Digestion of host immunoglobulin and activity of midgut proteases in the buffalo fly Haematobia irritans exigua

Abstract

The digestion of blood by the buffalo fly ( Haematobia irritans exigua) was monitored for 6 h at 33°C after a single meal. Following the meal, the concentration of soluble protein within the midgut increased to a peak at 2 hours then decreased steadily over the next 4 h. The magnitude of the increase in soluble protein at 2 h indicated a release of protein from another source; most likely from lysed red blood cells. The immunoglobulin (IgG) fraction of the blood meal was digested rapidly (50% within one hour of feeding) and fully digested within 4 h. This is indicative of its accessibility to digestive enzymes within the midgut. In contrast, when flies had continuous access to blood, the concentration of IgG in the midgut remained at a more constant level. The loss of antigen-binding activity of a specific antibody was more rapid than complete degradation of the IgG, with 70% of binding activity lost within one hour of feeding. The level of trypsin activity in the midgut increased from pre-feeding levels to reach a peak at 2 h before returning to basal levels after 6 h. The pattern of trypsin activity follows closely that of the concentration of soluble protein in the midgut ( r=0.88). The activity of leucine aminopeptidase in the midgut also increased immediately after feeding and remained elevated for 4 h before declining to a basal level after 6 h. The rapid digestion of IgG and subsequent loss of antibody activity suggests that for a specific anti-buffalo fly antibody to be effective it would need to be able to either evade the digestive system or induce a rapid response.