Physiology of the cat flea ( Ctenocephalides felis ) gut: serine protease activity, midgut pH, and intestinal barrier integrity

Fleas carry many bacterial pathogens, such as Bartonella henselae, Rickettsia typhi, and Yersinia pestis, which must adapt to the unique physiology of the flea digestive system to survive. Despite the importance of the gut environment in supporting pathogen persistence, key aspects of flea digestion and the intestinal barrier remain poorly understood. This study investigated the digestive system of the cat flea (Ctenocephalides felis), focusing on midgut pH, serine protease activity, and how aging affects intestinal integrity. Our results showed that trypsin activity is stable during early digestion but peaks around 24 h after a blood meal, during which the midgut temporarily becomes more alkaline before returning to acidic conditions. Bacterial infection disrupted the normal gut environment, leading to prolonged acidification and delayed but heightened trypsin activity. This suggests that fleas actively modify their gut conditions in response to pathogens. To assess structural integrity, we measured collagen IV denaturation in the basal lamina and used a non-absorbable food dye to evaluate barrier permeability. Denatured collagen was highest at 7 d post-emergence, but no dye leaked into the hemocoel, indicating that the midgut barrier remained functionally intact. Overall, these findings highlight a complex interaction between digestion, immune response, and aging in the flea gut, providing insights into how the midgut environment influences pathogen survival and transmission.