Efficacy of Native and Recombinant Cry1B Protein Against Experimentally Induced and Naturally Acquired Ovine Myiasis (Fly Strike) in Sheep

Abstract

Several hundred strains of Bacillus thuringiensis (Bt), isolated in New Zealand from samples of soil and sheep fleece, were tested for toxicity to larvae of the blowfly Lucilia cuprina Wiedemann. Characterization of the Bt strains revealed that three of the more active strains produced Cry1Ba (an insecticidal protein present in Bt mother cell crystal inclusion) that was toxic to blowflies. These strains were evaluated for the ability to prevent experimentally induced fly strike in a bioassay by using first instars. Results with undiluted spore/crystal preparations were variable, but they generally prevented fly strike on sheep maintained on pasture for 3-6 wk. Spore viability was satisfactory throughout the trials and environmental factors (e.g., precipitation and UV radiation) seemed to have minimal effect on persistence. The loss of fly strike protection in these experiments correlated with the movement of spore/crystal toxicity away from the skin as a result of wool growth. Solubilized protein preparations were not as potent as spore/crystal preparations and fly strike protection lasted only from 1 to 3 wk. Vegetative forms of the Cry1Ba-producing strains of Bt did not establish on the fleece of sheep, did not produce significant sporulation, and no protection against fly strike was achieved. Escherichia coli expressing recombinant Cry1Ba protein was toxic to larvae in vitro but did not effectively protect sheep from fly strike because blowfly larvae were able to establish readily 8 d posttreatment. In a single field experiment involving 80 sheep per group, a spore/crystal preparation from a Bt strain expressing Cry1Ba provided less protection from naturally acquired fly strike than afforded by a commercially available dip.

Diseases of the Integumentary System

“…35 In certain parts of the world, routine dipping or spraying with larvicidal compounds is used for prevention and control. Fly sprays should be used on and around any wounds.…”

Characterization of a newBacillus thuringiensisendotoxin, Cry47Aa, from strains that are toxic to the Australian sheep blowfly,Lucilia cuprina

“…The use of B. thuringiensis to control flystrike has been investigated by using B. thuringiensis strains applied to sheep fleece [5,6] resulting in protection from flystrike for 6-11 weeks. B. thuringiensis has also been shown to be common in sheep fleece [5][6][7]. These studies did not distinguish between insecticidal effect due to d-endotoxins and b-exotoxins which are general RNA synthesis inhibitors produced by some B. thuringiensis strains.…”

“…thuringiensis-based products are widely used as efficient bio-pesticides against a number of lepidopteran and coleopteran pests, and more than 29 million acres of B. thuringiensis genetically modified crop fields are grown globally. Recently, a number of B. thuringiensis strains that have specific activity against livestock insect pests including larvae of the sheep blowfly [7][8][9] have been identified. The use of B. thuringiensis to control flystrike has been investigated by using B. thuringiensis strains applied to sheep fleece [5,6] resulting in protection from flystrike for 6-11 weeks.…”

“…Further work is needed before such B. thuringiensis strains can be used commercially. Recently, a number of B. thuringiensis strains that have specific activity against livestock insect pests including larvae of the sheep blowfly [7][8][9] have been identified. Before such B. thuringiensis strains or their toxins can be used for commercial application, it is important to isolate and characterize the genes encoding the d-endotoxins.…”

Control of sheep flystrike: what's been tried in the past and where to from here

“…A number of field-isolated strains of Bt have been shown to produce toxins that are active against blowfly larvae in vitro and in vivo. [52][53][54][55] Heath et al 53 showed that Bt extracts applied to patches on sheep were able to protect sheep from experimentally induced flystrike (implants) for up to 6 weeks. A time-course study showed that protection from flystrike was not diminished by exposure of sheep to precipitation or sunlight, but, rather, the loss of protection over time was considered to be most likely due to movement of the toxin away from the skin as the wool grew.…”