Quantitative appraisal of murine filariasis confirms host strain differences but reveals that BALB/c females are more susceptible than males to Litomosoides sigmodontis

Litomosoides sigmodontis, a rodent filarial nematode, can infect inbred laboratory mice, with full development to patency in the BALB/c strain. Strains such as C57BL/6 are considered resistant, because although filarial development can occur, circulating microfilariae are never detected. This model system has, for the first time, allowed the power of murine immunology to be applied to fundamental questions regarding susceptibility to filarial nematode infection. As this is a relatively new model, many aspects of the biology remain to be discovered or more clearly defined. We undertook a major analysis of 85 experiments, to quantitatively assess differences in filarial survival and reproduction in male versus female and BALB/c versus C57BL/6 mice over the full course of infection. This large dataset provided hard statistical support for previous qualitative reviews, including observations that the resistant phenotype of C57BL/6 mice is detectable as early as 10 days postinfection (dpi). An unexpected finding, however, was that filarial survival was reduced in male BALB/c mice compared to their female counterparts. Worm recovery as well as the prevalence and density of microfilariae were higher in female compared with male BALB/c mice. Therefore, L. sigmodontis bucks the filarial trend of increased susceptibility in males. This could be partially explained by the different anatomical locations of adult L. sigmodontis versus lymphatic filariae. Interestingly, the effects of BALB/c sex upon microfilaremia were independent of worm number. In summary, this study has significantly refined our understanding of the host-L. sigmodontis relationship and, critically, has challenged the dogma that males are more susceptible to filarial infection.