Brugia pahangi: effects of duration of infection and parasite burden on lymphatic lesion severity, granulomatous hypersensitivity, and immune responses in jirds (Meriones unguiculatus).

The effects of Brugia pahangi infection duration and parasite burden on parasite-associated inflammatory and immune responses were determined over a 181-day period in jirds receiving from one to eight inoculations of infective larvae. Multiple infections did not produce a protective resistance to reinfection as determined by adult worm recovery at necropsy. Intralymphatic granulomatous lesions, lymph thrombi, were first seen at 48 days post initial inoculation (DPI). The numbers of lymph thrombi reached peak levels in singly inoculated jirds at 90 DPI and significantly decreased to low levels by 160 DPI. The ratio of lymph thrombi to adult worms recovered from the spermatic cord lymphatics followed a similar pattern. Sizes of renal lymph nodes, which drain lymphatics containing parasites, followed a temporal pattern of increase and decrease similar to that of lymph thrombi numbers. Peak granuloma areas around antigen-coated beads embolized in lungs were seen at 27 DPI. Granuloma areas around antigen-coated beads began to decrease after 69 DPI and reached sizes not significantly different from uninfected controls by 118 DPI. Multiple inoculations of infective larvae and increasing worm burdens did not affect the pattern of granulomatous response to antigen-coated beads. Eosinophilia of singly and multiply infected jirds peaked at 26 DPI. Eosinophilia of singly infected jirds returned to normal levels by 103 DPI but those of multiply infected jirds remained elevated until 160 DPI. Lymph node cell blastogenic responses to antigen were greater than those of splenocytes at all time intervals measured. However, significant differences in stimulation indexes between groups with different infection durations were not seen with either cell type. Antibody responses to somatic adult worm antigen as measured by ELISA reached near peak levels by 48 DPI and remained elevated for the course of the study in all infected jirds. The decrease in lymphatic lesion severity seen in chronically infected jirds temporally corresponds to the decrease in granulomatous reactivity measured around antigen-coated beads embolized in the lungs. This observation suggests that host and/or parasite factors associated with these two phenomena may be similar. Although these decreases may be the result of down-regulated immune responses, corresponding decreases in antibody levels and blastogenesis of lymphocytes stimulated by crude worm extracts were not observed in chronic infections.