The strain of Salmonella typhimurium isolated from the subcutaneous abscess of a runted mouse and used in this study was somewhat unusual, but not unique, in that it had a high virulence for young mice, yet low infectivity. This strain could mimic many of the features, signs, and symptoms of immunological runting when injected into neonates, either in pure culture, or when mixed with spleen cells, or when present in infected isologous or F1 hybrid spleen cells. Thus, the incidence of Salmonella runting was dose-dependent and related to the age of the neonate. Runts failed to gain weight, were sickly, and usually died within 30 days. They had a marked splenomegaly and hepatomegaly associated with areas of necrosis. However, in marked contrast to immunological runts they did not have lymphoid atrophy.

The incidence of runting was diminished when frozen-thawed spleen cell suspensions were used, but not with sonicated or heated suspensions or spleen cells from lethally irradiated mice. Runting could be prevented by immunizing breeders with S. typhimurium, and serum from mice immunized against S. typhimurium protected neonates injected with this organism. Isologous adult spleen cells did not protect against Salmonella runting.

It is suggested that in studies on runting only the intravenous route be used and that heated cells serve as a control. More rigid criteria should be applied to runting than those frequently accepted and mice should be autopsied whenever possible.

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