Tool 11.13 Diagnosis, control and eradication of footrot, lice, OJD and OB

Avoid introducing diseases like footrot, lice, Johne’s disease (JD) ovine brucellosis (OB) onto your property and seek professional advice on diagnosis and management.

Footrot

Diagnosis

Three broad classifications of footrot are benign, intermediate and virulent. Benign strains that cause mild lameness have little economic or welfare significance. By comparison, virulent strains can cause serious lameness, welfare and production losses.

Three important factors that can affect the expression of footrot must be considered when determining what strains are present. Environment is very important; wet, warm conditions allow footrot infection to be expressed. If conditions are dry, or sheep are run on poor-quality pastures or rough hilly country, footrot expression may be less than on good pastures. Recent footbathing or footrot vaccination can reduce the expression of virulence and British breed sheep tend to be more resistant to footrot than Merinos.

The following table outlines the key features of footrot.

A summary of the clinical characteristics of different footrot strains.

Footrot regulations vary between states with regard to diagnosis and management. Generally, intermediate and virulent strains are economically significant and worth eradicating. Always seek expert advice if unsure about the cause of lameness in your sheep or uncertain of the footrot status in sheep you are intending to purchase.

Footrot is a notifiable disease in most states, so check here to understand your obligations.  

Control

When conditions are green and pasture is growing, controlling the expression of footrot is the best option. The objective is to limit production losses and reduce the number of sheep with footrot. Control is best achieved with either foot-bathing, vaccination, or a combination of both, depending on labour resources and the severity of the strain of footrot. Control continues while conditions are green. Benign strains may not need control except for occasional footbathing in wet years.

The regulations surrounding the identification, treatment and notification of footrot differs between states, so visit your state department for more information:

• NSW Department of Primary Industries
• Queensland Department of Agriculture and Fisheries
• SA Department of Primary Industries and Regions
• Tasmania Department of Primary Industries, Water and Environment
• WA Department of Primary Industries and Regional Development
• Agriculture Victoria

Eradication

If virulent footrot is diagnosed on your property, eradication is an important priority. The first objective is to eliminate potential sources of re-infection. Benign footrot should not require intervention.

Two broad options exist for eradication: total destocking and replacement with footrot-free sheep or inspection and culling of all sheep with abnormal feet when conditions are dry. If high numbers are infected, initial salvage of infected sheep may be warranted. After a few inspections during the non-spread period, mobs should be quarantined and, once all sheep have spent a spring with no evidence of footrot, you can be confident that footrot has been eradicated. Always seek advice on footrot diagnosis and management from an experienced animal health adviser.

Lice

Diagnosis

Lice are diagnosed by visual examination of sheep. Lice are small orange/brown insects, 1‑2 mm long that live on the surface of the skin, often in small colonies. To inspect sheep before purchase or shearing, part the fleece in at least 20 places, particularly along the flanks. Target sheep with evidence of rubbing; the more sheep you inspect the more likely it is that you will pick up low level infections so at least 10 sheep should be inspected if none are found initially.

Treatment options need to be tailored, according to the situation, to either gain eradication or in long wool, effective control or eradication (different products and treatment methods must be used for this). An understanding of available products, the best way to apply treatments and factors need to be taken into account will help you ensure that treatments are effective.

Chemicals registered for lice control include organophosphate, synthetic pyrethroid, insect growth regulators, spinosad, macrocyclic lactone, neonicotinoid, isooxazoline, and a combination of magnesium fluorosilicate, rotenone and sulphur.

It is important to be aware of the characteristics of the different chemical groups, in particular their safety profile, residue consequences, withholding periods, modes of action and whether resistance has been detected. These are described in more detail on the LiceBoss Treating sheep for lice page.

The LiceBoss Products Tool lists all of the products currently registered for lice control in Australia, the chemical groups to which they belong, their method of application and the wool and meat withholding periods for each, together with their approximate prices.

When lice are found, economic and animal welfare considerations dictate the need for sheep to be treated with an effective product. The choice of product and the timing and method of treatment should be determined by a range of factors.

Products from several chemical groups are registered for the eradication of lice on sheep. Depending on the product chosen, the best time to treat sheep for lice is ‘off-shears’ (within the 24 hours after shearing) or in ‘short wool’ (2 to 6 weeks after shearing), or using the oral drench lice product.

These treatments should be able to eradicate lice if applied according to label directions and the appropriate other biosecurity controls are observed (e.g., boundary fences maintained, all introduced sheep quarantined, inspected and monitored, etc.). However, resistance has occurred to some chemical groups, rendering them ineffective against some lice populations. This needs to be considered when choosing appropriate treatment strategies as resistance reduces the number of control options available. Good management should include precautions that minimise the chance of resistance occurring.

When lice infestations are found between shearings, a decision to treat in long wool, to wait until the next shearing or to shear early is required. Left untreated, wool damage may become significant. Alternatively, the cost of treatment may not be warranted if wool damage is unlikely to be severe. Most long wool treatments (products applied to sheep more than 6 weeks after shearing) will reduce louse numbers but will not eradicate lice and regardless of whether or not a long wool treatment is applied, sheep will need to be treated again after their next shearing. There is one registered product (Flexolt®, active chemical fluralaner) which is registered to eradicate lice in long wool sheep and can be considered as a useful tool.

The Long Wool Tool in LiceBoss can help you to decide whether it makes better economic sense to muster the sheep and treat in long wool, or to wait and treat the sheep after they are shorn. Early shearing is rarely the most cost-effective choice but can be considered if the early shearing time is convenient and it is essential to eradicate lice immediately rather than wait until the usual shearing time.

Nearly all new infestations begin from contact with infested sheep. Major sources of lice are:

• infested sheep that stray onto a property
• sheep that stray from the property, come into contact with lousy sheep, and later return to the mob
• purchased sheep and sheep brought in from other properties (including sheep returning from agistment)

However, infestations can also result from sheep missed at muster and therefore left untreated in the previous year or from sheep not effectively treated after their previous shearing. Split shearings and treating ewes with lambs at foot or pregnant ewes due to lamb within 6 weeks of treatment are other causes of continuing infestations. Sheep lice (Bovicola ovis) do not breed on animals other than sheep (with the possible exception of goats in very rare instances). Birds do not carry sheep lice and they do not remain in wool rubbed onto fences, trees or other structures, so these are not sources of infestation.

A well-designed biosecurity plan that prevents contact with infested sheep is critical to keeping a property lice-free. Stock-proof fences are essential for good biosecurity. Without them, straying sheep can bring lice onto the property and isolation/quarantine of introduced sheep becomes impossible.

Control

If lice are detected in your sheep between shearings, control of fleece damage by jetting or eradication using fluralaner (Flexolt®) may be necessary before shearing. If there is at least six months to go before the next shearing and there are signs of wool damage or lice have been found, then long wool treatment is probably necessary to avoid excessive wool damage. If there is less than 2 months to go before shearing, the long wool treatment will probably not be economic unless wool damage is already severe, but treatment should be considered if sheep are showing signs of discomfort.

Between these times, the need for treatment depends on the severity of wool damage and your personal preference (see the LiceBoss Long Wool Lice Tool). Several chemicals are available for lice control. The choice depends on the time before shearing as withholding periods (wool harvesting interval and sheep rehandling interval) must be observed. Effective control or eradication will only be achieved if you follow recommended dose rates and application techniques and biosecurity protocols. Guidelines for managing sheep lice can be found in the AWI LiceSense booklet.

Eradication

Eradication of lice will only be achieved if all sheep are treated after shearing at the same time, correct dose rates used, and a backline or plunge dip correctly applied, or oral drench used. If ewes are treated within 8 weeks before lambing, using any backline treatment or dipped with an insect growth regulator (IGR), then the lice may not be eradicated before the lambs are born. Lambs may become infested and then reinfest the ewes. Dipping heavily pregnant ewes poses animal welfare risks, so avoid shearing within 8 weeks before lambing if lice treatment may be necessary. Seek advice about application technique and eradication from an experienced veterinarian or animal health advisor.

If no lice infested sheep are introduced and boundary fences are 100% stock proof, you will not have to re-treat sheep once lice have been eradicated. This will be a major cost saving, as well as reducing residues in the wool clip.

For more information, visit LiceBoss.

Johne’s disease (JD)

Diagnosis

OJD is an incurable, infectious, chronic wasting disease caused by the sheep strain of the bacterium Mycobacterium avium subsp. paratuberculosis. The bacteria are passed in the manure of infected animals, contaminating pasture and water supplies and spreading infection to other susceptible sheep. Once a flock is endemically infected with Johne’s, it is difficult to eradicate.

OJD has a long incubation period (the time from initial infection of the sheep until it shows signs of disease) and most infected sheep do not begin to show any signs of illness until after two years of age. From the onset of obvious signs of the disease, they start to rapidly lose condition and become emaciated and often scour (because the disease stops the absorption of nutrients) and usually die within the next three to six months. Annual death rates range from less than 1% to more than 20% in severely affected flocks.

OJD is often not diagnosed in a flock until a significant proportion of the flock is already infected and deaths are occurring. There is often no sign for the first few years – that is why it is often referred to as a ‘silent but costly disease’. Infected sheep can be shedding the bacteria in their manure for a considerable period (sometimes years), and even though the flock still looks healthy, they are contaminating the pasture and infecting other sheep.

The classic sign of the disease in a mob is a distinct ‘tail’, with sheep ranging in condition from good to very poor; then the sheep in the ‘tail’ start dying. The number of sheep in this ‘tail’ may be constant but is made up of different animals over time, insidiously eating away at flock numbers and profits.

When obvious OJD deaths are noticed, the disease is likely to be well established and it will take some years to get the situation under control, during which time deaths will continue.

Diagnosis can be made by post-mortem examination of affected sheep by an experienced veterinarian. Sheep with advanced OJD show characteristic signs of thickened gut wall around the ileocaecal junction and swollen lymph nodes around the intestines. Infection is confirmed by microscopic examination of gut sections that show presence of the OJD bacteria, using special staining techniques.

Faecal culture is the most common flock test to detect the presence of OJD bacteria. Individual samples can be cultured or faecal samples collected from 50 sheep, combined and cultured over a period of at least 12 weeks to grow the bacteria. Further testing (polymerase chain reaction – PCR) is used to confirm the presence of OJD bacteria and determine if the strain is a sheep or cattle strain. Blood testing (ELISA or CFT) can also be done, but these have been superseded due to poor sensitivity and cost.

As OJD is an incurable disease so prevention really is the best cure. To try to keep OJD out of your flock:

• Only buy in or agist sheep with a high level of assurance provided by either the SheepMAP or from recent testing of a closed flock.
• Always ask for a signed National Sheep Health Declaration before you buy any sheep and only purchase sheep from reputable vendors whom you trust.
• Vaccinate your flock if there is a risk of OJD coming in from straying stock from neighbours, or if you are in a district with high prevalence, as the risk from ‘lateral spread’ is high. Vaccination alone does not result in eradication of OJD but will suppress both the number of sheep affected and pasture contamination over time.
• Improve your flock’s resistance to disease through good nutrition and worm control.
• Where possible, use abattoir monitoring results from your adult sheep to monitor for infection.
• If you suspect that you may have OJD, ensure that you have cases investigated immediately and start vaccinating.
• The earlier you act, the smaller the impact of OJD on your business.
• Work with neighbours to minimise potential spread between farms and consider joining a regional biosecurity group.

Vaccination

For more information on the prevention and management of OJD, visit the National Johne’s Disease Program website.

Control

If OJD has been diagnosed on your property, management options are available to limit the impact of the disease. The main option available is to use Gudair® vaccine to help control the disease in your flock. Initially, if the prevalence of clinical disease is high, identify and remove sheep showing signs of wasting to help reduce death rates. Minimise stress due to worms and poor nutrition.

The other important issue is to minimise the risk of spreading OJD to other flocks. Ensuring boundary fences are 100% stockproof is important to limiting the spread. In addition, run low-risk sheep in boundary paddocks. Seek advice from your veterinarian or state government veterinarian. Information is also available from AWI, MLA and National Johne’s Disease Program.

Eradication

The disease has proven difficult to eradicate from infected properties. Affected sheep producers need to manage their flocks and properties to reduce the overall on-farm impact of the disease. Best practice advice includes:

• Vaccinate all animals as lambs and only buy approved vaccinates.
• Humanely slaughter any sheep showing signs of OJD. This is important on welfare grounds and it also helps to reduce contamination of your land.
• Use the dung test (pooled faecal culture test) to identify and cull heavily shedding mobs.
• Where possible, use abattoir monitoring results from your adult sheep to monitor disease control.
• Improve your flock’s resistance to disease through good nutrition and worm control.
• Reduce exposure of young sheep to OJD by placing them on ‘clean’ pastures or those grazed by other species. Avoid placing young sheep on pastures recently grazed by unvaccinated adult ewes or wethers.
• Clean up contaminated pasture by:
  • grazing with steers
  • rotating with crops
  • re-sowing pasture
  • grazing with terminal lambs and selling them straight to slaughter.
• Avoid feeding on ground – go for troughs or self feeders.
• Fence off low areas or creeks where water may be contaminated.

Ovine brucellosis

Diagnosis

Ovine brucellosis (OB) is an infectious bacterial sheep disease caused by Brucella ovis. Rams infected with B. ovis suffer from inflamed reproductive organs (particularly the epididymis), impacting semen quality, making them sub-fertile or sterile.

The effect on flock fertility depends on the percentage of infected rams and flock breeding management.

The effects of OB are often unrecognised, as rams often do not show signs of ill health, especially where marked fluctuations in fertility occur for other reasons.

Abnormalities will be detected by scrotal palpation of rams, and this should be undertaken at least once per year on your whole ram team. Testicles should be firm and springy on palpation with no abnormal lumps and scrotal circumference above 28 cm.

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. Alternatively, the blockage may break down and the 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. In some cases, the testes may also be inflamed. The disease 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.

If you feel any lumps or rams with poor testicular tone, call in your veterinarian for further investigation, including confirmation through blood tests.

Control

Movement of infected rams is the primary way that OB spreads between properties. This includes introduction of infected rams, but strays are also a major risk.

Infection is spread within the ram mob during joining when clean and infected rams mate with the same ewe, and also between rams during the non-joining period. When rams are not working they should be protected from stray sheep (or straying themselves) by being isolated in securely fenced internal paddocks, with age groups separated if possible.

Ewes, although fairly resistant to OB, can carry the infection for extended periods, resulting in abortions and discharge of the organism in the placenta, vaginal discharges and milk. There is the potential for introduced ewes from an infected flock to spread infection, but this is less likely and can be managed by isolating non pregnant ewes for one month before joining; or with pregnant ewes, lambing them in isolation and not joining them for four months post lambing.

Eradication

There is no preventative treatment available 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.

Therefore, preventing OB from entering your flock is your only option. An integrated approach to preventing OB includes:

• developing and implementing a farm biosecurity plan
• palpating rams’ testicles to check for lumps before purchasing and before joining
• seeking veterinary advice if you notice lower lambing percentages, longer lambing periods or abnormal lumps in the testes
• establishing the OB status of your flock before introducing OB-free sires
• only buying rams with an animal health statement from studs accredited as OB-free
• if not sourcing rams from accredited studs, keep newly purchased rams separate and test before integrating into the flock
• maintaining ram-proof fences to avoid visits from neighbours’ rams
• keeping rams in small groups and separating younger rams from older rams
• isolating rams from ewes until their disease status is established as ewes can spread OB