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Sheep External Parasites On The Rise?

12 June 2020

The winter of 2019/20 was exceptional, breaking records for both high temperatures and high rainfall, with the highest UK December temperature on record and the wettest UK February on record.  Coinciding with this there have been reports of more external parasites being found in sheep flocks e.g. lice, ticks and maggots.  These parasites will have enjoyed this mild winter and had good survival rates.


As a reminder, lice are more noticeable in the winter, but present throughout the year.  They are species specific, so sheep lice won’t infect cattle.  Being species specific, they only live for a few days away from the host e.g. on fence posts etc.  After hatching, the lice feed on debris on the surface of the sheep’s skin, which causes irritation, leading to sheep rubbing and biting their wool, similar to the typical signs of sheep scab.  It is extremely important to get a diagnosis prior to treatment, to ensure treatment will be effective.  This can be done by getting your vet to take a skin or wool sample.


This notifiable disease causes a huge threat to flocks both on welfare and financial loss.  Outbreaks are more common through the winter period but it is an all year round disease.  Due to the fleece drying out in the summer, there is less activity of feeding and laying of eggs in this period.  But if conditions are suitable e.g. dirty backsides, the females will continue with activity.

The scab mites spend their entire life on sheep, but if they are rubbed off on fence posts, they will survive for up to 17 days.  These posts can be a source of infection for other sheep.  If you suspect sheep scab, a skin scrape test is required for confirmation.  On diagnosis the best route of treatment should be chosen in discussion with your veterinary surgeon.


Ticks are usually found in hill and upland flocks, due to the dense vegetation.  They are most active in warm conditions, with peaks found in the spring and the late summer.  These blood-sucking parasites will feed on numerous hosts and are not species specific.  There are many species of tick, but the most common in sheep is Ixodes ricinus.  Its lifecycle is complex, requiring three hosts over three years of its life.

Ticks can transmit a number of diseases to sheep, and Lyme disease to humans and dogs.  As well as potentially being severely affected by untreatable conditions such as louping ill, exposure to tick-borne fever can make sheep more susceptible to other diseases.  Very young lambs are at risk of lameness and paralysis due to tick pyaemia.

There are numerous methods to reduce ticks on land including altering the habitat.  Ticks require a thick dense vegetation while not on their host, so by carrying out land improvements such as drainage, reseeding and increasing the grazing pressure e.g. cattle can help remove the thick vegetation.  Another method is to alter management of potentially infected hill ewes, when being brought in bye at a time of peak tick activity.  When the ticks are finished feeding on the hill sheep, they will drop off and only survive if they find thick vegetation such as field boundaries.  To minimise this risk hill sheep should be treated prior to being moved onto in bye pasture.  Any replacement stock being purchased, must also be treated on arrival.  Bought-in animals may have little resistance to disease such as louping ill, so consider protecting them with a treatment if they will be exposed to ticks.


Warm weather with occasional rain gives ideal conditions for the greenbottle to cause blowfly strike on sheep.  The greenbottle is attracted to the smell of decomposing matter (faeces, urine, wounds) giving a feed source before laying its eggs.  These eggs hatch within a few hours or days, the larvae, or as they are better known as maggots, will eat on the flesh of the sheep, causing a severe welfare and economic issue.

Measures can be taken to avoid flies being attracted, this includes, good worm control to prevent dirty wool including routine faecal egg counts, dead animals being lifted by the knackery promptly, treating and eradicating foot rot, crutching ewes to prevent dirty wool, treating open wounds and by preventing wound creation (ear tagging) during a high risk period.


The most effective control of ectoparasites is plunge dipping, provided the fleece is thoroughly soaked and the sheep are submerged.  Sheep need to have at least three weeks fleece growth for the insecticide to bind.  The sheep should be clean and dry before dipping to avoid excessive contamination of the dip which reduces the effectiveness if the dip.  Dipping will treat and control lice, scab, ticks, blowfly and keds.  To carry out dipping of sheep you must be trained and competent and either hold a certificate of competence in the safe use of sheep dips or be operating alongside someone who does.  There is a guide available for advice on sheep dipping from the HSE  Withdrawal periods must be adhered to.

More convenient pour on products are available, which are easy to use and can have a long period of protection.  The longest protection products are often insect growth regulator pour-ons, which can be applied early for almost whole season protection, but are only effective for blowfly. Pour-on chemicals are available with activity up to 12 weeks, and these often also provide protection against ticks and lice.  Such products dissolve in the wool grease and will be removed when animals are shorn, including crutching to remove dirty wool, so re-application may be required.  Pour on products should also be avoided in sheep with full fleeces, as this can lead to less effective treatment, potentially increasing the chance of resistance to the product.  The active ingredients vary with the products on the market and veterinary advice should be sought to ensure you select the control method best suited to your specific needs.  Again, remember to check the withdrawal periods on all products.

For further information and advice on controlling parasites, visit 

Kirsten Williams and Eilidh Corr

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