Blowfly, or flystrike, is a common and often devastating condition of sheep and other livestock, including alpacas and rabbits. It is a major welfare issue - to knowingly not treat affected animals would be an offence. The disease is distressing for both animal and owner; those of you that have seen a case will remember it vividly. Understandably, affected animals get knocked back in terms of growth and vigour, adding insult to injury.

Flystrike – how and why?

Flystrike is most often caused by greenbottle flies, although other species of fly can be responsible. Traditionally flystrike has been a disease of May to September, but mild springs and autumns may permit a wider season especially in lowland flocks. The severity varies from year to year, and the weather has an important role.

Female flies are attracted to open wounds or dirty fleeces, which are the perfect place to lay their eggs. The eggs hatch quickly – often within twelve hours and less if the weather is favourable. The maggots then eat their way through the surrounding flesh and fall out after about three days to complete their development. The first few maggots start a vicious cycle: the smell from a few maggots attracts other female flies to lay their eggs in the same place.

How to spot and treat flystrike early

The most important factor is vigilance as affected animals go downhill rapidly. Because eggs can be laid and hatch so quickly, ideally sheep (or alpacas) should be checked twice daily. The initial signs of flystrike are tail swishing, agitation and discoloured wool. Later cases may be down and appear more unwell.

Treatment of an affected animal requires a product that kills flies (see below), clipping the affected area, disinfection with a dilute solution (e.g Hibiscrub), and an antibiotic injection to tackle bacterial infection that often enters via the extensive skin wounds. Severely affected animals require veterinary attention. Where possible they should also be housed and nursed with care.

Get ahead of the flies – prevention of flystrike

Given it is such a gruesome condition, many keepers give a preventative product to sheep in anticipation of fly season. These work similarly to the ‘spot-on’ you might give your dog or cat for fleas. The timing of preventative treatments will change year to year with the weather – your vet is the best person to advise for your own farm and situation. Nothing is licensed for flystrike in UK camelids, so again consult your vet for the best advice on treating these.

Timing of preventative products also depends on the exact product used. There are many products that effectively prevent and/or treat blowfly and they vary in a number of ways. Careful consideration should be given to your farm and which product(s) might be most appropriate. They vary in:

• Active Ingredient: most are either an Insect Growth Regulator (IGR) or a pyrethroid (e.g cypermethrin, deltamethrin).

• Duration of Action: i.e. how long the product will work for.

• Withdrawal Time: an important consideration for animals that may be sold or slaughtered in the near future.

• Spectrum of Activity: some products act only against flies, others against ticks and lice too.

• Price: this can be calculated per dose or per day protected.

Choice of product can depend on if you are aiming to kill active maggot infection, versus prevention of infection in the first place. Some products do both, but the Insect Growth Regulators (IGRs) will not kill maggots that are already on the sheep. A great summary of the available products can be found at .

Remember to take care when using these products as they can be harmful to human health. Read the data sheet (which should be found in or on the packet) for exact instructions but as a rule of thumb eye protection, a face mask to cover the mouth, and gloves are all sensible precautions.

These products remain the mainstay of prevention but there are a couple of other things sheep keepers can do to be as proactive as possible. For example, poorly controlled gut worms lead to dirty fleeces, encouraging flies to lay their eggs. Therefore, talking to your vet to make sure your parasite control programme is working should also help with flystrike. Likewise, leaving shearing too late can also encourage flystrike, so book a shearer in good time and don’t have your sheep be the last ones unshorn in the county!


Kaz Strycharczyk is a veterinary surgeon working at Black Farm Sheep Health in Rothbury, Northumberland. Kaz also heads the practice Smallholder Scheme.