Blowfly strike
NADIS is a network of 40 veterinary practices and six veterinary colleges monitoring diseases of cattle, sheep and pigs in the UK. |
August 2004
By Neil Sargison BA VetMB DSHP FRCVS
NADIS Sheep Disease Focus
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 | 8 – 10 weeks | 12 days |
| ECOFLEECE SHEEP DIP | High cis cypermethrin | Plunge dip | 8 – 10 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. | ||||
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.
• While every effort is made to ensure that the content of this forecast is accurate at the time of publication, NADIS cannot accept responsibility for errors or omissions. All information is general and will need to be adapted in the light of individual farm circumstances in consultation with your veterinary surgeon.
Copyright © NADIS 2002
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