Density of infected carriers determines the burden of transmitted infection.

Five week old female FVB/n donor mice infected intranasally with S. pyogenes (1.63×108 cfu per dose) were introduced into a cage of naïve recipient female mice. The Donor: Recipient (D:R) ratio was varied between 4∶4, 3∶5 and 2∶6 between cages. Recipient mice were sampled at 4, 24, 48, 72 and 96 hours after the infection and introduction of the donor mice to a cage. Data show counts from direct nasal sampling from recipient mice only. Donor mice had >5000 cfu recovered at all time points (not shown). The burden of transmitted infection was significantly higher in cages with a D:R ratio of 4∶4 compared with cages with a 2∶4 ratio. (AUC analysis, followed by Kruskal Wallis with Dunns Post test p<0.05). Lines indicate the median and the interquartile range.