The responses of seven sheep to orf virus infection were
examined by cannulating the efferent supramammary lymphatic before
infecting the drainage area of the node. All animals developed
lesions typical of orf virus infection, and responded after an
initial lag period with an increase in total cell output paralleled
by an increased proportion of lymphoblast cells. The majority of
these lymphoblasts contained immunoglobulin, with a predominance of
the IgG class. When efferent lymph cells from four of the sheep
were cultured in vitro they were found to produce measurable
amounts of virus specific antibody. When assessed in two of the
sheep, the proportion of cells which stained with a T-cell-specific
monoclonal antibody was found to decrease as the response
developed. In contrast, analysis of afferent lymph (derived from
efferent prefemoral lymphatics cannulated eight weeks after removal
of the node) in four sheep revealed no marked cellular changes
following experimental infection.
The serological responses of naturally and experimentally
infected lambs to orf virus infection was analysed using an
enzyme-linked immunosorbent assay (ELISA) together with the Western
Blotting technique. The combination of these methods permitted a
quantitative and qualitative assessment of humoral responses, which
revealed considerable variation between animals. However, all
post-exposure sera reacted with a polypeptide which appears to be a
component of the surface tubules which are characteristic of the
virus. Despite high titres in some animals, there was no apparent
association between humoral responses and recovery or protection.
The heterogeneity of the orf virus population within the UK was
examined using the restriction enzyme EcoRI by digesting viral DNA
prepared from scab material derived from various outbreaks, and
separating the fragments by electrophoresis in polyacrylamide gels.
This technique revealed obvious genomic differences among the
isolates which were examined, extending even to those derived from
animals undergoing synchronous infections on the same premises.
By subjecting orf scab material to various contrived
environmental conditions both indoors and outdoors, it was shown
that exposure to rainfall can abrogate infectivity of the virus,
which questions the significance of virus persisting on pasture as
a source of infection.
Two rams which were shown to suffer from chronic orf infection
suggest that the virus is capable of surviving inter-epizootic
periods within the animal population. These animals had high serum
antibody titres, but exhibited reduced delayed hypersensitivity
responses to intradermal challenge with orf virus when compared
with control animals.
It was concluded that humoral responses play little or no part
in recovery and protection from of virus infection, and that
immunity is manifested by an accelerated reaction which is the
result of a delayed hypersensitivity response.