Bradford P. Smith

Enzyme-Linked Immunosorbent Assay for Salmonella Serology using Lipopolysaccharide Antigen

Enzyme-linked immunosorbent assay (ELISA) using Salmonella lipopolysaccharide (LPS) to measure specific IgG titers in cattle has proven useful. Serology can be used to assess vaccine responses and infection rates, to detect carriers, and to aid in epidemiologic studies. The objective of this study was to assess cross-reactions using sera from cattle vaccinated with different Salmonella serogroups. ELISA plates using lipopolysaccharide from serogroup B, C1, C3, D1 or E1 as the plate antigens were tested. LPS was extracted from Salmonella typhimurium (Serogroup B; somatic antigens 01, 4, 12), S. montevideo (C1; 06, 7), S. kentucky (C3; 08, 20), S. dublin (D1; 01, 9, 12) and S. anatum (E1; 03, 10) using the Westphal method. Fifteen cows were found to be seronegative for all 5 of these serogroup antigens. Each cow was then vaccinated 3 times at 2-week intervals with a killed Salmonella bacterin. The 15 different serotypes used for vaccination were chosen to represent a wide array of Salmonella serogroups with a wide array of somatic “O” antigens expressed, including somatic antigens 1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 18, 19, 20, 22, 23, 25, and 27. With each antigen tested, the highest ELISA titers were seen with sera from cattle vaccinated with homologous O antigens, indicating that reactions were highly O antigen-specific. Some cross reactions between serogroups sharing one O factor antigen were found; these titers were lower than those found with homologous serogroups sharing 2 or more antigens. Only serum from the cow vaccinated with S. anatum (group E; antigens 03, 10) cross-reacted at a low titer with group C1 (O somatic antigens 6, 7) and D1 (O somatic antigens 1, 9, 12) plate antigens, with which no somatic antigens were shared. We conclude from these results that Salmonella serology using LPS antigens is highly O antigen-specific and predictable.

The effect of Escherichia coli J5 and modified live Salmonella dublin vaccines in artificially reared neonatal calves

One thousand neonatal calves, allocated in a factorial design into four groups, were vaccinated subcutaneously with two doses each of either killed Escherichia coli (0111:B4) J5 bacterin or a UC Davis modified live, genetically altered (aro-) Salmonella dublin vaccine, or both, or with a placebo. In this prospective double-blind study to determine the immunogenicity and protective effects of both vaccines on bovine neonates in field conditions, calves were observed daily until 2 months of age, and serum samples from selected study calves were obtained at five different time points. No clinical adverse vaccine reactions were observed. Overall mortality was 7.5% (75 of 1000), E. coli and S. dublin infection being the most commonly associated aetiological agents of deaths. Both J5 (p < 0.01) and Salmonella (p = 0.05) vaccines were significantly effective in reducing the mortality rate but without an additive effect. The role of passive transfer was important in calf survival. The E. coli J5 and (aro-) S. dublin vaccination schedule employed significantly (p < 0.001) elevated J5 and Salmonella-specific serum ELISA antibody titres, respectively, by the sixth week of age.

Isotype-Specific Antibody Responses of Cattle to Salmonella Dublin Lipopolysaccharide and Porin Following Salmonella Dublin Vaccination and Acute and Chronic Infection

Stimulation of different T-cell subsets during antigen presentation influences the antibody isotype response to an antigen. Salmonella infection and Salmonella bacterin vaccination are likely to stimulate different T-cell subtypes. The objective of this study was to determine whether there are differences in the isotype response of cattle to Salmonella antigens following Salmonella infection and Salmonella bacterin vaccination. Sera from Salmonella bacterin–vaccinated, experimentally infected, and chronically infected (carrier) adult cattle collected during previous studies was used to evaluate the IgG1, IgG2, and IgM isotype responses of cows to Salmonella serotype Dublin lipopolysaccharide (LPS) and porin. Following vaccination and experimental oral infection, IgG1 titers to LPS and porin rose more quickly and persisted longer than did IgG2 titers. In contrast to Salmonella infection, bacterin vaccination stimulated a weak response to Salmonella porin. Salmonella infection also induced a higher IgG2:IgG1 titer ratio to LPS than did bacterin vaccination. Chronic Salmonella infection induced the highest LPS and porin IgG2:IgG1 titer ratios and the highest correlation between LPS and porin titers. Response operating characteristic curves for each isotype-specific enzyme-linked immunosorbent assay (ELISA) were determined to evaluate the effect of isotype on the sensitivity and specificity of Salmonella ELISA serology for distinguishing sera of Salmonella carriers from those of vaccinated and acutely infected cows. IgG2 titers to LPS and porin provide a more specific indicator of chronic Salmonella infection status than do IgG1 titers to the same antigens with little to no loss in sensitivity.

Serological Distinction of Bovine Salmonella Carriers from Vaccinated and Acutely Infected Cows

Identification of Salmonella carriers using lipopolysaccharide (LPS) ELISA serology in a Salmonella-infected herd requires distinction of chronically infected cattle from convalescent and vaccinated cows. Cows responding to Salmonella infection and vaccination produce titers to Salmonella LPS that overlap with the lower titers of some Salmonella carriers. The objective of this study was to determine if the LPS antigen specificity of the bovine humoral immune response to Salmonella LPS antigens differs following vaccination and acute and chronic Salmonella infection. The study focused on the nondiscriminatory area of Salmonella ELISA serology, specifically, peak-titered sera from Salmonella bacterin-vaccinated and experimentally infected cows and low-titered sera from Salmonella carriers. The LPS serogroup specificity of the IgG1 and IgG2 response following acute and chronic Salmonella serotype Dublin infection and Salmonella bacterin vaccination was evaluated using 5 Salmonella serogroup (B, D, E1, C3, and C1) LPS ELISA assays. IgG1 titers of carriers, vaccinated, and acutely infected cows were predominantly O antigen specific. Similarly, the IgG2 titers of acutely infected cows were also O antigen specific. In contrast, Salmonella carriers produced an IgG2 response to each of the heterologous LPS antigens (B, E1, C3, and C1) examined. The results of this study indicate that the bovine IgG1 isotype response to Salmonella LPS is serogroup specific. Conversely, production of IgG2 antibodies to core Salmonella LPS antigens shared across Salmonella serogroups is a feature of chronic Salmonella infections.