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A vaccine against Barber's Pole worm - the reality gets closer

Dr Brown Besier, Department of Agriculture & Food, Western Australia
December 2012

 

Barber’s Pole worm is generally considered the most important internal parasite of sheep and goats on a worldwide basis, and it is a major limitation on sheep and goat raising in many countries. Although a major problem in only part of the sheep areas of Australia, especially Northern NSW and in Queensland, it occurs in higher-rainfall and coastal zones in all states.

Not surprisingly, drench resistance is a major issue for Barbers Pole worm control, as frequent drenching has been the answer in most parts of the world. Unfortunately, this is a temporary solution, as we now see widespread resistance to almost all drench groups. For example, in Northern NSW resistance to moxidectin is reported from well over half of recent drench resistance tests.

Also not surprisingly, non-drench approaches to anthelmintics have been sought for many years. There have been some successes: effective pasture management routines have been developed by University of New England researchers, and breeding for worm resistance is an effective, if longer-term, strategy. However, the holy grail of a commercial Barbers Pole worm vaccine has eluded us despite research since the 1960s.

A new approach

During the early 1990s, an experimental vaccine was trialled by research groups in the UK, based on antigens (proteins that stimulate the immunological response in the gut) derived from the worm’s gut. Pen trials at the Moredun Research Institute, in Edinburgh, UK, showed high levels of protection against Barbers Pole worm. The majority of larvae were killed soon after ingestion by sheep, so significant worm burdens did not develop.

However, because it was not thought feasible to produce large quantities of Barbers Pole worms for vaccine production, it was considered commercial viability required the antigens to be produced in “recombinant DNA” systems. This has been used successfully for many biological products: the DNA responsible for the proteins of interest is inserted into bacteria, which are then cultured in industrial quantity, and the proteins removed and purified. However, despite many years of research using advanced techniques, it has not proved possible to produce Barbers Pole worm antigens by this method.

So, back to the drawing board: is it possible to produce enough Barbers Pole worms to extract the antigens directly?  The answer has turned out to be “yes”: a recent break-through by Moredun scientists showed that the amount of antigen needed for effective vaccination is very low. Discussions with scientists at the Department of Agriculture and Food Western Australia suggested that it would be possible to set up a commercial feedlot system, with sheep infected with a small dose of Barbers Pole worm larvae before being sent to an abattoir, and the part of the gut carrying the worms removed after slaughter. After a series of trials, it has proved possible to process large numbers of animals over a few days, with the worms recovered for use in vaccine production by Moredun scientists working in the DAFWA laboratories at Albany. Vaccine is now produced to a high level of quality certification using this system, and could potentially be used in any country.

Progress and next steps

A series of field trials in WA and NSW have shown that the vaccine could produce up to 90% protection on a flock basis, which is more than adequate to both prevent disease in sheep after vaccination and also the build-up of worms on the pasture. The vaccine will need to be given at 6-weekly intervals, probably 4 to 5 treatments over a Barbers Pole worm “season” in summer rainfall regions.

However, preventing sheep deaths in this environment requires either frequent drenching or the use of long-acting anti-Barbers Pole worm products.  On current trends, the long-acting option is likely to be increasingly less effective in coming years, as the first effect of drench resistance is to reduce the period of protection of these products. This would require greater reliance on those short-acting drenches that remain effective – putting further pressure on them for the development of resistance.

Further research is in progress to clearly demonstrate the vaccine’s effectiveness under commercial conditions. Although the vaccine gives no protection against other worm species, by removing Barbers Pole worm as a significant problem, control of the other species is simplified. Recent results suggest that a significant step forward has been taken towards an effective and affordable vaccine, and a new non-chemical answer to Barbers Pole worm has real potential.