Tool 11.14 Analysing the risk of potential sources of disease

This tool is designed to examine the risks of introducing important diseases that can have major economic consequences for a sheep enterprise.

General considerations to minimise disease introduction:

• 100% secure boundary fences — most diseases are prevented by secure boundary fences.
• Buying sheep — the more mobs of sheep you buy, the greater the risk of disease, so buy as few mobs as possible and get advice from your veterinarian or sheep health advisor regarding possible risks.
• Agistment off farm — always quarantine agisted sheep when they return to the home property as they can be exposed to several potential disease sources (roadways, stray or resident sheep on agistment property, trucks).
• Source of sheep — sheep from properties where regular trading occurs, with poor management or poor fences, are more likely to have disease problems.
• Sheep on roadways, in trucks — may be a risk if you are uncertain of stock movements with neighbours or stray sheep or in trucks (footrot is the major risk).
• Contractors — equipment or staff may spread diseases, such as lice or footrot, if hygiene procedures are not observed.

Specific disease issues

Footrot
As many sheep as possible should be inspected before purchase, especially lame sheep. Note that previous management, such as footbathing and dry seasonal conditions, may mask signs. Purchase sheep with a fully completed National Sheep Health Declaration. Seek advice if you are uncertain of footrot symptoms.

Place sheep in quarantine on arrival (see Tool 11.15). Small numbers of sheep, such as rams, may be inspected several times before introduction to ewes, but always follow quarantine periods with larger mobs. Footbathing sheep off trucks is only a superficial disinfectant and will not clear existing footrot infections. The risk of introducing footrot by contamination on boots is small, but if you or others have recently walked over ground or in trucks or yards where infected sheep have been, it is recommended you scrub your boots to remove all dirt and faecal matter, then walk through a footbath with a disinfectant such as Hibitane® or Stericide® between properties.

Lice
Thoroughly inspect sheep before purchase or on arrival at your property (whilst strictly quarantined) and assess relative risk based on trading activities and management. Lice can be difficult to find on sheep for several months after an infestation starts and treatment for eradication of lice in long wool is only possible with one currently registered active ingredient (fluralaner), so preventing their release onto your property is the best solution.

Two options currently exist for all introduced sheep (new purchases or stray or agistment sheep returned home) to your property:

1. Maintain all introduced sheep on the property in quarantine until they have been shorn and treated per the label instructions after shearing. Maintain quarantine for the required time (per the label instructions) after treatment (some chemicals do not kill all lice immediately); or
2. Maintain all introduced sheep on the property in quarantine, treat with fluralaner (Flexolt®, an oral drench which can be effectively used as a lice eradication tool in sheep of any fleece length and works from the inside out, but does not provide residual protection) per the label instructions and wait the indicated period on the label for the product to effectively eradicate the lice, then introduce the sheep to your flock per your existing biosecurity plan and protocols.

Shearers need to microwave their moccasins and change clothes before starting a new shed or group of sheep to kill any lice that may have contaminated them from previous sheds or quarantine mobs. If you have handled lousy sheep, change all clothes before handling clean sheep or sheep whose lice status you’re unsure of to avoid transferring lice between mobs.

Boundary fences and solid quarantine protocols for the introduction and return of sheep are a critical barrier to stop the introduction of lice. Many sheep producers with good boundary fences haven’t treated sheep for lice for years and have kept their properties lice free, even when neighbours have lousy sheep.

For more information, check out LiceBoss Treating sheep for lice.

Johne’s disease (JD)

Always buy sheep:

• With fully completed National Sheep Health Declaration
• From low-risk areas
• With an equivalent or higher SheepMAP MN score

The more mobs of sheep you buy, the higher the risk of introducing OJD. Current evidence suggests that many flocks are infected with OJD where the owner is unaware of the infection status. The exception is trade lambs, as they are less likely to be shedding bacteria before 12 months of age. Always run older sheep (CFA mob) or cattle on high-risk boundaries. Ensure boundary fences are 100% stock proof.

Information is available on prevention from your veterinarian, state government animal health officers, the MLA Johne’s disease page and National Johne’s Disease Program website.

Ovine brucellosis (OB)

OB is a bacterial disease that permanently infects the testes and epididymis of rams, rendering some animals infertile. It also temporarily infects the reproductive tracts of ewes, meaning that ewes can potentially spread the infection to uninfected rams. Rams can also become infected from other rams in the ram paddock.

There is no preventative treatment for OB and infected animals cannot be treated. Eradication of the disease is the only cure and requires identification of infected animals and culling of these animals accordingly.

Annual palpation as part of the pre-joining ram check 4 Ts check will assist with identification of suspect rams, and subsequent testing by a vet determines if rams have brucellosis.

• One tell-tale sign of OB is lower conception rates. These rates may not be exhibited in a complete failure, as not all rams may be infected, but you should treat lower conception rates as a signpost for ruling out OB.
• In infected rams, the obvious lesions that can be palpated occur in the testes and epididymides.
• The infection first affects the epididymides, causing inflammation, and swelling of the surrounding tissues.
• In most rams, the epididymides are completely blocked for a period, causing sperm to build up behind the blockage.
• In some rams, the blockage is permanent, and causes further swelling of the tail of the epididymis.
• The blockage may break down and swelling may decrease and the ram, although still infected, appears and feels normal.
• Detectable lesions first occur in the tail of the epididymis, which becomes enlarged and inflamed.
• OB can affect one or both testes, with the tail of the epididymis being the site most commonly affected. Rams with chronic infections may have a grossly enlarged tail of the epididymis and a shrunken testicle.
• In some cases, the testes may also be inflamed.

Each state Department of Primary Industries oversees a scheme to provide ram producers with accreditation to show that the farm is free from brucellosis. This involves inspection and blood tests carried out by a private veterinarian. Every ram is manually palpated and blood tested twice. All rams must pass with negative blood and physical tests before accreditation is given, then re-accreditation testing takes place after 12 months then biannually.

OB can cause severe effects on joining rates when it enters a flock so only buy rams from brucellosis-free accredited studs regardless of breed and quarantine all rams if they have strayed or been agisted and have them tested whilst being held in quarantine.

For more information and links to state brucellosis accreditation schemes, visit MLA Ovine brucellosis or Animal Health Australia.