Learning Objectives On completion of this topic you should be able to:
• describe the prevalence and importance of external parasitic disease of sheep; • define the effect on wool production of blowfly strike and body lice infestation; • understand the principles of control of blowfly strike and body lice infestation. Key terms and concepts Epidemiology, pathogenesis, resistance, resilience, life cycles, risk factors, preventive, strategic, tactical and curative disease control, integrated parasite management 18.1 Classification of external parasites of sheep While the single most important sheep disease problem in Australia is gastro-intestinal worm infection, the external parasites, sheep blowfly and body louse are also major multi-million dollar problems for the industry. Furthermore, unlike the worm problem, these problems are found throughout the industry and are not necessarily greatest in higher rainfall areas. As is illustrated in Figure 18.1 there are other external parasites of sheep such as nasal bots, sheep ked (perhaps now extinct in Australia), sucking lice and mites that are less important than blowflies and biting lice, but which nonetheless can cause severe problems on individual properties.
Figure 18.1 Classification of the main groups of external parasites of sheep in Australia. The most important groups are identified with an asterisk (Walkden-Brown and Besier 2006). 18.2 Blowfly strike (cutaneous myiasis) This is an acute disease caused by the feeding of blowfly larvae on the skin of sheep. The disease generally runs a short course (days to a couple of weeks) of varying severity. It has been estimated that up to 80% of fly strikes are small covert strikes causing minor illness that are not noticed by the producer. On the other hand severe strike can kill sheep in a matter of days. The sheep blowfly and its life cycle The primary strike fly is the Australian sheep blowfly Lucilia cuprina which initiates 80-90% of strikes. The fly is about the size and shape of a normal house fly but has a green body and foreleg (Figure 18.2). Its maggots are characterised by a smooth body. Flystrike initiated by L. cuprina may resolve after a few days or it may be complicated by secondary strike or bacterial infections. Existing strike sites
Class Insecta 3 pairs of legs, distinct head, thorax and abdomen, one pair of antennae
Class Arachnida 4 pairs of legs, cephalothorax and abdomen, no antennae
may be struck again by L. cuprina or secondary blowflies (such as Chrysomyia rufifaces, the hairy maggot blowfly) making existing wounds much worse. Secondary blowflies breed in decaying carrion and other material and are unable to initiate flystrike on their own. They tend to be larger and rounder than L. cuprina and have a variety of colours.
Figure 18.2: The major primary (Lucilia cuprina) and secondary (Chrsomya ruffifacies) blowflies of sheep in Australia (Levot 1999).
Figure 18.3 illustrates the lifecycle of the sheep blowfly. The female requires a protein meal before laying eggs, this being obtained from sources such as faeces and carrion. Over a 2-3 week lifespan, the female lays 2-3 batches of 50-250 eggs in the soiled or wet fleece (as well as carrion), being attracted by putrefactive odours and a “sheep odour”. Note that flies will only lay eggs on wounds or inflamed skin (dermatitis). They will not initiate a strike on normal skin.
Larvae pass through 3 larval instars in 4-6 days. The first instar requires a liquid protein meal but cannot damage the skin as they have no mouth hooks. Hence, the requirement for damaged or inflamed skin. The second instar has hooked mouth parts and also secretes proteolytic enzymes so is capable of physically and chemically damaging the skin. Larval products such as proteases cause an intense inflammatory response and set up a bacterial dermatitis that can lead to severe systemic disease characterised by toxaemia and septicaemia (“blood poisoning”). Mature larvae drop off at night after 4-6 days of feeding on the host and pupate in the soil. The pupal stage lasts only 3-7 days in summer but development is arrested by low temperatures and the fly usually over-winters in the pre-pupal stage. Adult flies can emerge as early as 7 days after dropping off the sheep. Thus the minimum time to complete the lifecycle is around 2 weeks. As with most parasitic lifecycles, in cooler weather the duration of the lifecycle is greatly extended.
Figure 18.3: The lifecycle of Lucilia cuprina, the Australian sheep blowfly (Levot 1999).
Types of blowfly strike There are a number of forms of flystrike, each differing in the causal factors and the control methods.
• Breech strike - flies strike the breech or tail skin with dermatitis due to wetting with urine and/or diarrhoea (scouring). This is the most common form of blowfly strike
• Body strike - flies strike the back or sides of the body (Figure 18.4), on skin with dermatitis (eg. fleece rot or lumpy wool). This is the second most common form of blowfly strike
• Poll strike - flies strike the head on rams at fighting wound sites, or where the horns grow close to the skull creating a microenvironment suitable for strike
• Pizzle strike - flies strike the urine-soaked area around the prepuce of males • Wound strike - flies strike wounds such as marking, mulesing and shearing cuts, as well as
those associated with footrot, foot scald and scabby mouth.
Figure 18.4: Sheep with fly strike of the body (body strike) (Agnote, Simpson 1990).
Effects of fly strike on wool production The effect of flystrike on wool production depends on the severity and frequency of strikes. Annual fleece weight may be reduced by up to 8% while up to 2% of the clip may be classified as stained, cotted or dead wool.
Figure 18.5 shows that individual fibre growth rate declined by around 17% by the end of an experimental infection period. However, there was an on-going reduction in growth rate after cessation of infection, with up to 27% reduction in growth rate during the post-infection period. The short term effects on wool follicle function tend to have greater effects on staple strength than overall wool growth as can be seen in Figure 18.6 in which moderate flystrike more than halved staple strength. The actual mechanisms underlying these effects on wool growth are not fully defined, in particular the roles of stress hormones, reduced feed intake and potential role of cytokines, along with their interaction (Walkden-Brown et al. 2000) However, recent work at UNE indicates that reduced feed intake accounts for only some 25% of the effects on staple strength and about 55% of the overall effects on wool growth.
Figure 18.5: Reduction in wool growth associated with fly strike (500 larvae/day for 8 days) as determined by autoradiography (Broadmeadow et al. 1983).
Figure 18.6: Staple strength (± SEM) in Control Merino wethers and those infected with 500 L. cuprina larvae daily for 8 days (Struck) or uninfected sheep pair fed with the struck animals (Pair fed). Two of the 5 sheep in the struck group had fleeces too tender to measure (“rotten”) so data for these animals is excluded (Walkden-Brown et al. 2000). Factors affecting the incidence of flystrike The occurrence of fly strike is influenced by a range of environmental (temperature, rainfall, wind) and host (genotype, phenotype, presence of dermatitis) factors. Environmental factors The major environmental factors affecting the incidence of flystrike are:
• Temperature – Flies are active at air temperatures between 17 and 35°C while soil temperatures between 13 and 30°C are optimal for pupal development. High summer temperatures induce larval mortality
• Rainfall - Both soil and fleece moisture are required. Humidity >70% is required for egg hatching
• Wind - Moderate wind speeds (5-30 km per hour) are ideal. Flies are less active during totally still conditions or strong winds
• Carrion - This allows maintenance of fly populations, although Lucilia is less efficient when using carrion instead of the host sheep. In carrion it is generally out-competed by secondary blowflies.
These environmental factors lead to a seasonal distribution in fly activity, with a peak in late spring, and a smaller peak in autumn in the southern areas but with sustained activity throughout summer in northern regions. Host factors 1 – Damaged skin The presence of liquid protein on skin is an absolute requirement for first instar larvae. Thus flies will strike wounds, sites of infection (eg. footrot) and dermatitis-affected areas. Fleece rot is the major predisposing factor for body strike. It is a bacterial dermatitis caused by Pseudomonas aeruginosa but is not a contagious disease, as the organism is present on the skin of all sheep. The enzyme phospholipase C is a virulence determinant for P. aeruginosa, producing a dermonecrotic toxin. When the fleece is wet to skin level for around 1 week there is rapid build up in the population size of the organism and dermatitis develops. Other Pseudomonas species and other organisms participate in the dermatitis but P. aeruginosa produces inhibitory compounds, thus becoming dominant. A pigment (pyocyanin) also results from this bacterial activity, changing from blue-green to brown over time. This is referred to as bacterial stain which, although partly scourable, attracts penalties. P. aeruginosa, also produces a specific blowfly-attracting odour making it the major predisposing factor in body strike.
Fleece rot is mainly seen in young sheep. However, predisposing factors to fleece rot in all classes of sheep include open fleeces with fringed staple tips, high suint content, low wax content and high diameter variation. Producers in high rainfall areas commonly cull young stock showing evidence of fleece rot (Figure 18.7). Medium and strong woolled Merinos are much more susceptible to fleece rot than finewool strains.
Lumpy wool, also referred to as dermatophilosis or dermo, is a bacterial dermatitis caused by the bacterium Dermatophilus congolensis. Unlike P. aeruginosa, this organism is not a normal inhabitant of the sheep skin and so lumpy wool is a true contagious disease. The disease is characterised by crusty scab formation and yellow discoloration in the fleece while infection of the lower legs causes strawberry footrot. It spreads from sheep to sheep by contact when sheep are wet or via contaminated dipping fluid. Although it can be readily controlled (antibacterials in dipping fluid, don’t yard wet sheep) and treated (antibiotics), when present it acts as a predisposing factor to body strike.
Figure 18.7: Inspection sites for examining sheep for fleece rot susceptibility (Murray and Mortimer 2001).
The skin scald associated with urine or faecal wetting is another form of dermatitis and predisposes to breech and pizzle strike. 2 – Phenotype There are a number of phenotypic factors influencing the degree of susceptibility of the individual to flystrike. Some of these have a genetic basis.
• The length of wool is critical, with the incidence of body strike being rare when there is less than 4 months wool carried by the animal. Short wool allows the fleece to dry quickly, preventing development of fleece rot. Thus the timing of annual shearing is an important control measure
• Fleece structure or type is also important, relating to the wetting and drying potential of the fleece environment (see fleece rot section)
• Conformation problems such as “devil’s grip” impede the ability of regions of the fleece to dry, while other attributes such as a wrinkly breech impede the breech area from drying, thus facilitating breech strike
• The use of the Mules operation and docking of the tails to the third joint can reduce the incidence of breech strike by up to 90%. These have been routine management operations for replacement sheep in Australia, but there is a strong industry commitment to phasing out mulesing and a number of alternatives have been investigated. The longer-term solution is undoubtedly genetic, but in the shorter term increased use of analgesia during mulesing, mulesing only those animals requiring it, and use of commercial skin clips rather than mulesing are all reducing the number of animals mulesed and the impact of the operation on them.
• Close-set horns in rams can also predispose the animal to poll strike. 3 – Genotype The genotype of the host influences the incidence of body strike. Figure 18.8 shows clear differences between the Merino strains in their susceptibility to both fleece rot and body strike, showing how the incidence of each disease is related. In general, fine wool genotypes are more resistant to both diseases than broader wool genotypes. There is also within-flock genetic variation in resistance to fleece rot (heritability of around 0.35) that can be exploited to increase resistance to body strike. There are also breed differences in susceptibility to body strike, with British breeds being more resistant than Merinos.
Figure 18.8: Differences in susceptibility to fleece rot and body strike within and between strains of Merino (Atkins and McGuirk, 1979).
Strategies for controlling fly strike Strategies for controlling blowfly strike can involve modification of the host or the environment. As it the case with worm control, integrated parasite management (IPM) approaches, using a range of control strategies in concert are ideal. The FlyBoss® web site contains a wealth of information about blowflies and blowfly control, including decision support tools.
Conventional control measures • Permanent modification of host phenotype. A number of conventional control strategies are
aimed at modifying the host phenotype permanently to reduce the incidence of breech strike in all sheep (mulesing and tail docking) and pizzle strike in wethers (pizzle dropping). Note the earlier comments about the phasing out of mulesing.
• Non-permanent modifications of host. These include shearing prior to the main fly period for control of body strike, and crutching of ewes and ringing of wethers and rams prior to fly waves for controlling breech and pizzle strike. As scouring can also facilitate breech strike, controlling parasitic and nutritional scours in stock will reduce the incidence of breech strike. In sheep treated with ivermectin capsules to control worms, the residual ivermectin in faeces, and dags inhibits breech strike establishment (Rugg et al. 1998).
• Chemical treatment of the host has been widely used in the control of flystrike via shortwool treatments (dipping by plunge or spray, or backline application) or longwool treatments (jetting by hand or jetting race, backline spray). There are a number of classes of insecticide available for use, varying in their mode and duration of action, the degree to which residues in wool are a problem, whether they control lice or not, and the extent to which resistance has developed. The main classes and their mode of application are summarised in Table 18.1 and discussed briefly below. The FlyBoss® tools are an excellent aid to optimising the number and frequency of treatments and the chemicals to use. - Organophosphates (eg. Diazinon). Now only registered for use as a dressing due to OH&S risk in shearers of transfer of OP from wool. - Insect growth regulators (eg. cyromazine, dicyclanil) act by inhibiting larval transitions in development. There are no residues issues with these two chemicals. Long term protection, no resistance, not used for lice control. - Insect development inhibitors (eg. diflubenzuron, triflumuron) also act by inhibiting larval transitions in development, but via a different mechanism. They tend to be also known as IGRs within industry and are used to control blowfly and lice. Resistance in flies has been reported for diflubenzuron. They provide relatively long term protection, but residues can be an issue.
- Macrocyclic lactones (eg. ivermectin). Can be suitable for long wool treatments (6-12 weeks withholding) and no resistance reported to date. Control lice as well.
- Synthetic pyrethroids (eg. cypermethrin, deltamethrin, alpha-cypermethrin). No resistance in flies, but common and high in lice. Residues are an issue with longwool treatments.
- Spinosyns (eg. Spinosad). They are suitable for long wool treatments for control of blowfly and lice. They have no withholding period and no resistance has been reported. They provide a relatively short duration of protection (4-6 weeks).
Table 18.1: Summary of major classes of chemicals that are available for both preventing and treating flystrike (FlyBoss.org.au)
• Genetic selection. Culling for susceptibility to body strike directly and indirectly on the basis of fleece rot, has been and continues to be widely used in high rainfall areas. Early attempts in the
3/02/12 10:37 AMChoosing The Right Chemical For FlyStrike
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Organophosphates (17 KB)
Insect Growth Regulators (30 KB)
Ivermectin (29 KB)
Chemicals registered to treat lice and flystrike on sheep (100 KB)
Choosing The Right ChemicalChemicals can be used strategically to reduce the risk of flystrike. The table below summarises the different classes of chemicals that are available for both preventing and treating flystrike.
In many instances the choice of chemical product will determine how the product must be applied to the sheep. Dicyclanil, for example, is only available as a spray-on product. Otherinsecticides may be available in several formulations and producers must choose products and application methods that suit their purpose. For example, cyromazine is available as ajetting fluid/dip concentrate that must be diluted in water prior to use; or as a ready-to-use spray-on product. Spinosad is available as an organic treatment option and will protect sheep from flystrike for 2-3 weeks. The most common application techniques are hand jetting, automatic jetting races, spray-ons and plunge or shower dipping. Dipping should only be regarded as an emergency flystriketreatment. The advantages and disadvantages of the various application techniques are compared in the following sections. Information is provided on best practice for dipping, jettingand spray on application in downloadable documents. The FlyBoss tools (http://www.flyboss.org.au/tools/flystrike-decision-support-tools.php) provide a detailed list of flystrike and lice treatment products currently available. The tools alsoprovide details on;
Commercial product name, active chemical and chemical group;Suitability as lice or fly treatments;Wool harvesting interval, Export slaughter interval and Meat withholding period;Size of pack;Estimated cost per pack and per dose.
The list can be sorted by application method. When applying flystrike and lice chemicals ensure that you are compliant with the Wool Harvesting Interval (WHI), Export SlaughterInterval (ESI) and Meat Withhold Period (WHP) relevant to the particular chemical. Failure to comply risks key markets for wool and meat products and the safety of workers andconsumers. The FlyBoss tools can also be used to assess the effectiveness of a particular chemical application in reducing risk within the flock. Use the FlyBoss tool to compare differentmanagement scenarios and optimise chemical treatments.For detailed notes on chemical groups download the following documents.
For more information relating to chemicals registered to treat lice & flystrike on sheep click here (http://www.flyboss.org.au/tools.php) to go to the FlyBoss Tools pages.
1970s at breeding easy care Merino’s that did not require mulesing were unsuccessful but with the phasing out of mulesing, genetic selection for resistance to breech strike is now a key strategy for long term blowfly control. SheepGenetics®/MerinoSelect® now offer Australian Sheep Breeding Values (ASBVs) for rams for the following breech traits
o Breech wrinkle score o Breech cover score o Dag score
Diagrams showing the scoring systems can be found at the MerinoSelect® or FlyBoss® web sites and the genetic and phenotypic associations between these traits and other fly strike traits is shown in Table 18.2. Table 18.2: Heritability (bold), phenotypic variance (Vp) and phenotypic correlations among breech strike and indicators (Smith et al. 2009).
Unfortunately genetic correlations between breech wrinkle score and production traits such as fleece weight and mean fibre diameter are negative so improvement in this trait needs to be part of multi-trait selection index (Brown et al. 2010).
• Monitoring or manipulating the environment. This is less widely practiced. Modelling studies have shown that fly populations are much more sensitive to factors affecting adult fly mortality than larval mortality, so the main strategy is to target the adult fly. Fly traps containing chemical attractants such as LuciTrap can reduce fly numbers by as much as 50% and may provide some reduction in strikes.
• Controlling carrion. The value is uncertain, given that secondary fly maggots out-compete and kill primary fly maggots in carrion. This strategy is unlikely to have a major effect on population size of Lucilia cuprina, although it would reduce the size of secondary fly populations.
Potential additional control measures Potential control measures that have been tried or considered include:
• Vaccination. There are two approaches, both of which are experimental at present. Firstly to vaccinate against fly antigens such as peritrophic membrane antigens in the gut of the fly (hidden antigen approach) or secondly to vaccinate against fleece rot, though this is hampered by the diversity of strains of P. aeruginosa. There is still substantial progress to be made in the development of effective vaccines
• Use of controlled release capsules containing ivermectin (eg. Ivomec Maximizer®) to control parasitic scours. In addition to the benefits in reducing scours, sheep that continue to scour are protected by the insecticide action of residual ivermectin in their faeces (Rugg et al. 1998).
• Use of bacillus thuringiensis toxins. This organism is a normal inhabitant of the sheep skin and there is scope for use of its toxins as a natural control of flystrike. However, it does not have the same degree of persistence in the fleece as do the chemical treatments
• Biological control of adult flies. The microsporidium Octosporea muscaedomesticae and various fungal toxins show some promise but there is still some way to go in the development of these methods as commercial strategies
• Releasing sterile males. This relies on the principle that female flies tend to mate once, such that if she mates to a sterile male, no viable eggs will be laid. Males can be sterilised by irradiation. This has been used to eradicate screw worm fly from some areas around the world but is prohibitively expensive to implement
Sheep - Wool I
Body and breech strike were not correlated (0.08). Body strike was correlated with fleece rot (0.23), but not wool colour (0.07). Table 1. Percentage flystrikes by year, selection line (UC=unselected control, CI = commercial improvement, PB = plain breech), and mulesing group (M=mulesed, UM=unmulesed)
n Body strikes# (%) Breech Strikes (%) Total strikes (%) Drop Line M UM M UM M UM M UM UC 105 111 6 0 0 7 6 7 CI 109 109 0 1 2 14 2 15 2005 PB 105 105 0 0 0 8 0 8 UC 70 72 3 6 17 90 20 96 CI 67 66 6 9 0 24 6 66 2006 PB 70 75 1 1 1 22 3 24 UC 38 38 8 13 5 71 13 84 CI 43 44 16 20 2 36 19 57 2007 PB 43 43 14 7 2 33 16 40 UC 52 55 4 0 4 25 8 25 CI 60 59 3 2 3 14 7 15 2008 PB 60 60 0 2 5 8 5 10
# includes poll strikes
Table 2. Heritability (bold), phenotypic variance (Vp) and phenotypic correlations among breech strike and indicators.
Trait Vp Body
wrinkle Breech wrinkle
Breech cover
Crutch cover Dag
Urine stain
Breech strike
Body wrinkle 0.38 0.25 (0.10) 0.29 0.03 0.11 -0.00 0.22 0.04 Breech wrinkle 0.57 0.36 (0.12) 0.02 0.09 0.06 0.21 0.22 Breech cover 0.45 0.23 (0.09) 0.29 0.01 0.05 0.01 Crutch cover 0.35 0.47 (0.14) 0.04 -0.05 0.09 Dag 0.43 0.09 (0.06) 0.00 0.22 Urine stain 0.40 0.30 (0.20) 0.04 Breech strike 0.06 0.32 (0.11)s.e. on all correlations 0.03-0.04 except for those with urine stain which were 0.06-0.07
DISCUSSION
Low breech wrinkle and dag were the characteristics with greatest effect on breech strike rate. There was no evidence that lower breech cover reduced breech strike. Breech wrinkle and dag have also been shown to be associated with breech strike in the Mt Barker, WA flock, but there is evidence in that flock that reduced breech strike rate is also associated with lower breech cover (Greeff and Karlsson 2009). The mean breech cover score in the WA flock is lower than that in the Armidale flock which may explain the different outcomes observed.
Breech strike and, with the exception of dags, all of the breech traits were moderately heritable. However, only breech wrinkles and dags were sufficiently correlated with breech strike to be useful selection criteria. Published heritability estimates for dags vary widely with environment and age (Greeff and Karlsson 1998, 1999; Woolaston and Ward 1999). Compared to winter rainfall areas, dags in the summer rainfall environment is a somewhat ‘transient’ trait. This may make dags of limited use as a selection criterion for breech strike in summer rainfall areas.
• Releasing males with defective genes. This has been used with success on a local scale in Australia but would require an expensive national effort to produce sustained results
• Genetic engineering. One approach is to engineer the host or skin bacteria to produce inhibitory products such as chitinase to disrupt the development and activity of insect pests.
18.3 Body louse infection (Bovicola ovis) The sheep body louse Bovicola ovis is an obligate parasite of sheep that can also infect goats for a short period. It feeds on the stratum corneum of the skin, causing irritation and a hypersensitivity reaction. This itchiness is the root problem with this disease. B. ovis is present in all states of Australia with 35-70% of properties being affected. It is a major disease of the wool industry costing an estimated $123 million per annum, primarily in the form of costs of control (Sackett et al. 2006). Lifecycle All stages of the lifecycle occur on the sheep. Adult females lay 2 eggs every 3 days. These eggs are attached to wool fibre 6-12 mm from the skin and hatch in 10 days (Figure 18.10). Eggs attached to fibre removed from the host, do not hatch. After hatching, the nymphs pass through 3 stages lasting 7, 5 and 9 days respectively. The pre-oviposition period in the female is 3-4 days. The minimum time required to complete the lifecycle is 34 days. Under good conditions it takes 4-5 months for a light infestation (0.3 lice per sq. cm) to develop into a heavy infestation (30 lice per sq. cm). As lice can only survive for 1-2 days off the host, transmission of the parasite is by direct contact between sheep.
Figure 18.10: Life cycle of Bovicola ovis ( Joshua et al. 2010).
Effects on wool production Lice infestation causes a reduction in greasy fleece weight ranging from 15-30% depending on the severity of the infestation. The main mechanism is irritation (due to local immune system hypersensitivity responses) causing rubbing and biting of the fleece by the host, leading to loss of fibre. Yield also tends to be reduced by up to 5% (absolute units) while cotting and discolouration results in increased classing of fleeces into cast lines. Fibre length and fibre diameter are unaffected. The data in Table 18.3 were collected over three years. The light infestation treatment commenced with around 100 lice per sheep while the heavy infestation commenced with around 1000 lice per sheep. While there was a relationship between severity of infestation and wool production, it is also worth noting that processing losses (carding and combing) also increased as severity increased. From the producer’s viewpoint, the lice infestation equated to a loss of $0.72 to $1.92 per lousy sheep. From the processor’s viewpoint, this equated to a loss of $21 -$32 per 100 kg wool processed into top.
2 PRIMEFACT 483, SHEEP LICE
Figure 2. Life cycle of sheep body louse – egg to egg in 34 days.
Females reach egg laying maturity within four days of moulting. Lice spend their entire life on the skin or wool of sheep. The life cycle takes about 34 days at a minimum. Female lice live about 27 days and males about 48 days. However, there are reports of lice surviving for over 120 days.
Factors affecting lice survival
Lice prefer to live at 37°C and 70–90 per cent humidity. They are susceptible to extremes in temperature and humidity and move up and down the wool fibre to accommodate these changes. Above 39°C the number of eggs laid is reduced, and at 45°C no eggs are laid. On a hot day the fleece temperature on exposed parts of a sheep, with less than 25 mm of wool, may range from 45°C near the skin to 65°C at the wool tip. These temperatures are too hot for eggs and young lice to survive. Also lice and eggs do not survive extended periods of very low temperatures.
Adults and nymphs can drown and eggs fail to hatch after saturation with water for more than six hours. This can occur if the fleece becomes saturated following heavy rain or if sheep are immersed in water. Lice and their eggs do not survive for very long off the sheep. Survival of lice in wool on fences and in yards is very short. This is due to lack of food, exposure to sunlight and desiccation as well as temperature fluctuations between night and day.
Generally sheep lice do not survive or breed on other animals or humans. Under experimental conditions they have survived on goats for a short time but will not reproduce. Similarly, lice on other species of animals will not infest sheep.
Lice do not like light and move rapidly into the fleece when the wool is parted. Most lice live close to the skin and shearing can remove 30–50 per cent of the total population. After shearing many lice die due to exposure to heat or cold or rain. However, some lice survive in areas of longer wool left on the sheep.
The lice population builds slowly following shearing until sufficient length of wool – usually about three month’s growth – is available to afford protection from the elements. However, it is unlikely that an infestation will be detected until the population growth enters the rapid increase phase about five months after shearing (James 1999). Lice numbers rise quickly at this time, unless conditions are particularly hot.
It takes 5–6 months for newly infested sheep to develop noticeable symptoms of an infestation. The first sign is likely to be sheep rubbing. When a sheep has developed a light infestation of lice – about one louse per 10 cm parting of the wool – there are already about 2000 lice present on the sheep (James and Moon 1999). It takes a further 2–3 months for severe wool derangement to occur. These times are approximate and may be longer when residual chemical is present in the wool, or if the sheep are shorn.
The time taken to show symptoms of lice infestation depends on the size of the population and the sensitivity of the sheep to irritation caused by lice. Lice numbers increase at a greater rate on sheep that have:
� a low immune system,
� are in poor condition, or
� are affected by disease.
These sheep can be targeted at the tail of a mob when inspecting for the presence of lice in a flock.
Lice transmission between sheep
Lice move from infested sheep to clean sheep when the sheep are in direct contact with each other. The warmth and shading of adjoining sheep allow the lice on one sheep to move up the wool fibres and across to another sheep. This occurs most commonly:
Table 18.3: Effects of different levels of lice infestation on wool production and processing performance (Wilkinson et. al. 1982).
Variable Level of infestation
None Light Heavy
GFW (kg) 5.1a 4.9ab 4.5b
CFW (kg) 3.4a 3.1ab 2.8c
MFD (um) 24.9a 24.7a 25.0a
Scouring yield (%) 66.8a 63.0b 62.6b
Card loss (%) 9.8a 12.3b 13.3b
Noil (%) 4.1a 4.8b 5.4c
Top and noil yield (%) 60.5a 55.7b 53.3b
Fibre length in top (cm) 8.3a 7.7b 7.2b abc Means within rows not sharing a common letter in the superscript are significantly different (P<0.05).
Factors affecting the incidence and severity of lice infestation Environmental factors
• Temperature - B. ovis is very sensitive to changes in temperature. Its optimum temperature is 37°C. Below 37°C egg production ceases while above 39°C egg production reduced. 45°C for 18 hr causes death irrespective of humidity
• Moisture - Lice are susceptible to drowning on a wet skin. Up to 90% of adults can drown during a thunderstorm. Eggs fail to hatch at a relative humidity of 90%.
Host factors
• Shearing removes most of the eggs, 30-50% of the adults and exposes the remainder to high skin temperatures. Summer skin temperatures in shorn sheep are often in the range 45-55°C
• Wool cover affects lice survival through effects on skin temperature. Less cover allows greater penetration of solar radiation and greater variation in skin temperatures. The longer the staple the better for lice.
Due to temperature and shearing effects there is a marked seasonal variation in numbers. Few lice are found after shearing, numbers remain low over summer, then increase from autumn through winter until the next shearing. Increases in numbers can be interrupted by soaking thunderstorms. Control measures B. ovis is theoretically easy to eradicate from farms, and individual producers often have no need for lice treatments for some years after a successful treatment, but eradication has never been achieved on a state scale. In theory (and practice) eradication can be achieved by a single effective treatment to all sheep, as lice do not survive well off the host. The LiceBoss® web site contains a wealth of information about lice and their control, including decision support tools. Control is based solely upon chemical treatment of all animals with the following chemical classes and methods of application available (Joshua et al. 2010). • Synthetic Pyrethroids – cypermethrin, deltamethrin, alphamethrin (pour-ons and spray-on) • Organophosphates – temephos (dip), diazinon (spray-on, dip under APVMA permit) • Benzoylphenyl urea IGRs (IDIs) – triflumuron (pour- on), diflubenzuron (dip and pour-on) • Ivermectin (jetting fluid) • Magnesium fluorosilicate/sulphur (dip) – approved for organics • Spinosad (dip and pour-on) – approved for organics • Imidacloprid (pour-on)
No matter what method or chemical is used, always follow label directions. The label is a legal document. It should be noted that IGRs and IDIs act only at the transitions during the lifecycle and have no effects on adult lice. Thus it can take up to 5 months for sheep to be completely free of lice after treatments with these chemicals. Resistance is a potential problems with the SPs and the BU IGRs. In terms of effectiveness, off shears treatments (backline, dip or spray) are more effective than short wool treatments (dip or spray), which in turn are more effective than long wool treatments (dip or jet). This reliance on chemical treatments raises issues of chemical residues and resistance. Withholding periods to shearing must be strictly observed. Short wool treatments are not only more effective, but they use less chemical and allow longer for it to break down prior to shearing so are preferable from a residue management point of view. Prevention of re-infestation is a major aspect of control. This requires good boundary fencing and care when purchasing new stock. Readings There are no prescribed readings for this lecture. Students are advised to access and read papers that interest them in the references section.
References Arundel, J.H. and Sutherland, A.K.. 1988, Animal Health in Australia Volume 10. Ectoparasitic diseases
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strike’, Wool Technology and Sheep Breeding, vol. 27, pp. 15-19.
Besier, B., Jacobson, C., Woodgate, R. and Bell, K. (2010). Sheep Health. In “International Sheep and Wool Handbook” (ed. D.J. Cottle) Nottingham University Press, Nottingham.
Brightling, A. 2006, Livestock Diseases in Australia, C.H. Jerram & Associates, Mt. Waverley Victoria, pp. 388
Broadmeadow, M., Gibson, J.E., Dimmock, C.K., Thomas, R.J. and O'Sullivan, B.M. 1983, ‘The pathogenesis of flystrike in sheep’, Sheep Blowfly and Flystrike in Sheep, (Ed. Raadsma, H.W.), University of NSW, Sydney, NSW Department of Primary Industries, Orange, NSW, pp. 327-332
Brown DJ, Swan AA, Gill JS (2010) Within- and across-flock genetic relationships for breech flystrike resistance indicator traits. Animal Production Science 50(12), 1060-1068.
Joshua, E., Junk, G., Levot, G. 2010, Sheep lice, Primefact 483, NSW Department of Primary Industries, Orange, NSW.
Levot, G. 1999, Life Cycle of the sheep blowfly, Agnote DAI-192, first edition, December, NSW Department of Primary Industries, Orange, NSW.
McLeod, R.S. 1995, ‘Costs of major parasites to the Australian livestock industries’, International Journal of Parasitology, vol. 25, pp. 1363-1367.
Murray, W. and Mortimer, S. 2001, Scoring sheep for fleece rot, Agfact A3.3.41, NSW Department of Primary Industries, Orange, NSW, pp. 4.
Rugg D, Thompson D, Gogolewski RP, Allerton GR, Barrick RA, Eagleson JS (1998) Efficacy of ivermectin in a controlled-release capsule for the control of breech strike in sheep. Australian Veterinary Journal 76(5), 350-354.
Sackett D, Holmes P, Abbott K, Jephcott S, Barber M (2006) Assessing the economic cost of endemic disease on the profitability of Australian beef cattle and sheep producers. Final Report of Project AHW.087. Meat and Livestock Australia, Sydney.
Smith J, Brewer H, Dyall T Heritability and phenotypic correlations for breech strike and breech strike resistance indicators in Merinos. In 'AAABG', 2009, pp. 334-337
Walkden-Brown, S.W., Daly, B.L., Colditz, I.G. and Crook, B.J. 2000, ‘Role of anorexia in mediating effects of blowfly strike on wool’, Asian-Australasian Journal of Animal Science, vol. 13, Supplement July B, pp. 76-79.
Wilkinson, F.C., de Channet, G.C. and Beetson, B.R. 1982, ‘Growth of populations of lice, Damalinia ovis, on sheep and their effects on production and processing performance of wool’, Veterinary Parasitology, vol. 9, pp. 243-252.
