Striking back at flystrike with vaccines… the battle continues

AWI is now three years into a major four-year preliminary collaborative research project to help develop a commercial vaccine that will help protect sheep right across the country from the Australian sheep blowfly.

Here, project leaders Tony Vuocolo (CSIRO) and Trent Perry (University of Melbourne) provide woolgrowers with updates on their respective research areas.

Flystrike vaccine research progressing, with hurdles

Tony Vuocolo, CSIRO

CSIRO scientist Tony Vuocolo vaccinating sheep with prototype flystrike vaccines.

CSIRO is about to complete the first three-year phase of research on the development of a flystrike vaccine. Much has been achieved in this time by extending our biological knowledge of the blowfly and identification of lead candidate protein antigens that have been tested in prototype vaccines. But there is still a lot more work to do before we can translate our exciting results demonstrated in the laboratory into a well performing vaccine that can be used on-farm for flystrike prevention.

The following is a summary of progress to date as we move into a phase of on-sheep efficacy testing of the lead vaccine prototypes being performed at the current moment along with our plans going forward.

During the first phase we investigated the proteins that the blowfly larvae make as they grow from just hatching out of the egg phase to mature larvae that are approaching the pupation phase to turn into flies. The availability of the blowfly genome that AWI and University of Melbourne were closely associated in developing was a very valuable tool in helping identify these genes and proteins.

From this analysis we were able to identify lead proteins that are now being targeted and produced as antigens for our vaccine. We shortlisted eight protein classes containing 35 candidate proteins and engineered and cloned these into bacteria or insect cells. We use these cells as mini-factories to make the specific protein antigens, allowing us to then use these antigens for inclusion and formulation into vaccines. We have made more than 50 prototype vaccines to date, vaccinating several hundred sheep and then tested all of these in the laboratory using serum from sheep. We are also keeping track and being informed about the potential implications of genome variations in the blowfly populations from around Australia by the population dynamics studies of the University of Melbourne (see following section).

From more than 50 prototype vaccines tested in the laboratory, we narrowed them down to two prototype vaccines that are being progressed to sheep-based larval challenges. In laboratory tests, these two vaccines were shown to reduce blowfly larvae growth rate by up to 75% (see the image below) and in some cases impact larval survival.

The difference in larvae growth after 72 hours between larvae exposed to the vaccine prototypes (top: native antigen vaccine; middle: recombinant antigen vaccine; and bottom: the control larvae, no vaccine) in laboratory tests.

“In laboratory tests, the two vaccines were shown to reduce blowfly larvae growth rate by up to 75%. The challenge now will be duplicating these results in trials on sheep.”

- Tony Vuocolo, CSIRO

The challenge now will be duplicating these results on the sheep’s back. Unlike the ‘in laboratory tests’ with larval feeding assays undertaken under controlled conditions in the laboratory, where serum, rich in antibodies generated from the vaccine is available in abundance to the feeding larvae, the on-sheep testing represents a different and uncontrolled environment. Early results from two vaccine prototypes in sheep trials returned a 15% reduction in larval growth compared to 75% in the laboratory.

We are now working to better understand the sheep generated immune response and to optimise vaccine protocols and formulation to ensure the larvae are exposed to adequate levels of antibodies generated from the vaccine for it to be effective.

Once we are able to demonstrate the success of the vaccine prototypes in reducing larval growth on sheep, we plan to approach animal health companies who have shown keen interest in being involved to secure their commitment in developing and commercialising an effective and affordable flystrike vaccine.

Woolgrowers have contributed to an improved understanding of blowfly populations

Trent Perry, University of Melbourne

University of Melbourne scientists Trent Perry (right) and Vern Bowles (top left) with PhD student Gothami Welikadage (bottom left) collecting blowfly larvae from an implanted sheep during their in vivo implant trial.

We are thankful to the many woolgrowers across the country who contributed blowfly samples to a recently completed three-year University of Melbourne project to determine if there are genetically different blowfly populations across Australia.

As a result, a total of 2,034 fly samples from more than 86 collections across Australia have been visually identified, had their DNA extracted, and then assessed to determine their genetic similarity or differences to each other

Comparing similarities and differences in genetic information from individual flies shows that there are three distinct blowfly populations in Australia (see Figure 1). Flies collected from Victoria, New South Wales, Queensland and South Australia are genetically similar to each other and interbreed, forming one large population, while those flies found in Western Australia and Tasmania form two other distinct populations of blowflies.

Figure 1. Genetic information from individual flies (each dot) demonstrates three distinct blowfly populations in Australia

“Genetic information from blowflies supplied to us by woolgrowers from across Australia demonstrates there are three distinct blowfly populations in Australia: those in WA, Tasmania and the eastern mainland states.”

- Trent Perry, University of Melbourne

This information is key to appraising the feasibility of new blowfly control strategies and will be important for improving the design of strategies to protect and sustain the effectiveness of chemicals currently used for control and others that may be registered in future. It will also be used to improve regionally targeted woolgrower management strategies for suppressing the spread of blowfly chemical resistance.

This information has already been used by CSIRO, helping them determine whether the candidate proteins they are examining for the blowfly vaccine project are similar for the different fly populations and therefore ensure the vaccine they are developing will be effective across Australia.

Another area of our project was a study that aimed to understand how flies establish a strike, particularly what proteins are critical in the early stages, just prior to and during the initiation of a wound. We conducted an in-vivo implant trial where sheep were infested with blowfly eggs at small, controlled sites, and samples were then collected from both maggots and the sheep at the time that the maggots were starting to create a wound.

This work has identified hundreds of fly proteins that are being excreted during strike initiation which is contributing to our understanding of the way sheep respond to strike and the battle between the maggots and sheep defence mechanisms. These studies will not only assist with current blowfly vaccine development research but will provide opportunities to identify additional novel vaccine candidates against this damaging pest.

This article appeared in the September 2022 edition of AWI’s Beyond the Bale magazine. Reproduction of the article is encouraged.