Inhibition of growth of Lucilia cuprina larvae using serum from sheep vaccinated with first-instar larval antigens.

Whole first-instar Lucilia cuprina larvae were homogenised and sequentially extracted with a series of buffers of progressively more severe solubilising power. The final extract, using a buffer containing 6 M-urea, was fractionated by preparative isoelectric focussing. At each step in this process, protein fractions were tested in sheep vaccination trials for their ability to induce immune responses affecting the growth of L. cuprina larvae which fed on the sera from vaccinated sheep. One isoelectric focussing fraction (pH 5.9-6.7) containing a number of larval proteins induced an immune response which inhibited the growth of larvae by a mean of 84 +/- 7% in an in vitro feeding bioassay. The recovery of larvae after feeding on sera from sheep vaccinated with this fraction was significantly reduced by 35 +/- 13%. This antilarval effect was shown to be mediated by ingested ovine antibodies. Immunofluorescence and immunogold localisations showed that the immune response was directed at proteins from the larval peritrophic membrane, larval cuticle and, to lesser extent, basement membranes and microvilli of digestive epithelial cells. Electron microscopic examination of larvae feeding on sera from sheep vaccinated with this fraction showed that the normally semi-permeable peritrophic membrane was blocked on the luminal side by an electron-lucent layer of undefined composition. It is postulated that this layer prevents nutrients from moving from the gut to the underlying digestive epithelial cells, thereby starving the larvae. The sera from sheep vaccinated with another isoelectric focussing fraction (pH 3.4-5.5) reduced the mean larval weight by 56 +/- 13% without significant effects on larval survival.