Ovine Observer

Managing virulent footrot in sheep and goats in Western Australia

Kirsty Moynihan
Veterinary Officer, Livestock Biosecurity
Department of Agriculture and Food Western Australia, South Perth
Email: kirsty.moynihan@agric.wa.gov.au

There are two forms of footrot in sheep and goats: virulent and benign. They are caused by infection with different strains of the bacterium, Dichelobacter nodosus.

Virulent footrot strains have the potential to cause more severe disease, whereas most benign strains cause a milder form that typically heals once the feet dry out. There are no quarantine restrictions for benign footrot, however producers must be vigilant and not present sheep with any footrot type lesions to saleyards.

Advanced virulent footrot is a painful, crippling condition. It is important to control or eradicate virulent footrot when it occurs in order to reduce production losses and to ensure animal welfare.

Tests for footrot
To test for footrot, a veterinarian or stock inspector takes scrapings of skin from lesions between the toes of infected sheep to be cultured in the laboratory for bacteria. If D.  nodosus bacteria grows in the culture media, isolates are subjected to the gelatin gel test. The gelatin gel test categorises the protease enzymes produced by D.  nodosus on their stability when heated into two groups:

• heat stable (S) strains, which cause virulent footrot
• heat unstable (U) strains that cause benign footrot.
   

• varying degrees of lameness
• separation or under-running of horny material at the junction of the skin and the horn of the hoof, starting at the heel
• usually more than one foot is affected
• both toes of each affected foot are often involved
• loss of body condition and decreased wool production.

Diseases which can be confused with virulent footrot include benign footrot, foot abscess, and scabby mouth. More than one of these diseases may occur in an animal at the same time.

Lameness in several animals should always be checked by a veterinarian as the exotic disease foot-and-mouth disease can make sheep lame. Detecting an exotic disease early will limit the impact of the disease on the livestock industries and Australia’s economy.

Severity of virulent footrot

The severity of virulent footrot lesions depends on three factors:

• the susceptibility of the animal
• environmental conditions
• the strain of bacteria.

If conditions are not favourable to the bacteria, then virulent footrot strains may only cause mild changes. Less severe forms may involve only the soft horn of the inside wall and the sole. This is why it is so important to take a skin scraping from between the toes and to use the gelatin gel test, rather than rely only on visual signs.

Spread of virulent footrot
Infected sheep or goats are the main source of new infections and some will maintain the infection from year to year. The bacteria can survive in deep, unexposed lesions in the foot for many years, but when away from the foot, survival time on the soil or in faeces is quite short. Hot, dry conditions cause bacterial death in a few hours, while in warm, wet conditions the bacteria can survive a maximum of seven to ten days. During dry periods there is minimal spread of disease. Most of the affected feet appear to heal; any bacteria that are shed will die, and they are unlikely to invade and cause damage to uninfected feet. When conditions become wet however, the previously ‘dried out’ lesions in the feet of the chronically infected animals may become active with large numbers of bacteria being shed. Wet feet of clean sheep are then easily infected.

The disease spreads rapidly during warm (greater than ten degrees Celsius) and moist conditions, usually in early spring to summer. Lesions may develop during autumn and summer in some years in high-rainfall areas or where there is an early start to the season.

Although reports are rare, the disease may be spread mechanically, either on the feet of other animals, or by people transferring contaminated mud on boots. This transfer is more likely within an infected property than between properties.  Unwashed livestock transport trucks and sheep handling equipment may occasionally contribute to the spread of footrot. In a small number of cases, cattle have been known to be infected with virulent footrot.

WA Footrot Control Program

Quarantine restrictions

DAFWA inspectors place flocks with virulent footrot under quarantine, restricting movements of sheep and goats from these properties until the infection has been eradicated. 

A permit to move (stating specific conditions) issued by a DAFWA inspector is required for all movements of stock from an infected property. Sales are permitted only if stock are to be sent direct to slaughter. Sheep that are not showing signs of lameness may be consigned for live sheep export under certain conditions.

Management options: eradication or control
Farmers with quarantined flocks first need to decide whether to eradicate or control footrot in their flock. They then, together with their DAFWA inspector, develop a footrot property disease management plan.

Eradication can be achieved by either:

• destocking the property of all sheep and goats
• repeat inspection and culling during the summer period.

If a farmer chooses to control virulent footrot for economic reasons, sheep can still be sold for slaughter under a permit to move. Farmers opting for control are subject to welfare and biosecurity audits.

DAFWA inspectors carry out quarantine release inspections on eligible properties. These occur after each spring until no virulent footrot is found. The inspector then releases the property from quarantine.

Use of footbathing, antibiotics or vaccine at any time before a release inspection must be discussed with a DAFWA footrot case manager as it may affect eligibility for release. Other conditions for eligibility for release from quarantine may also apply.

Footrot vaccine
Use of the serogroup specific vaccine in WA is limited to farms diagnosed with virulent footrot, where it is used to treat the infection, not to prevent footrot. The vaccine is currently only available under an Australian Pesticides and Veterinary Medicines Authority (APVMA) emergency permit and requires written permission from the WA Chief Veterinary Officer before use. Use of the vaccine must be carried out in accordance with the requirements of the Footrot Control Program and discussed with a DAFWA footrot case manager.

Prevention
The greatest risk of introducing footrot into your flock is by buying in infected stock, or having infected stray stock mix with your flock. Sensible precautions to avoid this include:

• buy direct from properties with a known health status
• inspect the feet of animals before buying
• ask for a Sheep Health Statement
• treat sheep returning from agistment as you would purchased stock
• keep newly introduced stock separate from existing stock for as long as possible and examine them regularly
• maintain boundary fences to prevent stray sheep from entering your property.