The "One Health" strategy envisions the control of all processes in the food production chain “from farm to fork.” The detection of biotoxigens, including enterotoxigenic Escherichia coli, can be carried out by various methods. However, none of them can be classified as rapid methods. The fluorescent antibody technique (FAT), which combines the objectivity of microscopic methods with the high specificity of immunological reactions, may be applied in such a role. Analysis of FAO reports shows that monitoring for the presence of enteropathogenic E. coli is a vital part of the "One Health" strategy. The development of rapid methods for the indication and identification of enterotoxigenic E. coli is a current topic of scientific investigation, and FAT is one of such method. The diagnostic effectiveness of FAT depends on the activity and specificity of Escherichia coli sera, which in turn depends on the donor animals, immunization schemes, and antigenicity of the vaccine preparations. Objective of the Study is to perform a comparative assessment of two immunization schemes in different animal species using a vaccine preparation from inactivated E. coli microbial cells. As the vaccine preparation, a 5-billion suspension of microbial cells from a 24-hour culture of an enterotoxigenic β-hemolytic strain of E. coli grown in tryptone-soy yeast broth was used. The cells were inactivated with formaldehyde (0.4 % of culture volume) and concentrated with Aerosil A-300 (3 mg/ml). Rabbits, sheep, bulls, and horses (6 animals of each species) were used as donor animals. They were selected based on similarity in age, body weight, housing, and feeding conditions. All animals were clinically healthy. Two immunization schemes were used. In the first scheme, the vaccine was administered subcutaneously three times, and in the second – four times, at 4-day intervals in twofold increasing doses. Blood samples for testing were collected on the 21st day after the last vaccine administration. Agglutinin levels were determined using the agglutination reaction (AR). A 2-billion suspension of inactivated E. coli cells was used as the antigen. The reaction was performed in a volume of 1 cm³. The titer was defined as the last dilution of serum that showed agglutination of at least 2 pluses. The highest agglutinin titers were found in rabbit sera – 1:7253±1389 (first immunization scheme) and 1:9387±853 (second scheme). Slightly lower antibody titers were found in sheep – 1:5547±1028 (first scheme) and 1:8533±1079 (second scheme), and horses – 1:5973±1428 and 1:6827±1079 respectively. The lowest titers were found in bull sera – 1:4267±540 (first scheme) and 1:4693±427 (second scheme). This indicates that different animal species react differently to the same antigen. It was established that the difference in agglutinin levels between the second and first immunization schemes was 35% in sheep, 22.8% in rabbits, 12.5% in horses, and 9.1% in cattle. Thus, the fourth administration of the vaccine enhances the humoral response in immunized animals. However, it also causes stress and sensitization in the animals. Therefore, three administrations of the vaccine are sufficient for producing active Escherichia coli sera. During immunization with Escherichia coli antigen, the highest agglutinin titers were re corded in the sera of rabbits (1:9387±853) and sheep (1:8533±1079), slightly lower in horses (1:6827±1079), and the lowest in bulls (1:4693±427). The immunization scheme involving three parenteral administrations of the vaccine at 4-day intervals in twofold increasing doses stimulates a high level of agglutinins without causing stress, and therefore can be used for obtaining highly active escherichiosis diagnostic sera.

Key words: enterotoxigenic Escherichia coli, immunization schemes, vaccine preparations, donor animals, agglutination reaction, bacterial antigens.