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This article was published in 1998
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OVINE FOOTROT IN THE NORTHERN TABLELANDS

JA Macfarlane, District Veterinarian & GC Green, Footrot Advisory Officer, Armidale

The good old days

The first area to be declared a PROTECTED area on account of footrot in sheep was the Armidale Rural Lands Protection Board in 1960 This was extended over all the New England Tablelands in 1969 and was known as the NE Footrot Protected Area NEFPA. The purpose of this Protected Area was to prevent the ingress of footrot onto the Tablelands. Pasture improvement was becoming extensive and footrot was showing up as a major disease problem.(1) The program was economically estimated to have a large positive net benefit to society using standard cost benefit analysis techniques.(2)

There was little formal definition of Virulent (v Benign) footrot, which has always been a problem. Essentially it was a field diagnosis based on assessment by the inspector of the day (of which there were many). Secondly inspections were restricted to owners who reported lame sheep, introductions from high risk areas that were notified and traces from saleyards and quarantined properties. To my knowledge no surveys were conducted to determine the real prevalence of the disease. Traditionally numbers of known infected properties have always been high after prolonged dry periods which was put down to introductions from other areas where the footrot prevalence was unknown (or more usually known to be high).

The NSW footrot strategic plan

The introduction of the NSW Footrot Strategic Plan in 1988, with the aim of declaring all NSW a Protected Area by 2000, put more formal methods into place such as surveys to demonstrate the prevalence of disease, and as well attempted to define Virulent footrot a bit more objectively.(4)

Armidale Rural Lands Protection Board was virtually drought declared from mid '91 through to the end of '95. During this period store sheep moved out of the area and a significant number of tracebacks for Virulent footrot were reported. Many of these when traced back to property of origin did not reveal any 'Virulent' footrot, on clinical inspection.

To clarify the situation Rob Walker (the NSW Footrot Co-ordinator) was asked to visit the area in 1996 and was shown a number of properties about which we were concerned. The conclusion was that our definition of Virulent footrot (based on clinical examination and sparing use of the elastase test) was not meeting 'best practice'. The reason for this was that the environmental conditions received in this area for the expression of clinical footrot lesions are not the same as the more classical footrot challenges that occur on Forbes irrigation country or good sub clover springs in the Riverina. In some situations where low grade Virulent footrot ie, the intermediate strains, was present infected stock were being classified as "NE Scald" ie benign, but the same strains in other areas were developing into a condition classified as Virulent footrot.

Footrot survey

The NSW Agriculture conducted a survey in February 1997 using an independent contractor (David Alley). The standard method of inspection of 100 older sheep and scoring all feet for a field clinical diagnosis, with Gelatin Gel lab tests for all suspect flocks used for confirmation.

Summary of results for ARLPB

Diagnostic Category Field Lab
No Footrot 59 1
Benign 29 19 (1 Virulent)
Virulent 5 4 (1 not isolated due to recent footbathing)
Intermediate 8 6 Virulent
2 Benign

Effectively Virulent footrot was found on 12 properties ex 101 inspected.

It should be noted that during the survey the weather was favourable for footrot expression with many inspections called off due to wet weather. Pasture during the survey was essentially the factor limiting expression.

On the 'downside' it also must be noted that the owner chose the sheep presented for inspection. It was noted that some sheep were recently footbathed, some had been previously inspected, and some owners had even purchased in special mobs. Hence it could be said there was owner bias against finding diceace on some properties.(3)

Current situation

These results were somewhat unexpected and ARLPB Directors asked that the NSW Footrot Committee to visit Armidale to discuss the implications, which they did on 6/12/97. Essentially they decided that the area should be re-scheduled to a Control Area on the basis of the results of this survey.

Directors and staff asked for clarification of diagnosis and it was confirmed that the flow chart (as given in Figure 2.1 of the Footrot Eradication Manual) for diagnosis should remain the effective means of diagnosis, is a positive lab test of itself was not sufficient information to make a diagnosis of Virulent footrot.(4)

The ARLPB has subsequently:

  1. employed a Footot Advisory Officer to co-ordinate inspections of all properties in ARLPB along with the usual extension activity, and promoted a Field Assistant to assist with inspections.
  2. purchased two Peak Hill Sheep Handlers for Footrot inspections. The Board has 2 Ezy Handlers for associated activities, and access to 2 others.
  3. organised a contractor to inspect 100 smaller holdings per year and this is expected to be on-going for 3 years.

Inspections to date have concentrated on studs, on properties that supply store sheep on circuit sales and so on. The reason for this was to allow vendors to issue a Vendor Declaration with some confidence, and thus allow clean properties to restock with clean sheep.

Inspection results to end of February

No footrot 195
Benign footrot 15
Virulent confirmed or suspect 55 Total 265

These are divided as follows:

Quarantine 28
Awaiting test results 6
Released from quarantine 8
Positive gel tests - awaiting further evaluation 9 Total 55

Method of detection of Virulent properties

Owner notified 9
Tracebacks 4
Traceforward 2
Saleyard detection 5
Alley detection 10
Routine inspection 6 Total 36

Expression of footrot

The question of the environmental conditions for footrot expression in NE needs addressing.

Moisture

The annual average rainfall in the ARLPB varies from 750mm to 1,500mm. It is basically higher in summer and the average for Armidale is given below. This represents a normal average for the district as Armidale is on the top of the Divide, and receives both easterly and western weather patterns.

Note the following need considering. The ARLPB varies in altitude from 600m to over 1,600m the lower country is on the western side and this markedly affects evaporation rate. Secondly the rainfall changes from a western pattern on the west side of the range (essentially a spring season) to a coastal pattern on the east (an autumn pattern). The east is also associated with period of 'Dorrigo Mist' in autumn and may remain 'gumboot' weather for months on end despite low recorded rainfall. Overall this means the eastern side is wetter with damper conditions persisting for much longer periods.

Temperature

Armidale temperature figures are given below, and the notes above also apply to temperature ie, it is hotter as well as drier in the west. Note that soil temperatures are lower in winter, higher in summer (compared with air temperatures) and usually lag a month or two behind ie, it usually doesn't get warm enough for footrot expression until December (all else being equal). Footrot is usually said to express when soil temperatures are above 10°C and below 20°C.

Combined Rainfall and Temperature The graph below demonstrates that moisture and temperature conditions suitable for footrot spread in an average year can occur from October through to April.

Pasture

Pastures range from native red grass/kangaroo grass/spear grass on bulldog granite country (butcher bird and goanna country) through to thick permanent perennial ryegrass / phalaris / fescue pasture on highly fertile heavy basalt soils (some of which are ex wetlands type soils) in the high altitude high rainfall country. Only 25 - 30% of the Board's area is considered to be reasonably improved.

In recent years clover has not been dominant in pasture, and clover dominant pastures are only seen in recently prepared pasture, or in special 'high performance pastures'.

Hence pasture when moisture and temperature are suitable is usually the factor limiting footrot expression.

Diagnosis of virulent footrot

The major diagnostic problem relates to situations where on field examination, the condition appears to be clinically benign ie, effectively no underrun noted despite reasonable environmental conditions, but Gelatin Gel positive on laboratory tests. I divide these into:

(i) properties on which conditions are such that expression will never be good enough for clinical expression of footrot (these are usually found to be positive).
properties on which footrot expression has been suppressed by owner action such as footbathing (again reinspections reveal these to be positive).
In the absence of any method of experimentally testing these sheep, tracebacks / traceforwards or reinspections usually reveal the same sheep express footrot in more favourable conditions.
(iii) properties on which conditions suggest that if virulent footrot was present, then clinical expression should occur. These cases are put under surveillance for further evaluation. I note the IWS report showed that a number of Gelatin Gel positive strains did not express as Virulent clinically, in pen trials. These belong in the Greentree "BIIK" category.

Eradication

Essentially the options we have adopted are:

(1) de-population of all infected mobs
(2) Blue's "scorched earth" policy - ie, all sheep inspected and all sheep with any lesions or deformed feet etc. removed and sold. Only 'clean' sheep retained and reinspected.
(3)The Alley modification - ie, all sheep segregated into clean, infected and suspects. Suspects are score 1 or 2 in young sheep with otherwise good feet. These are treated once with Zinc Sulphate bathing, LA Terramycin and held on grating, usually in the wool shed for 36 hours. These sheep remain in separate mobs and only clean sheep are kept in all follow up inspections.

We have always commenced eradication immediately (except for lambing ewes) and prefer conditions that allow at least some expression of footrot to ensure the pick up of all low grade lesions.

Long term aim

The plan is to inspect about a third of the properties each year, allow a year of grace, then do a repeat survey to see if there is any improvement. Assuming the situation has improved to less than 1% Virulent footrot then application for Protected Area status should be in 2001.

REFERENCES

  1. CLARK, F.L.
  2. CARMODY, J. (1981). An economic evaluation of the footrot eradication program on the Northern Tablelands of NSW. Dept. Agriculture Economics UNE
  3. WALKER, R (1977) New England Footrot Survey, 1997 NSW Agriculture
  4. NSW Footrot Eradication Manual (1995). NSW Agriculture

 


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