R.M. Sandeman

Tryptic and chymotryptic proteases released by larvae of the blowfly, Lucilia cuprina.

Proteases released by larvae of the sheep blowfly have been suggested to have a primary role in wound formation and larval nutrition. Assays were carried out on two larval products to analyse the substrate specificity of these proteases, their abundance and approximate molecular weights. Tryptic and chymotryptic activities were found in both products though there were more chymotrypsin-like enzymes in products from 48 h cultures (CESP) than in product collected direct from 48 h larvae (LESP). Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) gels incubated with azocasein showed plaques of major enzyme activity at molecular weights of 20,000 and 26,000 in LESP and at 20,000 in CESP, SDS-PAGE gels, when reacted with peptide substrates showed tryptic activity at 20,000 and 26,000 in LESP, whereas CESP showed only chymotryptic activity at 20,000 and higher molecular weights. The results suggest at least three enzymes, a trypsin and chymotrypsin in LESP, a chymotrypsin in CESP and a tryptic enzyme which is not stable to SDS-PAGE probably in both LESP and CESP. In addition, reactivity with elastase and plasmin substrates suggests the presence of enzymes with general effects on skin substrates and inflammatory pathways.

A scanning electron microscope study of L. cuprina larvae and the development of blowfly strike in sheep

Abstract Scanning electron microscopy was used to determine whether first instar larvae were capable of physically damaging sheep skin and thus initiating blowfly strike without predisposing conditions. In addition, damage at the lesion site was monitored throughout the infection period. It was found that first instar larvae do not possess the mouth hooks seen in second and third instars. They do however have sets of spines within the oral cavity which, with the spines on the creeping welts, may be able to abrade the outermost skin cell layers. Damage to the epidermis was observed within the first 8 h after larval implantation on sheep. The dermis was exposed within the first 24 h and after this time the normal skin architecture was rapidly destroyed. At 72 h areas of wool were missing and the collagen matrix of the dermis was clearly visible. Predisposing conditions such as fleece rot or wounding were not necessary for strike initiation. It is suggested that a combination of physical irritation and abrasion together with enzymes released by larvae enable them to initiate formation of the strike lesion. Thus, the only mandatory predisposing condition for fly strike may be wetted wool and skin.

Acquired resistance in sheep to infection with larvae of the blowfly, Lucilia cuprina

Abstract Acquired resistance in sheep to infection with larvae of the blowfly, Lucilia cuprina . International Journal for Parasitology 16 : 69–75. Resistance to blowfly larvae infections developed in sheep exposed to at least four consecutive infections. Half of the sheep treated showed significant levels of resistance, the others remained susceptible. This resistance took the form of a decreased yield of third instar larvae in comparison to controls and sheep which remained susceptible. In addition an increased sensitivity to larvae developed, as shown by the area of wound obtained per maggot recovered, by the early appearance in resistant sheep of exudate from the infection site and by skin reactions to larval products. Radioimmunoassays demonstrated high levels of serum antibody against larval excretory/secretory antigens, though the response did not peak until after four infections. Resistant animals showed somewhat lower antibody titres than susceptible sheep. Consecutive infections of only 50 larvae failed to induce resistance to larger challenge infections. It is suggested that consecutive infections of larger numbers of maggots induce a hypersensitivity response which may effect larval survival especially of first and second instar maggots.

Initial characterisation of the sheep immune response to infections of Lucilia cuprina

Assays for total serum antibody, histamine sensitivity and the presence of reaginic antibodies were carried out on sheep repeatedly infected with first stage larvae of Lucilia cuprina. Effects of the sheep response on the larvae were monitored at the final infection and compared with control animals by the recovery of larvae and measurement of the wound caused by the larvae. Overall larval survival was not significantly different in the pre-infected group though wound sizes were smaller. Histamine sensitivity appeared to correlate with wound size only in the control group. Thus recent infection experience may lead to immune responses which override non-specific inflammatory events and cause smaller wound sizes. A sub-group of the pre-infected sheep had lower larval survival and smaller wound sizes than the other animals and this correlated with increased levels of reaginic antibody and lower total antibody levels. The results suggest a genetic basis for resistance to fly strike and the possible involvement of reaginic antibody in protective responses.

Immunization of sheep against infection with larvae of the blowfly Lu cilia cuprina

Abstract Sheep were repeatedly exposed to a second stage excretory-secretory antigen preparation of Lucilia cuprina by intradermal injection (S group) or intranasal aerosol (I group) in an attempt to induce immunity to the larvae. Hypersensitivity responses to the injections were monitored and correlated with larval numbers at subsequent challenge. Intradermal injections showed that the immediate or IgE-mediated and the intermediate or Arthus response were the major skin hypersensitivity reactions to the larval antigens. At challenge there was no significant reduction in larval numbers between the S and the control group, however the Arthus reaction did show some correlation with larval recoveries in the S group. There was a significant reduction in larval numbers ( P

Sheep plasma protease inhibitors influencing protease activity and growth of Lucilia cuprina larvae in vitro.

The enzyme inhibitors alpha 2-macroglobulin (alpha 2M), anti-thrombin III (AT III) and alpha 1-proteinase inhibitor (alpha 1PI) were isolated from sheep plasma and tested for their ability to affect L. cuprina larval proteases and larval growth in vitro. Casein radial diffusion gels indicated that both alpha 2M and alpha 1PI completely inhibited the protease activity of a larval excretory-secretory preparation, while AT III had a partial effect. Casein zymograms revealed that alpha 2M inhibited all of the larval proteases, while AT III was able to modify the normal plaque pattern; alpha 1PI inhibited all plaques except a doublet present at pI 8.5. Larval growth in vitro was significantly inhibited by alpha 2M and AT III (P less than 0.05) when compared to albumin controls but was not affected by alpha 1PI. The levels of alpha 2M in sheep serum were monitored over the course of a larval fly infection. A significant increase in alpha 2M (P less than 0.05) was recorded in the serum of flystruck sheep. It is suggested that, under certain circumstances, these inhibitors may be involved in influencing flystrike through reducing the activity of larval proteases necessary for wound formation and larval nutrition.

Prospects for the control of sheep blowfly strike by vaccination

Research into vaccination against flystrike is aimed at either controlling the predisposing condition, fleece rot, or direct control of the fly maggots. A vaccine against the major bacterial species found in fleece rot lesions, Pseudomonas aeruginosa, is undergoing field trials and results suggest that this vaccine may reduce fleece rot incidence. Problems to be investigated include the existence of variants of P. aeruginosa in the field and the involvement of other species of bacteria in fleece rot. Strategies for direct vaccination include immunization with larval products involved in wound formation and larval nutrition and immunization against novel antigens usually from the gut of first instar larvae. Both methods have resulted in significant inhibition of larval growth. Analysis of larval products has revealed a number of active proteases which degrade skin proteins such as collagen. Inhibition of these enzymes with plasma enzyme inhibitors also affects larval growth in vitro. Antibodies raised against these enzymes are being tested for inhibitory effects against larvae and used to isolate cDNA clones from Lucilia cuprina libraries. Antigens from the gut are able to induce antibodies inhibitory to larval growth both in vitro and in vivo. Isolation of these antigens is proceeding in a number of laboratories. Problems still to be analysed include whether growth inhibition produces effective protection in the field and whether sufficient antibody will have early access to the larvae to significantly affect them.

The sheep antibody response to repeated infection with Lucilia cuprina

The specific serum antibody responses of sheep exposed to 10 consecutive infections of L. cuprina have been analysed by enzyme-linked immuno-sorbent assay and immunoblotting using monoclonal antibodies specific for sheep immunoglobulin isotypes. Recognition of a number of larval excretory-secretory products by IgM antibodies appeared to be non-specific. IgG1 was the major antibody class stimulated by the infection protocol and marked increases in antibody to specific excretory-secretory antigens were observed. Three molecules of 35, 30 and 25 kDa were particularly recognized although the extent of recognition of these molecules varied considerably between individual sheep serum. A pooled serum composed of sera collected after five to seven infections significantly inhibited larval growth in in vitro cultures when compared to a sera pool consisting of sera collected both prior to infection and after infections 1 and 2. The degree of inhibition was greater when serum with high specific antibody titre was used.

Hypersensitivity responses and repeated infections with Lucilia cuprina, the sheep blowfly

Sheep repeatedly infected with L. cuprina at 2- but not 4-week intervals developed partial resistance to infection after five infections, as measured by larval recovery. However, resistance did not persist for more than three infections. Skin weal responses were measured after injection of larval products simultaneously with each infection. The only correlation between weal size and larval recoveries occurred at infection 1 and indicated a relationship between skin sensitivity and innate rather than acquired resistance. The results suggest that resistance to L. cuprina can develop after repeated infections but that it is short lived and requires frequent larval exposure. A role for hypersensitivity responses was not confirmed by the weal responses but was suggested by the size of wound developed per larva recovered.

Antibody degradation in wound exudates from blowfly infections on sheep

Sheep were immunised with ovalbumin and then infected with the sheep blowfly, Lucilia cuprina in order to study immunoglobulin and specific antibody degradation at the wound site. Serum and wound exudates were collected over the infection period and the dry weight and protein content of the exudates were determined. Exudates were analysed by SDS-PAGE and immunoblotting for IgG degradation. Levels of IgG and specific anti-ovalbumin antibodies in the exudates were measured by ELISA. The total weight of exudates increased over the whole period of the infection, while protein content increased in the first 24 h and then remained relatively constant. Immunoglobulin was present 6 h after infection and levels increased with protein content. However, the levels of IgG measured were quite different depending on the secondary antibody used in the ELISA. A monoclonal antibody measured mainly intact IgG while a polyclonal anti-IgG measured intact and degraded IgG. This allowed an estimation that approximately 60% of the IgG in exudates was degraded from 6 h after infection. Assays in vitro showed that L. cuprina larval enzymes degraded sheep antibody. However, measurement of specific anti-ovalbumin levels in exudates suggested that although high levels of antibody were degraded this did not necessarily decrease the level of antigen binding. As a result, IgG degradation may assist and not hinder vaccine development by allowing antibody fragments to penetrate the peritrophic membrane and access gut cell antigens.