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Spiroplasma, small wall-less bacteria, are associated with the transmissible spongiform encephalopathies (TSE). Spiroplasma ribosomal DNA has been detected by PCR and DNA sequence analyses in TSE-affected brain tissues. Antibody against scrapie-associated fibrils, a consistent marker for TSE, has reacted with multiple spiroplasma antigens. A spiroplasma rabbit tick isolate, Spiroplasma mirum, has experimentally induced persistent brain infection in rodents characterized by spongiform encephalopathy. We hypothesized that ruminants as natural hosts for TSE were susceptible to experimental spiroplasma infection. Experiment #1, S. mirum was inoculated intracranially (IC) into sheep, goats and deer. Experiment #2, Spiroplasma spp. isolated from TSE-affected brains via passage in embryonated eggs into cell-free broth were inoculated IC into sheep and goats. Control animals were sham inoculated. All spiroplasma-inoculated ruminants developed spongiform encephalopathy. The deer developed clinical signs simulating chronic wasting disease (CWD). Spiroplasma spp. isolates from TSE brains when inoculated into sheep and goats induced lesions in brain stem, hippocampus and cerebellar cortex remarkably similar to naturally occurring TSE. These data have shown that spiroplasma induce TSE in ruminants, raising the issue that spiroplasma are possible candidate causal agents of TSE. By R01-NS044003 [ABSTRACT FROM AUTHOR]