G. A. H. Wells

Preliminary observations on the pathogenesis of experimental bovine spongiform encephalopathy (BSE): an update

Further preliminary observations are reported of an experiment to examine the spread of infectivity and the occurrence of pathological changes in cattle exposed orally to infection with bovine spongiform encephalopathy. Calves were dosed at four months of age and clinically monitored groups were killed sequentially from two to 40 months after inoculation. Tissues were collected for bioassay, for histopathological examinations and for the detection of PrP. Previous reported observations have included the presence of infectivity in the distal ileum of cattle killed after six to 18 months, the earliest onset of clinical signs in an exposed animal after 35 months, and diagnostic histopathological changes in the brain, in association with clinical disease, after 36, 38 and 40 months. In spite of the relative inefficiency of the bioassay of scrapie-like agents across a species barrier the new observations confirm that the onset of clinical signs and pathological changes in the central nervous system (CNS) occur at approximately the same time. The earliest pathological change, the presence of abnormal PrP 32 months after inoculation, coincided with the earliest detected infectivity in the CNS and occurred shortly before there was evidence of typical spongiform changes in the brain 36 months after inoculation. Infectivity has now been demonstrated in the peripheral nervous system, in the cervical and thoracic dorsal root ganglia 32 to 40 months after inoculation and in the trigeminal ganglion 36 and 38 months after inoculation. At the time of writing evidence of infectivity in other tissues is confined to the distal ileum, not only after six to 18 months but also after 38 and 40 months, but these findings may be supplemented by the results of further mouse assays. Nevertheless, they are in general agreement with current knowledge of the pathogenesis of scrapie.

The Neuropathology and Epidemiology of Bovine Spongiform Encephalopathy

Bovine spongiform encephalopathy (BSE), defined originally from its characteristic neuropathology, retains a place of particular interest in the scrapie‐like or prion disease group, presenting uniquely an example of such diseases occurring as a nationwide food‐borne epidemic in Great Britain. Comprehensive monitoring of the epidemic, both pathologically and epidemiologically, has facilitated our present understanding of the disease. BSE presents the classical neuropathological features of the transmissible spongiform encephalopathies. Although particularly similar to natural scrapie of sheep, BSE has, unlike scrapie, a stereotypic lesion profile from which it has been concluded that host and agent factors, including probably the strain of agent, which influence the profile, are constant in this disease. Neuronal loss in BSE may make an important but hitherto inapparent contribution to functional deficits. Preliminary ultrastructural studies have confirmed light microscopic features of brain changes in BSE but have as yet not established significant new findings.

Detection of disease-specific PrP in the distal ileum of cattle exposed orally to the agent of bovine spongiform encephalopathy

The immunohistochemical localisation of the disease-specific protein, PrPSc, was examined in the distal ileum of cattle up to 40 months after they had been exposed orally to the agent of bovine spongiform encephalopathy (BSE), in the intestines and mesenteric lymph nodes of an additional group of cattle, killed six months after a similar exposure, and in the distal ileum of naturally occurring clinical cases of BSE. PrPSc was detected, mainly in macrophages, in a small proportion of the follicles of Peyers patches in the distal ileum of the experimentally exposed cattle throughout much of the course of the disease. The observations are in agreement with the infectivity data derived from mouse bioassays of the distal ileum. At the later stages of the disease, the proportion of immunostained follicles increased as the total number of follicles decreased with age. In the additional experimental group of cattle, PrPSc was confined to the Peyers patches in the distal ileum. No immunostaining was detected in the lymphoid tissue of the distal ileum of naturally occurring clinical cases of BSE. In some of the clinically affected experimentally induced and naturally occurring cases of BSE there was sparse immunostaining of the neurons of the distal ileal myenteric plexus.

A cohort study to examine maternally-associated risk factors for bovine spongiform encephalopathy

This long-term cohort study, initiated in July 1989, was designed to examine maternally-associated risk factors for bovine spongiform encephalopathy (BSE), forming part of the epidemiological research programme to assess the risks of non-feedborne transmission of BSE. In this study, the incidence of BSE in offspring of cows which developed clinical signs of BSE is compared with that in offspring, born in the same calving season and herd, of cows which had reached at least six years of age and had not developed BSE. All offspring were allowed to live to seven years of age. The results indicate a statistically significant risk difference between the two cohorts of 9.7 per cent and a relative risk of 3.2 for offspring of cows which developed clinical BSE. However, there is some evidence that this enhanced risk for offspring of BSE cases declined the later the offspring was born, but was increased the later the offspring was born in relation to the stage of the incubation period of the dam. The results presented cannot distinguish between a genetic component and true maternal transmission or a combination of both risks, but they do not indicate either that the BSE epidemic will be unduly prolonged or that the future incidence of BSE in Great Britain will increase significantly.

Bovine Spongiform Encephalopathy: A Neuropathological Perspective

The occurrence of bovine spongiform encephalopathy (BSE), recognition that it is a new scrapie‐like disease epidemic in domestic cattle in the United Kingdom and concern of a remote zoonotic potential has, in four years, produced a plethora of documented information. While much of this information has been communicated outwith the scientific literature, this review attempts to summarise, from a neuropathological viewpoint, the main findings to emerge. The initial studies established the nosological homology of BSE with the subacute spongiform encephalopathies or “prion” diseases of animals and man. Epidemiological data are consistent with an extended common source epidemic originating from an abrupt change, commencing in 1981‐82, in the exposure of domestic cattle to a scrapie‐like agent in meat and bone meal incorporated into commercial animal feedstuffs. It is currently proposed that the method of production of meat and bone meal has contributed vital factors to the change in exposure. Invariability of the distribution pattern of vacuolar pathology in the natural disease and on primary transmission to cattle suggests a uniformity of the pathogenesis of BSE. Studies in mice suggest uniformity also of the biological properties of different BSE isolates but indicate that the properties differ from those of sheep scrapie isolates. Human health risks, although perceived to be negligible, have been addressed by various strategies including statutory measures and long term monitoring.

Pathogenesis of experimental bovine spongiform encephalopathy: preclinical infectivity in tonsil and observations on the distribution of lingual tonsil in slaughtered cattle.

The infectivity in tissues from cattle exposed orally to the agent of BSE was assayed by the intracerebral inoculation of cattle. In addition to the infectivity in the central nervous system and distal ileum at stages of pathogenesis previously indicated by mouse bioassay, traces of infectivity were found in the palatine tonsil of cattle killed 10 months after exposure. Because the infectivity may therefore be present throughout the tonsils in cattle infected with BSE, observations were made of the anatomical and histological distribution of lingual tonsil in the root of the tongue of cattle. Examinations of tongues derived from abattoirs in Britain and intended for human consumption showed that macroscopically identifiable tonsillar tissue was present in more than 75 per cent of them, and even in the tongues in which no visible tonsillar tissue remained, histological examination revealed lymphoid tissue in more than 90 per cent. Variations in the distribution of the lingual tonsil suggested that even after the most rigorous trimming of the root of the tongue, traces of tonsillar tissue may remain.

Rapid test for the preclinical postmortem diagnosis of BSE in central nervous system tissue

The efficacy of a rapid test for detecting PrPSc in central nervous system tissue was evaluated for the postmortem diagnosis of BSE at different times during the course of the disease. One hundred and six samples of brain, at the level of the medulla oblongata, and spinal cord, derived from the experimental study of the pathogenesis of BSE carried out in Great Britain between 1991 and 1995, were examined. PrPSc was detected in the samples from most of the exposed animals killed 32 months or more after they had been exposed to the agent, and before the onset of clinical signs which were first recorded at 35 months. Comparisons with the results of histology, fibril detection, PrP immunohistochemistry and mouse bioassay indicated that the rapid test is at least as sensitive as these conventional confirmatory diagnostic methods and its result can be obtained more quickly.

Risk of oral infection with bovine spongiform encephalopathy agent in primates.

The uncertain extent of human exposure to bovine spongiform encephalopathy (BSE)--which can lead to variant Creutzfeldt-Jakob disease (vCJD)--is compounded by incomplete knowledge about the efficiency of oral infection and the magnitude of any bovine-to-human biological barrier to transmission. We therefore investigated oral transmission of BSE to non-human primates. We gave two macaques a 5 g oral dose of brain homogenate from a BSE-infected cow. One macaque developed vCJD-like neurological disease 60 months after exposure, whereas the other remained free of disease at 76 months. On the basis of these findings and data from other studies, we made a preliminary estimate of the food exposure risk for man, which provides additional assurance that existing public health measures can prevent transmission of BSE to man.

Experimental infection of mink with bovine spongiform encephalopathy.

To determine whether the aetiological agent of bovine spongiform encephalopathy (BSE) is pathogenic for mink, standard dark mink were inoculated with coded homogenates of bovine brain from the U.K. Two homogenates were from cows affected with BSE. The third was from a cow that came from a farm with no history of having had BSE or having been fed ruminant-derived, rendered by-products, the proposed vehicle for introduction of the BSE agent. Each homogenate was inoculated intracerebrally into separate groups of mink and a pool of the three was fed to a fourth group. Signs of neurological disease appeared in mink an average of 12 months after intracerebral inoculation and 15 months after feeding. Decreased appetite, lethargy and mild to moderate pelvic limb ataxia were the predominant clinical signs, quite unlike the classic clinical picture of transmissible mink encephalopathy (TME). Microscopic changes in brain sections of most affected mink were those of a scrapie-like spongiform encephalopathy. Vacuolar change in grey matter neuropil was accompanied by prominent astrocytosis. Varying greatly in severity from one mink to another, the degenerative changes occurred in the cerebral cortex, dorsolateral gyri of the frontal lobe, corpus striatum, diencephalon and brainstem. Although resembling TME, the encephalopathy was distinguishable from it by less extensive changes in the cerebral cortex, by more severe changes in the caudal brainstem and by sparing of the hippocampus. The results of this study extend the experimental host range of the BSE agent and demonstrate for the first time the experimental oral infection of mink with a transmissible spongiform encephalopathy agent from a naturally infected ruminant species.

Limited detection of sternal bone marrow infectivity in the clinical phase of experimental bovine spongiform encephalopathy (BSE)

A RECENTLY published report (Wells and others 1998) updated interim findings in a sequential time point study which is examining the spread of infectivity and development of pathological changes in cattle exposed orally to infection with the agent of bovine spongiform encephalopathy (BSE) (Wells and others 1994, 1996). These previous results described the schedule of examination of cattle, killed from two to 40 months after oral exposure, and the development of clinical signs in cattle 35 to 37 months after the exposure. They also demonstrated infectivity by mouse bioassay in: distal ileum (sampled from cattle six to 18 months, 38 months and 40 months after exposure); central nervous system brain and spinal cord (sampled from cattle 32 to 40 months after exposure); and sensory ganglia dorsal root ganglia (sampled from cattle 32 to 40 months after exposure) and trigeminal ganglion (sampled from cattle 36 months and 38 months after exposure). No infectivity had been detected in any of the 35 remaining tissues for which assays were complete at June 1997 (that is, those sampled from cattle two to 22 months after exposure). Mouse bioassays of a large range of tissues from all sequential kill time points of the study have now been completed (at December 1998) and will be reported in full elsewhere. This short communication reports additional data on the bioassay in C57B1-J6 mice of bone marrow, completing results for this tissue from all cattle in the study (Wells and others 1998). Details of the experimental design of the study have been described previously (Wells and others 1996,1998). Bone marrow from the sternum (cancellous bone from the centre of the third or fourth sternebra) was sampled, as for each of the tis-