Qualitative and quantitative techniques have been used to compare the
follicle populations of control and Border disease affected foetuses and lambs
of the medium - fine fleeced Cheviot x Dorset Horn cross, and the coarse
fleeced Scottish Blackface breeds.
In the Cheviot cross, the main effect of the disease is to cguse primary
follicle hypertrophy, which is first apparent at 115 days' gestation and is
accompanied by an increased fibre size and an increased frequency of medullation.
These changes are not known to occur in any other infectious ovine disease.
The normal suppression of follicle activity which occurs perinatally and at
about the time of birthcoat shedding, and also during the :;inter months is present
in Border disease affected animals. Thus, the differences observed between. the
follicle populations of control and affected animals. at these times are less
obvious. A reduction in the number of secondary follicles which develop is also
apparent in Border disease, but this may be an indirect effect of the disease
process caused by increased primary. follicle size or possibly by impaired foetal
nutrition resulting from the placentitis which.is also a feature of the disease.
The developing primary follicles are susceptible to the effects of Border
disease following maternal inoculation at up to 80 days' gestation. The reason
for the loss of susceptibility thereafter is not clear, although it may be
related to the development of a foetal immunological response to the agent.
Studies of the literature on epidermal- dermal interaction and studies of
the disease process in the Cheviot qross suggest that the follicle papilla
determines the morphology of the follicle and fibre. Thus modification of the
cells of the papillae of primary follicles by the disease process probably
causes follicle hypertrophy. Post -natal studies.indicate that this effect is
durable if not permanent, and thus a new concept is introduced, namely that of
a developmental anomaly leading to ordered hyperplasia rather than the more
usual disordered hypoplasia.
The development of the follicle population of the control Scottish
Blackface resembled that of the Border disease affected medium - fine fleeced
Cheviot cross. Border disease in the Blackface did not cause further primary
follicle hypertrophy. As a working hypothesis to explain these findings, it
is proposed that Border disease prevents the function of an inhibitor substance
which normally controls primary follicle growth in medium and fine fleeced
breeds. Interference with the functioning of the inhibitor contributes to
the development of a coarse fleece.
An analogy exists between the effects of Border disease and the effects
of the mutant N and nr genes in the N -type Romney, which also cause primary
follicle hypertrophy, and the analogy underlines the hypothesis that Border
disease mimics a genetic effect in the skin.
Baselines can be established to determine abnormal medullation in fine
and medium fleeced breeds, and it is suggested that the frequency of medullae
or large medullae in central primary follicles should be used depending on the
breed studied. Peri -natal sampling and sampling during the birthcoat shedding
phase, during the winter months and during terminal illness should be avoided
if useful results are to be obtained. With these provisos, histological
examination of the skin could be a useful tool in medium and fine fleeced
breeds to detect adult carriers and affected lambs.