Development of a model of focal pneumococcal pneumonia in young rats

Abstract

BACKGROUND: A recently licensed pneumococcal conjugate vaccine has been shown to be highly effective in the prevention of bacteremia in immunized children but the degree of protection against pneumonia has been difficult to determine. METHODS: We sought to develop a model of Streptococcus pneumoniae pneumonia in Sprague-Dawley rats. We challenged three-week old Sprague-Dawley pups via intrapulmonary injection of S. pneumoniae serotypes 3 and 6B. Outcomes included bacteremia, mortality as well histologic sections of the lungs. RESULTS: Pneumonia was reliably produced in animals receiving either 10 or 100 cfu of type 3 pneumococci, with 30% and 50% mortality respectively. Similarly, with type 6B, the likelihood of pneumonia increased with the inoculum, as did the mortality rate. Prophylactic administration of a preparation of high-titered anticapsular antibody prevented the development of type 3 pneumonia and death. CONCLUSION: We propose that this model may be useful for the evaluation of vaccines for the prevention of pneumococcal pneumonia.

Figure 1

Hematoxylin-Eosin stain preparation of lung sections (original magnification 100×) obtained from autopsied rats following injection with a low (100 cfu per injection, panel A), medium (1000 cfu per injection, panel B) and high (10,000 cfu per injection, panel C) inoculum of type 6B pneumococcus in 0.5% low melting-point agarose. As the size of the inoculum increases, there is a clear progression from normal-appearing lung, focal pneumonia and diffuse pneumonia. Shown are 3 slides from a representative experiment.