Fly Strike is the thing a smallholder fears the most when they have sheep
Some people (usually your vet) will tell you that sheep are born to die, and sometimes I think its true.
Its summer, its August, and the lambs are growing fast. At 5 months old they are now weaned from their mums, and eating grass like there’s no tomorrow. The ewes and tups have all been sheared and there is enough fresh grass to go around. All the hard work has been done now and its time to relax. Take a holiday, or have BBQ’s in the garden.
Relax at your peril. Warm damp weather is ideal conditions for flies, blow flies in particular. They are lovely to look at, shiny and green, but if you see them near your sheep you are in trouble. If you don’t check the sheep regularly then you are in even bigger trouble.
Once you hear that someone else has had fly strike you immediately go onto full alert and check your own sheep. Then you worry about it until the first frosts. Or, like I did recently, you discover fly strike in one of your lambs. Having to deal with a struck sheep is a distressing thing for both animal and human, but it must be dealt with.
What is fly strike?
Someone will say that they have had fly strike when they discover that their animal has a wound that is infected with maggots from the blow fly. Most of the blowfly lifecycle occurs off the sheep and adult flies may travel large distances to find food.
“When given favourable conditions of humidity and warmth, the entire life cycle from egg to adult can occur in less than 10 days.”
Eggs hatch within 24 hours and first stage larvae penetrate the skin using their hook like mouthparts and secrete enzymes which liquefy and digest the tissue. Larvae are very active and cause 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.”
Once hatched, larvae feed and grow rapidly, undergoing two moults before they become mature maggots in three to 10 days. They then fall off the sheep and undergo further change on the ground, transforming to adult flies in about three to seven days.
“Female flies can lay 2,000 to 3,000 eggs in batches of hundreds, and live for approximately 30 days.”
Blowflies fall into two categories: primary flies are capable of laying eggs and starting a strike, while secondary flies cannot start a strike, but attack areas damaged by an existing strike, and make the injury worse.
When does flystrike occur?
Flystrike can occur at any time of the year, but in the UK animals are particularly at risk between April and October when the weather is warmer.
Why is fly strike bad for sheep?
“Blowfly strike is a major welfare concern and an important cause of ill thrift and death in affected animals.”
Flystrike occurs when certain species of fly (Lucilia sericata, Phormia terrae-novae and Calliphora erythrocephala flies) lay their eggs on another animal. These eggs hatch into maggots that then begin to eat the animal’s flesh. Flies are attracted by soiled or wet fur/fleece, often around the animal’s rear end. However, any area of the body can be affected, as can any wound, cut or scratch. Flystrike causes serious pain and suffering and it can be fatal.
How do I know if my sheep have fly strike?
“if its bad you will know just by looking”
There is a saying – ‘there are people who look at sheep but never see them’ – don’t let that be you. The quicker you spot something unusual the better. Lean on the gate and just watch them, get used to watching their normal behaviour. I’m told I have an obsession with poo – however I think you can tell a lot from the rear end of a sheep and their poo. What do you need to look out for?
- Fly struck sheep are usually restless and may bite or kick at the struck area. They may often turn back their head, biting nervously, or run in short gallops and lie down. 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.
- The fleece 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.
- Typically, they will also seek out shade, and may be lying under a tree or hedge. The affected area of the fleece may also appear damp and will be a darker colour.
- On closer examination, wool will be moist and may be stuck tightly together. The area may also be accompanied by a foul odour.
- In severe cases, the skin may be broken.
- If the disease progresses, septicaemia could result from secondary infection of the damaged area. If left untreated for long enough, the animal will die.
Sheep suffering from fly strike show obvious signs of distress. They spend less time grazing and more time tail wagging and rubbing the affected area and biting the struck areas of the fleece they can reach. If these signs go unrecognised and secondary strike occurs, the wounds can become very extensive and bacterial infection may lead to serious complications such as death from speticaemia and toxaemia. On examination of the sheep the result is often a foul smell from the wound and visible signs of maggots.
What do I do if I discover fly strike?
“Because the toxins released into the bloodstream by the maggots can cause the animal to go into toxic shock, death can result very quickly if flystrike is not spotted and treated rapidly.”
The recommended way to treat flystrike:
- Hand shear struck wool (where the maggots and eggs are) and a 5 cm barrier of clean wool around the strike, close to the skin to remove maggots. Unless wool is shorn off it is likely that maggot trails will be missed and sheep will remain struck.
- 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. Insect growth regulators (cyromazine and dicyclanil) are ineffective for the treatment of established flystrike.
- Weak and debilitated sheep with extensive flystrike wounds may require humane euthanasia, discuss this with your vet. You may have treated the maggots on the outside but what if there are maggots inside?
- Collect the maggot-infested wool into a maggot-proof (plastic) bag and leave the bag in the sun for a couple of days to kill all maggots. This breaks the life cycle. Don’t rely on registered flystrike dressings to kill maggots – some are incapable of killing large maggots and many maggots escape treatment by dropping from the sheep and burrowing into the soil before the insecticide can be applied. Unless maggot infested wool is collected and bagged, most maggots will survive and pupate and come back as adult flies.
- Apply a registered flystrike dressing to the shorn area to prevent re-strike.
- Remove struck sheep from the flock. Leaving struck sheep in the flock attracts blowflies. Move the struck sheep to a ‘hospital’ paddock (with a friend for company) this allows closer monitoring of recovery and reduces the risk to the rest of the flock.
- Protect your other sheep by checking, dagging, and applying a preventative treatment.
Pour-ons are normally what smallholders will use.
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 as it will simply be cut out – shearers may also insist that you do not apply treatments prior to shearing for their own health reasons. These chemicals are dangerous and there is the risk that without proper personal protective equipment humans may absorb some of the chemical through their skin when handling sheep.
How to do I treat my sheep after fly strike?
*within your emergency kit you should have the following – hand shears, stiff brush, maggot oil, spoton, crovect, stockholm tar, purple first aid or other antiseptic spray, tea tree oil, citronella or other fly repellant*
It will be just your luck that you discover fly strike on a Saturday afternoon when all the agricultural supply shops are closed for the weekend. Be prepared!
- Shearing can clear up most active strikes, we also found a stiff brush handy for getting rid of the exposed maggots. Remember to put as much of the fleece and maggots into a plastic bag as possible. This will stop the cycle.
- If you nick the skin when clipping apply an antiseptic spray and something such as stockholm tar (I like the sprayable horsey version) to seal the wound to prevent access to flies. Other recommendations are tea tree oil cream.
- I apply spoton directly onto the maggot infested area to help draw them out of the skin and kill them, others may use what they have to hand such as Crovect or Maggot Oil.
- Apply an insecticidal dressing (such as maggot oil) which protects the healing wound from re-strike. This will either prevent egg-laying female flies, or kill newly hatched larvae that emerge from eggs laid onto treated lesions.
- Treat any open wounds to prevent infection.
- The animal should also be treated with an effective broad-spectrum antibiotic. This is normally given by injection (if you do not have any antibiotic- contact your vet who will supply you with a syringe of appropriate medication – if you are not comfortable injecting yourself then you will either have to pay the vet or get someone who is skilled to do this task). Smallholders need to learn how to do these jobs – its tough – but its necessary. I have never had a problem with talking through a concern with my vet. The raw area attacked by the maggots is an open wound through which infection can enter and a vulnerable/stressed/ or sick lamb can also easily get pneumonia too.
- If weak and sickly your sheep may not eat, and this in itself could be fatal. If you are not sure that the animal is eating and drinking then it may be necessary to make sure that it gets adequate liquid and nutritional supplements. I do this by utilising a bottle and sturdy straw originally for administering nutrients for twin lamb disease, you may also be able to use a large syringe (without the needle) to give the sheep an oral application of water/salts/sugars/vitamins & minerals to keep it alive. This needs to be done gently and in little doses.
- Wash the open wound with an antiseptic solution every day to keep it clean (just syringe in or spray and let it soak in or run off). Keep an eye on the skin as it is vulnerable to sunburn or cracking and re-infection as the new skin grows underneath and the old skin falls off. Other recommendations include vaseline to keep skin moist, and or tea tree oil cream to sooth and prevent restrike.
- Check for more or hatching maggots and if necessary give a shot of invermectin to deal with any remaining deep seated maggots.
- Does your animal need wormed or fluked – was this the cause of the dirty bum and infestation? Even a change of food can upset a sheeps stomach e.g. from eating grass to being in a pen with hay and sheep feed.
After treatment, wounds can take several weeks to heal. The skin will start to grow under the dead areas within a few days and the peeling skin will fall off. During this time, your animal will be at increased risk of further bouts of flystrike and infection, so it will require careful nursing and additional preventative measures detailed above. Then when the skin heals and dries it might crack so keep it soft with creams such as udder cream, vaseline, or tea tree cream.
Herbal remedies to treat fly strike:
- Tea tree oil. Research in Australia has found that tea tree oil was highly successful in both preventing lice infestations and killing blowfly maggot larvae. “Tea-tree oil could be effective as a preventative treatment for any wound likely to be struck. It has also been shown to have antibacterial properties and is suggested to have wound healing effects”
How do I prevent fly strike in my sheep?
- keep on top of your worm burden especially lambs on new grass or who have not developed resistance to an acceptable worm burden
- if you have dirty bums then deal with it – cut away the soiled fleece (called dagging or crutching)
- keep on top of foot rot in your flock especially during wet weather – if your sheep are limping and there is no obvious problem then deal with it and consider vaccination for the long term (blow flies are attracted to the smell of foot rot)
- I use garlic licks and citronella sprays to discourage flies and reduce the risk
- use treatments at vulnerable times, some chemicals give you up to 10 weeks protection – a must if you are going away on holiday
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 dagging can further aid in the control of flystrike.
Which animals are most at risk of fly strike?
Flystrike (‘myiasis’) is a major welfare problem that mainly occurs during warm weather. It’s a painful condition that can affect rabbits, guinea pigs, cats and dogs as well as farm animals such as sheep, goats, llamas and alpacas.
“Even clean, well-kept animals can get flystrike.
It only takes one fly and one area of soiled fur/fleece or damaged skin”
Animals (not just sheep) that have a dirty rear end or generally dirty fur/fleece.
Causes can include:
- Long-haired animals that may not be able to groom themselves thoroughly without human intervention.
- Obese/overweight and older animals that cannot reach round easily to clean themselves.
- Animals with dental, spinal or balance problems, which make cleaning difficult or painful.
- Animals that are ill, as they may not feel well enough to clean themselves thoroughly and, depending on their illness, may also produce abnormally smelly urine or have diarrhoea, which will attract flies.
- Animals that have an inappropriate diet.
- Animals that have an internal parasitic infection.
- Animals with an open wound anywhere on the body.
- Unshorn sheep of woolly breeds.