Defend Your Cows Against Flies

An old proverb says "even the lion must defend himself against the flies" and for good reason. Flies are more than merely an annoyance; they can be a primary cause of production losses and are implicated in the transmission of disease.

In the UK, there are more than 20 species of 'nuisance' and biting flies which feed on the blood, tissue and excretions of cattle (e.g. sweat, tears, faecal and urine soiling). The sheer number of fly species which affect cattle makes them an important ectoparasite in the UK. These flies can cause severe irritation, causing cattle to inflict self-trauma and become a potential risk during handling (e.g. kicking in the parlour).

Time spent dealing with the irritation of flies, is time not spent eating. Productivity drops if cattle have to waste 'eating time' evading and attempting to alleviate the irritation they experience from flies. The welfare of livestock inflicted with this kind of irritation is also a concern.

Alongside welfare and direct production losses, flies are also implicated in the spread of diseases, such as 'summer mastitis' and 'pink eye'. 

Summer Mastitis 

'Summer mastitis' typically occurs in dry cows and heifers during the summer. However, it can also affect undeveloped udder tissue in young heifers and bulls. Caused by several types of bacteria acting synergistically, it is believed to be spread primarily by Hydrotea irritans. Despite its colloquial name, large numbers of this 'head fly' cluster on the abdomen and udder. These flies physically carry bacteria from animal to animal, i.e. they act as 'mechanical vectors'.

Hydrotea irritans lives in sheltered areas such as bushes and trees, and can only fly in calm, damp, humid conditions. The fly's habits cause the incidence of summer mastitis to be highly variable and a tendency to be associated with 'problem fields', e.g. near to woods or tall hedges.

The early stages of the disease are often subtle, with the only signs being progressive swelling or enlargement of the affected teats over about a week. If these signs are not spotted and managed early, the disease progresses, causing systemic illness. Cows affected by summer mastitis isolate from the herd, reduce or stop eating, may have swollen hocks which makes them stiff and they may also abort. Thick, clotted and often foul-smelling pus can be drawn from the affected teat. If this disease is not detected and managed early, affected cattle may die. Vigilance about checking your dry cows and young stock is crucial.

'Pink' or 'New Forest' Eye = Infectious Bovine Keratoconjunctivitis 

IBK is a highly contagious disease caused by the bacteria Moraxella bovis. It can spread rapidly during the summer months and is more common in youngstock. Flies act as mechanical vectors for the spread of this disease. The disease affects the eye, and lesions which damage the cornea are very painful. Pain disrupts grazing patterns and can result in production loss. 

Flies can also act as vectors for viral and parasitic diseases.

Fly Control 

An integrated approach to fly control is the best policy, including topical applications of medicines and management of the environment.

Topical Ectoparasiticides - Pyrethroids 

Products containing pyrethroids, e.g. Dectospot 10 mg/ml Spot-on Solution for Cattle and Sheep, which contains the ectoparasiticide deltamethrin, can aid in controlling biting and nuisance flies on your cattle. Dectospot can be used during lactation and pregnancy and has zero milk withdrawal. Other products are available. Your local vet or SQP can help determine the appropriate approach to meet your needs.

Environmental Control 

Targeting larval development is a highly important measure. Any moist, organic matter (e.g. manure or feed) can act as fly larvae 'nurseries'. Farm hygiene practices and manure removal (where possible) are important as they help reduce fly populations.

Pasture management is also relevant. Avoid grazing 'problem pastures' at peak risk times where possible and consider the use of alternative approaches, e.g. fly traps.

It is always advisable to speak to your local vet or SQP before introducing new practices to your farm. Your vet or SQP are in the best position to help you to design an appropriate fly defence strategy for your herd.

Author; Sharon Cooksey, BVSc., MRCVS. Professional Services Vet, Bimeda UK
Date editorial prepared: April 2020

Use medicines responsibly.

*Dectospot 10 mg/ml Spot-on Solution for Cattle and Sheep contains 10mg/ml deltamethrin and is a POM-VPS medicine. The SPC for Dectospot is hosted on the VMD website. For further information, please call Bimeda on 01248 725400 or see

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