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Footrot in the Western Riverina
D.D. Salmon, B.V.Sc., Veterinary Inspector, DENILIQUIN
INTRODUCTION
Footrot is a very important disease for the sheep producers of Western Riverina. The disease itself is fairly common, causing significant losses in production, as well as the work and expense of controlling or eradicating it. Footrot is also important in its absence; owners who do not have footrot are constantly faced by management decisions to decrease the risk of introduction of the disease. There are many examples where a particular practice is undertaken purely to avoid the introduction of footrot, where if that possibility did not exist more economical practises could be undertaken. The sort of things that we see include trucking sheep from one property to another instead of walking them; leaving paddocks free from sheep if there is a suspicion of the neighbour; and severe restrictions on trading of sheep to take advantage of seasonal fluctuations in feed or prices. Real or imagined fears of footrot can also effect saleyard activity. The saleyards of the Western Riverina are clearing houses for sheep as they move from Western New South Wales and Queensland into Victoria and South Australia. The fear of footrot at these saleyards can sometimes lead to major disruptions in this trade.
The plains of the Western Riverina are generally very flat, black grey soils with some red soils and a few windblown sand ridges. As its name implies, the Riverina is interlaced by creeks and rivers, sources of which are the Murrumbidgee River, the Billabong Creek and the Murray River. Rainfall is generally fairly low in the range of 12-17 inches per annum, rain can be received at any time of the year, but there is a winter bias. The summers are long and hot so that rain received in the summer rarely has any long term effect. Winters are usually short and mild with only a few frosts, and only rarely does the area see prolonged wet conditions even in winter. There is extensive irrigation development throughout the area. This development varies from small intensive completely irrigated properties to larger properties with significant irrigation development, to larger properties where irrigation is a relatively small part of the enterprise. Agriculture, particularly rice and wheat are prominent features of this irrigation development, and sheep enterprises are usually integrated with some cropping programme.
The main type of sheep enterprises practised on irrigation properties in the Riverina are, in order of importance, fat lamb breeding, cross breeding, that is joining Merino ewes with Border Leicester rams to produce cross bred ewe lambs for sale, Merino wethers - either for straight wool production or to grow out to sell to the live sheep trade, and Merino breeding.
Footrot is often a major problem on these irrigated properties. Pastures are irrigated in the Autumn, to get early growth of feed. This autumn watering gives ideal conditions for the spread of Footrot very early in the year. Most winters are quite mild, and there are usually only very short periods in which it is too cold for footrot to spread. The pastures are usually irrigated again in the spring, once more extending the period in which footrot will spread.
Very few sheep flocks on irrigated properties in the Riverina are self replacing; this means that introduction of sheep and the associated risk of introduction of footrot is a feature. These regular introductions decrease the value of a full scale eradication programme on any property when compared with sale of the sheep. Because most irrigation sheep enterprises rely on the sale of some sheep fairly regularly, it is very difficult to live with footrot and not fall foul of the Stock Diseases Act.
The fact that most sheep bred in the Riverina are destined for interstate sale further reduces the attraction of trying to live with footrot. Footrot infected sheep may only enter Victoria on direct consignment to specified abattoirs, and sheep destined for South Australia must be free from footrot and may not be vaccinated against the disease.
The other major sheep enterprise is the traditional grazing of merinos on native pastures. These are usually merino breeding enterprises, with quite a few studs. These properties usually range in size from 20,000 to 200,000 acres and run from 5,000 to 50,000 sheep. Stocking rates are quite low, usually two to four acres per sheep and with the low rainfall they have very little problem with footrot as a disease. These properties do have a problem with benign virulent footrot [sic]. Because of the generally dry conditions and low stocking rates virulent footrot can stay on a property for many years without being diagnosed. There is very limited spread from sheep to sheep, and very few manifestations of the disease. It is only in exceptionally wet years that the problem becomes visible, usually as a massive and disastrous outbreak of the disease. This lack of diagnosis is understandable if one considers the size of the properties and the large number of sheep which each man is required to supervise. The result of this is that some properties have problems with footrot every twenty years or so, other properties have a mob in a particular paddock infected every three or four years with no avenue for introduction.
These large dry properties also present unique problems in eradication of the disease: there may not be a spread period to challenge the effectiveness of an eradication programme for four to five years after the programme has been completed.
THE DISEASE
The epidemiology of footrot has been well described and on the whole it behaves as expected in the Riverina. There are major variations in the perceived virulence of footrot in this area, and not all of these variations can be explained by different strains of footrot. Infections from the same source often behave quite differently on different properties. These variations can be explained in various ways, differences in the moisture and temperature which prevail at a particular time will vary the amount of spread, and also the severity of individual infections. If the ground dries up, a large percentage of footrot lesions begin to resolve, leading to a difference of severity depending on when the paddock was last irrigated. Soil types also appear to play a role in the severity of footrot lesions; occasionally mobs have been split and put into different paddocks, showing marked differences in the severity of the infection. From my observations I believe that the lighter more acid soils are more conducive to the spread and development of footrot than the heavier alkaline soils. The other difference is of course sheep: different strains and breeds of sheep vary significantly in their susceptibility to footrot. The breed differences are well recognised but there has been little emphasis on the differences between the strains of sheep. I have not observed this closely enough to state categorically which strains are more susceptible than others but there are differences.
Research on differences between strains of Bacteroides nodosus is interesting, but I do not feel that it has any relevance from a diagnostic point of view. Difficulties associated with getting live B. nodosus to a laboratory for elastase testing and the time involved in getting a result, makes it much easier to just observe the infected sheep for a few days.
The pathological manifestation of footrot has been well described, but there are a few disease entities which sometimes mimic footrot quite closely, and they can be fairly difficult to differentiate - particularly if there is only a very low incidence of the disease. Penetration of the hoof, in the Riverina most commonly due to cathead (Tribulus terrestris) causes a small abscess between the horn and the sensitive laminae of the feet which may spread and appear very similar to a carrier state of footrot. Bruising of the sole of the hoof particularly due to running on bitumen roads can produce a very similar syndrome. Laminitis can result in shedding of the hoof, this does not mimic footrot as closely as the previous syndromes, but the extensive shedding of the hoof gives an under-run appearance which may be confusing. Burns caused by running on recently burnt ground produces a very similar shedding of the horn of the hoof and, while not so similar to footrot as other syndromes can lead to confusion.
Collection of smears for the detection of B. nodosus will be extremely successful so long as the smears are carefully taken. They must be taken from the junction between healthy and diseased tissue, the white material found there will show large numbers of B. nodosus. When taking smears I always inscribe the name of the owner and identification of the animal if appropriate on the slide to which the smears are applied. I do this with a tile cutter, and this reduces the chance of any mistakes being made.
CONTROL AND TREATMENT
Both footrot vaccines give very good control of the spread of footrot. Because the seasonal conditions and types of sheep enterprises in the Western Riverina do not lend themselves to living with footrot over a long period. I have not had any experience with footrot vaccines alone for a long term control programme. The vaccines have proven very effective in reducing the spread of footrot and the severity of the lesions which do occur during a spread period before attempting to get rid of the disease. As well as the reduced incidence and severity of lesions in a mob, it is easier to cure a vaccinated sheep than one that is not. On a few occasions paring alone has resulted in complete remission of footrot lesions even without foot bathing.
Footrot vaccination has also been used as a follow up to eradication programmes. Quite a few people vaccinate for a few years after they believe they have got rid of footrot, in case they have missed anything. The vaccination cover reduces the amount of persistent footrot, and also reduces the chance of it breaking out.
The eradication of footrot still involves a lot of work. In my experience proper handling equipment can take a lot of the back-break out of Footrot eradication.
A sheep handler of some description is essential if footrot is to be eradicated from any significant number of sheep. I have worked with most of the commercially available sheep handlers and quite a few farm manufactured ones, and in my opinion the best of them are the farm built ones in the style of the Gun-Crutcha. On these units the sheep walk up a ramp then they are tipped over a swinging gate onto a cradle. Other units are the V.E. machine which is expensive and noisy and there is a certain amount of work involved with tipping the sheep over. The Daken Unit which is cheap works quite well but can only be used for foot paring; and the Moffat Virtue Machine which uses the weight of the last sheep to lift the next sheep up, all of these machines certainly make the job easier but lack some of the flexibility of the first machine.
Once you have got a method of presenting the sheep on their backs at working height the next step in footrot work is the paring. In my experience the old steel handled Wilkinson Sword footrot shears are the best implement for this job. The new Japanese manufactured Wilkinson shears with alloy handles are quite good, but their first lot (with green painted handles) invariably broke when any sort of pressure was applied. The new batch of Japanese manufactured Wilkinson Swords with white painted alloy handles appear to be a lot better. It is also possible to get steel handled shears styled after the Wilkinson Sword and made in England, these appear to be quite good. All the other footrot shears which I have used vary from not very good to completely useless.
It is possible to get pneumatic assisted footrot shears.
These are very good, there is quite good fine control on the closure of the blade which makes it possible to carefully pare a foot. The only disadvantage to these blades is that they are extremely difficult to keep sharp, and they require an air compressor.
Foot knives are useful for some fine work, but I don't think that they are much use for the heavy paring involved with footrot eradication.
With all instruments used for paring feet it is absolutely essential that they be kept very sharp, footrot shears need to be touched up with a stone after every three or four sheep at the very minimum. There is a jig available which some people use to sharpen footrot shears, but however it is done it must be done often and thoroughly.
Paring feet is much easier if they can be softened beforehand. Quite a few people stand the sheep in wet straw for half an hour or so before they tip them up, and this works quite well. Jopp et al. in New Zealand use a water trough with a constant flow through of water to soften the feet, and this seems to have a lot of merit.
When paring for the first time a lot of feet are so badly infected that it is impossible to get adequate surgical preparation of the foot. In these cases an injection of 10cc's of Streptopen usually gives very good results so that these sheep can be pared effectively in a few weeks time.
For footbathing I now recommend 10% zinc sulphate. This is slightly cheaper than 10% formalin, and much more pleasant to work with. It is mildly corrosive, so that the solution can not be left indefinitely in steel footbaths. I have been involved with the use of quite a lot of Footrite and I have been very disappointed in the results. It does not appear to be any more effective than straight zinc sulphate despite the fact that it is more than three times the cost. Even with good surgical preparation of feet, standing of sheep in Footrite solution for one hour does not give results significantly better than straight zinc sulphate. I have now gone back to very, very strong emphasis on foot preparation followed by ten minutes or more in zinc sulphate bath.
Footrot eradication is a state of mind, once an owner has decided that it can be done, and that he is prepared to put the effort in, it is usually successful within one season. The fact that we almost invariably have a long and dry period in which this work can be carried out does make it much easier to eradicate Footrot in the Western Riverina than in many other parts of New South Wales or Victoria.
REFERENCE
Jopp, A.J. et al. (1984) New Zealand Veterinary Journal 32 : 189-190