Managing flystrike in sheep

Page last updated: Friday, 13 October 2017 - 12:23pm

Please note: This content may be out of date and is currently under review.

Flystrike is a significant health and welfare risk to Australian sheep and costs $280 million annually. Flystrike should be actively monitored and managed to prevent productivity losses and ensure good animal welfare.

Flystrike impacts the profitability of the enterprise, not only from loss of productivity from the individually struck animals, but also through the increased amount of time and cost of treating and preventing flystrike. Reducing the risk of flystrike has immense benefits to the health and wellbeing of sheep, the people who work with them and business/farm productivity.

There are five types of flystrike; both body and breech strike are seen as the most prevalent and important ahead of poll and pizzle strike. Risk of these types of flystrike will depend on environmental conditions as well as how susceptible sheep may be.

Predicting flystrike

Predicting your risk of flystrike will depend on environmental conditions as well as how susceptible your sheep are. Basic understanding of the Australian blowfly (Lucilia cuprina) and how blowfly larvae develop will also help in predicting flystrike.

Biology of the fly

The main species of blowfly that initiates about 90% of all strikes is the Australian sheep blowfly. It is a copper green colour with reddish eyes. The adult fly is approximately 10 millimetres (mm) long and produces a smooth skinned white maggot. The damaged tissue and body fluid that oozes from the flystrike wound caused by L. cuprina attracts other species of flies. The hairy maggot fly, Chrysomya rufifacies is the most important secondary fly. It does not initiate flystrike, but readily invades flystrike wounds started by L. cuprina. It is blue green in colour, 10mm long and produces the characteristic hairy maggots.

The Australian sheep blowfly. It is a copper green colour with reddish eyes. The adult fly is approximately 10mm long and produces a smooth skinned white maggot.

Adult flies usually live for approximately two to three weeks. Eggs generally hatch into larvae in 12-24 hours and larvae grow from pin head size to 10-15mm in length within about three days. They then drop off the sheep — usually at night or in the early morning when ground temperatures are coolest — and burrow into the soil to commence pupation a day or two later. This means that a large proportion will subsequently emerge as blowflies from around sheep camps.

Adult flies usually live for approximately two to three weeks with eggs generally hatching into larvae in 12-24 hours. The life cycle takes 17 days to maturity in warm weather.

Adult flies will normally not travel more than three kilometres from where they hatch during their life span. After hatching, the female fly needs a feed of protein for her reproductive organs to mature. She needs a further feed of protein before egg laying. Common sources of protein are carcases, manure and existing strikes.

What are the perfect environmental conditions for flystrike?

  • The presence of primary species (most commonly the Australian sheep blowfly).
  • Temperatures must be right (between 15–38 degrees).
  • Recent rain — enough to keep suitable sites on the sheep moist for about three days.
  • There must be suitable sites (wrinkles, urine, faeces) on the sheep to attract flies and sustain larvae.
  • Wind speeds below 9 kilometres per hour (km/h) as this gives flies the best opportunity to disperse.

Figure 1 below shows the relationship between the abundance of L. cuprina and strikes, and shows that even low numbers can cause significant strikes. The strike incidence at Mount Barker Research Station in September 1978 was 62% of the total for the year — yet the number of L. cuprina was extremely low. Blowfly traps caught only one to two blowflies per trap in an eight hour trapping period. This indicates that the presence of any L. cuprina flies in traps is ample warning that there are enough flies to cause a serious strike problem if all other conditions for strike are ideal.

The majority of blowflies are seen over spring and summer (16 flies trapped in summer compared to 7 at the end of autumn).
Figure 1
Studies have shown that L. cuprina are relatively inactive below 15°C and most active between 26°C and 38°C. The longer the temperature remains above 15°C the greater the chance of egg laying and dispersal. Wind speeds above 9km/h will reduce fight activity. They do not fly at all when the wind speed exceeds 30km/h.

How susceptible are your sheep?

Susceptibly depends on environmental conditions as well as sheep type and management strategies.

Best case scenario

  • plain bodied sheep with low incidence of fleece rot and body strike
  • hoggets have good worm control and rarely scour in spring
  • lambs have low wrinkle level on their breech
  • flocks have shorter (less than four months) wool during fly risk times in spring and autumn
  • paddock monitoring undertaken every two days during times of high risk and effective flystrike treatment on-hand.

Worst case scenario

  • highly wrinkled Merino sheep
  • hoggets are daggy and with poor worm control
  • flocks have long wool and are uncrutched/unshorn over high risk times
  • sheep are rarely monitored and left to fend for themselves.

There are many options that will help you reduce your risk of flystrike. Genetic options are long term and permanent, making them a valuable tool in lowering your risk. In the short term, a range of husbandry options are also available.

Preventing flystrike - management

Preventing flystrike is more economical than treating it. Short term management tools such as strategic chemical application, crutching and shearing, and controlling worms and dags will be beneficial in the short term, whilst producers looking for a longer term solution should investigate genetic options such as breeding for reduced dags and wrinkle.

Controlling worms and dags

Scouring and therefore dag formation is usually caused by sheep worms. Follow a plan to minimise worm burdens and resulting scouring. Use worm egg counts (WEC) to check whether worm numbers are reaching significant levels while sheep are on green feed.

A dag score 4 animal can be up to seven times more susceptible to breech strike than a dag score 1 animal in the same mob. Breech cover is also an important determinant of breech strike in winter rainfall areas and its importance increases with increasing dag and wrinkle score.

Dag score 1 indicates no dags, ranging to a dag score 5 animal which is extremely daggy down to its feet.

WECs will quickly determine whether scouring is due to large worm burdens so that drenching will quickly control it, or if it is due to a hypersensitivity to worm larvae where few worms are present and treatment will have little effect. (This often occurs in mature sheep at the first contact of the season with large numbers of worm larvae and scouring results from an intense immune reaction which removes the larvae.) Visit the WormBoss website for regional recommendations. Identifying the most heavily scouring animals and removing them from the flock is a positive start.

Crutching and shearing

Shearing and crutching are key management tools for controlling flystrike. Shearing and crutching can give up to six weeks protection from breech strike. If sheep are scouring, this protection can be reduced to three weeks.

In a non-mulesed flock the timing of crutching or shearing becomes even more important. Shearing or crutching time should be planned to coincide with the start, or just before the usual start, of the fly season — keeping in mind withholding periods and protection periods of chemical preventatives.

The main crutching (autumn for spring shearing; spring for autumn shearing) is a key time for tidying up sheep. Time crutching for just prior to the expected fly wave period and before the sheep become excessively dirty. With unmulesed lambs it is important to crutch prior to significant dags forming so that the operation can be carried out safely and easily. Crutching can be over the board or on crutching cradle, however, if sheep are excessively dirty and unmulesed it is suggested that over the board may be the preferable method.

If shearing in autumn then all sheep should be crutched in late winter; a couple of weeks before lambing is due. A pre-lambing crutching coincides with removing wool from around the udder to reduce the risk of udder strike and makes suckling easier. Consider a pre-shearing crutch to clean up any animals, particularly if shearing is at a time of high risk. This may catch any full-woolled animals that are at risk.

Correct tail length

Lambs should be docked at the third palpable joint or to the tip of the vulva in ewes and to the same length in wethers. Docking at the correct tail length has a significant benefit in reducing stain around the breech area, as well as other advantages, such as reducing cancers, particularly in lambing ewes. Docking at the correct length can reduce dag formation.

Tails should be docked to the third palpable joint

The rate of rectal prolapses increases when sheep are docked very short or 'butt tailed'. This is thought to be because of the impact tailing has on the surrounding muscles. Some studies have demonstrated that tail docking at shorter lengths reduces the ability of sheep to 'twitch' their tails which may in turn reduce the effectiveness of deterring flies.

The Te Pari Patesco knife is a relatively new gas knife that sears and removes the tail and stretches the woolly skin producing a bare area on the top of the tail. It works in a similar fashion to normal gas knives, except for a rotating anvil system that extends the skin on the woolly side of the tail before cutting, which results in more bare skin on the dorsal tail surface and tip of the tail. This leaves a greater bare area, where wool would normally grow and may possibly reduce urine stain and dags, and thus flystrike.

Mulesing

Mulesing may not be needed on properties that are in a low risk area and on properties where producers actively select and breed for fly and worm resistance.

It is intended that the Australian Animal Welfare Standards and Guidelines for Sheep will be regulated in Western Australia in the future. Under these Standards and Guidelines, the following standards apply:

  • A person performing mulesing must have the relevant knowledge, experience and skills, or be under the direct supervision of a person who has the relevant knowledge, experience and skills.
  • A person must not mules sheep that are less than 24 hours old or more than 12 months old.
  • A person must not mules sheep that are 6–12 months old without using appropriate pain relief.
  • A person must not mules sheep showing signs of debilitating disease, weakness or illthrift.
  • A person mulesing sheep must only remove wool-bearing skin.

For further information on this, please see our page on best practice marking of lambs.

Strategic chemical application

Blowfly prevention chemicals should be used to manage the risk of blowfly strike when environmental conditions are conducive to fly development and sheep are likely to be attractive to flies. Effective chemical use in the prevention or treatment of flystrike depends on understanding the biology of the blowfly, appropriate choice of chemical and correct application.

All registered products used to treat blowflies in a mob scenario (flock treatment) belong to one of three groups:

  • insect growth regulators (IGR) affect growth of immature stages by preventing external skeleton formation, does not kill adults and takes a couple of days to kill maggots, for example cyromazine, dicyclinil etc
  • macrocyclic lactones (ML) — affect the nervous system of insects (kill on contact) for example ivermectin
  • spinosyns — affect the nervous system of insects (kill on contact) for example Extinosad.

Applying chemicals

Of primary importance when applying chemicals is the need to ensure that the correct dosage for weight of the animal is applied by calculating the dose according to the heaviest animals in the mob.

It is important to consider wool withholding periods for shearing and meat and export slaughter intervals when applying a chemical.

The period of protection will depend on what chemical product is used. For more information on specific chemicals please download the chemical table for flystrike control document in the side menu.

Chemical treatment at marking

Applying a chemical application to the breech of lambs at marking may be beneficial in reducing breech strike incidence when wound healing from marking is taking place and through until weaning or first shearing.

Diazinon (OP) is no longer registered for flock treatment but can still be used for individual animal treatment of struck sheep. (Be aware that widespread resistance has been reported in blowflies.)  Extinosad® gives a limited period of protection on lambs as it relies on lanolin for binding and retention. As lambs have less lanolin compared to adults, binding is reduced. Breakdown of applied chemical is also aided by the open hairy nature of lamb fleece as it allows greater exposure of chemical to sunlight resulting in quicker breakdown.

When mulesing with pain relief consider using a preventative fly chemical that has a low volume dose required to avoid or minimise any wash-off or dilution effect that might occur when it is applied over the pain relief chemical.

Chemical treatments for flies can be applied using jetting equipment, hand-wand or by a spray-on method. Traditionally, either an automatic jetting race (AJR) or hand-jetting have been the more commonly used methods of application of preventative fly treatments. However, some IGR products are available as a spray-on application such as Vetrazin Spray-on® and Clik®.

Occupational health and safety

It is important that the product safety label is read and followed. You should also obtain the Safety Data Sheet (SDS, previously called a Material Safety Data Sheet) from your reseller for all chemicals that you use. Due to the likelihood of spray and splash occurring when jetting, wear waterproof long pants, steel capped gumboots and long sleeved waterproof gauntlets. Thin, inexpensive cotton gloves worn inside the gauntlets make the gauntlets easier to put on and take off.

When using concentrates to prepare the jetting fluid, wear a respirator and face shield for protection from fumes and splash. After use, this equipment should be washed, dried and stored safely. Soap, water and a towel should be available to wash off pesticide splashes, as well as clean clothes to change into at the end of jetting or during jetting if contamination occurs.

Withholding periods

Withholding periods require consideration when choosing a chemical and these include the wool and meat withholding periods and the export slaughter interval. The wool withholding period is the time between application of the chemical and when wool is harvested from the animal. The meat withholding period is the time between application of the chemical and slaughter for meat products. The export slaughter interval (ESI) is the period that must elapse between chemical application to livestock and their slaughter for export.

Do not misuse chemicals

Misuse of chemicals can have serious consequences. An unregistered chemical may be ineffective with their use encouraging the resistance development, causing residues and adverse side effects on livestock. Chemicals should only be applied according to label instructions. Using a firefighter to apply chemical is an example of misuse of chemical. With such an application there is little control of how much chemical is applied to individual animals.  Some animals may receive insufficient chemical resulting in ineffective treatment, while others receive substantially more chemical that will result in wool residues present at wool harvesting.

Blowfly chemical table

Choice of chemical will depend on whether treatment or prevention is required, withholding period restrictions, protective period, cost and compatibility with pain relief chemical applications where these are required.

The blowfly chemical table document, in the side menu, outlines details of the current registered products for mob treatment that have no reported resistance to blowfly.

Preventing flystrike using genetics

Breeding for resistance to breech strike offers the best long-term permanent solution. Selecting the best animals for your breeding flock requires selection of both superior rams and ewes using traits that are easily and accurately measured and heritable. Research shows that certain traits are associated with breech strike and these can be used to successfully breed for breech strike resistance. These are called indicator traits.

The traits that have been shown to be related to breech strike in winter rainfall environments are:

  • scouring and dags
  • breech wrinkle
  • breech cover (or alternatively bare area).

Low wrinkle, dags and breech cover are important as individual traits by themselves but are increasingly valuable where two or more of these traits are present on the same animal. The interaction between traits can also impact on the susceptibility of sheep to flystrike.

Genetic selection can be done on the lamb flock, the existing ewe flock and by choosing a ram source that uses genetic selection against flystrike as a breeding objective.

Contact information

Julia Smith
+61 (0)8 9892 8450