The Disease

Orf is a highly contagious eruptive skin condition of sheep and goats, which affects mainly young animals in the first year of their life. The more serious outbreaks are generally associated with intensive sheep husbandry where there is a build-up of infection in buildings (Reid, 1989). The virus is classified as a Poxvirus belonging to the genus Parapoxvirus.

The orf virus primarily affects the skin around the mouth, and under normal circumstances is resolved in four to six weeks. However, lesions may become extensive and spread to other parts of the body, and can be transmitted to the ewe's udder causing her to refuse to suckle. The lesions on the udder may cause acute staphylococcal mastitis in the ewe and starvation of the lambs. Furthermore, the starving lambs may spread the disease to other ewes when trying to steal milk. Corneal and veneral infections with orf virus have also been described (Doherty et al., 1996; Reid, 1989; Reid, 1991). Strawberry foot rot may be a form of orf together with a bacterial infection.

Severely affected young lambs may be unable to eat for days, becoming debilitated and prone to secondary bacterial infection of the lesion which may lead to prolonged disease or even death (Gumbrell and McGregor, 1997).

Orf virus spreads through direct contact with infected animals or virus-contaminated fomites, and infection will only establish at sites where skin is traumatised. Nuisance flies may also be involved in the spread of the virus. Infection spreads rapidly through the flock, with most animals becoming infected within a few weeks. Outbreaks can be particularly severe in housed sheep due to the high stocking rate. Contaminated livestock trailers may also be a source (Gumbrell and McGregor, 1997). Outbreaks usually last for between 6 and 8 weeks, and generally do not reappear until there is a fresh crop of susceptible lambs. Survival of the virus between outbreaks is thought to be in the form of virus contained in scabs. It can survive under cool, dry circumstances for years, but has been found to lose its infectivity quickly in cold, wet circumstances (McKeever and Reid, 1986). Protected from the elements, it can persist in buildings for many years. An alternative source of infection may be the existence of chronic orf infections of sheep (Mckeever, 1984; Nettleton et al., 1996a; Reid, 1989).

Sheep that have recovered from infection have only limited protection against reinfection, although subsequent infections tend to be milder and more short-lived than the initial one (Mckeever et al., 1988). Antibodies are produced against the virus, but even when present in large amounts they seem to have little effect on the course of the disease (Frerichs, 1980; Greig et al., 1984; Yirrell et al., 1989).