The pathological and molecular but not clinical phenotypes are maintained after second passage of experimental atypical bovine spongiform encephalopathy in cattle

Abstract

Background: Atypical bovine spongiform encephalopathies (BSEs), classified as H-type and L-type BSE based on the Western immunoblot profiles, are naturally occurring diseases in cattle, which are phenotypically different to classical BSE. Transmission studies in cattle using the intracerebral route resulted in disease where the phenotypes were maintained irrespective of BSE type but clinically affected cattle with a shorter survival time displayed a nervous form whereas cattle with a longer survival time displayed a dull form. A second transmission study is reported here where four cattle were intracerebrally inoculated with brain tissue from experimentally infected cattle presenting with either the nervous or dull form of H- or L-type BSE to determine whether the phenotype is maintained.

Results: The four inoculated cattle were culled at 16.5-19.5 months post inoculation after presenting with difficulty getting up, a positive scratch response (all) and dullness (three cattle), which was not observed in two non-inoculated control cattle, each housed with either group of inoculated cattle. Only the inoculated cattle had detectable prion protein in the brain based on immunohistochemical examination, and the Western immunoblot profile was consistent with the H-type or L-type BSE of the respective donor cattle.

Conclusions: Second passage of H-type and L-type BSE in cattle produced a TSE where the majority of cattle displayed the dull form regardless of clinical disease form of the donor cattle. The pathological and molecular phenotypes of H- and L-type BSE were maintained.

Figure 1

Time to clinical onset, duration and survival time in atypical BSE brain donors and recipients. Comparison of clinical onset, duration and survival times between H- and L-type BSE brain recipients L5, L6, H5 and H6 and the respective H- and L-type BSE donors L1, L4, H1 and H4, which were intracerebrally inoculated with atypical BSE brain from naturally occurring cases. The survival times of both H-type recipients H5 and H6 and L type recipient L5 were similar, with only 20 days between the first (H5) and the last (L5 and H6) culled animals, whereas animal L6 was culled two months after animals L5 and H6. Clinical duration and survival times were longer in the atypical BSE brain donors, whereas clinical onset was earlier in two of the atypical BSE donors, L1 and H1, compared to the recipients L5 and H5 respectively.

Figure 2

Clinical findings in L- and H-type BSE cases over time compared with controls. Difficulty getting up and gait abnormalities were seen in all four cases, dullness in three cases. Clinical findings were derived from observations and examinations. The horizontal axis represents the time points when neurological examinations were done; a symbol at a particular time point indicates that the sign was observed between this and the previous time point. The vertical, dotted orange lines indicate estimated clinical onset, the vertical, grey interrupted lines the time of cull. Head shy/restless in crush: e.g. head tossing or backing off when faced in crush. Over-reactive to tactile stimuli: e.g. head tossing in response to touching of the head. Abnormal tests of over-reactivity: startle at flash, clipboard test or hand clap, kicking on stick test. Abnormal gait: e.g. dysmetria.

Figure 3

Western immunoblot of brainstem samples from the four recipients detected with three different antibodies. A: mAb SHA31, B: mAb P4 and C: mAb SAF84. Lane Key: M Molecular mass marker. 1 & 7 Classical BSE control. 2 & 8 Classical scrapie control. 3 H5. 4 H6. 5 L5. 6 L6.