Pneumococcal meningitis threshold model: A potential tool to assess infectious risk of new or existing inner ear
Benjamin P C Wei1, Robert K Shepherd, Roy M Robins-Browne
1Department of Otolaryngology, University of Melbourne, Melbourne, Victoria, Australia. bwei@bionicear.org
Hypothesis:
A minimal threshold of Streptococcus pneumoniae is required to induce meningitis in healthy animals for intraperitoneal (hematogenous), middle ear, and inner ear inoculations, and this threshold may be altered via recent inner ear surgery.
Background:
There has been an increase in the number of reported cases of cochlear implant-related pneumococcal meningitis since 2002. The pathogenesis of pneumococcal meningitis is complex and not completely understood. The bacteria can reach the central nervous system (CNS) from the upper respiratory tract mucosa via either hematogenous route or via the inner ear. The establishment of a threshold model for all potential routes of infection to the CNS in animals without cochlear implantation is an important first step to help us understand the pathogenesis of the disease in animals with cochlear implantation.
Methods:
Fifty-four otologically normal adult Hooded Wistar rats (27 receiving cochleostomy and 27 controls) were inoculated with different amounts of bacterial counts via three different routes (intraperitoneal, middle ear, and inner ear). Rats were monitored during 5 days for signs of meningitis. Blood, cerebrospinal fluid, and middle ear swabs were taken for bacterial culture, and brains and cochleae were examined for signs of infection.
Results:
The threshold of bacterial counts required to induce meningitis is lowest in rats receiving direct inner ear inoculation compared with both intraperitoneal and middle ear inoculation. There is no change in threshold between the group of rats with cochleostomy and the control (Fisher's exact test, p < 0.05).
Conclusion:
A minimal threshold of bacteria is required to induce meningitis in healthy animals and is different for three different routes of infection (intraperitoneal, middle ear, and inner ear). Cochleostomy performed 4 weeks before the inoculation did not reduce the threshold of bacteria required for meningitis in all three infectious routes. This threshold model will also serve as a valuable tool, assisting clinicians to quantitatively analyze if the presence of a cochlear implant or other CNS prostheses alter the risk of meningitis.
Insights
A minimal number of Streptococcus pneumoniae bacteria are needed to cause meningitis, with inner ear infection requiring the lowest amount. Recent inner ear surgery did not alter this bacterial threshold for meningitis in rats.
Area of Science:
- Neuroscience
- Infectious Diseases
- Otolaryngology
Background:
- Cochlear implant-related pneumococcal meningitis cases have increased since 2002.
- The exact pathways of pneumococcal meningitis development are not fully understood.
- Bacteria can reach the central nervous system (CNS) via hematogenous spread or the inner ear.
Purpose of the Study:
- To establish a threshold model for Streptococcus pneumoniae infection routes to the CNS in animals without cochlear implants.
- To understand the pathogenesis of pneumococcal meningitis in animals with cochlear implants.
Main Methods:
- Fifty-four rats underwent either cochleostomy or served as controls.
- Rats were inoculated with varying bacterial loads via intraperitoneal, middle ear, or inner ear routes.
- Infection signs and bacterial presence in blood, CSF, and tissues were monitored over five days.
Main Results:
- The lowest bacterial threshold for inducing meningitis was observed with direct inner ear inoculation.
- No significant difference in the meningitis threshold was found between rats with cochleostomy and control rats.
- Bacterial counts required to induce meningitis varied significantly by inoculation route.
Conclusions:
- A specific bacterial threshold is necessary to cause meningitis, differing by infection route.
- Cochleostomy performed four weeks prior did not alter the bacterial threshold for meningitis.
- This threshold model can help assess the risk of meningitis associated with cochlear implants and other CNS prostheses.
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