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This article was published in 1992
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Benign Footrot Associated With Cathead Punctures
F. K. Nottle B.V.Sc. District Veterinarian, COONABARABRAN
'FOOTROT SPREADS FROM THE INTERDIGITAL AREA TO INVADE THE SOLE VIA THE HEEL' - so endeth the first lesson of the gospel. This is the constant theme through all footrot teaching and extension for classical outbreaks of the disease under moist conditions in lush pastures this clearly holds true. However there appears to be some special cases in which this standard explanation does not hold true. From field observations in the Coonabarabran district it would appear that at least benign strains of Dichelobacter nodosus can enter the foot via other means. These include through damage caused by foot abscess especially toe abscess and through puncture holes in the sole caused by the thorns on seeds of cathead (Tribulus terrestris) plants. The cathead puncture cases seen in May and June 1989 are presented in this paper.
CATHEAD (Tribulus terrestris)
Cathead, also known as caltrop, is a low, creeping, spreading, hairy annual herb with stems 30-70 cms long. The leaves are opposite on the stems with leaflets in 5-7 pairs, each 6-12mm long. The fruits (burrs) vary from almost smooth, awn-less segments to star-shaped capsules, each segment of which bears at least one sharp thorn. It grows profusely after summer rains. In the Coonabarabran district it germinates any time from late October. Depending on available moisture, it will persist until temperature drops and frosts commence in April.
WEATHER AND PASTURE CONDITIONS
The seasonal conditions during late 1988 and early 1989 were important in initiating the disease onset. Rainfall recorded at the Coonabarabran Post Office, which was typical for the district, and the distribution of that rain during those months is given below.
1988
October 6 mm
November 42 mm - numerous light showers
December 100 mm - 3 lots of heavy thunderstorm rain.
1989
January 32 mm - 21 mm fell on 1st
February 68 mm - 53 mm on the 5th, 12 mm on the 8th
March 125 mm - 124 mm from the 15th on 10 of 17 days to months end.
April 132 mm - 131 mm in first 14 days with rain recorded on 10 days
May 170 mm - rain on 14 days.
The pattern of this rainfall is a dry finish to spring followed by thunderstorm activity at widely spaced intervals and then persistent rain totalling 255 mm for 20 out of 31 days. this gave rise to the following scenario:-
The dry spring meant little ground cover was present early in summer. The thunderstorm activity in December meant excellent germination of cathead on the bare soils. The wide spacing between storms in January and February meant that very little apart from the cathead was surviving by mid-March.
The persistent rain from mid-March meant that the sheep were walking in a thin film of water for a month. It also meant there were no frosts to kill the cathead so it grew profusely making a great number of fresh green seed-heads. Over this period winter pasture species were germinating but naturally were very short and not of sufficient height to reach the interdigital area of the hoof.
The constant immersion of the hooves in the thin film of water made them extremely soft. This made penetration by the green cathead seed easy and provided a moist environment for any organisms they may have carried into the sole.
CATHEAD LAMENESS AT THE DISTRICT LEVEL
Along with many other areas of the state at that time there were numerous reports of a severe lameness in sheep resulting from injury to the foot caused by catheads. This was recorded by Special Veterinary Officer, John Plant, in the N.S.W. Agriculture Sheep Health Newsletter No 24 dated May 1989.
The description of the clinical signs was:'The lesions occur in the sole of the foot and vary in size up to 10 to 15 mm in diameter. They usually have rough frayed edges and are causing necrosis of the sole. In some cases there is a creamy pus present and the lesion may penetrate through the sole. In older lesions there is usually granulation tissue present at the base of the lesion. The disease can be compounded by wet conditions.'
Cases of this type were seen on numerous properties in the Coonabarabran Rural Lands Protection Board district from April to June 1989. One additional clinical sign was the presence of a foul smelling black exudate in the lesions and on the sole generally. To differentiate this smell from that of footrot it is best described as similar to that given off when dirty moist horses hooves are cleaned.
On two properties where footrot had been observed there were, as well as these typical signs of cathead lameness, well differentiated sole lesions that warranted further investigation.
PROPERTY HISTORIES
1. 'ND' Footrot diagnosed in January 1989. Prevalence of around 7% with severity being indicative of a strain of low intermediate virulence. Owner commenced eradication program based on combination of culling and treating by footbathing in formalin. At the time of the cathead problem in early May the sheep were being examined for the second time. They had been in a formalin footbath in late February - early March. The sheep were examined by me on the 9th and 25th May. Smears only were taken from infected feet on this property.
2. 'CL' Footrot diagnosed by private practitioner in May 1989 in another mob. Owner observed similar lameness in this mob of 2 year old wethers in late May. No treatment was undertaken on the property prior to my investigations. The sheep were examined by me on the 14th and 26th June. Both smears and cultures were taken on this property.
DESCRIPTION OF LESIONS
Foot lesions on both properties were similar. Sheep on 'ND' did not have any score 1 or 2 lesions in the interdigital skin. This was in keeping with the very short germinating winter pastures not reaching this area. Sheep on 'CL' had a few score 1 and 2 lesions on the first visit and the prevalence of these had noticeably risen at the time of the second visit.
With the 100% prevalence of cathead puncture wounds to the soles of the feet it was not possible to establish or even estimate the number of these that were infected with D. nodosus.
The types of lesion from which D. nodosus was found by smears or culture varied considerably. There were lesions somewhat similar to those often seen in chronic footrot in the summer with an eroded area and granulation in the central region of the sole extending across from the axial wall. These would not be considered all that unusual. The lesions of most interest were those with no connection to the axial wall of the hoof. These lesions were quite discrete. There were basically two types.
One was a small pit up to 3mm deep in the sole. These ranged from 1 to 10 mm in diameter and were primarily circular.
The other was a small pinhead puncture hole in the sole with a cavity below this. This cavity was mostly circular but could also be roughly semi-circular in shape. Again these cavities ranged from 1 to 10 mm in diameter.
Neither of these types of lesion penetrated the full thickness of the sole. They were confined entirely to the horn of the hoof.
Figure 1 illustrates the range of lesion types from which D.nodosus was smeared or cultured.
CULTURE OF THE LESIONS
Culture of the lesions was undertaken on sheep from 'CL'. As part of an evaluation by Frank Cockram of the Orange Regional Veterinary Laboratory of the two techniques, direct smearing onto agar with immediate anaerobic culture and the use of specific D. nodosus transport media were used. Cotton tipped swabs were used to collect material from the lesions. This was smeared onto the agar plates and the cotton tip was then broken off and put into the transport media. The agar plates were incubated within 1 hour of collection for 3 days and the transport media were kept at room temperature for the same period before both were forwarded to the laboratory.
The culture results were:
| Method | Number | D. nodosus isolated |
|---|---|---|
| Anaerobic agar | 11 | 6 |
| Transport media | 10 | 8 |
The 6 isolates on the anaerobic agar were all recovered from the transport media.
Elastase testing of the isolates gave no clearing after 21 days and they were therefore considered to be benign.