It's the Fly Season!

It’s that time of year where we are starting to see the flies again. Although they are not usually the direct cause of disease, in most animals they do cause both financial and welfare concerns. 


Head fly can be a major problem affecting UK sheep flocks during the summer months. The flies disrupt grazing, as animals will often spend their time itching and seeking shade. This distress has a huge impact on not only the animal’s welfare but also their body condition, which in turn can lead to decreased fertility. Pour-on products containing deltamethrin or cypermethrin should start to be used around this time of year. Fly control is especially important in animals with wounds or those that have been tagged recently.

Fly strike cases are most commonly seen in the UK between May and September. However, due to us having such a warm winter we saw cases through to December last year. Therefore we need to keep an eye out for flystrike in any animals with open wounds or faeces around their perineum. A recent survey found that 80% of UK flocks will see one or more cases of fly strike a year. The eggs hatch within twelve hours and the larvae eat away at the surrounding tissue. Usually the first signs you will see are restless animals and discolouration of the fleece, leading to extensive areas of traumatised tissue and can lead to death in untreated cases. Treatment involves clipping and applying a specific spot on to the affected area, anti-inflammatories and possibly nursing, depending of the severity of the lesion.

To reduce the risk of fly strike multiple measures can be taken. These include dagging, tail docking, fast disposal of carcasses and reducing the cases of footrot and scour. Chemical treatments can also be used as prevention, such as diazinon sheep dips and cypermethrin pour-ons. Care must be taken to follow instructions on application and dosing.


Flies are also a common problem in our cattle herds, and importantly flies act as vectors of disease. Nuisance and head flies spread the bacteria Moroxella bovis. They also spread the bacteria causing summer mastitis, most commonly seen in dry cows and heifers between May and September. You will see large numbers of flies on the affected quarter, which will be hot and swollen, have an abnormally large teat and she will likely be kicking at it. She can then become sick, and so will go off her food, isolate herself and appear extremely agitated. Common sequalae to summer mastitis are a loss of condition, permanently damaged quarter, abortion and even death. A vet should be called to any animal with suspected summer mastitis. Prevention includes pour-on fly control, fly sprays and grazing cattle away from areas with high fly populations.