Peter James, Queensland Alliance for Agriculture and Food Innovation (QAAFI), University of Queensland

Posted Flock & Herd March 2011

Impact of lice

The most recent estimate of the cost of lice (Bovicola ovis) to the Australian sheep industry was $123m p.a. of which $84m was due to control costs and $39m to production losses (Sackett et al 2006). By zone this equated to $1.04 per head in the high rainfall zone, $1.05 in the cereal sheep zone, $1.44 in the pastoral zone and $0.89 in prime lamb flocks. These values are based on estimated prevalence of 13% in the high rainfall and cereal sheep zones, 21% in the pastoral zone and 20% in prime lamb flocks. Clearly, if the prevalence of lice increases so does the industry cost of lice.

Prevalence of infestation

Long term, the nation-wide average prevalence of lice infestation appears to be reasonably steady at about 20% (James 2002). The most recent national survey of lice prevalence in 2004 covering more than 1300 growers nation-wide also suggested a national prevalence of 20% (Walkden Brown et al. 2006). This was variable across regions, ranging from 10% in the central and southern tablelands of NSW to 41% on the Darling Downs and Granite Belt in Qld. However, in recent memory there have been two periods where the prevalence of lice has surged to levels much higher than this. Surveys show a trend of gradually increasing prevalence during the 1980's which appeared to peak in the early 1990's, estimated above 50% in a number of States (see James 2002). After this time, through the late 1990's and into the next decade, louse prevalence fell to close to long term averages in most States (Plant and Dawson 1999, James and Riley 2004, Walkden-Brown et al. 2006). Grower experience and industry anecdote indicates another upsurge in 2009/10. Estimates of prevalence as high as 70% were made for WA (J. Cotter pers. com.) and NSW (E. Joshua pers. com.) and similar increases in prevalence, although probably not to quite the same levels were noted in South Australia (K. Hebberman pers. com.), Victoria (N. Campbell pers. com.) and Tasmania (B. Horton pers. com.).

Factors influencing prevalence

Identifying the reasons for this latest increase in prevalence is important to preventing similar problems in the future. Some possible reasons include:

Chemical resistance: Resistance has developed in Australian sheep lice populations to two groups of insecticides, synthetic pyrethroids (SPs) (Levot 1995) and more recently the insect growth regulators (IGRs), diflubenzuron and triflumuron (James et al. 2008, Levot and Sales 2008). Reduced susceptibility to organophosphates (OPs) was also reported in one louse population, but this appears to be rare (Levot 1994). Backline treatments are particularly prone to the development of resistance, especially when poorly applied, because of the mode of spread of chemical over the sheep which leaves gradients in the fleece.

From 1981 until 1993, all backline products on the market contained SPs. Since 1993 IGR backliners have commanded the major share of the louse control market, estimated in 2004 as 93% of all backline treatments (Walkden Brown et al. 2006). The first upsurge in lice coincided with the development and spread of resistance to the SP backliners. SP backliners were released onto the market in 1981. Anecdotal reports of poor effect from SP backliners were received as early as 1984 and increased after this with resistance confirmed in the late 1980's (Levot 1995). The level and prevalence of resistance in louse populations appears to have increased significantly after this time (see James 2002). Resistance factors in early reports were generally less than 25X but high level resistance (90x-642x) was subsequently reported in NSW, SA and Vic.  In 1988/9 in WA, 20% of isolates of lice collected and tested for susceptibility by in vitro methods gave results indicating resistance. In 1990/91 in SA 34% of market inspection samples and up to 68% of samples from flocks on Kangaroo Island showed resistance. In 1992/93 in WA 75% of randomly selected isolates of lice survived supervised treatment with a cypermethrin backline treatment and the prevalence of resistance was as high as 90% in samples collected from the western regions of Queensland in the late 1990s. Following the introduction of IGR backliners in 1993 and their subsequent increase in market share, lice prevalence appeared to drop back to more customary levels.

The second upsurge in lice appears to coincide with the development of resistance to IGR backliners. Concerns of reduced effectiveness of IGR backliners began to emerge in the early 2000s and pen trials suggested reduced louse susceptibility in at least some instances. Definitive evidence measured in assays based on both nymph moulting (James et al. 2008) and egg hatch success (Levot and Sales 2008) was subsequently reported. Strong cross resistance was indicated between triflumuron and diflubenzuron indicating that switching between products containing these two chemicals was not likely to improve effect. Testing by the nymph moulting assay of two strains where resistance had been validated in pen trials indicated resistance factors of between 67x and 94x (James et al. 2008). Unfortunately testing for IGR resistance is laborious, requires a high degree of expertise to conduct and methods are difficult to use for rapid screening. Although resistance has been documented there have been no surveys to assess the current prevalence of IGR resistance (anecdotally there are some areas where IGR backliners remain completely effective) and the extent to which resistance contributes to lice to control breakdowns is difficult to quantify.

Failure of a lice treatment to eradicate does two things. Firstly it contributes to ongoing infestation with associated management difficulties and production loss on individual properties. However it also increases the industry prevalence of lice. This means that the proportion of stray sheep and purchased sheep that are carrying lice is higher and the probability that a previously clean property becomes infested from these sources also increases.

Wool prices: It is often suggested that sheep owners decrease their spending on lice control and perhaps direct less attention to lice management during periods of poor wool returns. Morecombe et al. (1994) found a statistically significant association between wool prices and lice prevalence with up to 1.3% increase in prevalence over 21 months for every $1.00 kg-1 decrease in the wool market indicator. The magnitude of this association suggests that although the poor wool prices experienced in recent years may have contributed to increased louse prevalence, the effect is unlikely to be large.

Drought conditions: Lice populations generally increase more rapidly on animals with poor nutrition or under stress and more rapid build up and spread of lice during the recent widespread drought conditions could also have contributed to increased louse prevalence.

Suspension of diazinon: Concerns about health risks from occupational exposure of shearers, wool handlers and farm workers led to the suspension of diazinon-based products for these uses from May 2009. Diazinon was cheap and effective and the most widely used chemical for dipping, showering and jetting. Its suspension for these uses may also have contributed to increased louse prevalence. (Diazinon is still available as an offshears spray on product and under permit for use in cage dips by licensed operators).

Lice on other breeds: Higher lamb and mutton prices in recent years have increased use of 'exotic' breeds such as Dorpers and Damaras, particularly as terminal sires in Merino flocks, and the number of meat breeds and crossbred

sheep generally. This has raised concern that these animals could contribute to the spread of lice. Shedding breeds are often represented as being 'resistant', but as noted by Fourie and Horak (2000) they can carry low numbers of all three species of sheep lice. As these breeds are often not shorn and lice are likely to cause little economic effect on them, they are commonly not treated for lice. It is clear that they could provide a source of new infestations.

There appear to have been no tests of the efficacy of louse control products on shedding breeds and recommendations for the best means of control are uncertain. Most labels for backline and dipping treatments specify use within a certain period from shearing. If sheep are not shorn, according to the label instructions, a post-shearing treatment should not be used. Long wool treatments do not claim to eradicate lice. Even if the sheep are shorn and treated, because of differences in skin structure, the amount of lanolin and other potential differences, efficacy cannot be guarantied. Most treatments are not ovicidal, and as there is less wool and lanolin to hold the chemical, a second treatment 10 – 20 days later to kill nymphs that later hatch from eggs may be necessary to ensure eradication with some products.

Integrated control

Lice were controlled through much of the last century predominantly by the routine application of louse treatments after shearing and a relatively high regulatory input. Although regulatory programs are still conducted in some States and routine annual treatments are still carried out on many properties, recent years have seen a reduction in regulatory inputs by State authorities and a general movement away from routine annual treatment in favour of more integrated approaches. In 2004 only 41 % of growers reported that they treated annually (Range 26% and 28% in New England and Central and Southern Tablelands of NSW to 78% and 81% in Granite Belt/Darling Downs and Southern Queensland, respectively) (Walkden-Brown et al. 2006)

The key elements of integrated programs are:

• Prevention of new infestations

• Monitoring and detection for the presence of lice

• Strategic use of chemicals to eradicate or control infestations when they occur.

The detail of these elements is well described on the LiceBoss website and on various State Government and Veterinary chemical company websites. This paper discusses some areas where new information and tools are available

Preventing new infestations

Preventing new infestations can be one of the more difficult aspects of good lice control and this is particularly so during times of high lice prevalence. New infestations come from two main sources, from purchased or other introduced sheep and from strays. As already noted, the chance of introducing lice, either with purchases or strays, is heavily influenced by the industry or district prevalence of lice. For example, if the prevalence of lice is 20%, on average the chance of strays or purchases having lice is about one in five. If on the other hand the prevalence is above 50%, there is a better than even money chance that when new sheep come onto the property they will have lice.

Although strays continue to be a problem for growers, the procedures for preventing the introduction of lice are clear cut – sheep proof fences and agreed practices with neighbours for the return of stray sheep.

With purchases the situation is more difficult, particularly when sheep are purchased through saleyards. Often the purchaser has little knowledge the lice history of the property of origin, and often sheep are purchased relatively soon after shearing. Finding lice on recently shorn sheep is difficult, particularly in the early stages of an infestation, because thorough inspection is more difficult and signs of rubbing will not be evident. If sheep have been treated, but the treatment not completely effective because of poor application or resistance, only low numbers of lice will be present and residual chemical will suppress the rate of build up. In this situation it can take many months for the infestation to become obvious.

Probably the best course of action is to quarantine purchased animals from other sheep on the property for as long as possible so that lice are not spread if the sheep prove to be carrying lice. Some growers choose to treat all animals that come onto the property. This can be successful if sheep can be shorn and treated, but if a long wool treatment must be used, although treatment reduces the risk, long wool treatments do not guarantee to eradicate lice. Knowledge of the lice history of the property of origin is the best option. Properties that have not had lice for some years and which have not treated are probably the safest source of sheep from a louse control perspective.

Lice can also be transmitted on the clothing or footwear of shearers or other sheep handlers moving from an infested property to a clean property (Crawford et al. 2001). While this is unlikely to be a major cause of new infestations, if it is known that shearers or other workers have recently had contact with sheep on an infested property, precautions should be taken to guard against inadvertent lice transmission.

Although not strictly new infestations, many infestations also result from lice carried over from previous infestations, for example from incomplete musters or split shearings, or from failure to eradicate. Because lice spread very slowly in the early stages of an infestation it can take many months from initial infestation before lice become obvious. For example in a recent study in the pastoral zone of western Queensland, where one sheep in a spring-shorn mob was infested 6 weeks after shearing and subject to very high levels of solar radiation through summer, we found that lice persisted in the mob through a full year at levels that would have been unlikely to be detected on most sheep properties.

Detection and monitoring

Lice infestations generally commence from one or a few sheep in the mob. The infestation then increases by build up in numbers of lice on individual sheep and spread between sheep. Detection of lice in a mob requires two things, firstly an infested sheep must be included in the sample for inspection or testing and secondly, lice must be found on the infested sheep. Clearly, if the infested sheep is not included in the sample tested or inspected lice will not be found. For this reason, the sensitivity of live sheep inspections for detecting early infestations is low, even when relatively high numbers of sheep per mob and parts per sheep are inspected (James et al. 2002). The new lice detection test (LDT) (P. Young and N. Sales pers. com.) , now available from NSW I&I uses antibody technology to detect lice antigens in wool grease and debris collected from combs and cutters. It can be highly sensitive because it theoretically allows for all woolled sites on all sheep in a mob to be sampled and can overcome the sampling difficulties noted above. Unfortunately, it is also expensive ($0.52) per head, more than the cost of some louse backliners. For this reason growers will probably not want to test all sheep and the most economical option may be to test those mobs thought to be most at risk (for example, mobs with rubbing sheep but where lice could not be found by visual inspection, recent purchases or sheep in paddocks where strays have been seen).

However, the LDT is not applicable to sheep that are not going to be shorn (for example purchases, strays and suspected mid-season infestations). In these situations fleece derangement can be a relatively early indicator of new infestations and correlates well with lice numbers present during development of an infestation. In studies with artificial infestations, some sheep exhibited deranged fleece as early as 5 weeks after initial infestation with 5 lice (James et al. 2007). However, because many things can cause deranged fleece, even when rubbing sheep are present confirmation by finding lice is still necessary. As some sheep start rubbing and wool-biting with relatively low numbers of lice, a relatively detailed inspection (20 wool parts or more) may be necessary to find lice even when they are the cause of rubbing.

Strategic use of chemicals

The third element of integrated approaches for sheep louse control is the strategic use of chemicals. As there is an abundant literature and web material available on proper application of backliners and good shower and plunge dipping practice, this section focuses on resistance management and some areas where relatively new tools are available.

Resistance management

Backline products command by far the major section of the louse control market because of their ease of application and significant management advantages. It would appear that the continued availability of effective backline products and the management advantages that they offer is heavily dependent on the prevention of further resistance.

Management of resistance in sheep lice should theoretically be easier and have a greater likelihood of success than with many other parasites. This is for two reasons. First registration of post shearing treatments, including backliners, requires that they must be able to reduce susceptible lice to non detectable levels if applied according to label instructions. Even if a resistant genotype of lice emerges, eradication should be possible by use of a different chemical group following next shearing. Second, lice have no off host stage. If all sheep are treated effectively the population should be eradicated. This is in contrast to other parasites such as gastrointestinal parasites and sheep blowflies where complete eradication from a property is not expected and where there is likely to be continual re-challenge from the off-host stages.

As noted earlier, the development of resistance has been associated with heavy reliance on products from one chemical group over an extended period time – first the SPs and later the IGRs. However we presently have sheep lousicides available that contain chemical actives from a wide range of chemical groups. In particular, we have backline or spray-on products from five different groups (SP, IGR, OP, spinosyns and neonicotinoids). This provides unprecedented possibility for the development of robust resistance management strategies, even without the necessity of recourse to dipping or showering. The promotion of regular rotation of chemical groups for post shearing treatments and the design of resistance management strategies which take account of long wool and flystrike treatments should be a priority. Clearly, careful application according to label instructions will be a critical element in such programs.

A relatively new tool that can help with the design of resistance management programs is the 'Products' module in LiceBoss and FlyBoss ( The Products module enables the sorting and selection of lice and flystrike control products according to chemical group, simplifying the design of effective resistance management strategies.

Treatment of pregnant ewes or ewes with lambs at foot

An area of chemical treatment where problems often arise and where providing advice can be difficult, both for post shearing and long wool situations, is in the treatment of ewes with lambs at foot or ewes soon to lamb. The difficulty here is that lambs, particularly young lambs, will probably not be shorn at the same time as ewes, treatments suitable for ewes may not be suitable for lambs, or the ewes may lamb before a louse treatment has killed all lice allowing transmission to the new lambs and establishment of a carry over infestation. There are now suitable chemicals and strategies that can be used in most situations. However, because of the number of different possible scenarios choosing the best option to suit a particular set of circumstances can be difficult. The Ewe/lamb module in LiceBoss is a simple decision tree model that guides the user through a number of questions and can greatly assist the choice of the best treatment option.

Long wool treatments

Long wool treatments are expensive, can lead to high chemical residues in the wool and can contribute to resistance development. Furthermore, they do not claim to eradicate lice and so all sheep will need to be treated again after their next shearing. Therefore optimising the effect of long wool treatments and avoiding their unnecessary use has significant advantages for growers. It is theoretically possible to extend the concept of economic thresholds, a central tenet of many IPM programs, to the use of long wool treatments. Simply stated, the economic threshold is the point at which a control should be applied to prevent the cost of losses due to lice from exceeding the cost of the treatment. Therefore the question for growers when confronted with a midseason infestation is whether they will be better to apply a long wool treatment or to withhold treatment until after next shearing when eradication can be achieved. The Long Wool decision support module in LiceBoss can assist with this decision. This module estimates the current level of infestation from the proportion of sheep in the mob rubbing (as estimated by the user). It then predicts the rate of build up in lice and wool loss at next shearing if a long wool treatment is not applied, and compares the cost of possible treatments with the predicted loss if the mob is not treated. As different products have different wool withhold periods it also provides information on which products can legally (according to label directions) be used at different times before shearing. This module can greatly assist growers or consultants providing advice to determine the most cost effective means of dealing with a midseason infestation.

Selling wool into low residue markets

Many growers are now seeking to access markets that specify low levels of chemical residues. The Wool Res model (Campbell and Horton 2002), now available on the LiceBoss and FlyBoss websites, provides an interactive tool to assist growers design lice and fly control programs to meet particular wool residue requirements (for example EU Ecolabel). Users enter information including product, method of application, location and time to shearing and the WoolRes model provides predicted residue levels at shearing and a probability that a particular course of action will meet different residue requirements.

LiceBoss :

LiceBoss provides an overview of the main features of a good integrated control program for sheep lice and access to a comprehensive suite of detailed information sheets covering most aspects of sheep lice biology and control. LiceBoss also provides access to two other interactive decision support systems (DSS) in addition to those noted above that can help audit louse control practices and which can be particularly useful in agricultural education or training programs. The Short Wool DSS can help identify where a control breakdown may have occurred or to determine possible areas of improvement as well as to assist in deciding whether or not to treat for lice after shearing. The Treaments DSS helps check backline, plunge dipping or shower dipping technique to help identify possible deficiencies or improvements. LiceBoss can be accessed at


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