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Transmissible Encephalopathies in Animals

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Transmissible Encephalopathies in Animals Transmissible Encephalopathies in Animals Richard H. Kimberlin ABSTRACT their physicochemical stability and vacuolar degeneration of grey matter their apparent failure to stimulate any areas of the brain and spinal cord Scrapie in sheep and goats is the kind of protective host response to (hence the generic term, "spongiform best known of the transmissible infection. The biochemical nature of encephalopathies"). In addition brain encephalopathies of animals. The these agents is still a matter of extracts contain large numbers of combination of maternal transmission controversy but many workers con- characteristic amyloid fibrils seen by of infection and long incubation sider them to be outside the known negative stain electron microscopy (4). periods effectively maintains the range of viruses and viroids. For this These fibrils are known as scrapie- infection in flocks. A single sheep gene reason the term "unconventional associated fibrils (SAF: sometimes (Sip) controls both experimental and viruses" is often used. The terms called "prion rods") but they are an natural scrapie and the discovery of "virinos" and "prions" have entered important diagnostic feature of the allelic markers could enable the use of the literature as synonyms for the related diseases as well (5). sire selection in the control of the scrapie family of agents but this may Scrapie-associated fibrils are easily natural disease. Studies of experimen- be premature. Such terms are best purified from clinically affected brain tal rodent scrapie show that neuroin- used to identify the specific hypo- (6). They consist mainly of a mem- vasion occurs by spread of infection thetical concepts about the nature of brane glycoprotein (7) PrP, whose from visceral lymphoreticular tissues these agents that prompted their messenger RNA is present in many along nerve fibers to mid-thoracic introduction. types of cell (8) but is found in cord. The slowness of scrapie is due to The most radical concept, embo- relatively high concentrations in restrictions on replication and cell-to- died in the much published "prion" normal neurons (9). In the course of cell spread of infection affecting hypothesis (1), is that the scrapie agent infection, this protein is modified neuroinvasion and subsequent neuro- is an infectious protein which some- posttranslationally (8,10), accumu- pathogenesis. Probably both stages in how directs its own replication. The lates in brain (11) and acquires the mice are controlled by Sinc gene, the "virino" hypothesis (2,3) draws upon ability to form SAF. Sometimes, murine equivalent of Sip. The glyco- the large body of evidence for the modified PrP is deposited extracellu- protein PrP may be the normal existence of a scrapie-specific genome, larly to form large cerebral amyloid product ofSinc gene. Posttranslation- which is likely to be nucleic acid. It plaques visible by light microscopy ally modified PrP forms the disease suggests that the putative scrapie (12,13). In appearance, these plaques specific "scrapie associated fibrils" nucleic acid is very small, not resemble the amyloid plaques asso- and may also be a constituent of the translated and therefore dependent on ciated with Alzheimer's disease (AD) infectious agent. Scrapie-like diseases a host-coded protein to form an but the latter are formed from a have been reported in mink and infectious agent. Taxonomically, protein that is quite different from several species of ruminants including "virinos" would fit in between viruses modified PrP (14). There are other cattle. All of them may be caused by and viroids. fundamental differences in the patho- the recycling of scrapie infected sheep Several naturally occurring diseases logy of AD and the spongiform material in animal feed. The human are caused by members of the scrapie encephalopathies and it is emphasized health implications are discussed. family of agents. Typically these that AD is not known to be diseases are associated with very long transmissible. incubation periods (often several Scrapie in sheep and goats is the INTRODUCTION years). Overt clinical signs have a best understood member of the chronic progressive course over weeks transmissible spongiform encephalo- The title of this paper refers to or months and invariably culminate in pathies (15). Two related diseases of scrapie and a group of related diseases death. Pathological lesions occur only humans are kuru and Creutzfeldt- of the central nervous system (CNS). in the central nervous system (CNS). Jakob disease (CJD) but neither These diseases are caused by a class of The characteristic lesion seen by light appears to be epidemiologically infectious agents which are notable for microscopy is a noninflammatory, related to scrapie (16). This is not true Institute for Animal Health, AFRC & MRC Neuropathogenesis Unit, West Mains Road, Edinburgh, EH9 3JF, United Kingdom. Present address: SARDAS, 27 Laverockdale Park, Edinburgh, EH13 OQE, United Kingdom. Submitted June 5, 1989. 30 Can J Vet Res 1990; 54: 30-37 of transmissible mink encephalopathy endemic infection of cattle as scrapie is evidence of scrapie replication in the (TME), a rare disease of ranch-reared of sheep. At the moment there is no CNS. This pattern is consistent with mink which occurs when animals are evidence one way or the other but if it infection via the alimentary tract. The sufficiently exposed orally or by did occur, endemic BSE would be as early and persistent infection of scarification to scrapie infected feed difficult to control as scrapie. extraneural tissues may play a role in (17). Transmissible mink encephalo- The occurrence of BSE has reem- initiating neuroinvasion (especially pathy can be experimentally transmit- phasized the importance of scrapie with less neuroinvasive scrapie strains: ted from mink to mink (for example research from both fundamental and see 34). In addition the lymphoreticu- by injection of infected tissue) but practical points of view. Considerable lar system (LRS) is likely to be there is no evidence that this occurs progress has been achieved in many important epidemiologically in pro- naturally. In other words, mink are areas of scrapie research. What viding a reservoir from which infec- "dead-end" hosts for exogenously follows is a brief, selected synthesis of tion can be spread maternally and acquired scrapie infection (18). recent findings which reveal some- horizontally. An infected ewe can Transmissible mink encephalo- thing of the way these diseases work. transmit scrapie to several successive pathy provides an important prece- Two recent books give a more lamb crops before she develops the dent which is relevant to the appear- comprehensive account of current disease (15). This is an efficient way by ance of scrapie-like diseases in five knowledge of the subject (26,27). which scrapie infection is maintained species of captive ruminants in the last in a flock. In contrast, studies of ten years. These are chronic wasting NATURAL SCRAPIE experimental TME in mink indicate disease (CWD) of mule deer (19) and There are no conceptual difficulties very low levels of infectivity in the Rocky Mountain elk (20) in the USA, with the epidemiology of scrapie LRS which could be a major reason single cases of spongiform encephalo- infection in sheep and goats. Infection why there is no natural spread of pathy in nyala and gemsbok in the UK is most commonly transmitted from infection from mink to mink (18). (21), and a large scale epidemic of ewe to lamb, both up to time of Although scrapie is caused by an bovine spongiform encephalopathy parturition and afterwards when ewe infectious agent, host genetic factors (BSE) in the UK (22,23). and lamb run together (15). The age- have a considerable influence on the Of these diseases, BSE is the most incidence curve (with a peak in the development of disease in sheep (but, important. It has the pathological fourth year) reflects the incubation interestingly, not in goats). Selective hallmarks of the whole group (vacuo- period of maternally transmitted breeding studies have been carried out lation, and SAF formed from modi- infection. There is also horizontal with Cheviot (35), Herdwick (36) and fied PrP; 22,24). Transmissibility to spread of infection between unrelated Swaledale (37) sheep injected with mice has recently been demonstrated adults and this can account for some either Cheviot or Swaledale sources of (25). Epidemiological evidence of the scrapie cases in old sheep (28). scrapie. Together these studies suggest strongly suggests that BSE is caused However the mechanisms of maternal that all sheep carry a gene, Sip, which by scrapie infection getting into the and horizontal spread of infection are controls scrapie incubation period cattle population via meat and bone not fully understood. Transplancental (38). This gene has two alleles, sA and meal supplements to concentrated infection and infection via milk are pA. Most sheep carrying the sA allele feed stuffs (23). Differences in the level two possibilities. The placenta from an will develop experimental scrapie (i.e. of concentrate feeding can account for infected ewe is a good source for the sA is dominant for susceptibility) but the 30-fold lower incidence of BSE in spread of infection to unrelated incubation periods are usually longer beef suckler herds compared to dairy animals (29) and the high physico- in the heterozygotes than in the herds. If, like mink, cattle are "dead- chemical stability of the scrapie agent homozygotes (34). Sheep homozygous end" hosts for scrapie infection, then (30) is a major factor in the buildup of for pA are far less susceptible. They the ban (introduced in the UK in mid- contamination on farm premises. rarely develop the disease after 1988) on the feeding of animal derived There is no direct evidence that semen subcutaneous injection of scrapie but protein to ruminants should lead to or ova can become infected and there they can be susceptible to intracere- the disappearance of BSE. However, is much interest in the possibility that bral injection, after extremely long the long incubation period, estimated embryo transfer may provide a basis incubation periods.
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