Sheep Diseases

  • Internal parasites

    The current Integrated Parasite Management ('IPM') program for controlling internal parasites ('worms') of sheep in south-eastern Australia is based on research conducted over the past 30 years. 

    Background to worm control programs

    Larval populations of the common worms (Ostertagia and Trichostrongylus) are highest in the cold, wet months of winter, and lowest during the hot, dry months of summer. Eggs deposited during late summer and autumn contribute the largest number of larvae available to sheep during next winter (Jun-Aug). These months are cold and wet, with limited pasture growth. The additive effects of low pasture availability, high nutritional demands and high numbers of infective larvae cause ill-thrift, clinical worm infections and deaths. In Merinos this is more severe in weaners and maiden ewes.

    Consequently, worm control programs in the high winter rainfall areas are based on reducing pasture contamination with worm eggs during summer and autumn. The simplest way of achieving this is a strategic drench in early and mid-summer to all classes of sheep, using a highly effective combination of drench compounds.

    The timing of these two treatments varies from district to district, and from year to year, according to local conditions that can either limit or favour the availability of larvae. In hot summers, the second treatment may not be necessary, or required only in more susceptible classes of sheep such as weaners and ewes. However, in wet summers the effectiveness of the treatments is reduced by rapid re-infection after each drench.

    To overcome the problem of re-infection over summer, and hence increase the reliability of the summer treatment program, especially for weaners, a highly effective system of grazing management, known as 'SMART GRAZING', can be integrated with drenching.

    Briefly, Smart Grazing requires a period of intensive grazing, at 2.5 to 3 times the usual stocking rate, for 30 days after each of the summer treatments, followed by de-stocking. This allows the pasture to recover and, more importantly, ensures that very few worm eggs are deposited on the prepared paddocks between November and April. At the autumn break the 'Smart Grazed' paddocks are set-stocked with weaners treated with an effective drench. They can remain there for the rest of the year, usually without additional treatments, although regular worm egg count (WEC) monitoring is still an important part of this program.

    IPM program for worms

    Each farming system is quite different and so a 'general' worm control program must be tailored to each farm.

    Critical elements of a successful program are:

    1. Testing for drench resistance every 3 years - this will show which drenches are effective

    • Useful tests are the Worm Egg Count Reduction trial (WECRT), DrenchRite„¢ test or a 'DrenchCheck' (monitoring worm egg counts at drenching, then again 10-14 days after drenching a mob)

    2. Monitoring worm egg counts at critical times:

    • Weaners should be monitored every 4-6 week after the autumn break
    • Other classes of sheep should be monitored as follows:
      • All mobs in late Jan/ Feb - to see if a second summer drench is needed in Feb or can be delayed. This will also indicate which paddocks may be more 'high-risk' the following winter.
      • All mobs in July/ August - to check on peak winter egg counts and whether or not a pre-lambing treatment of ewes may be needed. There is a big interaction with ewe condition score and available pasture for the decisions taken at this time.
      • All ewe mobs at marking in October - this is a check on how effective the control program has been in ewes, and can highlight some mobs that need an additional drench (only needed in high worm risk areas).
      • At the first summer treatment in Dec - this gives an indication of any higher risk paddocks from the 'spring' generation of worms that might occur next winter (only needed in high worm risk areas).

    3. Giving introduced sheep a Quarantine drench - this consists of at least 2 effective drench groups, but preferably 4

    4. Use grazing management to provide low risk pastures for weaners in their first winter:

    • Smart Grazing, or
    • Alternate grazing between sheep and cattle

    5. Breeding sheep for increased resistance to worms (low worm egg count) and less scouring (a low dag score)

    6. Having targets for ewe condition score at lambing (eg. CS = 3) and weaner bodyweight over summer-autumn-winter, and monitoring progress against these targets.

    7. Some farms in shorter growing season environments (7 months or less) are testing the use of 'controlled contamination' to reduce selection for drench resistance. This is not routinely recommended for farms in longer growing season areas, and should only be done with additional monitoring in close consultation with a veterinary adviser.

    'Controlled contamination'

    A tactic to reduce selection for drench resistance is being tested in areas with a shorter grower seasons, such as WA and Kangaroo Island. This includes 'controlled contamination', by grazing extremely low worm risk areas, such as crop stubbles, with an undrenched mob of wethers in late summer, or leaving a percentage of sheep (2-5%) untreated at the second (or only) summer drench.

    It is important to understand that these strategies have been designed for use in Mediterranean climates, and have not been systematically tested in areas with longer growing seasons. Research in these areas, including western Victoria, has consistently shown ample carry-over of 'unselected' worm larvae from the spring before the summer drenches are given. In fact, this is often the reason why worm control programs in these areas fail.

    Consequently, it is risky for most farms in western Victoria to adopt these strategies without first seeking advice from an experienced veterinary adviser. This can spread more larvae over the farm in summer-autumn, potentially causing severe worm problems the next winter.

    Liver fluke

    Live fluke (Fasciola hepatica) can infect both sheep and cattle. However, cattle are more resistant to infection and signs of infection are usually restricted to young cattle (less than 12 months age).

    The fluke has an indirect life cycle, the major intermediate host being a freshwater snail (Lymnaea tomentosa). Thus, the areas most prone to liver fluke infection are where the snail lives, such as soaks, creeks or irrigation areas.

    In south-eastern Australia the risk periods for fluke correspond with the snail's breeding season in autumn and spring. This means strategic (preventive) treatments are best given immediately before these breeding periods - July and March.

    Liver Fluke can cause two different diseases in sheep:

    • An acute disease which corresponds with immature fluke migrating through and damaging the liver. This can also initiate deaths from Black Disease (caused by a Clostridia bacteria, prevented by 5-in-1 vaccination)
    • Chronic fluke disease. This corresponds with the presence of mature fluke in the bile ducts of the liver. Signs include loss of condition, anaemia and bottle jaw.

  • Footrot

    Footrot is a highly contagious disease caused by bacteria that infect the feet of sheep, causing severe pain and lameness. It can have an enormous impact on flock productivity and profitability by reducing wool production, causing poor growth rates, lowering ewe fertility and restricting sale options for infected sheep.

    What is Footrot?

    Footrot is an infectious disease of sheep, cattle and goats. It occurs thoughout Australia but is most prevalent in the medium to high rainfall areas.

    Footrot is caused by a bacteria called Dichelobacter nodosus (previously called Bacteroides notodus) which thrives in moist, warm conditions. The footrot bacteria can live indefinitely in pockets of infection inside the feet of affected animals, even under dry conditions. However, the bacteria cannot survive outside the foot (say in soil or sheepyards) for more than 7 days.

    There are many strains of the footrot bacteria which vary in their ability to cause disease. Some strains never cause serious footrot in sheep, regardless of the environmental conditions - these cause 'BENIGN FOOTROT'.

    Other strains cause severe footrot lesions and spread rapidly in warm, moist environmental conditions. These cause 'VIRULENT FOOTROT'.

    In between these two extremes, there is a range of strains that produce from mild to severe footrot. All of these bacteria look identical under a microsope.

    Diagnosis of footrot

    Before undertaking a footrot eradication program, it's essential to get the correct diagnosis of what strain of footrot is present.

    This requires a thorough inspection of 50-100 sheep to determine the number affected and assess the severity or progression of any lesions present.

    Treatment of footrot

    BENIGN FOOTROT is easy to control, but difficult to eradicate - in fact it's rarely economically worthwhile to attempt eradication of BENIGN FOOTROT. There will be little evidence of lameness, especially when it's dry. However, during spring it may be nessary to control the spread of infection by footbathing.

    VIRULENT FOOTROT is quite different - CONTROL and then ERADICATION is the only course of action. This disease not only causes significant production loss, but compromises animal welfare and causes a lot of worry for owners of affected flocks.

    Eradication can take up to 2 seasons and requires commitment, good facilities and adequate labour - a specialist footrot contractor is often the best option.

    Poor planning is ususally the reason why eradication attempts fail. In particular, pay attention to reinspection of clean sheep, and don't muck about with sheep with severe lesions - "IF IN DOUBT, CHUCK IT OUT". Ideally, 2 clean inspections are required for each mob, then don't mix mobs until after the next spread period (usually spring).

    CONTROL - Before attempting eradication, Footrot needs to be controlled during the spring and autumn spread periods. This usually means regular footbathing and sometimes vaccination. Good control reduces the number of affected sheep that have to be treated or culled during the eradication phase. Eradication is undertaken during the non-spread period (summer) and requires inspection of ALL feet of ALL sheep at least twice during the dry period.

    Footrot eradication programs are expensive and must be tailored to each flock. If you think your sheep have footrot, contact us and we will develop a control and eradication strategy for your individual situation.

  • Lice

    To prevent the introduction of Footrot, lice and OJD, purchased and stray sheep should be carefully inspected and kept isolated from other mobs on the farm.

    If lice are detected and significant fleece derangement is occurring it may be necessary to use a long-wooled treatment to control lice until the next shearing. All sheep should then be treated after the next shearing to eradicate lice.

    A decision support tool developed by AWI (LiceBoss®) is available to help with these decisions .

    The broad Integrated Parasite Management ('IPM') strategies for lice are:

    • Monitor for lice before shearing, crutching, and at other times when sheep are yarded
    • Institute a lice biosecurity plan, which includes:
      • Checking the lice history and quarantining purchased or agisted sheep.
      • Preventing strays with stockproof fences.
      • Guarding against the introduction of lice by contractors or shearers who have recently been on a lice-infested property.
    • If lice are detected, treat with an effective chemical at next shearing, regardless of whether or not a long wool treatment has been used. ALL sheep must be mustered and treated.

  • Blowfly strike

    The Australian sheep blowfly (Lucilia cuprina) breeds preferentially on sheep and initiates the majority of flystrike of sheep in Australia. It is thought to have arrived in Australia from South Africa in the mid- to late-1800's, causing major outbreaks of flystrike in many areas of south-eastern Australia in 1897.

    An integrated parasite management (IPM) approach to controlling blowfly strike combines the use of insecticide with methods of making sheep less susceptible to both breech and body strike, such as:

    • effective worm control programs to reduce scouring
    • mulesing & tail docking
    • genetic selection against fleece rot, skin wrinkle and breech soiling ('dag').

    The proposed phase out of mulesing in 2010 is a major challenge to the industry and a number of alternatives are being explored through research sponsored by Australian Wool Innovation (AWI).

    In contrast to control programs for internal parasites ('worms'), the application of insecticides to control blowflies is based more around farm management practices, such as shearing, lambing and cropping.

    The use of insecticides has been associated with the development of serious insecticide resistance in L.cuprina - this is summarised in Table 1. Fortunately resistance to the IGR (insect growth regulator) compound cyromazine (Vetrazin® and other brands) has not yet been detected in field strains of Lucilia cuprina. However, field strains resistant to another member of the IGR class of insecticides (Diflubenzuron) have occurred.

    Integrated parasite management ('IPM') strategies to control flystrike include:

    Correct tail length (3rd palpable joint to cover the tip of the vulva) and mulesing technique (or a replacement technology when it becomes available)

    • Selecting sheep with reduced susceptibility to body strike (less fleece rot and body wrinkle) and breech strike (plainer breech, larger bare area, less scouring)
    • Control of scouring:
      • in young sheep through effective worm control.
      • in older sheep by genetic selection against 'hypersensitivity scouring' - use rams that have a low dag score and cull daggy ewes.
    • Other methods that may be applicable:
      • Choosing low risk paddocks for the most susceptible sheep (weaners).
      • Using flytraps to monitor fly populations - this will define low and high risk paddocks and identify patterns of fly activity.
      • Strategic application of pesticides with regard to fly biology. The timing of strategic treatments can be early in the season (Sep-Oct), to control the first generation of flies, or late in the season (Apr-May) to limit the population of over-wintering larvae.

    Table 1. Summary of insecticides used for the treatment and prevention of flystrike

    Insecticide

    Group

    Currently Registered

    Compounds

    Major Period used

    Trade name

    Period of protection

    (weeks)

    Resistance detected

    Arsenicals

    No

     

    (1919 - 1946)

      

    No

    Organo-

    chlorines

    No

    DDT & BHC

    Dieldrin & Aldrin

    (1946 - 1955)

    (1955 - 1958)

    Various

     

    Yes - in 1957 (a single major resistance gene)

    Organo-

    phosphates

    Yes

    Diazinon

    (1957-)

    4-in-1

    Di-jet

    Jet-Dip

    KFM

    Kleen-Dok

    Others

    < 6 weeks

    Yes, in 1965 - two resistance genes involved

    Carbamates

    No

    Butacarb

    (1966 - 67)

      

    Not defined, but 2 factors suspected

    Insect growth regulators

    Yes

    Cyromazine

    (1979-)

    Cyro-fly

    Venus

    Vetrazin

    Virbazine

    11-14

    Yes, but only in laboratory bred strains of flies which produce non-viable larvae

     

    Yes

    Dicyclanil

    (2000-)

    Clik

    Magik

    20-22

    Yes - some cross resistance to lab strains bred for resistance to Diflubenzuron

     

    Yes

    Diflubezuron

    (1992-)

    Fleececare

    Crusader

    Duodip

    Strike

    Magnum

    12

    Yes, low resistance detected in field populations of flies in Qld

    Spinocyns

    Yes

    Spinosad

    (1999-)

    Extinosad

    4-6

    No

    Ivermectin

    Yes

    Ivermectin

    (2000-)

    Paramax

    12

    No

  • Fleece Rot

    Fleece rot is caused by a bacterium called Pseudomonas aeruginosa. It predisposes sheep to body strike, as the exudate from the fleece rot lesions is extremely attractive to the female sheep blowfly (Lucilia cuprina). Culling sheep with fleece rot is a simple way of reducing the prevalence of body-strike within a flock. However, to make reasonable genetic progress, rams with known increased resistance to fleece rot should be selected as well.

  • Weaner ill-thrift and mortality

    Ill-thrift and deaths of sheep between weaning and 12 months of age is a widespread and important problem on Australian woolgrowing farms.

    Death rates of 10% to 25%, or more, occur in Merino flocks over a wide range of environments.

    High mortality occurs amongst apparently healthy weaners, without trace element deficiency, and is not restricted to particular weaning ages or seasonal conditions. These high rates of weaner mortality are a significant animal welfare issue and can significantly affect farm productivity.

    Mackinnon research

    The Mackinnon Project, through a PhD study conducted by Dr Angus Campbell, recently completed a detailed analysis of the cause of weaner mortalities. Low weaning weight and low bodyweight gain through the summer-autumn period were shown to be major risk factors. Further research is planned with assistance from AWI funding provided through the Australian Sheep CRC.

  • OJD (Ovine Johnes Disease)

    Ovine Johne's disease (OJD) is a chronic, wasting disease of sheep caused by Mycobacterium paratuberculosis bacteria. Johne's disease was initially considered a disease of cattle, but some strains of the bacteria have become adapted to sheep. These 'sheep' strains can also infect cattle and goats, although this is still quite rare. Sheep can also be infected with the cattle strain, and this is more common than cattle being infected with the sheep strain.

    Signs of OJD

    The most common sign of OJD is the gradual onset of a 'tail' in a mob of sheep. This can become more obvious at times of nutritional stress, such as during or after drought, late pregnancy or lactation.

    Young sheep (lambs, weaners) are more susceptible to infection than adults. Infection causes the lining of the bowel to become thickened. This process can take years, but eventually reduces the absorption of nutrients and causes the gradual loss of bodyweight. Scouring is less common with OJD than with Johne's disease of cattle.

    Control

    Control of OJD is centred around the vaccination of lambs with Gudair® and removing clinically affected sheep as soon as possible.

    In Victoria, a conservative estimate of the proportion of flocks infected with OJD in early 2007 was at least 6%, but most owners do not know whether their flocks are infected or not. However, if you are reasonably confident that the disease isn't present, it makes good sense to try and keep it out of your flock. Good fencing to exclude strays, and taking the time to check the OJD status of purchased sheep, are important strategies. This is the same as for other important infectious diseases, such as footrot.

    More information on the legal and sheep trading aspects of OJD is available on the NSW or Victorian DPI websites.

  • Vaccination against clostridial disesases

    Clostridial diseases are a significant cause of deaths and economic loss in unvaccinated flocks, so clostridial vaccintation is simple and cheap form of stock insurance.

    Control by vaccination

    Initially two doses of 5-in-1 vaccine, given at marking then weaning, followed by a booster 12 months later, will give a solid life-long immunity against most clostridial diseases. These include Black Disease, Blackleg, Malignant oedema, Pulpy Kidney ('PK' or enterotoxaemia) and Tetanus.

    Additional vaccinations are only required in special circumstances, such as when sheep are run on irrigated pastures. In this case Black Disease, associated with immature liver fluke infection, can be a problem.

    In addition, immunity to Pulpy Kidney is not as long-lived and may wane in older ewes and wethers (although there are very few of these around at the moment!).

    This may be a problem for older ewes and their lambs, particularly with high levels of grain feeding or when sheep are moved onto lush pasture, such as lucerne.

    Consequently, if you have had Pulpy Kidney deaths in lambs born from older ewes in the past, it is advisable to give a booster vaccination to older ewes. In addition, if older sheep are being fed grain rations for an extended period (such as during drought), a 5-in-1 booster is worthwhile insurance.

  • 'Smart Grazing' For Weaner Worm Control

    Introduction

    Smart grazing is an improved, yet simple & reliable, strategy for the control of worms in weaner sheep during their first winter. It can counter the negative effects of summer rainfall that reduces the effectiveness of the '2-summer treatment strategy' in the winter rainfall areas southern Australia.

    The strategy hasn't been evaluated in the summer rainfall areas.

    The why & how of 'Smart grazing'

    Merino weaners are very susceptible to worms in their first winter. Consequently, they need to graze pastures that have as few worm larvae as is practicable. 'Smart grazing' combines intensive grazing for 30 days with each of the 2 'summer' drenches to ensure that virtually no worm eggs are deposited on a chosen pasture from the first summer drench (November) until after the autumn break (March-April), when the weaners are put into these pastures.

    Intensive grazing means using 2½-3 times the normal stocking rate, for no longer than 30 days after each of the summer drenches is given. Not exceeding 30 days is critical because it takes 3 weeks for a worm larvae (eaten off pasture) to develop into an egg-laying adult in the sheep. After the intensive grazing period, the paddocks are de-stocked to allow the pastures to re-grow. This means that the total stocking pressure for the 'Smart grazed' paddock will be the same as that for a paddock continuously stocked at the farms normal stocking rate.

    The intensive grazing will reduce pasture residues to around 800-1000 kg DM/ha after the first summer drench, and around 600 kg DM/ ha after the second. If there is insufficient feed, the periods of intensive grazing can be reduced. On the other hand, if there is excess feed the summer drenches can be 'staggered' for different mobs so as to provide a longer intensive grazing period, or cattle can be used as well.

    Finally, the weaners must be drenched with an effective drench before they start grazing the 'Smart grazed' paddock after the autumn break.

    Smart grazing on a typical farm

    A typical self-replacing flock of 5,000 DSEs in southern Australia is made up of 1,500 ewes, 1,500 wethers and 1,000 weaners, running at a winter stocking rate of 15 DSE/ Ha.

    70 Ha of 'Smart grazed' paddocks must be prepared for the weaners. Thus, 2600 DSE (70 x 15 x 2.5) are needed to stock the 70 Ha at 2½ times the normal stocking rate for each of the two intensive grazing periods - this is all of the wethers and 70% of the ewes on the farm.

    A timetable for 'Smart grazing'

    OCTOBER: Select the 'Smart grazing' paddock - choose one with a history of good winter pasture

    NOVEMBER: Give the first summer drench (MUST be with an effective product), then intensively graze the paddock at 2½-3 times the normal stocking rate.

    DECEMBER: Remove the sheep to another part of the farm after 30 days intensive grazing. Ideally, the pasture residue should be 800-1000 kg DM/ ha (2.2-3 cm).

    JANUARY: Paddock remains unstocked until the second summer drench.

    FEBRUARY: Give the second summer drench, then intensively graze the 'Smart grazing' paddock with the drenched sheep (again, not > 30 days).

    MARCH: Paddock remains de-stocked until the autumn break.

    AUTUMN BREAK (MARCH-APRIL): Drench weaners and set-stock on the 'Smart grazing' paddock when pasture > 600 kg DM/ha (1.5 cm). Weaners can remain there until spring but monitor their worm egg counts every 4-6 weeks.

    Why does smart grazing work?

    The intensive grazing periods:

    • Reduce the amount of pasture dry mater, making the pasture less suitable for the survival of worm larvae.
    • Ensure that there is no deposition of worm eggs on the pasture from the time of the first summer drench until the autumn break.
    • Probably allow the drenched sheep to 'vacuum' up infective larvae, in much the same way as cattle do when they are used in alternate grazing programs with sheep.
    • Have the same cumulative stocking pressure from November to March as set-stocked paddocks grazed continuously by wethers.
    • Are quite flexible. What must not be changed is the need (i) not to exceed 30 days grazing after each summer drench, and (ii) for a fully effective compound at the summer drenches.

    What are the benefits?

    Results from a controlled experiment over 2 years in western Victoria show that, compared to weaners grazing paddocks prepared the usual way (grazed by wethers over the summer/ early autumn), weaners grazing 'Smart grazing' plots:

    • grew 13% more clean wool (2.29 vs. 2.03 kg) which was 3.5% broader (17.1µ vs. 16.5µ)
    • were 3 kg heavier in October (46.5 vs. 43.2 kg).

    During winter, the egg counts from the 'Smart grazed' weaners didn't go higher than 250 epg, a trigger for drenching weaners used by many farmers and their advisers. In contrast, the weaners on the paddocks prepared by set-stocked wethers exceeded 400 epg in both years.

    The numbers of worm larvae on the 'Smart grazed' pastures in winter were from one-half to a one-third of those on pastures in paddocks prepared by grazing with set-stocked wethers.