The U.S. Department of Agriculture and Genex Corp. of Rockville have produced a genetically engineered substance that may lead to a vaccine against a parasite that costs the poultry industry $300 million a year.

Agriculture Secretary John R. Block said yesterday the department and Genex, cooperating in a joint research venture, have achieved "a very significant breakthrough" in the effort to control coccidia parasites, which attack the digestive systems of chickens and prevent the fowl from gaining weight.

The poultry industry spends about $100 million a year to treat coccidiosis, either through medication or by vaccination using live, virulent cells, Block said. Chickens quickly develop resistance to the drugs currently used, and the live cells create the danger of infection, he said. The effects of the disease reduce the value of infected flocks by another $200 million a year.

Genex and the Agriculture Department have used one type of coccidia parasite to produce the first genetically engineered antigen to coccidia. An antigen stimulates the production of specific antibodies. If this antigen is successfully developed into a vaccine, the antibodies produced will make the vaccinated chickens immune to the parasite.

A vaccine against coccidiosis may be developed within the next five years, said Richard Roblin, Genex's director of technology marketing, who attended the press conference with Block. He added that there are "technical uncertainties" upon which the estimate is conditioned. The antigen developed stimulates antibodies to only one species of coccidia parasite, whereas an effective vaccine would require antigens to as many as six species, he said.

USDA's Agriculture Research Service has spent about $550,000 over the last four years on the project. Roblin declined to disclose Genex's investment.

Scientists at USDA's research center at Beltsville used cell fusion techniques to produce a pure source of antibodies to the particular parasite used. In 1982, USDA selected Genex, through a competitive bidding process, to use the antibodies to create a genetically engineered antigen.

Genex and USDA said they have injected chickens with the antigen, and found that the antigen stimulated the production of antibodies and conferred partial protection against one species of the parasite.

The research partners will continue to work together on development of a vaccine. Further testing is required to determine whether the protection can be made complete and whether the same approach is effective against other species of coccidia, and to develop the best method of delivering the antigens, they said.

The results could have wider applications in agricultural medicine. The same techniques are being used to develop a vaccine to foot-and-mouth disease, and may be used to conquer other livestock diseases, said Orville Bentley, USDA assistant secretary for science and education.

"This research is a good example of how USDA scientists work with industry to solve agricultural problems," Block said.