- Infectious Diseases of Livestock
- Part 2
- Orf
- GENERAL INTRODUCTION: PARAMYXOVIRIDAE AND PNEUMOVIRIDAE
- Rinderpest
- Peste des petits ruminants
- Parainfluenza type 3 infection
- Bovine respiratory syncytial virus infection
- Hendra virus infection
- Paramyxovirus-induced reproductive failure and congenital defects in pigs
- Nipah virus disease
- GENERAL INTRODUCTION: CALICIVIRIDAE AND ASTROVIRIDAE
- Vesicular exanthema
- Enteric caliciviruses of pigs and cattle
- GENERAL INTRODUCTION: RETROVIRIDAE
- Enzootic bovine leukosis
- Jaagsiekte
- Visna-maedi
- Caprine arthritis-encephalitis
- Equine infectious anaemia
- GENERAL INTRODUCTION: PAPILLOMAVIRIDAE
- Papillomavirus infection of ruminants
- Papillomavirus infection of equids
- GENERAL INTRODUCTION: ORTHOMYXOVIRIDAE
- Equine influenza
- Swine influenza
- GENERAL INTRODUCTION: CORONAVIRIDAE
- Porcine transmissible gastroenteritis
- Porcine respiratory coronavirus infection
- Porcine epidemic diarrhoea
- Porcine haemagglutinating encephalomyelitis virus infection
- Porcine deltacoronavirus infection
- Bovine coronavirus infection
- Ovine coronavirus infection
- Equine coronavirus infection
- GENERAL INTRODUCTION: PARVOVIRIDAE
- Porcine parvovirus infection
- Bovine parvovirus infection
- GENERAL INTRODUCTION: ADENOVIRIDAE
- Adenovirus infections
- GENERAL INTRODUCTION: HERPESVIRIDAE
- Equid herpesvirus 1 and equid herpesvirus 4 infections
- Equid gammaherpesvirus 2 and equid gammaherpesvirus 5 infections
- Equine coital exanthema
- Infectious bovine rhinotracheitis/infectious pustular vulvovaginitis and infectious pustular balanoposthitis
- Bovine alphaherpesvirus 2 infections
- Malignant catarrhal fever
- Pseudorabies
- Suid herpesvirus 2 infection
- GENERAL INTRODUCTION: ARTERIVIRIDAE
- Equine viral arteritis
- Porcine reproductive and respiratory syndrome
- GENERAL INTRODUCTION: FLAVIVIRIDAE
- Bovine viral diarrhoea and mucosal disease
- Border disease
- Hog cholera
- Wesselsbron disease
- Louping ill
- West nile virus infection
- GENERAL INTRODUCTION: TOGAVIRIDAE
- Equine encephalitides caused by alphaviruses in the Western Hemisphere
- Old World alphavirus infections in animals
- Getah virus infection
- GENERAL INTRODUCTION: BUNYAVIRIDAE
- Diseases caused by Akabane and related Simbu-group viruses
- Rift Valley fever
- Nairobi sheep disease
- Crimean-Congo haemorrhagic fever
- GENERAL INTRODUCTION: ASFARVIRIDAE
- African swine fever
- GENERAL INTRODUCTION: RHABDOVIRIDAE
- Rabies
- Bovine ephemeral fever
- Vesicular stomatitis and other vesiculovirus infections
- GENERAL INTRODUCTION: REOVIRIDAE
- Bluetongue
- Ibaraki disease in cattle
- Epizootic haemorrhagic disease
- African horse sickness
- Equine encephalosis
- Palyam serogroup orbivirus infections
- Rotavirus infections
- GENERAL INTRODUCTION: POXVIRIDAE
- Lumpy skin disease
- Sheeppox and goatpox
- Orf
- Ulcerative dermatosis
- Bovine papular stomatitis
- Pseudocowpox
- Swinepox
- Cowpox
- Horsepox
- Camelpox
- Buffalopox
- GENERAL INTRODUCTION: PICORNAVIRIDAE
- Teschen, Talfan and reproductive diseases caused by porcine enteroviruses
- Encephalomyocarditis virus infection
- Swine vesicular disease
- Equine picornavirus infection
- Bovine rhinovirus infection
- Foot-and-mouth disease
- GENERAL INTRODUCTION: BORNAVIRIDAE
- Borna disease
- GENERAL INTRODUCTION: CIRCOVIRIDAE AND ANELLOVIRIDAE
- Post-weaning multi-systemic wasting syndrome in swine
- GENERAL INTRODUCTION: PRION DISEASES
- Scrapie
- Bovine spongiform encephalopathy
- Transmissible spongiform encephalopathies related to bovine spongiform encephalopathy in other domestic and captive wild species
Orf
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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.
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.
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...
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