Species: Sheep

1. INTRODUCTION

Orf is a highly contagious disease of sheep and goats caused by a parapoxvirus. The virus causes pustular and scabby lesions on the non-woolly areas of the skin and occasionally in the mouth and oesophagus. Abrasion of the skin is required to establish infection, for example, through rough grazing.

The orf virus can survive for years in shed scabs in a dry environment such as in lambing sheds (Robinson & Balassu, 1981) but rapidly loses infectivity when exposed to rain on pasture (McKeever & Reid, 1986). Sheep may also carry the virus without showing lesions and introduce the disease into susceptible flocks (Nettleton, et al., 1996).

The clinical manifestations of orf in sheep are very variable (Lewis, 1996; Reid, 1991).

(a) In ewes, lesions are usually found on the udder and teats. This may result in acute

secondary staphylococcal mastitis leading to loss of udder function or death of the ewe.

(b) Lambs sucking infected teats commonly develop orf. Lesions in suckling lambs are most

frequently seen around the month and nostrils and may extend into the mouth and

oesophagus. The pain caused by these lesions may prevent the lamb from feeding and it

may die from dehydration and starvation. Ewes with painful lesions on the udder may also

prevent their lambs from suckling with the same consequences.

(c) Grazing and finishing lambs may develop the mouth form of orf or more rarely, a

condition known as strawberry footrot. In the latter condition, lesions occur on the lower

part of the legs and may cause lameness. Lambs with either form of orf may suffer

reduced growth rates.

(d) A venereal form of orf occurs in ewes and rams. Affected rams become reluctant to mate.

This disrupts the breeding season with a consequent long drawn out lambing.

There is no effective treatment for orf in sheep and present control strategies are considered inadequate (Reid, 1995). The main method of control in affected flocks is immunisation using a live vaccine, which itself causes orf in sheep.

The orf virus is transmissible to humans, in whom it causes painful skin lesions.

Although orf is considered to cause major economic loss in the sheep industry (Reid, 1995), we have been unable to find any published studies on the cost of orf in Great Britain.

2. PREVALENCE/INCIDENCE

2.1 Population at risk

All sheep flocks in Great Britain are potentially at risk of contracting orf. The sheep populations most at risk are breeding ewes and suckling and growing lambs.

The incidence of orf in sheep flocks in Great Britain is not known, although the disease is considered widespread.

In a study that we recently concluded for MAFF on the health and management of slaughter lambs in 49 flocks in Great Britain (unpublished data), orf was reported in lambs on four (8.2%) farms. However, this was not a random survey of the national sheep flock, nor was it specifically directed at orf.

It has been suggested that the prevalence of orf has increased over the last decade (Lewis, 1996) but this is not borne out by the number of cases seen at the Veterinary Investigation Centres (VICs) over this time (Table 1).

Table 1: Number of diagnoses of orf made at VICs from 1984 to 1995 as a proportion of total

A possible value for the prevalence of orf in lamb flocks in Great Britain is 10% but this is based on our one very limited study of the health of slaughter lambs.

To determine the economic cost of orf, information is also required on the proportion of sheep flocks in Great Britain that experience the various clinical manifestations of orf as these affect the type of losses incurred. However, this information does not seem to have been collected.

2.3 Prevalence/incidence of orf within flocks

Once a flock contracts orf, the disease spreads rapidly among exposed groups in the flock (Robinson & Balaussu, 1981). For example, if pregnant or nursing ewes become infected, most ewes in the group together with their lambs are likely to develop orf.

Suggested values for prevalence of orf in ewes/lambs within flocks are 90 to 100%.

3. CONTROL METHODS

3.1 Vaccination

Currently, two orf vaccines are licensed for use in the UK: Scabivax (Mallinckrodt Veterinary Ltd) and Vaxall Orf Vaccine (Willows Francis Veterinary). Both are live vaccines and work by infecting sheep with a ‘mild field strain’ of the orf virus. Because their use introduces orf onto a premises, they are only recommended for flocks which already have a problem with orf. Neither vaccine produces a strong long-lasting immunity, but their use will reduce the severity of clinical outbreaks of orf in a flock.

The usual practice is to vaccinate pregnant ewes eight weeks before lambing to allow scabs to heal and fall. It is recommended that growing lambs are vaccinated six to eight weeks before an expected outbreak. Vaccination may also be used in the face of an outbreak of orf.

There is no published information available on the level of usage of orf vaccines in sheep flocks in Great Britain.

In the study that we recently concluded for MAFF of the health and management of slaughter lambs in 49 flocks in Great Britain (unpublished data), lambs on four (8.2%) farms were vaccinated against orf. Three farmers used Scabivax. The fourth farmer did not specify the brand of vaccine used. These farms were not those that reported clinical cases of orf in lambs.

against orf annually.

4. CASE TREATMENT

In uncomplicated cases of orf, lesions typically heal between 24 and 30 days. However, if lesions become infected with secondary bacteria, recovery may take much longer.

Being a viral infection, there is no specific treatment for orf. Secondary bacterial infections are treated with antibiotics. Surgery may be necessary to remove lesions from the mouths of lambs.

There does not appear to be any published information on the number of sheep that require such treatments annually, nor on the average cost of treating severe clinical cases of orf.

In severe outbreaks of orf in nursing ewes and suckling lambs, it may be necessary to artificially feed the lambs. This is a very labour-intensive activity.

No information was found on the amount of time spent treating cases of orf and the flow-on effects of the disease.

5. DISEASE EFFECTS

5.1 Mortality

The mortality rate in cases of orf is usually less than 1% but secondary complications can increase this rate to around 20 to 50% (Robinson and Balassu, 1981).

5.2 Premature culling

Ewes, which suffer loss of mammary function due to orf, may have to be culled.

5.3 Reduced growth rates

Growth may be retarded in lambs affected with the mouth form of orf or with strawberry footrot.

5.4 Reproductive performance

There may be a reduction in the reproductive performance of a flock if rams affected with the venereal form of orf become reluctant to mate. Lambing percentages may be reduced and/or the breeding season and lambing may be extended.

5.5 Human infections

Humans may become infected with the orf virus by handling infected sheep, wool or skins, or accidently when vaccinating sheep against orf. Farmers, abattoir workers, veterinarians and shearers are at greatest risk of infection.

The virus causes painful lesions on the the hands, fingers and face that respond poorly to treatment. In rare cases, the virus may cause a systemic reaction sufficiently severe to require hospitalisation.

Around 50 cases a year are reported in the UK (HSE, 1993). These are likely to be the most serious cases. In uncomplicated cases, lesions resolve in 5 to 8 weeks after infection. However, due to the location of lesions on fingers and hands, they may prevent the affected person from working during this period.

5.6 Disruption to farm management

Outbreaks of orf in a flock prevent sheep being marketed, slaughtered and going to shows.

5. 7 Animal welfare

Orf is considered a serious animal welfare problem (Reid, 1995). The lesions of orf are painful and cause much distress, particularly to ewes and suckling lambs.

6. REFERENCES