EFFECT OF SPLENECTOMY ON THE SUSCEPTIBILITY OF MICE INOCULATED WITH DIPLOCOCCUS PNEUMONIAE

An experimental model is described which demonstrated increased susceptibility of mice to infection with D. pneumoniae following splenectomy. It was necessary to use small numbers of a particular strain of pneumococcus (D. pneumoniae type 6), intravenous infection and a particular strain of mouse (pathogen-free NCS strain). The increase in susceptibility persisted for at least 4 months after splenectomy. With modifications in experimental design such as use of large numbers of organisms, a different strain of pneumococcus, the intraperitoneal route of infection or a different mouse strain no increase or a much less impressive increase in susceptibility was demonstrated. Following intravenous injection of small numbers of D. pneumoniae Type 6 bacteremia tended to persist in all NCS mice. Multiplication of pneumococci subsequently occurred in a higher proportion of mice with splenectomy and at a more rapid rate than in control animals. Mice with splenectomy usually had more D. pneumoniae per ml of blood than per gram of any tissue. This suggested that in these mice multiplication of microorganisms occurs primarily in blood. In control mice higher concentrations of bacteria were present in spleen than in blood, and higher concentrations were found in blood than in other tissues. These results suggested that in normal mice infected intravenously with small numbers of D. pneumoniae Type 6, the spleen protects by removing and killing small but critical numbers of D. pneumoniae which are circulating in the blood. No evidence was found to suggest that the altered susceptibility is mediated by an effect of splenectomy on numbers of circulating leukocytes or on the antibacterial activity of mouse blood

Influenza and the rates of hospitalization for respiratory disease among infants and young children

Background: Young children may be at increased risk for serious complications from influenzavirus infection. However, in population-based studies it has been difficult to separate the effects of influenzavirus from those of respiratory syncytial virus. Respiratory syncytial virus often circulates with influenzaviruses and is the most frequent cause of hospitalization for lower respiratory tract infections in infants and young children. We studied the rates of hospitalization for acute respiratory disease among infants and children during periods when the circulation of influenzaviruses predominated over the circulation of respiratory syncytial virus. Methods: For each season from October to May during the period from 1992 to 1997, we used local viral surveillance data to define periods in Washington State and northern California when the circulation of influenzaviruses predominated over that of respiratory syncytial virus. We calculated the rates of hospitalization for acute respiratory disease, excess rates attributable to influenzavirus, and incidence-rate ratios for all infants and children younger than 18 years of age who were enrolled in either the Kaiser Permanente Medical Care Program of Northern California or the Group Health Cooperative of Puget Sound. Results: The rates of hospitalization for acute respiratory disease among children who did not have conditions that put them at high risk for complications of influenza (e.g., asthma, cardiovascular diseases, or premature birth) and who were younger than two years of age were 231 per 100,000 person-months at Northern California Kaiser sites (from 1993 to 1997) and 193 per 100,000 person-months at Group Health Cooperative sites (from 1992 to 1997). These rates were approximately 12 times as high as the rates among children without high-risk conditions who were 6 to 17 years of age (19 per 100,000 person-months at Northern California Kaiser sites and 16 per 100,000 person-months at Group Health Cooperative sites) and approached the rates among children with chronic health conditions who were 6 to 17 years of age (386 per 100,000 person-months and 216 per 100,000 person-months, respectively). Conclusions: Infants and young children without chronic or serious medical conditions are at increased risk for hospitalization during influenza seasons. Routine influenza vaccination should be considered in these children.Link_to_subscribed_fulltex