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3.1
Evidence for infection
3.2 Nature of the infectious agent
The aetiology of BSE has never been much in question. BSE is a neurological disease with distinctive microscopic lesions in the central nervous system, exactly like scrapie (see Histopathology, p. 33). Historically, the recognition of the characteristic scrapie-like picture in other diseases led to the studies which demonstrated the experimental transmissibility of TME, kuru, CJD and chronic wasting disease (CWD) (see Table 1). It seemed almost certain that BSE would also be transmissible.
Extracts of BSE-affected brain contain abnormal fibrils very similar to SAF. These fibrils are derived from a normal host-coded protein PrP, which has undergone an abnormal post-translational modification. The fibrils obtained from BSE-affected brain are made up of the same modified host protein (see Molecular pathology, p. 35). The presence of SAF is another characteristic feature of the transmissible spongiform encephalopathies which indicated an infectious aetiology for BSE.
Because of the long incubation period of these diseases, formal proof of transmissibility came about two years after BSE was first recognized.
The transmissibility of BSE was first demonstrated in mice in which a disease very like murine scrapie developed about 300 days after the combined intracerebral and intraperitoneal injection of affected cattle brain homogenates (Fraser et al., 1988). The same material was injected both intracerebrally and intravenously into cattle and produced cases of BSE after 500 to 650 days (Dawson, Wells and Parker, 1990; Dawson et al., 1991).
Other studies showed that BSE can be transmitted to mice by feeding BSE affected cattle brain (Barlow and Middleton, 1990a; 1990b), thus reproducing what the epidemiological studies show to be the natural route of transmission in cattle (see The vehicle of infection, p. 20). Studies initiated in 1979, for reasons unconnected with BSE, provide some direct evidence for the transmissibility of scrapie to cattle by the injection of infected brain from cases of experimental scrapie in sheep and goats (Gibbs et al 1990). There is no doubt that BSE is caused by a scrapie-like infectious agent after long incubation periods.
3.2 Nature of the infectious agent
The physicochemical nature of the scrapie agent is a subject of enduring fascination and considerable controversy. From a practical point of view, knowledge of the chemical structure of the agent would be of enormous importance in providing a diagnostic test of infection. In the absence of sufficient knowledge (and a test), little can be said which is of relevance to the animal health problems posed by BSE. The following is a very brief summary of current information and hypotheses on the nature of the agent which are discussed more fully elsewhere (Kimberlin, 1990a)
Most information has come from studies of the scrapie agent. The agent is small enough to pass through bacteriological filters, thus demonstrating that it is virus-like or subviral in size. But the agent has other properties which are atypical of viruses. The first is that infectivity is highly resistant to many physicochemical treatments, such as heat, and exposure to ionizing or ultra violet radiation. It is no surprise that some infectivity can survive rendering processes (see The vehicle of infection, p. 20, and The start of the BSE epidemic, p. 22). Second, scrapie infection neither induces an immune response nor impairs the immunological responsiveness of the host to other infections. This is in keeping with the non-inflammatory nature of the central nervous system lesions and it is a major reason why there are no laboratory diagnostic tests for the infectious agent. The combination of long incubation period, unusual stability and immunological neutrality explains why the scrapie group of agents have long been known as the "unconventional slow viruses".
There are still major uncertainties about the chemical nature of the scrapie agent. Part of the problem is that studies are critically dependent on bioassays of infectivity in laboratory animals such as hamsters and mice, with the attendant long incubation periods. But the greatest problem has been the inherent "stickiness" of the scrapie agent which has bedevilled attempts at purification and impeded its chemical characterization.
Ignorance about the structure of the scrapie agent has not prevented other research which has led to a good understanding of the biological properties of the agent, the pathogenesis of the disease and the underlying reasons for its long incubation periods.
Many different strains of scrapie can be identified by their incubation periods, under standard conditions of experimental infection, and by the severity and distribution of histological lesions in the central nervous system. About ten different strains of scrapie are easily recognized by their biological properties in mice and at least three have been identified in hamsters. Mutation of the scrapie agent is well documented in both hamsters and mice and it is clearly not a rare event.
Therefore, scrapie closely resembles other microbial infections in exhibiting strain variation and mutation. This means that the infectious agent has a strain-specific genome. On a priori grounds, the genome is likely to be nucleic acid, even though it has not yet been identified. Some authors maintain that the agent is probably a virus, however unconventional it may be.
The ultraviolet irradiation properties of the scrapie agent indicate that the putative nucleic acid genome is very small. Its estimated target size to ionizing radiation is less than a molecular weight of 100 000. This may be too small for it to code for the protein which, as studies with proteases have shown, is a necessary component of the infectious agent. This has given rise to the "virino" hypothesis which proposes that the protein needed to protect the genome is host-coded. The lack of an immune response to infection could then be explained simply by the absence of foreign antigens. Taxonomically this places virinos between conventional viruses and viroids (a class of plant pathogens which neither need nor code for proteins to be infectious).
The purification of SAF is associated with the partial copurification of infectivity. However, a large amount of modified PrP accumulates in clinically affected brains and it is not easy to prove that the association of infectivity with purified SAF is other than a fortuitous consequence of a very sticky agent. Even if some modified PrP is a component of the infectious agent, there is evidence that much of it is not.
Nevertheless, the association between infectivity and modified PrP has led to further speculation. One possibility is that modified PrP may be the host protein which, according to the virino hypothesis, protects the putative nucleic acid genome. This hypothesis is intellectually attractive because it immediately provides a basis for the interaction between the sip/PrP gene and the infectious agent in natural scrapie.
However, modified PrP is the only molecule to have been identified in preparations containing high infectivity, and another possibility, the "prior" hypothesis, is that modified PrP is itself the infectious agent.
The central issue is the nature of the scrapie genome; on this depends a diagnostic test for infection. Supporters of the virus and virino hypotheses are waiting for a scrapie-specific nucleic acid to be found. Proponents of prions need evidence to explain how scrapie strain variation and mutation can be based on a post-translationally modified, normal protein.