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Beyond the Bale : Jun - July 08
By Dr Gio Braidotti P roviders of agricultural aid to developing countries have long known of the potential for benefits to flow back to Australia. And the Australian sheep industry is the latest recipient of such a foreign aid ‘boomerang’ since a footrot vaccination program in Nepal, led by the University of Sydney, succeeded in eradicating the disease. In the process, the scientists made a discovery of particular importance to Australia’s sheep industry. Footrot is caused by the bacterium Dichelobacter nodosus, which has a number of strains. Benign forms usually cause transient lesions and minimal economic loss, but virulent strains cause a debilitating hoof infection that reduces wool growth and quality, ewe fertility and lamb growth. Environmental factors also play a role in the development of footrot in sheep. Dr Om Dhungyel, a Research Fellow at the University of Sydney, says all this variability complicates control and eradication efforts. Of particular concern on the vaccine front are proteins on the surface of the bacteria, called antigens, which serve as the contact point with the animal’s immune system. While antibodies to any one bacterial antigen can kill that particular type of bacterium (or serogroup), D. nodosus comes with the ability to vary its antigens and evade the antibody. What that means for vaccine makers is that the more bacterial serogroups present in a paddock, the more antigens that need to be included in a vaccine to ensure protection. “There are nine serogroups of footrot bacteria, and the currently available commercial vaccine contains all nine antigens,” Dr Dhungyel says. “The problem is that these vaccines are not very effective.” Previously under the leadership of Professor John Egerton and now under Professor Richard Whittington, the footrot team showed that including so many antigens in a single vaccine diminishes its efficacy. The antigens tend to compete for the immune system’s attention, causing some responses to shut down to the point where the vaccine fails to protect an animal. The predicament Australia finds itself in is that footrot- affected flocks and paddocks often involve a high number of serogroups, which can consequently render a vaccine useless. Sydney University’s footrot team was approached by the Australian Centre for International Agricultural Research (ACIAR), the agency responsible for providing agricultural assistance to developing countries. ACIAR was responding to a request from Nepal and Bhutan for help establishing a disease-control program. Fortunately, the number of serogroups in footrot-affected Nepal was just two – a situation that permitted a custom-made vaccine. As a native of Bhutan, with experience as a veterinary officer in Asia, Dr Dhungyel took on a key role with the ACIAR project. Using DNA engineering he created a ‘bi- valent’ (two-antigen) vaccine specifically for Nepal. The results using the custom-made vaccine were spectacular. Within two years, Dr Dhungyel’s vaccine helped the ACIAR team completely eradicate the disease from Nepal. The same thing happened in Bhutan, where a one- off outbreak of the disease was stopped in its tracks. In both countries, further vaccination was shown to be unnecessary. “Our trials overseas proved that if you target just two strains at a time, the vaccine works really well. So in Australia, where multiple serogroups are quite common, the ACIAR project raised the possibility of eliminating footrot by sequentially targeting the serogroups, two at a time.” The idea caught the attention of woolgrowers, especially in Tasmania, who set about obtaining AWI support for an Australian trial. As part of this project, the team tested and compared the immune response to vaccines that contain one antigen, all the way through to nine antigens. The experiment formally demonstrated that including more than two antigens reduces the vaccine’s efficacy dramatically. The project’s on-farm trial began in 2005, with the involvement of 12 footrot-affected properties in Australia. The ongoing trial involves isolating bacteria from each property, typing them and then targeting the first vaccine to the two most prevalent and virulent bacterial types. Vaccination started in 2006 on six properties, with the remaining participants vaccinated for the first time in 2007. “The results are quite promising,” Dr Dhungyel says. “After the first round of vaccination, the prevalence of the targeted bacteria went down dramatically on almost all the farms. Where other serogroups are present, one of two things is happening: either the prevalence of the remaining serogroups also goes down, or the remaining serogroups take over, causing disease.” To cover multiple serogroups, several rounds of vaccination are necessary, raising the spectre that years may be required for full vaccination. To forestall that possibility, the trial is also evaluating the minimum interval between vaccines. On this front the footrot team has made another important discovery: “We found we can go back with a new vaccine in just three months. That means we can speed up targeting all the serogroups on a property, covering six within a year.” The vaccine is also capable of curing, as well as preventing, disease, so that it can be provided at any time after making a diagnosis, with the curative impacts visible within two weeks of vaccination. No vaccine is 100 per cent effective, but the number of non-responders that need to be culled from a flock does not exceed five per cent. “The current AWI project aims to confirm that with care, and approached from a regional perspective, the new vaccines make eradication achievable, given supportive legislative and quarantine issues. At the very least, a far better level of disease control really is possible.” ú More information: www.vetsci.usyd.edu.au/research/ farmanimal_health Aid ‘boomerang’ brings home new footrot tactic A new footrot vaccination system is under trial after an unexpected discovery in Nepal during an aid project involving Australian scientists 10 ANiMAl HeAltH BeyonD the BAle Footrot reseArch the footrot genome has been analysed to identify new classes of proteins for use in vaccine development. the new, high- throughput technology called ‘reverse vaccinology’ is being applied in an AWi-funded project led by Professor Julian Rood of the Australian Research Centre of excellence in Structural and Functional Microbial Genomics, at Monash University. With reverse vaccinology, the genome is scanned for genes likely to encode molecules that reside on the surface of the living pathogen. these are the proteins that are ‘visible’ to the host animal’s immune system and can be used in a vaccine to prime the production of antibodies capable of killing the bacteria. With a total of 1300 genes, the footrot genome has delivered 90 genes that are undergoing further testing in animals by collaborators at the University of Sydney, headed by Professor Richard Whittington. Never before has the footrot bacterium been subject to such a systematic analysis of its cell-surface molecular structures. “the strategy is to first see whether we get any disease protection in animals immunised with the purified protein,” Professor Rood says. “then we screen different bacterial isolates and ask two questions: first, do all bacterial isolates produce this protein, and second, how variable are these proteins in the different isolates.” On the diagnostic front, benign bacterial strains that have falsely tested positive as virulent in the gelatin gel test can now be identified in 90 per cent of cases using a new test created by Dr Brian Cheetham of the University of New england. “it was created for those situations when the field data and the gelatin gel test indicate a false positive,” Dr Cheetham says. “the new test relies on PCR (polymerase chain reaction) technology to detect a bacterial gene present primarily in virulent strains.” the PCR test has been found to work well in NSW, where it is now commercially available, and is undergoing testing in South Australia and Victoria. More information: www.microbialgenomics.net; www.une.edu.au/staff/bcheetha.php On-farm footrot inspection. PHOtOS: UNiVeRSity OF SyDNey Hooves of a sheep badly affected by footrot.
Jun - Jul 08 Supplement
Aug - Sep 08