Male and Female Cat Fleas (Ctenocephalides felis) Differ in Bacterial Control, Survival, and Circulating Hemocyte Responses Following Systemic Challenge.

Although both male and female cat fleas (Ctenocephalides felis) consume vertebrate blood throughout adulthood, previous studies of flea immunity have focused primarily on females, leaving whether systemic immune responses differ between the sexes largely unknown. To address this knowledge gap, we compared bacterial persistence, survival, circulating hemocyte abundance, and phagocytic activity in male and female C. felis following microinjection of Escherichia coli into the hemocoel. Females exhibited significantly lower infection prevalence and bacterial burdens than males 24 h post-challenge. Consistent with these differences, bacterial infection reduced survival in both sexes but had a substantially greater effect on males, while sterile injury alone significantly reduced male survival but had little effect on females. Across all treatment groups, females possessed significantly higher circulating hemocyte abundance than males and were the only sex to exhibit an increase in circulating hemocyte numbers following injury or bacterial challenge. Additionally, a significantly greater proportion of female hemocytes participated in phagocytosis, although the number of bacteria internalized per hemocyte did not differ between the sexes. Together, these findings demonstrate sex-specific differences in the systemic antibacterial responses of C. felis, with females exhibiting lower bacterial burdens, greater survival, higher circulating hemocyte abundance, and a greater proportion of phagocytic hemocytes than males. These results identify sex as a biological variable influencing systemic immune responses in cat fleas and provide a foundation for determining whether similar differences occur following natural routes of microbial exposure.