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Pathology associated with meningoencephalitis during bacterial kidney disease of salmonids
D J Speare1, V E Ostland, H W Ferguson
1Fish Pathology Laboratory, Ontario Veterinary College, University of Guelph, Canada.
Abstract:
The neural pathology associated with spontaneous cases of bacterial kidney disease (BKD), in five species of commercially reared salmonids, was investigated histopathologically and with immunofluorescence. Patterns of localisation of the causative organism of BKD within the central nervous system suggest that haematogenous spread to the meninges, particularly the tela choroidea posterior, the tela choroidea and vascularised capsule of the saccus dorsalis and epiphysis of the epithalamus, and the saccus vasculosus of the hypophysis, appears to be a frequent route by which the central nervous system becomes infected. Retrograde extension from the posterior uvea to the floor of the diencephalon along the epineurium and perineurium of the optic nerve also may be a mechanism of neural invasion. Extension appeared to occur from these sites into adjacent areas of the meninges, the neural parenchyma and ventricles. Demonstration of bacteria within salmonid ependymal cells, as well as the apparent ability of salmonid ependymal cells to respond metaplastically suggest a similarity to mammalian type III ependymal cells (tanycytes). Based on this study, it is apparent that teleosts can survive protracted severe brain damage. This, combined with the apparent similarities of neural response to infection between the salmonids used in this study and higher vertebrates, suggests that teleosts may be a useful lower vertebrate model for studying the pathogenesis and sequelae of bacterial meningitis.
Insights
Bacterial kidney disease (BKD) in salmonids can infect the central nervous system via the bloodstream, spreading to the brain. Teleosts show neural responses similar to mammals, suggesting their use in studying meningitis.
Area of Science:
- Veterinary Pathology
- Fish Neurology
- Infectious Diseases
Background:
- Bacterial kidney disease (BKD) is a significant concern in commercially reared salmonids.
- Neural involvement in BKD has been observed but routes of central nervous system (CNS) infection are not fully elucidated.
Purpose of the Study:
- To investigate the neural pathology of spontaneous bacterial kidney disease in salmonids.
- To identify the routes of central nervous system infection by the BKD pathogen.
- To compare teleost neural responses to infection with those of higher vertebrates.
Main Methods:
- Histopathological examination of affected salmonid brains.
- Immunofluorescence techniques to localize the causative organism of BKD.
- Comparative analysis of neural responses.
Main Results:
- Haematogenous spread to meningeal sites (tela choroidea, saccus dorsalis, epiphysis, saccus vasculosus) is a frequent route for CNS infection.
- Retrograde extension along the optic nerve from the uvea is another potential invasion mechanism.
- Salmonid ependymal cells exhibit metaplastic responses, similar to mammalian tanycytes, and can survive severe brain damage.
Conclusions:
- Teleosts can tolerate significant brain damage, indicating resilience.
- Similarities in neural response to infection suggest teleosts as a valuable model for studying bacterial meningitis.
- Further research using teleosts could advance understanding of meningitis pathogenesis and outcomes in vertebrates.