Litomosoides sigmodontis: Dynamics of the survival of microfilariae in resistant and susceptible strains of mice

Litomosoides sigmodontis in the BALB/c mouse is the only model of filariasi s which allows the observation of the complete development in an immunocomp etent mouse. In this study, we injected microfilariae (mf) intravenously, a s well as into the pleural cavity, the site of natural release of mf from a dult female worms, and followed the kinetics of elimination within the host . In susceptible BALB/c mice, mf circulated at high levels in the blood. In contrast, in C57BL/6 mice, which are refractory to full development, mf we re eliminated rapidly from the peripheral blood. However,6 days after intra pleural injection, viable larvae could be found in the pleural cavity and l ung capillaries of both susceptible and resistant strains. The numbers of m f in the pleural cavity and lung capillaries in individual mice were signif icantly correlated, but not dependent on strain or peripheral microfilaraem ia. Thus, although C57BL/6 mice showed enhanced production of nitric oxide by pleural exudate cells and a faster change in the numbers of circulating leukocytes after injection, rapid killing of mf by cell or nitric oxide-med iated mechanisms were not the reason for the different outcome. Furthermore , 3 h after iv injection, only a small percentage of mf could be recovered from the peripheral circulation, indicating the presence of a reservoir for mf containment. In conclusion, injected mf showed disparate dynamics of pe rsistence within susceptible and resistant hosts, which is similar to the d isparate outcome of natural infections with L. sigmodontis. This difference became obvious within 1 day after injection. The lung capillary system pla ys obviously a crucial part in regulation of microfilaremia. Our model also provides a possible means to explain frequent cases of occult infections i n human filariasis. (C) 2000 Academic Press.