Resistance of C3H/HeJ mice to the effects of Haemophilus pleuropneumoniae

Comparisons were made in the mortality associated with an inhaled dose of viable Haemophilus pleuropneumoniae type 5, strain J45, between adult C3H/HeN and C3H/HeJ mice. Mice of both strains were also challenged with Escherichia coli strains O111:B4 and J5. The 50% lethal dose (LD₅₀) of H. pleuropneumoniae in C3H/HeN mice was calculated to be 10^6.5 CFU. At a mean dose of 10^6.7 CFU a 46% mortality rate occurred in C3H/HeN mice, whereas only 10% of the C3H/HeJ mice died (P < 0.01). Deaths occurred significantly earlier in C3H/HeN mice (P < 0.01). No deaths occurred later than 12 h postinfection in either group. Pulmonary lesions in the mice that died were similar to those in pigs that die during the acute phase of H. pleuropneumoniae infection. In surviving mice of both strains, a mild resolving interstitial and bronchopneumonia was present which was not typical of subacute H. pleuropneumoniae infections in swine. Quantitative bacterial isolations from the lungs, liver, and spleen indicate that H. pleuropneumoniae did not multiply in the lungs, was rapidly cleared, and did not become systemic. No deaths occurred in the mice inoculated with E. coli J5 or O111:B4 at mean doses of 10^6.3, 10^7.2, and 10^8.5 CFU, and 10^6.4, 10^7.5, and 10^8.2 CFU, respectively. The difference in the mortality rate between the C3H/HeN and C3H/HeJ mice suggests that endotoxin may be involved in acute deaths in pigs infected with H. pleuropneumonia. As indicated by the E. coli challenge, however, other factors are also likely to be involved. Because of differences in the pathology and microbiology following H. pleuorpneumoniae pulmonary infections in mice and pigs, mice do not appear to be an accurate model of the overall disease in swine.