease ("Walkabout disease")

C. R. Toop

Follow this and additional works at: https://researchlibrary.agric.wa.gov.au/journal_agriculture3

Recommended Citation Recommended Citation

Toop, C. R. (1958) "Kimberley horse disease ("Walkabout disease")," Journal of the Department of Agriculture, Western Australia, Series 3: Vol. 7: No. 4, Article 4.

Available at: https://researchlibrary.agric.wa.gov.au/journal_agriculture3/vol7/iss4/4

This article is brought to you for free and open access by the Agriculture at Digital Library. It has been accepted for inclusion in Journal of the Department of Agriculture, Western Australia, Series 3 by an authorized administrator of Digital Library. For more information, please contact [email protected].

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A horse suffering from "Walkabout Disease" [Photo, by A. L. Rose.]

KIMBERLEY HORSE DISEASE

("Walkabout Disease")

By C. R. TOOP, B.V.Sc, Chief Veterinary Surgeon

F

ROM the earliest stages of its development, the cattle industry in the Kimberleys has been seriously handicapped by mortality in horses. A plentiful supply of good horses is essential for the successful operation of a cattle station, and when losses occur suitable replacements are often difficult to obtain and they are always costly.

The average station requires about 200 horses for mustering, droving and allied activities, and on one large concern near Fitzroy Crossing t h e total exceeds 600 head. Conditions for horses in this region are h a r d a t the best of times. Tem- peratures are usually high and humidity is often excessive. Horses are not only required to work under these conditions but must also live off the country, and as the dry season advances a n d the quality of the grazing deteriorates, they steadily lose condition.

A large reserve of horses is thus essential to enable mustering and droving plants to be spelled and replaced by fresh horses as their condition declines.

HEAVY MORTALITY

A further and more serious cause of wastage is the mortality resulting from Kimberley Horse Disease or t h e so-called

"Walkabout Disease" which is regarded by

The two blocks used in this article were made available by courtesy of the C.S.I.R.O.

cattlemen as the principal cause of loss amongst horses in the n o r t h e r n areas.

Official records date back to 1897 when the loss of 122 horses was reported as having occurred on a single property in five months, with a total of 300 deaths in 397

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six years. These losses have continued down t h e years a n d it was revealed by Army veterinary officers stationed in the Kimberleys during t h e War, t h a t 1,419 horses died of t h e disease on 16 properties between 1942 a n d 1946.

Cases m a y be reported a t any time of t h e year, but t h e majority of deaths occur d u r i n g t h e wet season between J a n u a r y a n d April, a n d losses are heaviest in unusually wet years.

SYMPTOMS

T h e most characteristic symptom is an a b n o r m a l desire to walk, but this is not observed in all cases. I n a typical case t h e animal appears sleepy a n d depressed a n d adopts a peculiar stance with the h e a d depressed, t h e lips hanging loosely, t h e eyes half closed a n d t h e weight of the body t h r o w n forward over the forelegs.

Periodically it commences to walk—aim- lessly a n d generally in a straight line, and m a y s t a n d pushing its h e a d against obstacles such as trees or fences which impede its progress.

These symptoms may continue for several days until d e a t h occurs, and in the final stages t h e a n i m a l becomes prostrate a n d is unable to regain its feet.

In t h e more chronic form of t h e disease, t h e r e is a slow but progressive loss of condition a n d t h e horse becomes emaciated a n d lethargic. Some of these cases are r a t h e r indefinite b u t a diagnosis can always be established by t h e microscopic e x a m i n a t i o n of t h e liver. A post mortem e x a m i n a t i o n reveals much evidence of liver damage. T h e pathological changes include necrosis or d e a t h of certain groups of cells, h a e m o r r h a g e s into t h e tissues, a n d depending on t h e type of case, a variable degree of fibrosis, but t h e feature of most significance is a marked enlarge- m e n t of t h e liver cells which is described a s megalocytosis a n d is diagnostic.

Investigations by D e p a r t m e n t a l officers a t t h e t u r n of t h e century h a d suggested t h a t t h e disease was due to impaction of t h e stomach, resulting from t h e consump- tion of t h e coarse herbage a n d the heavy infestations with stomach and bowel worms which were frequently observed, were also t h o u g h t to play a part.

POISON PLANTS SUSPECTED The possibility of p l a n t poisoning h o w - ever, was recognised almost from t h e out- set, and although feeding experiments with suspect plants including t h e species eventually incriminated were a t t e m p t e d , they were too limited in design and scope to provide any information of value.

WHITEWOOD

The first scientific investigation was undertaken in 1927 a n d as the result of a series of feeding experiments, they h a d conducted, it was reported by Murnane and Ewart t h a t Whitewood—a shrub or small tree with a wide distribution in t h e n o r t h had been incriminated. This find- ing was not, however, accepted by Kim- berley cattlemen who were quick to point out t h a t the disease was absent on some properties where Whitewood grew in great abundance, a n d t h a t losses were heavy on others where it did not occur a t all.

It was subsequently shown t h a t White- wood leaves could be consumed by horses in large amounts with impunity, whereas t h e seeds even in very small amounts were highly toxic, and it is obvious now t h a t seeds had been included in t h e p l a n t material fed, and t h a t the condition produced had been mistaken for Walkabout disease. Whitewood seeds are unpalatable and are never eaten voluntarily by horses, so the m a t t e r is of no practical significance.

CO-OPERATIVE INVESTIGATION There the problem rested for more t h a n 20 years and losses still continued, until in 1949, a co-operative investigation by t h e C.S.I.R.O., the Animal Industry Division, Northern Territory a n d the De- p a r t m e n t of Agriculture, Western Austra- lia, was arranged.

The first p a r t of this investigation was concerned with a field survey to define the geographical distribution of t h e disease and to make a further study of its epidemiology. It ranged over 20,000 square miles a n d extended from t h e Vic- toria River in t h e Northern Territory to Wyndham and Fitzroy Crossing in t h e Kimberley Division. This was combined with a botanical survey which involved a vast amount of detailed work in t h e col- lection and identification of p l a n t speci- mens.

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These surveys revealed that the disease occurred on some properties but not on others, that its distribution coincided with the major river systems, notably the Fitzroy, Ord and Victoria, and that its incidence was particularly high on areas subject to periodical flooding.

By a comparison of the species listed on affected and non-affected properties and a process of elimination, it was possible to reduce the number of plants that might Tie incriminated to four, including two

species of Indigo and two of Crotalaria.

The next step was to conduct feeding trials to determine their toxicity and it was necessary in these experiments to use horses which had never before had access to the vegetation of the area and to ship them, together with supplies of chaff from Fremantle, to Fitzroy Crossing where the work was conducted.

Two of the four suspect plants, Indigo- fera trita and Crotalaria trifoliostrum were fed in large quantities for a long period but produced no ill effects and could thus be excluded.

The second species of Indigo—Indigo- Jera enneaphylla, commonly known as Birdsville indigo—had already been incriminated as the cause of "Birdsville disease" in Queensland and the Northern Territory, and when fed to a horse at Fitzroy Crossing it produced typical symptoms. It was thus established that Birds- ville disease may occur in the Kimberleys as well as in other parts of northern Aus- tralia. It grows abundantly in some areas and may cause some deaths but does not appear to be a serious cause of horse mortality in the Kimberley Division.

The only suspect plant now remaining for investigation was Crotalaria retusa and because of its geographical distribution it fitted into the picture much more

closely than any of the other species studied. Popularly known as the wedge- leaved rattlepod, it reaches a height of about three feet and has clear yellow pea- shaped flowers, and where the soil is suitable for its growth, it occurs in great pro- fusion on areas which become flooded along the river systems. It was abundant

•on all properties where heavy and fre- quent losses in horses had occurred and where it was not found in horse paddocks,

there was always evidence that affected horses had previously gained access to it elsewhere on the run.

It was also known that closely related species of Crotalaria had produced a liver disease in horses in the United States which appeared to be identical with Walk- about disease.

Crotalaria retusa or Wedge-leaved Rattlepod.

FEEDING TRIALS

The plant was fed to two horses during the wet season of 1952 and after consum- ing about 100 lb. during a period of 70 days, one of them developed typical symptoms of the disease and a microscopic examination of its liver revealed the characteristic pathological changes.

Th second horse did not develop symptoms, but an autopsy revealed similar liver damage.

A disease had thus been produced which was similar in every respect to the natur- ally occurring condition, and Crotalaria retusa had been incriminated beyond all 401

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doubt as t h e p l a n t responsible. This r e - sult was subsequently confirmed in a comprehensive series of feeding trials conducted a t K a t h e r i n e in the Northern Territory in which it was shown t h a t horses m a y need to consume the plant for two and some times three seasons before t h e disease occurs, a n d t h a t deaths may n o t commence u n t i l weeks or months after feeding h a s ceased.

Analyses revealed t h e p l a n t to contain t h e toxic alkaloid, monocrotaline, which slowly but progressively damages the liver u n t i l symptoms appear and d e a t h occurs.

T h e information gained from these studies h a s enabled preventive measures to be employed. T h e m a t u r e plant is not palatable to horses but it is freely eaten in t h e seedling stage, a n d this occurs d u r i n g t h e wet season when the majority of station horses a r e spelling in the horse paddocks.

PREVENTION

Thus by a re-location of horse paddocks or the erection of fencing so as to exclude low lying areas where Crotalaria retusa grows abundantly, it is possible to p r e - vent mortality. This practice h a s been adopted on a number of Kimberley Stations, and it has been reported t h a t the severe losses which occurred in former years have been eliminated.

Access to the plant cannot be prevented during mustering and droving, but by t h a t time it has reached t h e flowering or seed- ing stage when it is not eaten and no risk is involved. Several other species of Crotalaria in t h e Kimberleys might also produce t h e disease, but because of their distribution or t h a t they are unpalatable, they are probably of little importance.

(From an A.B.C. Country Hour broad- cast made available by courtesy of the Australian Broadcasting Commission).

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