Sheep Skin Diseases and Schmallenberg Virus update

Friday 09 November 2012
Sheep Skin Diseases and Schmallenberg Virus update

Last month the Allen family of Barker Knott Farm, Windermere kindly agreed to host a meeting for the Farmer Network on sheep skin diseases with guest speaker Dr Peter Bates, Veterinary Medical Entomology Consultant.

An update on the Schmallenberg virus was also given by vet and independent sheep specialist Judith Charnley, who later spoke on the same subject at two October meetings organised by Myerscough College for farmer groups in Stockport and Burnley.

Click on the links below to go direct to that information: 

Sheep Scab >
Chorioptic Mange / Forage Mites / Chewing Lice / Blowfly Strike / Head Fly / Midges / Ticks >
Control and Treatment >
 
Schmallenberg Disease >
What can be done whilst waiting for a vaccine? >

 

 

Ectoparisites

Ectoparasites live on the skin or fleece of an animal and have a significant effect on sheep production through:

 

Sheep Scab (Ovine Psoroptic Mange)

Sheep Scab 2Scab needs wet cold conditions to survive and so it’s main spread time is winter. It can only survive for 17 days when it is off the back of the sheep. The mites can suck blood but not on the sheep’s back as the skin is too thick. Initially there are very few visible symptoms and not always fleece loss. It starts as small sub clinical lesions and takes time to develop, twenty days for the lesions to grow and can take between 3/4 weeks to 5/6 months to appear. These lesions are caused by an immune response to an allergy to the mite faeces. After twenty days the mites increase in number and after about 60 days the mites will start to die off as the fleece disappears but will not completely disappear. It has been found that the host animal will start to produce antibodies in defence to kill off the mite which would indicate that a vaccine could possibly be developed.

Certain factors affect the growth of the lesions:

Ear mites – Obscure outbreaks of scab?

Ear mites will not cause sheep scab but the sheep scab mite has been recorded in the ear canals of 38.6% of sheep presenting with lesions extending over 220.9% to 100% of the body and if these mites present in the ear canal were taken out and put on the backs of sheep they would cause scab. They live in the ear canal but are very difficult to see. The mites will migrate to the ear 28 days after infection and so when dipping it is vital to ensure complete submersion of the head, however, often not all are killed due to an air lock within the ear.

An injection of ivomectin will kill ear mites.

 

Symptoms of Scab

 

Adult mites can be seen as tiny white dots but often go unnoticed until the infection is well established, 60% of the flock can be affected by the time the vet is called out as symptoms occur long after the infection has taken hold. The mites pass from sheep to sheep and also via mites deposited in the environment.

40% of lesions tend to occur on the neck.

Sheep scab mite can also affect cattle. In cattle the mite suck blood and migrate upwards through the coat. There have been cases in Belgium where resistance to ivermectin have been shown in the treatment of cattle affected by scab. In these cases pour on treatments are still effective. These mites are the same mites as sheep mites but are not currently showing the same resistance problems in sheep. Pour on treatments work well in cattle as they have a lighter coat than a sheep whose fleece is also greasy. Injection still remains one of the most effective ways of treating scab in sheep.

The sheep scab order 1997 gives local authorities the means to improve the control of sheep scab when owners of affected sheep do not take appropriate measures voluntarily. This applies where clinical signs of scab are present.

 

The cost of sheep scab

sheep scabThe Sheep Veterinary Society produced results from a study done in 2007 which indicated potential financial losses within a hypothetical  lowland flock as follows:

Overall a potential profit of £5.27 per ewe was reduced by £18.84 to an overall loss of £13.57 per ewe.

 

Chorioptic Mange

Lives on the skin surface and feeds on the skin and wool debris. It infests sheep, goats, cattle, camelids and horses. It is becoming more widespread in the UK and commonly affects the feet and the scrotum.  The heat caused by the mange can cause the sperm count to decrease in rams thereby affecting fertility. Once treatment for mange is given the condition is reversible.

 

Forage Mites

These are often confused with sheep scab mite. They live in the hay and feed and disappear as soon as the affected forage is removed.

 

Chewing Lice

These cause a mild dermatitis and fleece derangement. Heavy infestations can be the cause of wool loss. They are capable of surviving off sheep for up to 17 days. There are just as many sheep infected by lice now as scab but the sheep don’t appear to be as irritated by lice as with scab. The lice prefer hot dry conditions so tend to reduce in number when its wet and cold. Lice is a big problem in Australia and shepherds will microwave their moccasins after clipping to kill the lice!

The lower the body condition the more likelihood of contracting lice, ill animals such as those with fluke or worms often have more lice. Lice do not like short fleece and so treatment is most effective 1 hour after clipping.

 

Blowfly Strike

This is an infestation of the tissue by the larvae of the flies of the Calliphoridae family. In the UK three species regularly attack sheep – Greenbottle, Blackbottle and Bluebottle. They like wet but not cold conditions and thrive in wet summers. Within 36 hours of blowfly strike, the maggots excrete ammonia through the skin and can cause blood poisoning. They tend to strike around the tail and eggs are laid at the tail head which is where pour on treatments should be applied.

 

Head Fly

Eggs are laid on dead and decaying vegetable matter, there is only one cycle per year  and so this can produce large swarms. They feed on body sweat and secretions from the eye and base of horn.

 

Biting Midges 

These are not a parasite but suck blood and transmit other diseases such as blue tongue and potentially Schmallenberg. Only the females bite and need the blood to develop ovaries.

 

Ticks

Ticks lay 5 or 6 eggs when engorged with blood. They spend only a short while on the host to feed, the rest of their lives are spent in vegetation digesting their blood meal and moulting. They like cool damp weather and so seem to be on the increase with climate change.

They are responsible for transmitting various diseases including Tick borne fever, Louping lll and tick pyaemia (cripples).

 

Mycotic Dermatitis

This is not actually mycotic at all but an allergy to the waste products produced by bacteria. It is found in the soil and multiplies in damp fleece. It needs wet, moist conditions in which to thrive. The scabs produced are seen to move up with fleece growth. Dipping with sulphates (same product as used for foot rot control) is the best treatment.

 

Differential Diagnosis and the need for accurate diagnosis

It is vitally important to get a proper veterinary diagnosis to ensure correct treatment and to avoid unnecessary costs. Similar symptoms to those produced from scab can be present due to other parasites or illnesses but require different treatment. Sheep can carry more than one ectoparasite and non-parasitic skin conditions simultaneously.

Inappropriate use of treatment can potentially increase resistance creating an even bigger problem.

 

 

Control of sheep Ectoparasites         

Ensure a proper diagnosis at first onset of symptoms.

 

Farmers have a legal responsibility to control parasites under the code of recommendations for the welfare of livestock (14th August 2000) and can be liable for prosecution if they have failed to follow the guidelines.

Effective parasite control depends on whether the parasite is

PERMANENT – spending its entire life on the sheep (eg scab, earmites, chewing lice, chorioptic mange) or

SEMI-PERMANENT – spending some of the time off the animal (ticks, blowfly, biting flies, bots)

 

Plunge dipping

Currently the best treatment for parasite control is plunge dipping. This is the most effective way of ensuring complete coverage of the entire animal and penetration of the fleece through to the skin.

The current cost is around 50/60p per sheep for materials and chemical disposal. It also covers the biggest range of parasites.

 

Injection (mectins)

This will only cure present sheep scab but will not prevent it from reoccurring. Inject 3 times at 2 week intervals for a 30% reduction.

 

Pour ons

These are not effective with scab as they do not kill mite but are with blowfly strike by means of interfering with the growth hormone of the larvae.

 

Spot ons

These are effective with lice, blowfly and ticks but will only cure not prevent or protect from further outbreaks.

 

Sheep showers

These are not effective against scab as it is difficult to penetrate the fleece and the sheep would be required to spend too long in the shower (20 mins) to be effective.

 

Vaccines

A vaccine for scab is still some way of but some trials have been done in Scotland.

 

Schmallenberg Disease – Orthobunya viruses

Schmallenberg2In the summer of 2011, dairy cows in the Netherlands and Germany started showing Winter Dysentery type condition symptoms:

In the early autumn problems were being reported in early lambing flocks, goat herds and calving cows.

SBV was first identified in the UK in January 2012 on 4 farms, 2 in Norfolk, 1 in Suffolk and 1 in East Sussex.

The disease is thought to be transmitted via infected midges blown across the channel. As yet there is no evidence that it can be caught by humans (zoonosis) and so is considered a disease of “low impact” by the European Food Safety Authority, however if present within your flock whilst in lamb, 30% losses can be expected.

There is no evidence of antibodies being produced in humans who are in contact with affected stock, however strict hygiene should be adhered to and pregnant women and immunosuppressed people should avoid contact.

It can take two to three weeks to spread through a herd or flock, then usually a full recovery is made and the virus disappears. Problems occur when the disease is present in pregnant animals, up to 30% abortion and birth deformities can be expected.  If the dam is infected during the first third of pregnancy, the foetus is likely to have severe brain damage. Infection during the middle third, limb deformities are seen and in late pregnancy, infection shows brain swelling leading to ataxia, recumbency, inability to suck and sometimes fits.

Vulnerable gestation periods are thought to be around days 28 to 56 in sheep and days 62 to 173 in cattle.

In twins, often only one lamb is affected.

SchmallenbergThe deformities are often so severe that giving birth is impossible. It is strongly recommended that veterinary assistance is required.

Blood tests should be carried out in suspected cases and results are usually returned within 7 days. In areas with as yet unconfirmed cases, there is no charge for blood testing.

A vaccine is currently being worked on by three pharmaceutical companies and is expected around January. There is currently no treatment.

If animals have the virus in their blood for 2-5 days and a midge bites the anial, it can transmit the disease to another animal. Deer and camels have also been shown to be able to become infected.

Only the number of premises infected are currently recorded, not the number of individual animals.

Imported animals could bring the disease into the UK but this is currently unproven.

 

Cases in the UK up to November 2012

 

What can be done whilst waiting for a vaccine?

Consider:

SBV is currently a non notifiable disease as it does not affect humans and currently does not affect trade.

If you suspect the disease, report it. The more we learn about it the quicker we can find a solution.

 

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