Features
Why is Orf such a Persistant Problem?
by Dr Peter Nettleton
For centuries the orf virus has inflicted pain and misery on sheep flocks and shepherds. Vaccines, developed in the 1920’s can provide control but the virus still thrives whenever it gets the chance. This article describes why the virus is so successful, the variability of the disease, what can be done to minimise its effect in your flock, and human orf.
The Virus
Orf virus is a member of the poxvirus family, and as viruses go it is one of the biggest. As well as containing genes essential for its rapid replication it has genes coding for proteins which interfere with the hosts immune and inflammatory responses. Orf virus only grows in the cells of superficial layers of the skin and upper digestive tract and is not capable of generalised spread through the body. This restricts the host from using its full armoury of protective immune mechanisms against the virus. Antibody against the virus is produced in large amounts but seems to have little effect on the disease. Animals that have recovered from the disease have only a limited protection against re-infection although subsequent infections tend to be milder and shorter-lived than the initial episode. It is “cell-mediated” immune mechanisms that have an important role in protection but these are hampered by the specific evasive strategies evolved by the virus. Much of the recent research at Moredun is now focused on characterising the viral products responsible for immune evasion with the aim of establishing methods to counteract them and improved methods of immunisation
Disease Variability
Orf virus can only establish infection where the skin is damaged. Animals, therefore, most commonly develop painful scabby lesions on their lips and nostrils. Under normal circumstances these resolve in four to six weeks, but lesions may become extensive and spread to other parts of the body. Lambs can transmit virus to the ewe’s udder making her reluctant to let them suckle. Mastitis and lamb starvation are likely sequels. The worst orf outbreaks involve very young lambs which develop rapidly growing lesions inside the mouth involving the gums, palate and tongue. Groups of such lambs are a pitiful sight and can drive their flockmasters to despair. Similar proliferative strawberry-sized lesions can occur anywhere in occasional animals but outbreaks in older lambs involving the feet and lower limbs occur on fields with a lot of thistles or stubble. Why the disease varies so much is not fully understood. Lambs born into an environment contaminated with a lot of virus are more likely to develop severe oral orf, and any environment causing skin abrasions will exacerbate disease. Variability of virus strains has been demonstrated and there is field evidence that flocks that have suffered only mild orf for years may undergo more serious disease when a new strain is introduced. There is anecdotal evidence of differing breed susceptibilities, but strong evidence of variation in an individual animal’s ability to overcome the virus. Some individuals can have exuberant dry scabby growths which shed virus for months making them chronic carriers of infection.
Control
Virus in dried scabs which have fallen off can be infectious for years if maintained in a cool dry environment. Lambing sheds should therefore be thoroughly cleaned and disinfected to prevent build-up of infection. Outside, the virus survives for up to 6 months so again cleaning and disinfecting troughs, racks and creep feeders can help prevent introduction of virus to a susceptible group.
On affected farms vaccination is an important aid to control. The vaccine (ScabivaxTM – Schering Plough Animal Health) is a suspension in blue/green coloured glycerine of scab material obtained from sheep infected with a selected strain of orf virus. This vaccine virus cross-protects against all field strains of orf virus. Where orf is a problem in ewes and/or lambs in a lambing flock, vaccination of both ewes and lambs may be necessary. The live vaccine virus can be shed for up to 8 weeks after administration and can be a source of infection to susceptible animals. Ewes must therefore not be vaccinated during the last 8 weeks of pregnancy nor allowed access to the lambing area until all scabs have been shed. Lambs can be vaccinated at any time from birth once the birth fluids have dried but delaying vaccination until turn-out will avoid contamination of the buildings. Vaccination should be done strictly according to the manufacturer’s instructions; the safest routine vaccination site is the skin between the fore-leg and the chest wall.
Vaccination can be useful in the face of an outbreak as a way of artificially exposing all animals and ensuring they experience only mild disease. In an outbreak, all affected animals should be isolated from the main flock and the remainder vaccinated as soon as possible. Any further animals developing orf lesions should be drafted into the affected group as the disease becomes apparent.
Treatment
Treatment of affected animals is aimed at reducing bacterial invasion of lesions using topical antibiotic sprays and/or astringents. Antibiotics should be used prudently to prevent the selection of resistant bacteria, but in severe infections injectable antibiotics may be appropriate. In lambs which have difficulty sucking attention to feeding will prevent debilitation and aid survival. There are a variety of treatments that have their advocates and devotees. It is difficult to assess their efficacy under field conditions and evidence from controlled trials is lacking.
Human Orf
Orf is a common infection in people handling sheep. Virus infection will establish through broken skin and thus the fingers of those bottle feeding lambs are particularly at risk. A single, painful raised lesion is seen first which increases in size over 2-4 weeks with a weeping surface and a white centre. The lesion then crusts but will bleed if knocked. Only after 6-8 weeks will the crust dry up and detach leaving no scar. Care must be taken to prevent spread to other sites or to other people. Cases of generalised malaise have been reported and generalised skin reddening (erythema multiforma) also occurs in some patients. Extensive lesions can occur in immunosuppressed patients and people with atopic dermatitis may be more vulnerable to severe infection. Recovered patients will have antibody to orf virus but immunity is short lived and re-infection may occur. Treatment of orf in humans is also aimed at preventing complications through secondary bacterial infection but anti-viral compounds hold realistic promise for future treatment. One such drug, Cidofovir, has recently been used successfully to treat an immunosuppressed patient with severe orf.
Footnote: this article fist appeared in the Sheep farmer magazine and is reproduced with their permission