Spring is here – and so are the flies, ticks and midges!
The warm spring, coupled with a mild winter will result in earlier activity of flies, ticks and midges this year. All of these can be transmitters of disease, as well as causing disease themselves.
Fly strike is caused by a variety of blowfly species. They lay eggs in soiled fleece, footrot-affected feet and wounds. The maggots hatch out and damage the skin. Mature maggots drop from the sheep to pupate and develop into flies. There are several different species of fly, some of which start the flystrike process and others which are only attracted to established cases.
Death in flystruck sheep is caused by a combination of water and protein loss through the wounds, bacterial infection and the absorption of toxins, chiefly ammonia, produced by the maggots.
This chart shows the lifecycle of Lucilia sericata, the main blowfly species affecting sheep (Courtesy of Wall & Lovatt, 2015)
The development of the pupa to the adult fly occurs at a speed determined by the temperature. In warm years, where spring is early, there may be several generations of flies in the same year, with increasing numbers in each subsequent generation. As spring has come early this year, the flystrike season might begin sooner than in previous years. Don’t be caught out! The occurrence of the first cases in an area can be found here: https://farmanimalhealth.co.uk/tracker
Early season flystrike is often on ewes, and only the most susceptible animals are affected. As fly numbers rise, strike becomes more common and the risk shifts onto lambs as their fleece becomes longer and build-up of worms on pasture leads to fleece soiling. Ewe risk falls after shearing.
As flies do not like strong winds, fly numbers are higher in sheltered or wooded areas and lower in exposed fields.
Prevention of flystrike can be optimised through the following measures:
Reduce the susceptibility of sheep - good worm control and dagging to prevent soiled fleece, good footrot control, correct timing of shearing.
Reduce fly numbers - ensure that early cases are prevented to reduce the size of the first wave of fly population, eliminate fly-breeding sites (e.g. ensure carcasses of both livestock and birds, rabbits etc. are disposed of promptly), and use traps. Use of fly traps in a study in SW England reduced the incidence of flystrike five-fold.
Protect at risk sheep with suitable products - there are many pour-on products for fly strike control. They can be divided into two main classes - Insect Growth Regulators, e.g. CLiK, and Synthetic Pyrethroids, e.g. Spotinor. The former spread in the wool grease to cover the whole animal, but cannot be used to treat cases of established strike, whereas the latter protect only where they are applied, and have worse environmental impacts, but can be used to treat cases. Finally OP dips can be used to both treat and prevent flystrike. The different products of each class have different durations of action - make sure you pick one which is correct for the purpose. Please don’t hesitate to speak to us for further advice.
Ticks become active around 8 -10⁰C.. They can cause disease through the transmission of tick-borne fever (which can cause abortion, fever and immunosuppression), and tick-bite pyaemia - where skin bacteria are introduced into the body as they bite, causing abscesses and joint ill in lambs. Fortunately, we don’t have louping ill in the area, but if you are buying sheep from moorland areas, do speak to us about quarantine to prevent its introduction.
Prevention of tick bite involves use of dips or pour-on products (not all products effective against flystrike are effective against ticks), and removal of dense undergrowth that supports the ticks when the humidity falls in mid-summer. Reducing other host populations (e.g. deer, rabbits, birds) will also help reduce tick numbers.
Midges do not cause disease in their own right (apart from the occasional unfortunate sheep that is allergic to them), but they do transmit diseases like Schmallenberg Virus and Bluetongue Virus.
SBV is still circulating in the area (we found antibodies against it in several flocks in 2018), so if you will have sheep in early pregnancy in the summer/early autumn, especially if you buy in females from further north where SBV did not reach, you should consider vaccination.
BTV 8 was circulating in France last summer and the summer before - watch this space to see whether it returns this year. If it does reach the French coast, then vaccination should be considered as it only takes the wind to be in the wrong direction and the infected midges can cross the Channel.
Housing animals, or siting at-risk animals in low midge risk areas (windy, a long way from standing water and woodland), will reduce the risk of transmission. No products are fully effective at preventing midge bite, so vaccination against the diseases they carry is the most important control measure.