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Vet's View: The importance of remaining vigilant against fly diseases

Flies and maggots have been a particular problem for livestock in recent months. Louise Hartley talks to vet Polly Gratwick about three of the main fly diseases farmers should be looking out for.

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This year, like the last, has been another bad year for flies. Demand for fly treatment products has been so high there have been shortages of some products.


With warm and wet weather providing optimum conditions for the fly life cycle, the recent hot, thundery weather has provided perfect conditions for their reproduction, says Miss Gratwick, a Devon-based independent veterinary adviser.


There are three main families of flies which cause problems with livestock – horseflies (tabanidae), which are large flies with painful bites; blowflies (calliphoridae), which are blue, green and black bottles; and ‘nuisance flies’ (muscidae), which include the housefly, stable fly, face fly and head fly.


As well as pain from flies biting, and general irritation which can cause reduced feed intake and loss of meat and milk production, flies are implicated with the spread of infections and certain diseases, including fly strike, conjunctivitis (New Forest eye) and summer mastitis.

Fly strike

Fly strike

Fly Strike is caused by the larvae (maggots) of blowflies.


Miss Gratwick says: “Blowflies lay their eggs on wounds or soiled fleece and once these hatch, the maggots feed on the skin secreting enzymes which breakdown the tissues further.


The decomposing tissue then attracts further flies.”


Because of the large numbers of maggots, the lesion can become deep very quickly and if not treated, will lead to septicaemia and even death.


Fly strike lesions can be small and not easy to spot at first, but affected sheep will often be dull, lethargic, anorexic and rapidly lose condition.


“Treatment includes the use of a larvicidal topical product, along with cleaning of the wound,” says Miss Gratwick.


“Antibiotics and pain relief are also usually necessary, but good worm control and crutching or dagging sheep can help reduce the likelihood of strike by reducing faecal contamination of the wool.”

Conjunctivitis and New Forest eye

Conjunctivitis and New Forest eye

The face fly is the main fly which causes problems to animals at pasture. This fly feeds on the secretions from the eyes, nose and mouth.


Miss Gratwick says: “The irritation can cause a general conjunctivitis, but more importantly, these flies are the main vector for spreading the infection moraxella bovis which causes New Forest eye [NFE].”


NFE starts in the early stages as weepy eyes with a small white spot or ulcer on the cornea. If caught at this early stage, treatment with topical antibiotic ointment is very successful.


But as the disease progresses, the ulcer develops and can involve the whole surface of the eye. Repeated antibiotic treatments (usually sub-conjunctival injections) are required to treat this stage.


This year Miss Gratwick has had eye problems within her own cattle. She says: “Instead of the usual NFE, they have been affected with a really nasty conjunctivitis infection. They had very red, swollen conjunctiva with a purulent discharge. The surface of the eye itself had no ulceration.


“The response to antibiotic treatment was very good, within a few days you could not tell which ones had been affected, but this was certainly the most severe conjunctivitis I have seen in 12 years in practice.”

Summer mastitis

Summer mastitis

The head fly is thought to be one of the more important flies involved in the transmission of summer mastitis in cattle, and the main bacteria causing it have been isolated in these flies.


Summer mastitis is usually seen in non-lactating cattle and is often associated with specific problem fields.


Miss Gratwick says: “A classic presentation is typically a hot painful udder containing very thick pus. This pus is very difficult to strip out and the infection will often burst out of the udder.


“Loss of the quarter is usually the result, especially if teat amputation is necessary. Secondary infection of the wound due to further flies is also common. Treatment is generally by broad spectrum antibiotics, but success is limited.”

Prevention options

Ensuring good fly control as soon as you see the flies is the best way of preventing or limiting problems. Options range from simple insect repellent through to products which kill flies and their larvae.


Miss Gratwick says: “The most common products used are synthetic pyrethroids. These are useful as they have a repellent effect as well as being a contact insecticide, and are especially effective for prevention of NFE.


“They also persist well in the coat and skin, being absorbed into the skin but not penetrating into deeper tissues. These are commonly used in fly tags for cattle and most of the topical spot-on/pour-on products available.”


Another common prevention tool are insect growth regulators. These prevent the larvae from developing into mature maggots or flies and are very good at preventing fly strike if applied in time, says Miss Gratwick.


Organophosphates are still available for use in sheep dip, and these give very good protection and treatment of fly strike as well as other ectoparasites such as scab, she adds.


“With all of these products, it is important to use them correctly and safely. All the products are designed to be absorbed into the skin so wear gloves and protective clothing and wash off any splashes immediately.


“They are very toxic to aquatic life so animals should be kept away from water courses for at least one hour after treatment and the products should not be applied in wet weather. With sheep, the persistence in fleece means most products are recommended to be used after shearing.”

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