NADIS is a network of 40 veterinary practices and six veterinary colleges monitoring diseases of cattle, sheep and pigs in the UK.
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.
Fly Control in sheep
Blowfly strike is a major welfare concern and an important cause of ill thrift and death in affected animals. Last year, the first cases were reported throughout the UK at the beginning of May, and the incidence remained high during the summer. Problems were compounded by wet weather, which made chemical preventive management difficult. There have been no NADIS reports of flystrike so far this year (22nd May 2005), but this situation will change rapidly following the past few days’ warmer and wetter weather.
SEVERAL SHEPHERDS COMMENTED DURING AUGUST 2005 ON THE EXTREMELY RAPID PROGRESS OF BLOWFLY STRIKE AND ON AN UNUSUAL LESION DISTRIBUTION OVER THE SHOULDERS OF LAMBS
In the UK, blowfly strike results in the opportunistic invasion of living tissues by maggots of Lucilia sericata, Phormia terrae-novae and Calliphora erythrocephala flies. Unlike the situation for sheep scab and lice, blowflies are not obligate parasites, being involved with many environmentally useful tasks. Most of the blowfly lifecycle occurs off the sheep and adult flies may travel large distances between farms. When given favourable conditions of humidity and warmth, the entire life cycle from egg to adult can occur in less than 10 days.
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. Eggs hatch within 24 hours and 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 with secondary bacterial infection as they develop. Maggot development can take as little as 5 days before they fall off and burrow into the soil to form pupae. Struck areas often attract other blowflies and further waves of strike.
On close examination, the wool overlying struck areas is discoloured, moist and 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. Unless promptly recognised and treated, tissue degradation products and maggot secretions can result in toxaemia and death.
ABOUT 80% OF FLYSTRIKES IN UK LAMBS OCCUR ON THE BREECH. THE INCIDENCE OF FLYSTRIKE ON THE BREECH AND BODY ARE SIMILAR IN EWES
THE WOOL OVERLYING STRUCK AREAS IS DISCOLOURED, MOIST AND HAS A DISTINCTIVE SMELL
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
Despite a good understanding of blowfly biology, the effective prevention of flystrike remains problematic and is seldom achieved by the use of insecticides alone. Blowflies can travel for several miles, so unlike lice and scab mites, they cannot 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. 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, while the choice of application method depends on the product used and facilities available.
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.
Plunge dipping in diazinon or high-cis cypermethrin can provide protection from blowfly strike for 3 to 8 weeks. However, effective plunge dipping is a complex and precise procedure and failure to control blowfly strike is commonly associated with poor dipping practice rather than inefficacy of the chemical used.
Sheep should have at least 3 weeks’ fleece growth for the insecticide to bind. The volume of the dipper should be worked out and the correct dilution rate must be accurately calculated. When sheep are dipped in organophosphate solutions, they do not simply remove dip solution from the bath, they also absorb emulsified fat soluble dip particles from the solution into their wool grease. It is therefore necessary to replenish the dip bath according to the manufacturer’s instructions, using a higher chemical concentration than that used for the initial charge.
Care should be taken to avoid excessive faecal contamination of the dip, which can bind some of the active chemical. To achieve best results, dags should be removed, sheep yarded overnight and feet cleaned by running sheep over slats before dipping.
HYGIENE STANDARDS NEED TO BE HIGH TO AVOID THE BREAKDOWN OF DIP, AND BECAUSE DIPPING IN A SOLUTION OF DILUTE CHEMICAL AND FAECES ONLY MAKES SHEEP MORE ATTRACTIVE TO FLIES
Dips are hazardous substances which can be absorbed through the skin, orally or by inhalation of vapour or aerosols. A certificate of competence from the National Proficiency Test Council is required for the purchase of both organophosphate and pyrethroid dips and personal protective equipment must always be worn when dipping sheep. Disposal of dip solution left in the bottom of the dipper at the end of a session can prove problematic. Permission from the relevant local environmental agency is required before it can be spread on pasture and extreme caution taken to ensure that it cannot enter a watercourse.
None of the plunge dip solutions available in the UK is 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. However, many contractors now offer shower dipping services. 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.
Automatic jetting races.
Automatic jetting races can prove useful for blowfly control, although it is important that they are correctly maintained and used. Jetting races don’t recycle dip, so avoid stripping and faecal contamination problems and don’t incur some of the problem associated with disposal of used dip. Contractors using plunge dips off-license in automatic jetting races are unlikely to be supported in cases of lack of efficacy or adverse effects.
When applied correctly to potential areas of strike over the back and breech, pyrethroid 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.
THE USE OF POUR-ONS SHOULD BE AVOIDED IN LONG-WOOL SHEEP DUE TO THE RISK OF UNDERDOSING AND SELECTION FOR RESISTANCE
PRODUCTS FOR BLOWFLY CONTROL LISTED IN THE 2004 NATIONAL OFFICE OF ANIMAL HEALTH 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 (IGR2) Pour-on 10 weeks 3 days
CLIK POUR-ON Dicyclanil (IGR2) Pour-on 16 weeks 20 days
1. Organophosphates dips and ganglion blocking anthelmintics should not be used within 14 days of each other.
2. Insect growth regulator
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.
Headflies (Hydrotea irritans) gather around the heads of sheep in large swarms and cause considerable annoyance. The flies feed on wounds, for example around the base of horns of young growing sheep, and can result in the loss of large areas of skin. Headflies are active from June to September, especially given still, moist and warm weather conditions and nearby woodland.
Synthetic pyrethroid plunge dips only afford limited protection against head flies. Prevention is best achieved using a high-cis cypermethrin or alphacypermethrin pour-on, which may need to be repeated at 3 or 6-weekly intervals respectively during high risk periods. Severely affected sheep may need antibiotic treatment in conjunction with protection of the wounds with Stockholm tar or insecticidal cream.
PRODUCTS FOR HEADFLY CONTROL LISTED IN THE 2004 NATIONAL OFFICE OF ANIMAL HEALTH COMPENDIUM
Product Chemical Application method Protection Meat withdrawal
SPOT ON INSECTICIDE Deltamethrin Pour on5 ml to head 2 weeks 7 days
CROVECT High-cis cypermethrin Pour on5 ml to head ~3 weeks 3 days
DYSECT Alphacypermethrin Pour on5 ml to head 6 weeks 28 days
Your vet can provide advice about the management of flystrike and headfly in your flock.
Copyright NADIS 2005
• 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.
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