Überwachung von TSE bei kleinen Wiederkäuern und Schweinen: Eine Pilotstudie

Experimental Transmission of the Chronic Wasting Disease Agent to Swine after Oral or Intracranial Inoculation

Chronic wasting disease (CWD) is a naturally occurring, fatal neurodegenerative disease of cervids. The potential for swine to serve as hosts for the agent of CWD is unknown. The purpose of this study was to investigate the susceptibility of swine to the CWD agent following experimental oral or intracranial inoculation. Crossbred piglets were assigned to three groups, intracranially inoculated (n = 20), orally inoculated (n = 19), and noninoculated (n = 9). At approximately the age at which commercial pigs reach market weight, half of the pigs in each group were culled ("market weight" groups). The remaining pigs ("aged" groups) were allowed to incubate for up to 73 months postinoculation (mpi). Tissues collected at necropsy were examined for disease-associated prion protein (PrPSc) by Western blotting (WB), antigen capture enzyme immunoassay (EIA), immunohistochemistry (IHC), and in vitro real-time quaking-induced conversion (RT-QuIC). Brain samples from selected pigs were also bioassayed in mice expressing porcine prion protein. Four intracranially inoculated aged pigs and one orally inoculated aged pig were positive by EIA, IHC, and/or WB. By RT-QuIC, PrPSc was detected in lymphoid and/or brain tissue from one or more pigs in each inoculated group. The bioassay was positive in four out of five pigs assayed. This study demonstrates that pigs can support low-level amplification of CWD prions, although the species barrier to CWD infection is relatively high. However, detection of infectivity in orally inoculated pigs with a mouse bioassay raises the possibility that naturally exposed pigs could act as a reservoir of CWD infectivity.IMPORTANCE We challenged domestic swine with the chronic wasting disease agent by inoculation directly into the brain (intracranially) or by oral gavage (orally). Disease-associated prion protein (PrPSc) was detected in brain and lymphoid tissues from intracranially and orally inoculated pigs as early as 8 months of age (6 months postinoculation). Only one pig developed clinical neurologic signs suggestive of prion disease. The amount of PrPSc in the brains and lymphoid tissues of positive pigs was small, especially in orally inoculated pigs. Regardless, positive results obtained with orally inoculated pigs suggest that it may be possible for swine to serve as a reservoir for prion disease under natural conditions.

Neuropathological survey reveals underestimation of the prevalence of neuroinfectious diseases in cattle in Switzerland

Neuroinfectious diseases in livestock represent a severe threat to animal health, but their prevalence is not well documented and the etiology of disease often remains unidentified. The aims of this study were to generate baseline data on the prevalence of neuroinfectious diseases in cattle in Switzerland by neuropathological survey, and to identify disease-associated pathogens. The survey was performed over a 1-year period using a representative number of brainstem samples (n=1816) from fallen cattle. In total, 4% (n=73) of the animals had significant lesions, the most frequent types of which were indicative of viral (n=27) and bacterial (n=31) etiologies. Follow-up diagnostics by immunohistochemistry, PCR protocols and next-generation sequencing identified infection with Listeria monocytogenes (n=6), ovine herpesvirus 2 (n=7), bovine astrovirus CH13 (n=2), bovine herpesvirus 6 (n=6), bovine retrovirus CH15 (n=2), posavirus 1 (n=2), and porcine astroviruses (n=2). A retrospective questionnaire-based investigation indicated that animals' owners observed clinical signs of neurological disease in about one-third of cases with lesions, which was estimated to correspond to approximately 85 cases per year in the adult fallen cattle population in Switzerland. This estimate stands in sharp contrast to the number of cases reported to the authorities and reveals a gap in disease surveillance. Systematic neuropathological examination and follow-up molecular testing of neurologically diseased cattle could significantly enhance the efficiency of disease detection for the purposes of estimating the prevalence of endemic diseases, identifying new or re-emerging pathogens, and providing "early warnings" of disease outbreaks.

Atypical transmissible spongiform encephalopathies in ruminants: a challenge for disease surveillance and control

Since 1987, when bovine spongiform encephalopathy (BSE) emerged as a novel disease in cattle, enormous efforts were undertaken to monitor and control the disease in ruminants worldwide. The driving force was its high economic impact, which resulted from trade restrictions and the loss of consumer confidence in beef products, the latter because BSE turned out to be a fatal zoonosis, causing variant Creutzfeldt-Jakob disease in human beings. The ban on meat and bone meal in livestock feed and the removal of specified risk materials from the food chain were the main measures to successfully prevent infection in cattle and to protect human beings from BSE exposure. However, although BSE is now under control, previously unknown, so-called atypical transmissible spongiform encephalopathies (TSEs) in cattle and small ruminants have been identified by enhanced disease surveillance. This report briefly reviews and summarizes the current level of knowledge on the spectrum of TSEs in cattle and small ruminants and addresses the question of the extent to which such atypical TSEs have an effect on disease surveillance and control strategies.

Surveillance and simulation of bovine spongiform encephalopathy and scrapie in small ruminants in Switzerland

BACKGROUND

After bovine spongiform encephalopathy (BSE) emerged in European cattle livestock in 1986 a fundamental question was whether the agent established also in the small ruminants' population. In Switzerland transmissible spongiform encephalopathies (TSEs) in small ruminants have been monitored since 1990. While in the most recent TSE cases a BSE infection could be excluded, for historical cases techniques to discriminate scrapie from BSE had not been available at the time of diagnosis and thus their status remained unclear. We herein applied state-of-the-art techniques to retrospectively classify these animals and to re-analyze the affected flocks for secondary cases. These results were the basis for models, simulating the course of TSEs over a period of 70 years. The aim was to come to a statistically based overall assessment of the TSE situation in the domestic small ruminant population in Switzerland.

RESULTS

In sum 16 TSE cases were identified in small ruminants in Switzerland since 1981, of which eight were atypical and six were classical scrapie. In two animals retrospective analysis did not allow any further classification due to the lack of appropriate tissue samples. We found no evidence for an infection with the BSE agent in the cases under investigation. In none of the affected flocks, secondary cases were identified. A Bayesian prevalence calculation resulted in most likely estimates of one case of BSE, five cases of classical scrapie and 21 cases of atypical scrapie per 100'000 small ruminants. According to our models none of the TSEs is considered to cause a broader epidemic in Switzerland. In a closed population, they are rather expected to fade out in the next decades or, in case of a sporadic origin, may remain at a very low level.

CONCLUSIONS

In summary, these data indicate that despite a significant epidemic of BSE in cattle, there is no evidence that BSE established in the small ruminant population in Switzerland. Classical and atypical scrapie both occur at a very low level and are not expected to escalate into an epidemic. In this situation the extent of TSE surveillance in small ruminants requires reevaluation based on cost-benefit analysis.

Transmissibility studies of vacuolar changes in the rostral colliculus of pigs

Background

Histopathological examinations of brains from healthy pigs have revealed localised vacuolar changes, predominantly in the rostral colliculus, that are similar to the neuropil vacuolation featured in the transmissible spongiform encephalopathies and have been described in pigs challenged parenterally with the agent causing bovine spongiform encephalopathy (BSE). Feedstuff containing BSE-contaminated meat and bone meal (MBM) may have been fed to pigs prior to the ban of mammalian MBM in feed of farmed livestock in the United Kingdom in 1996, but there is no evidence of the natural occurrence of a transmissible spongiform encephalopathy (TSE) in the domestic pig. Furthermore, experimental transmission of BSE to pigs by the oral route has been unsuccessful. A study was conducted to investigate whether the localised vacuolar changes in the porcine brain were associated with a transmissible aetiology and therefore biologically significant. Two groups of ten pigs were inoculated parenterally with vacuolated rostral colliculus from healthy pigs either born before 1996 or born after 1996. Controls included ten pigs similarly inoculated with rostral colliculus from New Zealand-derived pigs and nine pigs inoculated with a bovine BSE brain homogenate.

Results

None of the pigs inoculated with rostral colliculus developed a TSE-like neurological disease up to five years post inoculation when the study was terminated, and disease-associated prion protein, PrP^d, was not detected in the brains of these pigs. By contrast, eight of nine BSE-inoculated pigs developed neurological signs, two of which had detectable PrP^d by postmortem tests. No significant histopathological changes were detected to account for the clinical signs in the PrP^d-negative, BSE-inoculated pigs.

Conclusion

The findings in this study suggest that vacuolation in the porcine rostral colliculus is not caused by a transmissible agent and is probably a clinically insignificant change. The presence of neurological signs in pigs inoculated with BSE without detectable PrP^d raises the possibility that the BSE agent may produce a prion disease in pigs that remains undetected by the current postmortem tests.