PRIONS PETER H. RUSSELL, BVSc, PhD, FRCPath, MRCVS Department of Pathology and Infectious Diseases, The Royal Veterinary College, Royal College Street, London NW1 OTU. Web siteWeb site

Objective Students should be able to: define the characteristics of a prion including the proteinase- resistance, fibre formation and vacuolation in the brain. contrast how ruminant-derived protein transfers BSE between cows and how maternal leucocytes transfer Scrapie. describe how to suspect Scrapie and BSE and the means of diagnosis. outline the other transmissible spongiform encephalophies (TSE’s) and the links between variant Creutzfeldt Jacob Disease of man and BSE.

INTRODUCTION These are ALL notifiable diseases, including scrapie. The agent is found at highest titre in the brain. Scrapie and BSE are separate agents.

Characteristics of Prions :  Proteinaceous infectious particles (Proins termed Prions) have not been visualised.  Prions are assayed for infectivity by mouse inoculation by the intracerebral route, mice develop clinical symptoms several months later.  Scrapie infectivity for mice can be co-purified with prion-protein, PrP of scrapie (PrP sc ).  Normal neuronal cells express cellular PrP c on their cell surface.

Characteristics of Prions (cont.):  Infectivity is not destroyed by ultraviolet light or nucleases which destroy nucleic acid as if any nucleic acid is highly protected or absent.  The catalysis of PrP c into PrP sc, may depend on solely upon the interaction between the infective PrP sc and host PrP c without any nucleic acid replication..  An antiserum and now a mAb to PrP sc cross- reacts to CJD PrP

SCRAPIE Pathogenesis and clinical signs: Sheep Following experimental inoculation the agent is first recoverable from lymphoid tissues (tonsil + mesenteric LN). PrP is seen by immuncytochemistry in dendritic cells and macrophages of 3rd eyelid germinal follicles and 2-3 months later from the medullary region of the mid-brain. In the mid-brain the neurones become vacolated and then shrink. This is accompanied by the formation of amyloid plaques. These plaques contain fibrils of PrP sc. The vacuolation may be caused by the intracellular action of PrP sc. No antibody nor inflammation is produced in vivo.

Genetic susceptibility to Scrapie: The PrP gene determines incubation period which can be the difference between health at 70m and death at 25m. Amino acids at 3 positions, 136, 154 and 171 are important. Resistance is dominant and 171 is the most important amino acid.

Immunity and epidemiology, maternal transmission: In 1990 a questionnaire to flock owners suggested that 33% of British flocks contained infected animals and at the rate of 1-10% with clinical symptoms. The national average was 0.5%.

Diagnosis:  Clinical symptoms hyperaesthesia to final ataxia.  No test for animals incubating the disease.  Post mortem pathology and histology (see above).  The 27-30K protease-k-resistant band of PrP Sc can be detected by Western blotting of solubilised medulla using a mAb to PrP Sc. This is being developed as an automated diagnostic test for BSE (Prionic).

BSE

Pathology Cattle:

Pathology Sheep:

Epidemiology and immunity:

(cont.)

Early-indoor lambers and creep-fed lambs are fed RDP concentrate. However Scrapie /BSE did not increase in sheep during the BSE outbreak in cattle although MAFF now need to ensure that sheep are free from BSE in case lambs can be infected with BSE in the same manner as Scrapie and represent a biohazard.

Epidemiology and immunity: (cont.)

Epidemiology and immunity: Other species:

Case against BSE in man: No cases of human SE have been associated with handling or eating sheep or BSE-infected cattle or milkers eating cattle cake. Transgenic mice which contain the human PrP c gene in place of their own are no more susceptible to BSE than normal mice whereas they developed CJD faster then normal mice (213d instead of 420d).

The effect on food consumption : The consumption of beef dipped 30% after the start of BSE but then recovered although beef consumption is on a slow long-term downward trend. Certain food conglomerates once bought beef only from outside the UK.

Diagnosis:

Control:

Other UK legislation

MINK ENCEPHALOPATHY

References: BSE.Vet.Rec.,(1988) 123, , ; (1996), 139, 126; (1996), 138, BSE.J.Path., (1990) 160, Feline spongiform encephalopathy. Vet.Rec., (1992), 131, BSE in mice; macaques; preclin diag: Nature (1995) 378, ; (1996) 381, ; (1996), 381,563 BSE/CJD epidemiologyNature, Aug 29th1996 BSE, vert transmission,Vet Rec 1997, Aug 16 th, BSE, beef bones now safe, VetRec 1998, 143, Dec5th, pp Variant CJD = BSE?, Nature, 1997, 389, pp , , PrP genotyping for selection of low scrapie rams, Vet Rec, (1998) 142 (23) pp Prionics WB, Shaller et al., Acta Neuropath, (1999), 98,

Summary  Scrapie is maternally-transferred although selective breeding using resistant rams reduces disease.  BSE is transferred by bovine brain being fed as ruminant-derived protein. This was discontinued in July 1988 but the feeding of some BSE-contaminated ruminant-derived protein continued until at least  BSE also occurs in felidae which are fed RDP.  BSE cannot be distinguished from variant CJD of man.  BSE is a single pathotype and bovines lack genes for length of incubation period. Both sources of variation exist for Scrapie in sheep.

Summary (cont.)  BSE and Scrapie have incubation periods of several years. Animals do not make any immune response and die with a spongiform encephalopathy in which fibrils of PrP sc coat the neurones and appear as amyloid.  Public health concern is high because of BSE-brain entering the human food chain and some florid cases of CJD since  Whether BSE is in sheep is being researched