NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Blowfly strike

Blowfly strike occurs annually in most UK sheep flocks, the incidence being highest in warm and humid areas of south west England. It is estimated that about 12,000 sheep die each year in the UK as a result of flystrike. Flystrike is a major welfare concern and an important cause of ill thrift in affected animals. Furthermore, the disease results in economically significant fleece and hide damage.

BLOWFLY STRIKE

The incidence of Blowfly strike during the first two weeks of august was exceptionally high. Many shepherds commented on the extremely rapid progress of the disease and unusual lesion distribution over the shoulders. The problem was compounded in many cases by wet weather, which made chemical preventive management difficult.

Unlike the situation for sheep scab and lice, blowflies are not obligate parasites, being involved with many environmentally useful tasks such as carcase decomposition. Most of the blowfly lifecycle occurs off the sheep and adult flies may travel large distances between farms.

Blowfly populations are greatest during the summer months, when given favourable conditions of humidity and warmth, the entire life cycle from egg to adult can occur in less than 10 days.

DEBILITATING SKIN DISEASE AND ATTRACTION OF SECONDARY FLIES

First stage larvae penetrate the skin using their hook like mouthparts and secreting enzymes which liquefy and digest the tissue. Larvae are active and voracious, causing further skin and muscle liquefaction as they develop, with secondary bacterial infection and attraction of secondary blowflies. Toxins released by decomposing tissues cause systemic illness which often leads to death.

LIQUEFACTION OF THE SKIN AND UNDERLYING MUSCLE

Affected sheep are usually restless and may bite or kick at the struck area. Affected areas are usually sites of faecal contamination or infected wounds and are, therefore, usually over the hindquarters and perineum or elsewhere on the body at wound sites. About 80% of flystrikes in UK lambs occur on the breech associated with faecal soiling. The incidence of flystrike on the breech and body are similar in ewes. On close examination, the wool overlying struck areas is discoloured, moist and distinctively foul-smelling. During the early stages, the maggots, which are approximately 1.5 cm long, are only visible, end-on, when the wool is parted, but as the disease progresses, the wool falls out to reveal the underlying affected tissue.

Feet affected by footrot and septic wounds, such as shearing cuts or dog bites commonly become flystruck.

Flystrike control

Effective prevention of flystrike remains problematic. Blowflies can travel for several miles, so unlike lice and scab mites, they can not be eradicated from a farm. Furthermore, while modern insecticides are extremely effective, in practice correct application of these drugs to achieve satisfactory residual activity is difficult.

Farm management. The effective control of flystrike is seldom achieved by the use of insecticides alone. Freshly dead animals, faecal material and rotting vegetation provide protein for blowflies, so the prompt disposal in an approved manner of carcasses and attention to general farm hygiene can aid in their

control. Blowflies prefer a warm, moist and sheltered environment, so the risk of strike can be reduced by moving sheep to more exposed pastures. The smell of wool grease and the presence of foot rot, urine soaked wool, skin diseases, scour, or infected cuts attract blowflies to sheep. Established strike lesions attract even more blowflies. Recently shorn sheep are seldom struck and effective control of gastrointestinal parasites and footrot, general animal health care, crutching and trimming around the pizzle can further aid in the control of flystrike.

Overseas, large-scale trapping of blowflies, using an insecticide free trapping system, has been evaluated for possible control of the Australian blowfly, Lucilia cuprina. The results have been encouraging, but it is unlikely that this method will prove useful in the UK, where different species of blowfly predominate.

Plunge dipping. Plunge dipping in diazinon or high-cis cypermethrin can provide protection from blowfly strike for 3 to 8 weeks, depending on the product used.

PLUNGE DIPPING

For optimum control sheep should have at least 3 weeks fleece growth at the time of dipping. Care should be taken to avoid excessive faecal contamination of the dip, which can bind some of the active chemical and even enhance the attraction of dipped sheep to blowflies. To achieve best results, dags should be removed, sheep yarded overnight and feet cleaned by running sheep over slats before dipping. The products are relatively expensive, disposal of spent dip solution problematic and residues in wool toxic to aquatic environments.

Shower dipping. Many contractors now offer shower dipping services. Shower dipping can be used effectively for the control of flystrike, although it is not effective for the control of scab. When using a shower dipper, it is essential that the machinery is checked beforehand and that sheep are dipped for long enough to ensure that the chemical reaches skin level. As a rough guide sheep should be showered for one minute per week off shears. Saturation dipping requires at least 2 - 4 weeks wool growth for the insecticide to bind and faecal contamination of the dip solution must be minimised. None of the plunge dip solutions available in the UK are licensed for use in shower dippers and there is no available information concerning their efficacy or operator safety when applied other than by plunge dipping. Contractors using these products off-license in shower dippers, therefore, have no legal back-up in cases of lack of efficacy or adverse effects.

Hand jetting. Overseas, the use of hand jetting is gaining popularity for the control of flystrike. Dip solution is applied using a high pressure pump and specially designed handpiece to the breech and rump of lambs and also to the back and flanks of adult sheep. Jetting does not require expensive equipment, uses less dip solution than plunge or shower dipping, does not recycle dip so avoids stripping and faecal contamination problems and does not incur the problem of disposal of used dip. It is important to ensure that apparatus is working correctly and that the dip chemical is carefully applied. Alternatively, automatic jetting races can prove useful for blowfly control, although again it is important that they are correctly maintained and used.

AUTOMATIC JETTING RACE

Pour-ons. When applied correctly to potential areas of strike over breech and rump, high-cis cypermethrin or insect growth regulator pour-ons can provide effective control of blowfly strike. High-cis cypermethrin pour-ons provide protection for about 6 weeks and alphacypermethrin for 8 - 10 weeks, while the insect growth regulator pour-ons, cyromazine and dicyclanil, provide protection for 10 and 16 weeks respectively. Pour-on chemicals dissolve in the wool grease and are removed when animals are shorn. In the case of high-cis cypermethrin, this may lead to wool residue problems. Furthermore, the use of pour-ons in adult animals before shearing may be wasteful.

POUR ON TREATMENT

The choice of dip chemical is partly governed by the length of protection required, which varies from as little as 2 weeks for some organophosphate plunge dips up to 16 weeks for the insect growth regulator pour-on, dicyclanil. The choice of application method depends on the product used and facilities available.

Saturation dipping requires at least 2 - 4 weeks wool growth for the insecticide to bind. The procedure is laborious and dips must be regularly replenished to maintain an effective concentration. Hygiene standards need to be high to avoid the breakdown of dip, and because the dipping in a solution of dilute dip and faeces only makes sheep more attractive to flies.

Pour-ons are best applied to short-wooled sheep and can provide effective control of flystrike. No expensive equipment is required and there are no problems of used dip disposal. To avoid wool residue problems, pour-ons and saturation dips should not be applied to crossbred wool less than 60 days before shearing.

PRODUCTS FOR BLOWFLY CONTROL LISTED IN THE 2004 NOAH COMPENDIUM

Product

Chemical

Application method

Approximate protection

Meat withdrawal

COOPERS ECTOFORCE SHEEP DIP

Diazinon¹

Plunge dip

3 - 6 weeks

35 days

OSMONDS GOLD FLEECE SHEEP DIP

Diazinon¹

Plunge dip

3 - 6 weeks

35 days

PARACIDE PLUS

Diazinon¹

Plunge dip

3 - 6 weeks

35 days

ROBUST

High cis cypermethrin

Plunge dip

8 - 10 weeks

18 days

AURIPLAK FLY AND SCAB DIP

High cis cypermethrin

Plunge dip

Up to 8 weeks

12 days

ECOFLEECE SHEEP DIP

High cis cypermethrin

Plunge dip

Up to 8 weeks

12 days

CROVECT

High-cis cypermethrin

Pour-on

6 – 8 weeks

3 days

DYSECT SHEEP POUR-ON

Alphacypermethrin

Pour-on

8-10 weeks

28 days

VETRAZIN POUR-ON

Cyromazine

Pour-on

10 weeks

3 days

CLIK POUR-ON

Dicyclanil

Pour-on

16 weeks

20 days

1. Organophosphates dips and ganglion blocking anthelmintics (levamisole and morantel) should not be used within 14 days of each other.

Insecticide resistance has not been demonstrated in UK blowflies. However, in Australia, New Zealand and South Africa, strains of L. cuprina have demonstrated an ability to develop resistance to organochloride and organophosphate insecticides. L. cuprina is a particularly unpleasant obligate parasite, which unlike the common UK blowfly species, breeds mostly on the bodies of living sheep attracted to the smell of moist fleeces. This life cycle may increase the risk of development of insecticide resistance.

Treatment of flystrike

SHEEP REQUIRE DAILY INSPECTION FOR FLYSTRIKE DURING HIGH RISK PERIODS

Flystruck sheep need to be treated immediately. Struck areas are sensitive to sunburn, so should not be clipped other than to gain access to the wound. A good soaking with an insecticidal organophosphate or high-cis cypermethrin dressing will then kill the maggots and protect the surrounding skin from secondary strike. It may be necessary to massage the dressing into the damaged tissue using a gloved hand. If plunge dip solution is used, it should be diluted to normal dip strength. Weak and debilitated sheep with extensive flystrike wounds require humane euthanasia.

Insect growth regulators (cyromazine and dicyclanil) are ineffective for the treatment of established flystrike.

Neil Sargison BA VetMB DSHP FRCVS