I. VIII CONFÉRENCE DE LA

I.
A. - RAPPORTS (suite)
REPORTS (continuation)
PONENCIAS (continuación)
2.
Maladies virales des ovins causées par des « slow viruses ».
Viral diseases of sheep caused by « slow viruses ».
Enfermedades de ovinos producidas por « slow viruses ».
VIIIe CONFÉRENCE
DE LA
COMMISSION RÉGIONALE
DE L'O.I.E.
POUR L'EUROPE
Hambourg (République Fédérale d'Allemagne)
4-7 juillet 1978

Bull. Off int. Epiz., 1978 , 89 (7-8), 429-436.
Slow virus diseases of sheep
in Great Britain
by
W.A. WATSON (*)
In 1954 SIGURDSSON introduced the concept of slow infections and
suggested that the slow infectious diseases of sheep included Rida
(Icelandic form of Scrapie), Visna, Maedi (progressive pneumonia)
and Infectious Adenomatosis (Jaagsiekte). Although the accepted
criteria for the classification of diseases in the slow virus group have
changed since that time with a greater understanding of the aetiology
and pathogenesis of many of these diseases, Visna/Maedi, Scrapie
and Jaagsiekte will be reviewed in the context of their importance to
the sheep industry in Great Britain.
VISNA/MAEDI
Visna/Maedi manifested as a chronic pneumonia or paraplegic
disease of sheep has been reported in various breeds in several coun-
tries but never in Great Britain. It has been suggested that certain
breeds, strains or crosses may be more resistant than others to infec-
tion with this virus but undoubtedly husbandry practices, particu-
larly the close housing of sheep, predispose to spread.
The popularity of the Texel breed in Great Britain as a fat lamb
producing sire has justifiably resulted in pressure from the industry
for Texel importations and many sheep of this breed have been
imported from France in the past 3 years. The problem of
Visna/Maedi in the Texel is well recognised and it is particularly dif-
ficult to formulate import certification requirements to afford com-
plete protection against such an insidious disease.
(*) State Veterinary Service, Ministry of Agriculture, Fisheries and Food, Tol-
worth, England.

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Our present import certification requirements are :
a) a period of flock freedom from signs and symptoms suggestive
of Visna/Maedi;
b) negative results to serological tests on the sheep to be imported
and on the adult sheep in the flock of origin, and
c) the flock from which sheep are taken is required to have been
closed to imports other than from flocks in the country concerned
or Great Britain for a period of 5 years. Serological examinations
have utilised the complement fixation test (GUDNADÓTTIR and KRIS-
TINSDÓTTIR, 1967; DE BOER, 1970).
It is, however, essential in view of recent suggestions that specific
antibodies may not be demonstrable in infected sheep, for a tho-
rough appraisal to be made of the CF test and the more recently
developed indirect immunofluorescent test (DE BOER, 1970) and
agar gel immunodiffusion test (TERPSTRA and DE BOER, 1973 ; CUT-
LIP et al., 1977) to reduce to the minimum the risks associated with
Visna/Maedi. Appropriate O.I.E. recommendations regarding the
international trade in sheep should be based on these findings.
SCRAPIE
Scrapie is a natural infection of sheep and goats which has been
recognised in this country for nearly 250 years (McGowan, 1914).
The disease was recorded as being of particular importance in 1795
by CLARIDGE, and the prevalence appears to have fluctuated since
that time, particularly within breeds.
The disease is responsible for considerable financial loss but the
characteristic reluctance of sheep farmers in the past to disclose and
discuss Scrapie has prevented an accurate assessment of its economic
importance to the industry. Direct loss may be due to the death of
sheep or result from the widespread culling of the progeny of affec-
ted sheep and from the loss of reputation of individual breeders,
with a resultant fall in sales. It can disrupt the international trade in
sheep and damage genetic improvement schemes, limiting the selec-
tion potential in those breeding programmes which seek to avoid
Scrapie.
Scrapie is the best known example of the « subacute spongiform
encephalopathies », a group of diseases which includes transmissible
mink Encephalopathy, Kuru and Creutzfeldt-Jakob Disease in man.

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This similarity has stimulated intense research effort, much of it
aimed at the identification of the causal agents. The relation of
Scrapie to human disease and ageing has recently been reviewed by
FIELD (1976). The acceptance of the essential unity of these diseases
(GIBBS and GAJDUSEK, 1971; GAJDUSEK, 1977) and the continued
emphasis on their transmission to primates has provoked public
health controls on the disposal of Scrapie-affected and in-contact
sheep in the United States.
The causal agent of Scrapie, which is highly resistant to a wide
range of physicochemical treatments including irradiation, is trans-
missible, filterable and capable of replication. The experimental
transmission to a variety of animal species which develop characte-
ristic disease after varying periods of incubation is the only way in
which the agent can be recognised and quantified.
The literature on the nature of the Scrapie agent has been revie-
wed by HUNTER and MILLSOM (1977) and from research to date has
emerged the widely held hypothesis that the informational molecule
is likely to be a small membrane bound nucleic acid. Such a small
DNA molecule has been isolated from mouse and hamster Scrapie
brain and the main objective of further work is to determine whe-
ther Scrapie-specific information is coded by this molecule (ARC
Report, 1977). The clinical syndrome of Scrapie can vary in diffe-
rent breeds and species reflecting not only genetic differences bet-
ween the hosts but also strain differences of the agent. The incuba-
tion period and the rate of progression of clinical symptoms is rela-
ted to the strain used. Moreover, experimental evidence in mice sup-
ports the view that different strains of the Scrapie agent can com-
pete for a limited number of multiplication sites and the inoculation
of « slow » agents can prevent « faster » agents injected later from
producing disease.
The utilisation of genetic selection for resistance in sheep enabled
two lines differing in susceptibility by 90% to be established in a
group of Cheviot sheep (DICKINSON et al., 1968); a similarly resis-
tant line of Herdwick sheep was also established (NUSSBAUM et al.,
1975). However, the occurrence of an outbreak of natural Scrapie
recently in sheep in susceptible lines of both breeds unrelated to a
component of the SSBP/1 challenge strain emphasises the duplicity
of strains of the agent. Nevertheless, HOARE et al. (1977) have esta-
blished a nucleus breeding flock of Swaledale sheep comprising
those surviving a Scrapie challenge and these in turn produced off-