Contact: Your local PIRSA office, local private veterinary practitioner or phone Biosecurity - SA Animal Health at Glenside on 8207 7900.
Footrot in Sheep
Footrot is a contagious disease of sheep caused by the bacterium, Dichelobacter nodosus (D. nodosus). Other bacteria are often present in an outbreak.
Virulent footrot is a most serious disease. It generally causes severe lameness resulting in significant economic loss from reduced wool growth, poor ewe fertility, reduced growth rates and lower prices for sale sheep.
Footrot
The Cause
For the development of footrot in sheep it is essential that both the correct infective factor and environmental factors are present.
Infective Factor
The infective factor is the presence of D. nodosus. However for infection to occur there must be dermatitis present between the claws (interdigital dermatitis). The bacteria will not invade healthy feet and will not survive more than 5 to 7 days in the environment under ideal conditions. The organism can persist for many years in the feet of infected sheep, even under dry conditions. Footrot is generally introduced into a flock via introduced infected sheep. However, in some cases a clean flock can be infected by walking on pasture where infected sheep have recently grazed.
Environmental Factors
The three main environmental factors required for the establishment and transmission of footrot are:
· An average temperature of 10 degrees C or higher for 4 – 5 days; and
· Adequate moisture; and
· Adequate pasture length and density to make feet susceptible to infection.
Footrot is a problem in areas with greater than 500 mm/year of average annual rainfall. In South Australia, high risk environmental areas include the South East, Fleurieu Peninsula, Adelaide Hills, Kangaroo Island, parts of the mid-North and the lower part of the Eyre Peninsula.
In most years suitable conditions exist from September to November for the spread of the disease. In some years, where there has been an early autumn break, suitable conditions for expression and spread can exist in April and May.
Page | 9
Symptoms
Footrot is classified as either virulent or benign.
A clinical diagnosis is based on the Edgerton Foot Scoring System whereby a clean (non-infected) foot has a Foot Score of 0 and a severely affected foot has a Foot Score of 5. Examples of each score are shown on page three.
For regulatory purposes in South Australia, virulent footrot is defined as a flock having greater than 1% of Score 4 and 5 foot lesions. However, where significant levels of Score 3 lesions are present, virulent footrot may be a possibility.
Resolution of such suspect cases may depend on laboratory virulence testing.
Benign Footrot
Benign footrot generally causes minimal economic losses. However in some situations it can predispose animals to grass seed penetration which can cause severe lameness.
The interdigital skin between the claws becomes moist and inflamed. This is known as interdigital dermatitis. A high percentage of the flock may be affected under favourable conditions and more than one foot is generally affected.
When affected sheep are grazed on dry pastures under dry weather conditions, the disease can spontaneously disappear without any treatment.
Early stages of virulent footrot will have identical symptoms to benign footrot.
Virulent Footrot
Virulent footrot starts with interdigital dermatitis and loss of hair between the claws. The disease then progresses to the sole of the foot and separation of the horn of the hoof from the underlying tissues.
In some sheep, the disease develops rapidly in warm moist conditions to the whole of the sole (Score 4) and can extend up the wall (Score 5).
Usually both claws are affected and often in more than one foot.
Severe lameness is frequently a feature of virulent footrot.
When footrot is caused by a milder strain, there are fewer sheep affected and these may be infected in only one or two feet.
In dry conditions, or following footrot treatments, all of the above typical signs can be supressed.
Detection of virulent footrot in these circumstances can be very difficult.
Chronically affected hooves may become blackened, distorted and overgrown before the shell peels off completely. Flystrike is common at this stage, especially during summer. Severely affected sheep become very lame and may suffer extensive skin damage to the knees and brisket due to grazing on their knees. If the disease progresses to this stage, it can have severe animal welfare implications.
Distinguishing Between Footrot and Foot Abscess
In a field situation it is very important to distinguish between footrot and foot abscess. Foot abscess is caused by the bacteria Fusobacterium necrophorum and Actinomyces pyogenes. Both footrot and foot abscess can cause severe lameness. Table 1 shows some identifying characteristics of footrot, heel abscess and toe abscess.
Table 1
Clinical characteristics of footrot, heel abscess and toe abscess
Footrot / Heel Abscess / Toe AbscessUsually affects more than one foot. / Usually affects one hind foot which is carried. / Usually affects front feet.
Usually affects both claws / Generally one claw affected / Generally one claw affected
No swelling. / Swelling – usually spreading to the toes. / Swelling is not a feature.
No pus discharge, but a black-grey slime like substance may be present. / Creamy-white pus discharge. / May be pus/fluid from point of the toe when pared/trimmed
Heat may be present in some cases / Hot to touch, particularly at the site of the swelling. / May be hot to touch.
Lesions have a putrid odour and may be flyblown. / Lesions can have a slight odour, but are rarely flyblown. / Odour distinct from footrot. Lesions may be flyblown.
Spreads rapidly to sheep of all ages, including lambs, when conditions are favourable. / Usually confined to heavy sheep such as rams and pregnant ewes. / Affects all classes of sheep.
No break in the coronet skin, but separation of the inner sensitive horn and outer hard horn. / Abscess usually breaks out at the coronet or in the interdigital skin. Resolving abscess- defined area granulated. / Abscess usually occurs under the horn at the front of the toe. May open in a line above the coronary band. Can cause separation of the front half of the sole in chronic cases.
Regulation of Footrot
Virulent footrot is a notifiable disease in South Australia. Once virulent footrot has been diagnosed in a flock, quarantine Order issued under the Livestock Act 1997. This Order, once imposed, prohibits the sale of sheep to other graziers, allowing sheep to stray and the selling of sheep from the quarantined flock in a public market. Animals may be sent direct to an abattoir for slaughter. The aim of these restrictions is to prevent the spread of virulent footrot to other graziers.
The quarantine Order will be released 10 days after all sheep are sold for slaughter. With flocks undergoing treatment for eradication purposes, the quarantine Order will be released in the following spring if there is no evidence of virulent footrot present after animals have grazed during a suitable spread period.
It is an offence under the Livestock Act to put sheep with virulent footrot in a public market. Detection can lead to a fine or prosecution.
On-farm Eradication Programs
Important Points to Consider for an Eradication Program
· Rely on the experts. An on-farm eradication program that is undertaken under close supervision of an experienced person (such as experienced veterinarians or Biosecurity SA - Animal Health Advisors), or with the assistance from an experienced footrot contractor has the greatest probability of success.
· There are no short cuts to eradication. Relying on cheap treatments or missing treatments will result in failure. Retaining infected animals that do not respond to early treatment will result in a breakdown of the eradication program.
· The eradication program must be planned. Plan your eradication program as early as possible in conjunction with an experienced veterinarian or Biosecurity SA - Animal Health Advisors. Allocate suitable periods to undertake control and eradication programs in the flock. Prepare contingency plans following the eradication phase in case the disease breaks out again in one or two mobs. Allocate separate areas on the property for clean mobs and infected mobs with separate handling areas wherever possible.
· Maintain Facilities. Eradication is assisted by having sound fencing and good facilities for the handling, inspection and treatment of sheep.
· Do not attempt an eradication program while the disease is still spreading. The disease will spread at a faster rate than it can be cured. The disease can spread if there is moisture on your boots when walking through pasture and the average temperature is above 10 degrees C.
· Footrot is eradicated mainly by culling. The sooner treatment of infected sheep can be terminated in non-spread conditions and replaced by culling, the better the chances of eradication.
· Slaughter all infected sheep as soon as possible. While infected sheep remain on the property, they are a threat to clean sheep. Many eradication programs fail because infected sheep stray into a clean flock or because graziers spend too much time trying to cure infected sheep and not enough time ensuring that non-infected sheep are, in fact, clean.
· Footrot can be elusive! Under dry conditions, some apparently normal animals can carry virulent strains of the bacteria as small pockets of disease in the claws of their feet. It is also important to understand that treatments such as antibiotic and footbathing in zinc sulphate can make the detection of sheep that are not totally cured, more difficult to detect. Footrot cannot be considered eradicated until the flock has gone through a season conducive to spread of footrot, without showing evidence of the disease.
· Prevent reinfection. It is important not to neglect the likelihood of reinfection from introduced sheep or from neighbouring properties. Quarantine all introduced sheep for as long as possible to ensure freedom from footrot.
Voluntary Destocking
Before undertaking an on-farm eradication program, destocking the whole flock or part of the flock should be considered.
Voluntary destocking of the whole flock is the most certain way of eliminating footrot as long as care is exercised in purchasing clean replacement sheep. However, if this option is exercised, it must be thought through very carefully. Consider the age of sheep, flock structure and cost of purchasing replacement sheep/cattle. From the time that the last sheep has left the property, a minimum of 10 days must have elapsed before the quarantine Order is released.
A partial destock is often worth considering to reduce the number of sheep that need to be treated. This may be a mob or mobs where infection is most severe or in older age groups where the animals are not so valuable.
If eradication via an inspection and a treat/salvage/cull program is the preferred option, there are three phases:
· Control Phase. This is undertaken before and during the spread period in order to reduce the level of infection in the flock to the stage where eradication becomes feasible.
· Eradication Phase. This involves the detection and removal of all infected sheep in the flock during the non-spread period
· Surveillance Phase. This involves careful surveillance of the whole flock to ensure the disease has been successfully eradicated, and preventing reinfection.
Control Phase
The control phase should be used during a spread period to reduce the effect of footrot on infected animals and to minimise the number of animals that need to be culled during the eradication phase.
This phase generally involves foot bathing in a 10% zinc sulphate solution (1 kg zinc sulphate heptahydrate dissolved in 9 litres of water). The frequency of foot bathing required will vary depending on the severity of the footrot. It can be as often as weekly, but mostly is undertaken as often as necessary to control lameness.
Bathing time should be a minimum of a slow walk through a 6 metre footbath. Standing in a footbath for 1 – 2 minutes is even better.
An alternative to zinc sulphate is a Radicate®. This is a copper-based footbath solution. Sheep should be held in this solution for 15 minutes and then their feet allowed to dry for an hour on concrete or grating. Treatment must be repeated in 1 – 2 weeks. Good cure rates can be expected in a significant number of sheep. This product has the advantage of creating an artificial non-spread period between treatments, allowing inspections to continue even when environmental conditions are conducive to spread. As this product is copper based it can stain the wool on the lower parts of the body. Over time this staining will disappear.
Formalin has been widely used in the past. However due to it being an occupational health safety and welfare (OHS&W) risk, it is no longer recommended.
A bi-valent footrot vaccine has been used on a trial basis in South Australia. It can help with footrot prevention and as a cure. It does require the footrot organisms to be serotyped first and the vaccine to be “tailor-made” to that flock. It is not widely used at present due to the cost of serotyping. It works best if only one or two serogroups are present in the flock. It is not economic or practical if more than 4 serogroups are present.