Orf

R P KITCHING

GENERAL INTRODUCTION: POXVIRIDAE

Introduction

Orf is a contagious disease of sheep and goats characterized by vesicles, pustules and scabs, particularly around the mouth, caused by strains of parapoxvirus.⁹ The disease is infectious to humans. Orf is found in all sheep-rearing countries of the world, and the morbidity rate in all but well-isolated flocks approaches 100 per cent in yearling sheep and goats; in severe outbreaks the mortality rate can be greater than 10 per cent,² particularly when associated with secondaryinfections such as those due to staphylococcus and fly strike. The disease tends to have a seasonal appearance, being particularly destructive when it appears around lambing. Depending on the husbandry practised, it is also common when weaned lambs and kids are put out to graze on coarse pasture, from which the term ‘thistle disease’ is derived.

Aetiology

The orf virion, as seen under the electron microscope, is oval in shape and covered in what appears to be a continuous filament (see the introduction, Poxviridae, and Figure 111.1), unlike the virions of other pox genera which are brick-shaped and covered in short tubular elements. The external dimensions measure 260 by 160 nm. The orf virion is morphologically and antigenically indistinguishable from other members of the parapoxvirus genus which cause pseudocowpox, bovine papular stomatitis and camel orf, and certain other parapox infections of wildlife. The genome is double-stranded DNA. Individual strains of orf virus can be characterized by the pattern produced on agarose gel by the restriction enzyme digests of the DNA. These distinguish orf virus from the parapoxviruses of cattle and other species, but do not exclude the possibility of some strains having the ability to infect both cattle and small ruminants. Monoclonal antibodies have been prepared against orf virus which identify a major neutralizing site, and may be of value in distinguishing between strains of parapoxvirus derived from different species.¹

Traditionally, it was considered that orf virus was extremely resistant to inactivation in the environment, and this was used to explain how it persisted in a flock to infect each year’s lamb crop. Certainly the virus is more robust than most other viruses. It can retain some infectivity after 30 minutes at 55 °C, and scab material stored dry and in the dark at 7 °C remains infective for many years.

Figure 111.1 Parapoxvirus. Note continuous regular arrangement of filaments on the surface, giving the impression of a double helix

Figure 111.2 Orf lesions on the lips

However, when exposed to sunlight or temperate environmental conditions, scab material loses infectivity in a few weeks, and the significance of orf virus persisting on pasture or on gateposts has probably been overstated. The virus can be destroyed by detergents containing lipid solvents, which destroy the virus membrane.

Epidemiology

Although the infection occurs mainly in young animals, both sexes and all breeds and ages of sheep and goats are susceptible to infection.⁸Wild sheep and goats may also become infected. Natural transmission occurs by direct contact or indirectly from the environment or fomites.⁸ Transmission is effected by the orf virus entering the host through abrasions in the skin or mucosa. The presence of persistently infected animals in a flock can explain its annual reappearance, and its introduction to a flock or herd often coincides with that of new, apparently uninfected stock.

Figure 111.3 Orf lesions on the lips

Figure 111.4 Lesions on the ears of a goat

Figure 111.5 Severe gingival lesions below the incisors

Figure 111.6 Lesions in the oesophagus

Spread may also occur from wild ruminants accessing common pasture. Transmission between ewes and suckling lambs is common and results in the greatest losses, and is often associated with inability of the lambs to feed due to the presence of labial and/or buccal lesions and secondary mastitis in the ewes. Lambs at pasture may also be exposed to infection via buccal mucosa damaged by rough or sharp plant material, or skin softened by damp and muddy conditions. Other bacterial or viral infections which compromise the mucosal integrity can also predispose to orf infection, as does skin damage following shearing or rough handling. Vitamin A de- ficiency may contribute to susceptibility to infection. Poor hygiene, particularly in hand-milked flocks or herds, ensures the rapid spread of the virus.

In southern Africa it is believed that abrasions and other superficial injuries caused when feeding on dry stemmy or spiny feed and grass awns are the usual portal of entry of the virus. Goats tend to browse on thorny shrubs and trees, and this habit seems to be responsible for the severity of outbreaks of orf in Boergoats in Namibia and other semi-arid areas. Epidemics of orf in lambs sometimes occur during good rainy seasons when lambs are allowed on pastures with tall, wet grass or shrubs. The wet conditions give rise to softening and small wounds of the skin, particularly of the feet, with the result that the lesions are mostly confined to the coronet of the hooves.

Pathogenesis, clinical signs and pathology

Primary skin lesions develop two to six days after infection at the portal of entry of the virus to the body.⁸ Secondary lesions are formed by local...