Q&A: How to prevent and treat blowfly strike in sheep

Farmers are being urged to strike first against blowfly, as the unseasonably mild weather could pave the way for a sharp increase in the UK fly population.

The Met Office believes this past winter may have been the warmest ever recorded, prompting fears that these favourable conditions will result in a more abundant blowfly population.

Blowfly strike is a familiar annual problem, with 88% of farmers recently saying it is the most widespread ectoparasite affecting sheep in the UK, with strike occurring on more than 80% of UK farms.

See also: Q&A: How to tackle scald in sheep

With 49% of farmers agreeing prevention is the best practice when it comes to blowfly, Elanco Animal Health is encouraging farmers to take control and strike first against the parasite using preventative products.

Elanco technical consultant Matt Colston explains what the dangers of blowfly are and how farmers can best ensure they aren’t caught out in this Q&A.

1. What is blowfly and how is it caused? 

Blowfly strike is caused by the larvae of the greenbottle fly, which damages the skin and soft tissues of the animal as it feeds on those soft tissues.

2. When are sheep at highest risk from blowfly?

From the end of March until we get a frost in the autumn. It’s quite variable depending on the weather each season. The most common time to see it is in mid-summer.

In addition, any sheep with an injury, or area with damaged or soiled fleece, becomes more attractive to blowfly, so this is something to watch out for.

Specific locations on the farm can also cause particular problems, such as areas where there are relatively high levels of humidity and low wind speeds – for example, sheltered, damp spots on the farm.

3. Is there anything that can be done to prevent it? 

Yes. Insect growth regulators (IGRs) are preventative products. IGRs work by preventing the first-stage larvae developing into the second-stage larvae – it’s this stage of larvae that starts to damage the skin and cause problems.

Historically, people have used organophosphate (OP) dips, but these tend to have a shorter protection period and are a bit variable.

You also need to have a certificate of competence before using these products. Other preventatives include the synthetic pyrethroid (SP) pour-ons. They are insecticidal and provide a degree of prevention to the areas they touch when applied with a fan spray.

Applying pour on treatment to sheep © Tim Scrivener

© Tim Scrivener

4. What symptoms should farmers look out for?

Keep an eye out for staining of the fleece and sheep nibbling themselves.

However, by the time you see these symptoms the damage is already done, so it is better for a farmer to strike first and prevent the damage rather than treat it once it has already happened.

Don’t wait until it’s too late, as by that point you will probably have lost a couple of lambs.

5. Is it treatable? 

In the event of strike, treatment is via a SP pour-on, but because of the degree of damage and the speed at which it happens, you may need more treatment. This is because the SPs kill the maggots, but you may need to treat the wound as well.

6. What product should farmers use?

IGRs are narrow-spectrum products against blowfly, so they are ideal to use when only blowfly protection is needed.

Correct application for all medicines is important to ensure you get the most out of them. Follow packaging instructions and use the correct applicators in all cases.

7. If left untreated, what economic and welfare implications could blowfly strike have? 

It is a horrible condition and has a huge effect on animal health and welfare, as well as emotional and financial implications for the farmer. If left untreated, it will cause death.

The animals that survive take a long time to recover, which in turn has a long-term effect on growth rate and productivity. For the farmer, the stress and worry of having potential welfare issues in the flock are extreme.