31 March 2000

WORM VACCINE CHALLENGE

By Shelley Wright

GROWING problems of resistance in sheep to anthelmintics, as well as mounting public concern about chemical residues in food, are among the driving forces for a team of scientists in Edinburgh who are trying to develop vaccines to tackle roundworms and sheep scab.

David Smith, project leader on worm vaccines at the Moredun Institute, says that work has already shown that a vaccine against Barbers Pole worm – Haemonchus contortus – is effective.

The challenge now is to develop methods of producing the vaccine in quantities that make it economically viable. Initial vaccine production has involved extracting a protein from parasites harvested from a sheep.

"But using that method, wed only get enough of the protein from each sheep to prepare vaccines for about 50 sheep. That obviously would not make economic sense," says Dr Smith.

Instead, the researchers are trying to create a recombinant DNA equivalent in the laboratory, which would enable bulk vaccine production. "We know the genes needed to encode the appropriate proteins, but we have yet to synthesise them in a form that will work," he says.

Unfortunately, there is no way of knowing how long that will take. "The trouble is that these things are so unpredictable," Dr Smith says. "We may crack it next week, or it could be years, or we might never get it to work."

Work is also under way at Moredun to develop vaccines against the more widespread damage done by the roundworm species Ostertagia and Tricho-strongyles.

Although some early results from trials on Ostertagia look promising, developing a vaccine is likely to be less straightforward than in the case of the Barbers Pole worm.

"It – Haemonchus contortus – feeds on blood. What we are doing is to inject proteins taken from the parasites guts into sheep. They then produce high antibody levels and when the parasite then eats the blood in the sheep, its own gut is attacked, killing it."

Theoretically though, it should eventually be possible to produce a vaccine to control a range of different roundworm species, Dr Smith says.

As with clostridial vaccines, protecting sheep against worms would probably have to be done at least once a year.

"Anthelmintics are fine when they work. But resistance has been reported in the UK to all three anthelmintic groups. Based on the experiences seen in the southern hemisphere, theres every indication that the problem will spread," he says, adding that there are no signs of any new wormer compounds on the horizon.

So vaccines would offer farmers an excellent alternative in the treatment of roundworms, he believes. "But we need to produce a good vaccine that can offer prolonged protection and is competitively priced. Thats our challenge."

Another potential benefit that vaccines could offer producers is in countering growing consumer resistance, particularly in Western Europe, to the use of chemicals in primary food production, Dr Smith says.

"Vaccines have a greener image than, say, anthelmintics," he says, pointing out that they could also be used by organic producers.

While work on worm vaccines has been done at Moredun for the past 15 years, research into a vaccine against sheep scab is in its infancy – and getting a product to the market to control scab could take as long as 10 years, Dr Smith believes.

A vaccine against worms is possible, but needs to offer prolonged protection and be competitively priced to succeed.

VACCINE PROMISE

&#8226 Against worms and scab.

&#8226 Commercial production?

&#8226 Some time to market.