Blowfly strike – approaches to seasonal management in sheep

Sheep blowfly strike (ovine myiasis) is a seasonal problem throughout the UK. The economic costs of strike to the UK sheep industry are considerable, including animal deaths, production losses, hide and wool damage, treatment and control costs, as well as time and labour involved in frequent inspections.

The main species of blowfly responsible in most of the UK is the common green bottlefly, Lucilia sericata (Wall, 2012). However, in the north and west, an increase in the numbers of secondary fly species may be present, predominantly Lucilia caesar and Protophormia terraenovae (Morris and Titchener, 1997).

Blowfly life cycle

The life cycle of all myiasis species is relatively similar. Adult females deposit batches of eggs in the wool. After hatching, first-stage larvae migrate down the wool fibres to the skin, where they feed. The larvae moult twice during their development. When fully fed, mature, third-stage larvae cease feeding and enter a dispersal (wandering) phase, where they drop from the host and migrate across the ground for two or three metres, before pupating in the soil (Figure 1).

After pupation, new adults emerge and resume the cycle. For Lucilia, at sheep skin surface temperatures, eggs hatch after 12 to 24 hours and the larvae feed for about 3 days before wandering. This high rate of development on the host accounts for apparently unaffected sheep seen one day being heavily infested one or two days later, which necessitates daily flock inspection.

Strikes initiation

Most strikes initiate in the anal/perineum area (breech), but strike to the neck, shoulders, back and withers (body) is also important, particularly later in the summer. The larvae from a single oviposition may produce few overt clinical signs and are often difficult to detect (Figure 2).

Significant clinical damage results from the fact, once initiated, an infestation of feeding larvae creates a powerful odour that attracts other gravid female flies, and multiple oviposition can then occur rapidly with high numbers of larvae present. When infested by large numbers of larvae, sheep experience increases in temperature and respiratory rate, accompanied by a loss of weight and appetite, anaemia and chronic ammonia toxaemia, leading to depression followed by death, if left untreated (Figure 3).

In the UK, the first flies emerge from winter diapause and start initiating strikes in southern lowland areas, in March to May each year. The first seasonal strikes occur later in more northerly and upland areas because of their lower temperatures.

Blowfly eggs and early stage feeding larvae require high humidities to hatch and survive and, therefore, desiccate and die in dry conditions. In the soil, wandering larvae and pupae require temperatures greater than 9°C to continue development, while adults require temperatures greater than 11°C to lay eggs (Wall et al, 1992).

This combination of high humidity and temperature requirements drives the seasonal pattern of strike incidence observed each year and, as a result, weather conditions, acting in concert with other predisposing factors, are known to have a major influence on the incidence of strike.

The requirement for high humidity by the larvae means strike incidence is affected by a range of physical attributes, particularly the condition of the fleece and its capacity to retain moisture after rain. In general, sheep breeds with a more open fleece would be expected to have lower humidity at the skin surface, to dry more rapidly and have a lower susceptibility to strike.

Furthermore, shearing is associated with a 95% reduction in the risk of ewe strike by L sericata, as a result of reduced humidity in the fleece of a shorn animal (Broughan and Wall, 2007).

The accumulation of faeces around the tail and anus (faecal soiling) has long been recognised as a primary risk factor for breech strike.

The odour of faecally soiled fleece attracts gravid blowflies to oviposit and this area provides a warm, humid environment for the blowfly larvae to develop. The degree of faecal soiling in the breech region of the lamb is affected by at least two groups of factors: those affecting the consistency and those encouraging its accumulation in the breech region, such as fleece length.

Faecal consistency appears to be the most important cause of faecal soiling; lambs with watery faeces were found to be 8.5 times more likely to be struck than lambs with solid faeces (French and Morgan, 1996). The importance of gastrointestinal nematodes in relation to faecal soiling and its effect on breech strike has been demonstrated (Broughan and Wall, 2007).

An intact tail is also an important risk factor for strike, again through its effect on faecal soiling; strike incidence was been shown by French et al (1994) to be more than five times higher in lambs where the tail had not been removed, compared to docked lambs.

Seasonally, at the start of the summer, relatively low L sericata abundance may be the main factor limiting strike incidence and oviposition will occur only on the most highly susceptible animals, which are those most heavily faecally soiled.

During the summer, as fly abundance increases, ewes are sheared, which reduces their susceptibility. However, for lambs, their wool grows increasingly susceptible and they begin to scour as they ingest increasingly large nematode burdens. Hence, the high fly challenge is focused increasingly on lambs and the incidence of both breech and body strikes increases (Table 1).

As fly abundance and activity are strongly influenced by weather conditions, strike incidence would be expected to be highly sensitive to even relatively small changes in climate (Rose and Wall, 2011). In temperate environments, such as the UK, the range of elevated temperatures predicted by climate change scenarios are likely to result in elongated blowfly seasons, with earlier spring emergence and a higher cumulative incidence of strike leading, ultimately, as is already seen in some parts of Australia, to the year-round presence of blowflies.

Strike risk is also likely to increase for sheep grazed at higher altitudes and latitudes, where lower temperatures offer protection. The combination of warm, wet winter conditions and fully fleeced wet sheep could potentially lead to very significant increases in myiasis incidents (Rose and Wall, 2011).

Strike control

Reductions in sheep susceptibility may be affected by reducing the suitability of the fleece for oviposition and larval survival. At its simplest, this may be achieved by maintaining breeds of sheep with more open, hairy or self-shedding coats.

The reduction in susceptibility to strike may also be brought about by mechanical means. Dagging, the removal of faecally soiled wool, and crutching, the regular shearing of wool from around the breech, may both reduce susceptibility to strike by eliminating suitable oviposition sites. Similarly, strike susceptibility is reduced in ewes following annual shearing. Surgical practices, such as tail docking, will also reduce the incidence of strike.

Strike is controlled primarily through the prophylactic and therapeutic use of neurotoxic insecticides (Bates, 2004). A range of treatments exist to help manage strike problems and characteristics of each treatment need to be carefully matched to the specific husbandry requirements of the farming system (Table 2).

Of increasing importance, however, are the insect growth regulators (IGRs), such as cyromazine and dicyclanil. Other IGRs, such as diflubenzuron, are not available for blowfly control in the UK. IGRs are arthropod-specific compounds that interfere with cuticle formation and chitin synthesis, and disrupt the larval moulting process. On IGR-treated sheep, egg batches are still deposited, eggs hatch and first-stage larvae start to feed, but they die once they reach their first moult.

Hence, while they can provide effective strike prevention, IGRs are not an effective treatment for established strikes. Dicyclanil has in vitro activity against dipteran larvae and can provide substantially longer protection against fly strike than older IGR compounds, such as cyromazine (Schmid et al, 1999) due, in part, to the oily vehicle that binds it to wool grease. A 100% reduction in blowfly strike for up to 16 weeks after application was recorded in the Netherlands after sheep were treated with dicyclanil (Schmid et al, 1999).

Although increasing numbers of farmers use IGRs for strike control (40% in 2003; Bisdorf and Wall, 2008), large numbers still use synthetic pyrethroids (35% in 2003) and a proportion summer dip in organophosphate (12% in 2003), despite increasing public and legislative concern for operator and environmental safety.

For many sheep farmers, spring strikes represent a particular practical problem in both ewes and early season lambs, and many farmers traditionally have preferred to treat early season strikes on a case-by-case basis. This is because many lambs born in January and February will be close to market weight by the time the blowfly season starts in April/May, so any product with a lengthy withholding period will result in unacceptable delays in getting lambs to market.

For ewes, farmers may try to wait until after shearing before applying treatment to benefit from the reduced susceptibility conferred by the shorter fleece. Subsequent protection for ewes post-shearing can be provided by the use of a longer acting product. However, this means ewes and early lambs are particularly vulnerable to fly strike in the spring and early summer and, if no prophylactic treatment is used, a warm spring will leave these animals exposed to high strike challenge.

This is clearly a significant welfare and animal husbandry issue. As a result, products with relatively short periods of activity should be considered for ewes in early spring, during the pre-shearing at-risk period, particularly on farms with a known history of high fly strike incidence. Earlier shearing is also likely to be helpful in a warmer spring.

Strategic, early season treatment may also be particularly important because it reduces the fly population at its lowest point and because this has an effect that persists beyond the residual activity of the treatment, particularly if the entire at-risk population is treated. This is because treatment of lambs and ewes simultaneously removes available oviposition sites, resulting in a substantial reduction in the Lucilia population, lowering fly challenge for a period after the direct protective effect of the insecticide has ended until the fly population can recover.

For later season lambs, the residual activity and withholding period of the treatment selected should be carefully selected to match the anticipated growth rates of the lambs. A late season treatment for ewes may also be required if there is a particularly warm autumn.

To date, no resistance in L sericata populations in the UK to any insecticides has been formally recorded, despite anecdotal farmer reports of maggots appearing to survive direct application of synthetic pyrethroid. It is likely sufficient reservoir populations exist to minimise this risk.

Fly strike is a preventable condition. Good fly strike control depends on understanding the seasonal pattern of strike incidence and the interaction of the predisposing factors: fly abundance, weather (warm and wet), scouring and faecal soiling and fleece length – and thinking ahead.