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Experiences With Benign Footrot in Sheep & Goats

K. Hart, B.V.Sc., Veterinary Inspector, BRAIDWOOD

Many forms of footrot exist and will be discussed today. I intend only to talk about benign footrot - a condition at the extreme end of the footrot pathology spectrum, sometimes euphemistically called 'Scald' in the field.

Scientifically, I think benign footrot is best described by the comprehensive definition given by David Stewart in December 1983 in the Post Graduate Foundation Proceedings No. 67 on Sheep Production and Preventive Medicine i.e.

'Benign footrot (scald) is a non-persistent condition in which interdigital skin lesions only may be observed or there may be in addition mild under-running lesions to the abaxial wall and/or toe (with a minimal amount of necrotic exudate) in a small proportion of feet.'

A simple definition to describe benign footrot from a laboratory point of view would be as follows:

Benign footrot strains are strains of Bacteroides nodosus with a low proteolytic index that are elastase negative after 21 days incubation.

PREVALENCE

I believe that benign footrot is widespread in New South Wales, even on properties where footrot is not suspected. The best survey I know of that illustrates this point was reported by Morgan et al. in 1972, using randomly selected Victorian flocks.

These workers examined 35,000 sheep from 90 flocks not known to have virulent footrot.

1. Organisms similar to B. nodosus were present in smears from 80 sheep in 34 flocks (i.e. 38% of the flocks visited).

2. 31 of these 34 flocks were re-visited after a month or more. All sheep previously yielding B. nodosus on smears had been ear tagged to allow re-examination. Virulent footrot had developed in only one of the re-visited flocks.

3. Only benign footrot appeared to be present in the 30 remaining flocks. The results were:

a. 2 flocks - all tagged sheep normal.

b. 9 flocks - interdigital lesions persisting in some sheep, no B. nodosus demonstrated on smears.

c. 19 flocks - B. nodosus found on smears from re-sampled sheep.

These survey results suggest that benign footrot is fairly widespread in Victoria, and I submit that a similar situation exists in N.S.W.

PROPERTY EXPERIENCE WITH BENIGN FOOTROT

Three properties in my district have been investigated in some detail in an attempt to determine if benign strains of B. nodosus were the only types present.

PROPERTY A

A well managed property running mainly merino sheep. Closed flock except for the annual introduction of merino rams, using the owner's truck.

1. 1976 (Summer) :

• Mild lameness noted in a mob of merino ewes. The lesion was hair loss and inflammation of the interdigital skin. Smears were positive for B. nodosus.

• Owner conducted a very thorough footrot eradication programme, using formalin.

• No recurrence of lameness problems until 1981, in spite of several apparently ideal footrot spread seasons.

2. 1981 (Winter) :

• Lameness problem (about 1% prevalence) in merino ewes, running on a very wet, pasture improved paddock for 6 weeks. Clinically, all cases examined were typical foot abscess. However, smears of the interdigital skin area were positive for B. nodosus.

• Property investigated two weeks later by Peter Claxton from Veterinary Research Station, Glenfield, N.S.W. in an attempt to isolate the B. nodosus strains responsible on hoof agar. 9 ewes with typical foot abscess were sampled.

• All were negative for B. nodosus on smear and culture. One merino ram examined had a typical benign footrot lesion in one foot, smear positive for B. nodosus. B. nodosus cultured on hoof agar from this ram proved elastase negative at 21 days.

• No further lameness problems until 1983. The property was drought affected for 18 months.

3. 1983 (Late Winter) :

• Lameness developed in a mob of merino rams after period of wet weather. Interdigital lesions from lame rams resembled benign footrot, and organisms resembling B. nodosus were present on smears.

• To resolve the situation once and for all, the mob of 150 merino rams was isolated on a wet, pasture improved paddock for two months under ideal footrot spread conditions.

At the end of the period, all rams were examined in a sheep handler. Only 9 ex 150 revealed foot lesions of any description, the most severe being typical benign footrot with slight separation at the skin/horn junction.

• Lesion material from the 8 most promising feet were plated on the hoof agar. 2 out of 8 feet showed B. nodosus on culture, elastase negative at 21 days.

DIAGNOSIS : Benign Footrot

PROPERTY B

Well managed property running 8,000 merino and crossbred sheep. Some exchange of merino rams with Property A in the 1970's.

1. 1982 (Summer) :

• Acute lameness in 8 ex 300 crossbred ewes running in a swampy paddock. Lesions were typical of foot abscess. Organisms resembling B. nodosus found on smears.

• No further lameness problems on property during drought conditions.

2. 1983 (Spring) :

• Lameness in a small number of merino ewes running under ideal footrot spread conditions. The most advanced foot lesion seen was typical interdigital inflammation with caseation and some erosion of the soft horn. There was no under-running.

• Only 1 of 22 smears taken showed B. nodosus organisms.

3. 1984 (Late Spring) :

• 5% lameness problem in a mob of 280 crossbred ewes grazing a lush, pasture improved paddock. Examination of lame ewes revealed typical 1 and 2 score benign footrot lesions in one to three feet. All smears were positive for B. nodosus.

• Revisited property 8 days later for culture. Smears and lesion material inoculated onto hoof agar were collected from 10 feet ex 7 ewes (tagged to allow individual identification). One was from a typical foot abscess, with 8 samples from typical 1 and 2 lesions. The tenth foot sampled had a 3 score lesion with some under-running at the sole. There was very little necrotic material in the under-run area, and an unusual odour not typical of footrot.

• All smears revealed organisms typical of B. nodosus.

• B. nodosus colonies were cultured from all feet except the abscess case and the 3 score lesion. All isolates examined were elastase negative at 21 days incubation.

4. 1985 (Late Summer) :

• The same mob of ewes were rechecked after 3 months in the same paddock under fairly dry summer conditions without treatment.

• 6 of the 7 ewes were re-examined. All foot lesions had completely resolved except the foot with the 3 score lesion, which was misshapen and still slightly under-run. A smear from this foot showed one organism resembling B. nodosus. This animal was classified as a benign footrot carrier.

DIAGNOSIS : Benign Footrot.

PROPERTY C - GOATS

1984 (Spring)

• A mob of 100 feral goats was introduced to a previously goat free property. A few sheep were running on the property prior to introduction. The sheep were not lame and footrot was not suspected in the mob.

• After several months running on lush, pasture improved country, 10% of the goats were clinically lame. The foot lesions were typical of footrot, with interdigital hair loss, necrosis and odour but no significant under-running. One 2 month old kid was showing severe lesions of this type in all feet.

• The condition was classified as virulent footrot on clinical grounds, and smears and lesion material on hoof agar were sent to the laboratory for confirmation.

• All smears were positive for B. nodosus. Colony morphology of all isolates was suggestive of benign footrot. Only 2 isolates survived due to a laboratory problem, but these were elastase negative at 21 days.

DIAGNOSIS : Probably Benign Footrot.

* From limited observations in this flock, I suspect benign footrot to be a more severe clinical disease in goats than sheep.

RECOMMENDATIONS

Where benign footrot is suspected in a large sheep flock with no recent virulent footrot history:

1. Isolate the suspect mob, preferably in a moist paddock where there is suitable environment for footrot spread.

2. Obtain hoof agar plates from the laboratory and take smears and cultures from the 10 worst feet in the mob (sheep selected on the basis of clinical lameness). Sheep sampled should be individually identified - a. Brucellosis tag around the front leg is useful for this.

3. It takes a month to complete the elastase test (cultures checked at 7, 14 and 21 days). If elastase positive at any stage, treat as virulent footrot.

4. If nothing grows on culture but smears are positive for B. nodosus, keep the mob isolated in a wet paddock and repeat the exercise after 4 to 8 weeks.

5. If isolates are elastase negative at 21 days, the problem is benign footrot.

If advice on eradication of benign footrot is requested, consider the following points:

a. Benign footrot will clear up under dry conditions without treatment. Even a move to a drier paddock may be sufficient.

b. Formalin probably cannot eradicate benign footrot. It is also unlikely that vaccination would be effective against benign strains.

c. Zinc sulphate could be worth a try. Minimal foot-paring should be necessary.

d. It is arguable whether the cost of a programme to eradicate benign footrot can be justified. Benign footrot is an innocuous disease, producing only transient lameness. Pregnancy toxaemia could perhaps be a sequel to benign footrot in ewes running in very wet conditions, but apart from this the major importance of the disease is its similarity to virulent footrot.

REFERENCES

1. Thomas, J.H. (1962) Australian Veterinary Journal 38 : 159

2. Egerton, J.R. and Parsonson, I.M. (1969) Australian Veterinary Journal 45:345

3. Morgan, I.R. et al. (1972) - Australian Veterinary Journal 48 : 23

4. Stewart, D.J. (1983) - University of Sydney Post Graduate Committee in Veterinary Science. Proceedings No. 67 - Sheep Production and Preventive Medicine. p. 487

5. Claxton, P. (1984-1985) pers. comm.