Salmonella typhimurium infection in calves: cell-mediated and humoral immune reactions before and after challenge with live virulent bacteria in calves given live or inactivated vaccines

Salmonellosis in adult cattle in Central Argentina: case series

Salmonellosis is an infectious disease caused by bacteria belonging to the Salmonella genus. Bovine salmonellosis is more frequent in young cattle under intensive overcrowd husbandry conditions, and therefore uncommon in adults. We report four outbreaks of clinical salmonellosis due to Salmonella Typhimurium, Salmonella Newport and Salmonella Dublin provoking outbreaks of diarrheic/septicemic disease in adult cattle of Central Argentina. Anamnesis information, clinical, pathological, and bacteriological findings were retrospective analyzed. This report emphasizes the importance to include salmonellosis among the differential diagnosis of clinical enteric/septicemic disease in adult cattle under different husbandry conditions in Argentina. The source of Salmonella could not be established in these outbreaks.

Improved Tolerability of a Salmonella enterica Serovar Typhimurium Live-Attenuated Vaccine Strain Achieved by Balancing Inflammatory Potential with Immunogenicity

A notable proportion of Salmonella-associated gastroenteritis in the United States is attributed to Salmonella enterica serovar Typhimurium. We have previously shown that live-attenuated S Typhimurium vaccine candidate CVD 1921 (I77 ΔguaBA ΔclpP) was safe and immunogenic in rhesus macaques but was shed for an undesirably long time postimmunization. In mice, occasional mortality postvaccination was also noted (approximately 1 in every 15 mice). Here we describe a further attenuated vaccine candidate strain harboring deletions in two additional genes, htrA and pipA We determined that S Typhimurium requires pipA to elicit fluid accumulation in a rabbit ileal loop model of gastroenteritis, as an S Typhimurium ΔpipA mutant induced significantly less fluid accumulation in rabbit loops than the wild-type strain. New vaccine strain CVD 1926 (I77 ΔguaBA ΔclpP ΔpipA ΔhtrA) was assessed for inflammatory potential in an organoid model of human intestinal mucosa, where it induced less inflammatory cytokine production than organoids exposed to the precursor vaccine, CVD 1921. To assess vaccine safety and efficacy, mice were given three doses of CVD 1926 (109 CFU/dose) by oral gavage, and at 1 or 3 months postimmunization, mice were challenged with 700 or 100 LD50 (50% lethal doses), respectively, of wild-type strain I77. CVD 1926 was well tolerated and exhibited 47% vaccine efficacy following challenge with a high inoculum and 60% efficacy after challenge with a low inoculum of virulent S Typhimurium. CVD 1926 is less reactogenic yet equally as immunogenic and protective as previous iterations in a mouse model.

Vaccination against Salmonella Infection: the Mucosal Way

Salmonella enterica subspecies enterica includes several serovars infecting both humans and other animals and leading to typhoid fever or gastroenteritis. The high prevalence of associated morbidity and mortality, together with an increased emergence of multidrug-resistant strains, is a current global health issue that has prompted the development of vaccination strategies that confer protection against most serovars. Currently available systemic vaccine approaches have major limitations, including a reduced effectiveness in young children and a lack of cross-protection among different strains. Having studied host-pathogen interactions, microbiologists and immunologists argue in favor of topical gastrointestinal administration for improvement in vaccine efficacy. Here, recent advances in this field are summarized, including mechanisms of bacterial uptake at the intestinal epithelium, the assessment of protective host immunity, and improved animal models that closely mimic infection in humans. The pros and cons of existing vaccines are presented, along with recent progress made with novel formulations. Finally, new candidate antigens and their relevance in the refined design of anti-Salmonella vaccines are discussed, along with antigen vectorization strategies such as nanoparticles or secretory immunoglobulins, with a focus on potentiating mucosal vaccine efficacy.

Animal Models for Salmonellosis: Applications in Vaccine Research

Salmonellosis remains an important cause of human disease worldwide. While there are several licensed vaccines for Salmonella enterica serovar Typhi, these vaccines are generally ineffective against other Salmonella serovars. Vaccines that target paratyphoid and nontyphoidal Salmonella serovars are very much in need. Preclinical evaluation of candidate vaccines is highly dependent on the availability of appropriate scientific tools, particularly animal models. Many different animal models exist for various Salmonella serovars, from whole-animal models to smaller models, such as those recently established in insects. Here, we discuss various mouse, rat, rabbit, calf, primate, and insect models for Salmonella infection, all of which have their place in research. However, choosing the right model is imperative in selecting the best vaccine candidates for further clinical testing. In this minireview, we summarize the various animal models that are used to assess salmonellosis, highlight some of the advantages and disadvantages of each, and discuss their value in vaccine development.

Use of an attenuated live Salmonella Typhimurium vaccine on three breeding pig units: A longitudinal observational field study

The study examined the effects of a licensed live Salmonella Typhimurium vaccine, administered to sows and gilts on three commercial pig units experiencing clinical salmonellosis associated with S. Typhimurium or its monophasic variant. After vaccination, clinical salmonellosis resolved and shedding of S. Typhimurium declined markedly and persistently on all breeding or breeding-finishing units, during the one- to two-year monitoring period. On two finishing units supplied in part by one of the vaccinated herds, pigs from the vaccinated herd were less likely to shed Salmonella than those from non-vaccinating herds, and Salmonella counts in faeces were also lower from the vaccine-linked animals. Non-Typhimurium Salmonella serovars were isolated typically in fewer than 10% of samples, and showed no clear temporal changes in frequency. Vaccination of dams alone with S. Typhimurium was associated with reduced shedding of closely-related serovars among all age groups in this commercial setting.

Salmonella Vaccination in Pigs: A Review

The control of Salmonella enterica in pig production is necessary for both public and animal health. The persistent and frequently asymptomatic nature of porcine Salmonella infection and the organism's abilities to colonize other animal species and to survive in the environment mean that effective control generally requires multiple measures. Vaccination is one such measure, and the present review considers its role and its future, drawing on studies in pigs from the 1950s to the present day. Once established in the body as an intracellular infectious agent, Salmonella can evade humoral immunity, which goes some way to explaining the often disappointing performance of inactivated Salmonella vaccines. More recent approaches, using mucosal presentation of antigens, live vaccines and adjuvants to enhance cell-mediated immunity, have met with more success. Vaccination strategies that involve stimulating both passive immunity from the dam plus active immunity in offspring appear to be most efficacious, although either approach alone can yield significant control of Salmonella. Problems that remain include relatively poor control of Salmonella serovars that are dissimilar to the vaccine antigen mix, and difficulties in measuring and predicting the performance of candidate vaccines in ways that are highly relevant to their likely use in commercial production.

Generation of a safe Salmonella Gallinarum vaccine candidate that secretes an adjuvant protein with immunogenicity and protective efficacy against fowl typhoid

We constructed a live, attenuated Salmonella Gallinarum (SG) that secretes heat-labile enterotoxin B subunit protein (LTB), and evaluated this strain as a new vaccine candidate by assessing its safety, immunogenicity and protective efficacy against fowl typhoid. An asd(+) p15A ori low-copy plasmid containing eltB encoding LTB was transformed into a ΔlonΔcpxRΔasd SG (JOL967) to construct the candidate, JOL1355. In Experiments 1 and 2, birds were orally immunized with JOL1355 at 4 weeks of age, while control birds were inoculated with sterile phosphate-buffered saline. In Experiment 2, the birds of both groups were orally challenged with a virulent SG at 8 weeks of age. In Experiment 1, examination for safety revealed that the immunized group did not show any bacterial counts of the vaccine candidate in the liver and spleen. Birds immunized with the vaccine candidate showed a significant increase in systemic IgG and mucosal secretory IgA levels in Experiment 2. In addition, the lymphocyte proliferation response and the numbers of CD3(+)CD4(+) and CD3(+)CD8(+) T cells were also significantly elevated in the immunized group, which indicated that the candidate also induced cellular immune responses. In the protection assay, efficient protection with only 16% mortality in the immunized group was observed against challenge compared with 76% mortality in the control group. These results indicate that the live, attenuated SG secreting LTB can be a safe vaccine candidate. In addition, it can induce humoral and cellular immune responses and can efficiently reduce mortality of birds exposed to fowl typhoid.

Construction of an attenuated Salmonella delivery system harboring genes encoding various virulence factors of avian pathogenic Escherichia coli and its potential as a candidate vaccine for chicken colibacillosis

An attenuated Salmonella (deltalon, deltacpxR, and deltaasdA16) delivery system containing the genes encoding P-fimbriae (papa and papG), aerobactin receptor (iutA), and CS31A surface antigen (clpG) of avian pathogenic Escherichia coli (APEC) was constructed, and its potential as a vaccine candidate against APEC infection in chickens was evaluated. The birds were divided into three groups designated group A (nonvaccinated control), group B (given a single immunization), and group C (administered prime and boost immunizations). Prime and booster vaccinations with the constructions were administered to 1-day-old and 14-day-old birds, respectively. Immune responses were measured postimmunization, and the birds were challenged via an intra-air sac route with a virulent APEC strain at the second, third, and fourth weeks of age. Group B birds were partially protected against the challenge and showed increased levels of plasma immunoglobulin (Ig)G, mucosal IgA antibodies, and lymphocyte proliferation. Group C birds showed greater protection against the challenge, with significantly stronger immune responses compared with the birds in the other groups. Overall, our data suggest that the Salmonella delivery system with recombinant constructs is capable of inducing robust immune responses and induces effective protection against colibacillosis caused by APEC.

Immunogenicity of a Salmonella Enteritidis mutant as vaccine candidate and its protective efficacy against salmonellosis in chickens

A novel Salmonella Enteritidis (SE) vaccine candidate strain, JOL919 was constructed by deleting the lon and cpxR genes from a wild-type SE using an allelic exchange method. The study was carried out to evaluate the strain as a vaccine candidate against salmonellosis. The strain showed the enhanced macrophage invasion, early bacterial clearance and higher immune responses as compared to the other mutants, JOL917 (Δlon) and JOL918 (ΔcpxR), and the wild type. In further analysis, the chickens immunized with JOL919 showed a significant increase in plasma IgG and intestinal secretory IgA levels, which was an indication of robust humoral and mucosal immune responses induced by the candidate. The lymphocyte proliferation response and CD45(+)CD3(+) T cells, associated with an activation of T helper and cytotoxic cells, were also significantly increased in the immunized group, which indicated that the candidate also induced cellular immune responses. The immune cell influx into caecal tissues analyzed by immunohistochemistry showed that CD8(+) T cells were predominated in the immunized group, suggesting that the candidate can clear the invaded pathogen in the intestines by a more direct way involving cytotoxic activity. By the examination of the protection efficacy measured by observations of gross lesions in the organs and bacterial recovery, the candidate can provide an efficient protection upon virulent challenge.

Producing Salmonella-free pigs: a review focusing on interventions at weaning

Salmonella infection in pig production is typically endemic and largely asymptomatic. It is a cause of substantial concern among food safety bodies, prompting voluntary and legislative responses aimed at monitoring and reducing the number of Salmonella-infected animals entering the human food chain. Elimination of the problem at an early stage of production is highly desirable, and to this end the present review examines published evidence on the carriage of Salmonella by piglets before and after weaning, as well as evidence on the dynamics of Salmonella infection in the weaner and grower stages of pig production, the effects of maternal immunity, and risk factors for Salmonella excretion after weaning. Various interventions to reduce or eliminate Salmonella infection in young pigs have been tried, such as vaccination, competitive exclusion, treatments in feed and water, antibiotic administration, disinfection of animals, and segregated weaning to clean accommodation. The evidence on the effectiveness of these is considered, and the last is examined in some detail, as it appears currently to offer the best chance of eliminating Salmonella from growing stock.

A clinical field trial to evaluate the efficacy of vaccination in controlling Salmonella infection and the association of Salmonella-shedding and weight gain in pigs

A clinical field trial was performed to determine the effectiveness of an autogenous Salmonella Typhimurium bacterin compared with a commercial live S. Choleraesuis vaccine in pigs. The association between Salmonella shedding and weight gain was also investigated. Nine cohorts of weaned pigs, (330 to 350 pigs per cohort), were randomly assigned to 1 of 3 treatment groups (injection with S. Typhimurium bacterin, vaccination via water with S. Choleraesuis vaccine, or a control group receiving no vaccine). In each cohort, the average daily gain was calculated for a selected pen throughout the production stage. Pen (pooled) fecal samples were collected bi-weekly and cultured. The odds of Salmonella shedding in both vaccinated groups was higher than in the control group (P < 0.05). The prevalence of Salmonella shedding declined overall as pigs aged (P = 0.04). However, the control pigs showed the smallest decrease in Salmonella shedding over the entire production stage, while prevalence of Salmonella shedding in the vaccinated groups decreased twice as much as the control group over the entire production stage. Salmonella Typhimurium var. Copenhagen DT104, S. Cerro, and S. Agona, which had been isolated on the study farm previously, were recovered from pigs in this study. Shedding of S. Typhimurium var. Copenhagen decreased over time in both vaccine treatment groups. On the other hand, S. Cerro shedding rate was lower in the control pigs compared with vaccinated pigs and S. Agona could be recovered only from the samples collected from S. Choleraesuis vaccinated pigs. The pigs from pens with a higher Salmonella recovery rate experienced slower growth compared with pigs from pens where Salmonella was not isolated. This latter finding indicates that there might be an economic incentive for producers to try to control endemic salmonellosis if effective programs could be developed.

The probiotic paradox: live and dead cells are biological response modifiers

Probiotics are usually defined as products which contain viable non-pathogenic micro-organisms able to confer health benefits to the host. There are specific gastrointestinal effects of probiotics such as alleviating inflammatory bowel disease, reducing acute diarrhoea in children, inhibitingSalmonellaandHelicobacter pylori, removing cholesterol, secreting enzymes and bacteriocins and immunomodulation. However, many of the effects obtained from viable cells of probiotics are also obtained from populations of dead cells. Heat-killed cells ofEnterococcus faecalisstimulate the gastrointestinal immune system in chicks. Dead bifidobacteria induce significant increases in TNF-α production. Administration of heat-killedE. faecalisto healthy dogs increases neutrophil phagocytes. The probiotic paradox is that both live and dead cells in probiotic products can generate beneficial biological responses. The action of probiotics could be a dual one. Live probiotic cells influence both the gastrointestinal microflora and the immune response whilst the components of dead cells exert an anti-inflammatory response in the gastrointestinal tract. This is quite analogous to a proposed mode of action of antimicrobial growth promoters in animal production. This has several implications for the production and application of probiotics, as it will be difficult to assess the relative proportions of live and dead cells in a probiotic culture. Variable amounts of dead cells might contribute to the variation in response often seen with live probiotic cultures. However, the use of dead probiotics as biological response modifiers has several attractive advantages; such products would be very safe and have a long shelf-life.

The incidence of salmonellosis among dairy herds in the northeastern United States

The objectives of this study were to estimate the incidence of salmonellosis among a large sample of dairy herds in the northeastern United States (both at the animal level and the herd level), to describe the serotypes and antimicrobial resistance profiles of the positive samples, and to determine whether various herd-level factors were important predictors of incidence. Participating veterinarians enrolled 831 dairy herds and submitted fecal samples from 2,565 female dairy cattle for Salmonella culture because of suspicion of clinical disease. Estimates of animal-level incidence rates were calculated for each age group as the number of cases per animal time at risk, and an estimate of herd-level incidence rate was calculated as the number of positive herds per herd time at risk. Descriptive analysis of serotype data and level of antimicrobial resistance was performed, and Poisson regression analysis was used to study associations between the within-herd incidence of salmonellosis and certain predictor variables (herd size, housing type, vaccination status, and prior history of Salmonella infection). Salmonella was isolated from 576 (22.5%) samples representing 93 herds. The animal-level incidence rates for preweaned female calves, heifers, and adult cows were 8.1, 0.04, and 1.8 cases per 1,000 animal-years, respectively. The herd-level incidence rate was 8.6 positive herds per 100 herd-years. Salmonella Newport was the predominant serotype, accounting for 41% of the cases, followed by Salmonella Typhimurium. Over 68% of all isolates were resistant to 5 or more antimicrobial agents. Herd size was the only significant predictor of the incidence of salmonellosis in a multivariable model; herds with at least 400 female dairy cattle had a higher incidence rate than smaller herds. Our results shed light on the impact of salmonellosis on the dairy industry in the northeastern United States, and they help clarify the role of dairy cattle as a source of Salmonella serotypes that are also important human pathogens.

Live Salmonella enterica serovar Typhimurium (S. Typhimurium) elicit dendritic cell responses that differ from those induced by killed S. Typhimurium

The immune response of bovine monocytes-derived dendritic cells (DC) exposed to either live or killed Salmonella enterica serovar Typhimurium was compared. Both live and killed bacteria induced changes in morphology with distinctive formation of processes and up-regulation of the ability of DC to stimulate allogeneic T-cell proliferation. Also, both live and killed bacteria up-regulated the expression of MHC-I, MHC-II and CD80. However, live bacteria induced greater up-regulation of the expression of CD40 and CD86 than killed bacteria. Live bacteria also induced greater up-regulation of transcription for IL-6, IL-12 and GM-CSF than killed bacteria as measured by quantitative RT-PCR. These data suggest that blood-monocyte-derived DC may follow distinct maturation pathways following exposure to live or killed bacteria. These differences are likely to have consequences for the priming of the adaptive immune responses.

Intranasal immunogenicity of a Deltacya Deltacrp-pabA mutant of Salmonella enterica serotype Typhimurium for the horse

The aim of this study was to investigate the intranasal immunogenicity for the horse of a Deltacya Deltacrp-pabA mutant (MGN-707) of Salmonella enterica serotype Typhimurium (S. typhimurium). MGN-707 caused no sign of disease, was not detected in feces and a single administration induced strong Salmonella-specific serum and nasal mucosal antibody responses. All ponies had made strong salmonella specific serum IgGa, IgGb, IgA and IgM antibody responses by day 25 after the first immunization. IgM responses to salmonella lipopolysaccharide (LPS) were short lived whereas salmonella specific serum IgGa and IgGb persisted at high levels in all ponies until 83 and 140 days, respectively. Specific nasal mucosal antibody responses dominated by IgA and IgM were evident by day 25 in all ponies except one in which only specific IgGa and IgGb were evident. Specific nasal mucosal IgA persisted in most ponies until day 69. A second immunization on day 140 boosted antibody responses, and stimulated a strong nasal mucosal IgA response in the pony that failed to make an IgA response after primary immunization. At the termination of the experiment, IgA and IgGb dominated jejunal antibody responses whereas vaginal responses were mainly IgA. The latter response unequivocally confirms the existence of a common mucosal immune system in equids. The results indicate that a S. typhimurium Deltacya Deltacrp-pabA mutant has potential as an intranasal vaccine against salmonellosis in the horse.

Auxotrophic, plasmid-cured Salmonella enterica serovar typhimurium for use as a live vaccine in calves

Calves were vaccinated orally, subcutaneously or intraperitoneally with a smooth, plasmid-cured strain of Salmonella enterica serovar typhimurium, strain 81. Oral vaccination was not effective, as only 1/5 calves survived challenge with virulent S. typhimurium. Strain 81 was attenuated for calves, as only a slight rise in rectal temperatures was detected after vaccination. The organism was excreted by some calves in the faeces, but no signs of diarrhoea were observed after vaccination. After parenteral vaccination, strain 81 was able to reach the intestines, gastric associated lymphoid tissues and other internal lymphoid tissues and remained viable for up to 14 days in the bovine host. After oral challenge with a virulent strain, 9/10 vaccinated calves survived challenge as opposed to 4/10 control calves (p<0.5). Diarrhoea was present in all calves of the control groups, but in only 4/10 of the vaccinated calves. The clinical reactions of the vaccinated calves were milder than in the control calves, as the rises in rectal temperatures were lower, diarrhoea was less severe, and the challenge strain was present in fewer organs from vaccinated calves than control calves. This study showed that parenterally administered Salmonella vaccines can induce both mucosal and systemic immunity, and it is postulated that this capability of strain 81 is related to its colonisation of lymphoid tissues and other systemic and intestinal tissues. This study confirmed that plasmid-cured strains were attenuated in the bovine host and conferred significant protection after parenteral vaccination, but not oral vaccination.

Intranasal immunogenicity of a Delta cya Delta crp-pabA mutant of Salmonella enterica serotype Typhimurium for the horse

The aim of this study was to investigate the intranasal immunogenicity for the horse of a Deltacya Deltacrp-pabA mutant (MGN-707) of Salmonella enterica serotype Typhimurium (S. typhimurium). MGN-707 caused no sign of disease, was not detected in feces and a single administration induced strong Salmonella-specific serum and nasal mucosal antibody responses. All ponies had made strong salmonella specific serum IgGa, IgGb, IgA and IgM antibody responses by day 25 after the first immunization. IgM responses to salmonella lipopolysaccharide (LPS) were short lived whereas salmonella specific serum IgGa and IgGb persisted at high levels in all ponies until 83 and 140 days, respectively. Specific nasal mucosal antibody responses dominated by IgA and IgM were evident by day 25 in all ponies except one in which only specific IgGa and IgGb were evident. Specific nasal mucosal IgA persisted in most ponies until day 69. A second immunization on day 140 boosted antibody responses, and stimulated a strong nasal mucosal IgA response in the pony that failed to make an IgA response after primary immunization. At the termination of the experiment, IgA and IgGb dominated jejunal antibody responses whereas vaginal responses were mainly IgA. The latter response unequivocally confirms the existence of a common mucosal immune system in equids. The results indicate that a S. typhimurium Deltacya Deltacrp-pabA mutant has potential as an intranasal vaccine against salmonellosis in the horse.

Salmonella: immune responses and vaccines

Salmonella infections are a serious medical and veterinary problem world-wide and cause concern in the food industry. Vaccination is an effective tool for the prevention of Salmonella infections. Host resistance to Salmonella relies initially on the production of inflammatory cytokines leading to the infiltration of activated inflammatory cells in the tissues. Thereafter T- and B-cell dependent specific immunity develops allowing the clearance of Salmonella microorganisms from the tissues and the establishment of long-lasting acquired immunity to re-infection. The increased resistance that develops after primary infection/ vaccination requires T-cells cytokines such as IFNgamma TNFalpha and IL12 in addition to opsonising antibody. However for reasons that are not fully understood seroconversion and/or the presence of detectable T-cell memory do not always correlate with the development of acquired resistance to infection.Whole-cell killed vaccines and subunit vaccines are used in the prevention of Salmonella infection in animals and in humans with variable results. A number of early live Salmonella vaccines derived empirically by chemical or u.v. mutagenesis proved to be immunogenic and protective and are still in use despite the need for repeated parenteral administration. Recent progress in the knowledge of the genetics of Salmonella virulence and modern recombinant DNA technology offers the possibility to introduce multiple defined attenuating and irreversible mutations into the bacterial genome. This has recently allowed the development of Salmonella strains devoid of significant side effects but still capable of inducing solid immunity after single oral administration. Live attenuated Salmonella vaccines have been used for the expression of heterologous antigens/proteins that can be successfully delivered to the immune system. Furthermore Salmonella can transfer plasmids encoding foreign antigens under the control of eukaryotic promoters (DNA vaccines) to antigen-presenting cells resulting in targeted delivery of DNA vaccines to these cells. Despite the great recent advances in the development of Salmonella vaccines a large proportion of the work has been conducted in laboratory rodents and more research in other animal species is required.

Cattle immune responses to tetanus toxoid elicited by recombinant S. typhimurium vaccines or tetanus toxoid in alum or Freund's adjuvant

Cattle were immunised orally, nasally or subcutaneously with either S. typhimurium 4/74 aroA(-) aroD(-) or S. typhimurium 4/74 htrA-based live vaccines expressing Fragment C (TetC) of tetanus toxin from plasmid pTetnir15. Oral inoculation with S. typhimurium 4/74 aroA(-) aroD(-)- (pTetnir15) elicited mucosal anti-TetC IgA but no measurable systemic humoral responses to TetC. Subcutaneous inoculation with the same strain elicited both mucosal IgA and systemic anti-TetC IgG1 responses. Nasal inoculation did not elicit any detectable anti-TetC responses. Oral delivery of S. typhimurium htrA(-) proved fatal in inoculated animals. None of the animals inoculated with either mutant S. typhimurium developed detectable T cell proliferative responses to the guest antigen. Cattle were also inoculated with tetanus toxoid adsorbed in alum or emulsified in Freund's complete adjuvant. Animals inoculated subcutaneously with Ttox emulsified in FCA developed systemic IgG1 and IgG2 antibody, while animals inoculated with Ttox adsorbed in alum developed systemic IgG1 but little IgG2 to Ttox. Both of these groups of animals developed measurable TetC-specific proliferative T cell responses that were associated with the production of IFNgamma.

Attenuation and immunogenicity of Deltacya Deltacrp derivatives of Salmonella choleraesuis in pigs

Six different isogenic Deltacya Deltacrp derivatives of a strain of Salmonella choleraesuis var. kunzendorf-chi3246 virulent for pigs were constructed by transposon-mediated deletion mutagenesis. These strains were evaluated for virulence and ability to elicit a protective immune response in young weaned pigs after oral administration and were compared to a commercially available vaccine which lacks the 50-kb virulence plasmid (vpl(-)). These derivatives were Deltacya Deltacrp vpl(+), Deltacya Deltacrp vpl(-), Deltacya Delta(crp-cdt) vpl(+), Deltacya Delta(crp-cdt) vpl(-), Deltacya Deltacrp pmi-3834 vpl(+), and Deltacya Delta(crp-cdt) pmi-3834. In experiments to evaluate safety, no significant adverse effects of any of the vaccine constructs were observed, except that two of the strains which carried the virulence plasmid (vpl(+)) caused a small, short-term elevation in maximum temperature compared to pretreatment temperature values. Orally immunized animals, except for those vaccinated with the Deltacya Deltacrp pmi-3834 vpl(+) strain or SC-54, developed significant serum antibody responses 21 days postvaccination as measured by enzyme-linked immunosorbent assay. No cell-mediated immune responses to heat-killed S. choleraesuis were noted at the same time point as measured with heat-killed bacteria as antigen in a lymphocyte proliferation assay. In an oral challenge exposure model with a highly virulent heterologous strain of S. choleraesuis, the Deltacya Deltacrp strains with deletions in pmi were not protective. As measured by morbidity scores, the responses to challenge of the pigs vaccinated with the other four Deltacya Deltacrp derivatives were significantly better than those of the nonvaccinated, challenged group. With the exception of temperature elevation and slight differences in diarrhea scores postchallenge, none of these strains differed significantly from each other in the other clinical parameters analyzed. While the commercial vaccine was protective by most of the parameters measured, it was not fully protective against challenge with virulent S. choleraesuis as judged by diarrhea scores and temperature elevation. Collectively, these data demonstrate that Deltacya Deltacrp derivatives, with or without the virulence plasmid but not with deletions in the pmi gene, are candidates for vaccines for protection against salmonellosis in pigs.

Evaluation of an aroA mutant Salmonella typhimurium vaccine in chickens using modified semisolid Rappaport Vassiliadis medium to monitor faecal shedding

In groups of chickens vaccinated orally or intramuscularly with a live aroA mutant Salmonella typhimurium vaccine strain and then experimentally inoculated with 10(8) CFU of wild type S. typhimurium or 10(9) CFU of S. enteritidis, faecal shedding of the vaccine and wild type strains was monitored by the buffered peptone water-modified semisolid Rappaport Vassiliadis medium method, which detected less than 10(2) CFU per gram of faeces. The vaccine strain was shed in the faeces for up to 26 days. Vaccination failed to reduce the faecal shedding of wild type S. typhimurium or S. enteritidis. The variation in the shedding patterns of chickens within each group was greater than between treatment groups.

Seroreactivity of Salmonella-infected cattle herds against a fimbrial antigen in comparison with lipopolysaccharide antigens

The IgG seroreaction of Salmonella-infected cattle herds against a fimbrial antigen (SEF14) was compared with that against lipopolysaccharide (LPS) antigens. Sera from 23 dairy herds (n = 205) from an island with no occurrence of salmonellosis, four herds (n = 303) with recent outbreaks of S. dublin and four herds (n = 168) with recent outbreaks of S. typhimurium, were tested in a SEF14-ELISA, S. dublin LPS (0:1, 9, 12) ELISA and S. typhimurium LPS (0:1, 4, 5, 12) ELISA. At a cut-off OD of 0.5, only one of the animals tested from the salmonellosis-free island showed significant seroreaction against the SEF14 antigen, which was confirmed in a Western-blot analysis. Three out of the four S. dublin-infected herds had several seroreactors in the SEF14-ELISA, whereas all the four herds were positive in the 0:1, 9, 12-ELISA. All but two samples (both from the same herd) in the four S. typhimurium-infected herds, positive in the 0:1, 4, 5, 12-ELISA, had OD values below 0.5 in the SEF14-ELISA. The results indicate that cattle can produce detectable specific antibodies against fimbrial antigens which may be used for screening of S. dublin-infected herds, particularly in areas with low prevalence of salmonellosis, increasing the predictive value of serology.

Comparison between persisting anti-lipopolysaccharide antibodies and culture at postmortem in salmonella-infected cattle herds

Herds with recent clinical outbreaks of Salmonella dublin (7 herds) and S. typhimurium (4 herds) infections were followed serologically in O-antigen ELISAs over about one year, divided in four equal sampling phases. Animals found to be persistent high-reactors or seronegative at the end of the study were slaughtered and subsequently cultured for salmonella in a selected number of organ samples. Approximately 3% of all animals had high seroreactions up to 17 months after the outbreaks, and less than half of the seropositive animals in the S. dublin-infected herds were salmonella culture positive at slaughter (14/31). However, one persistently seronegative animal was also culture positive. Furthermore, as much as 70% of the male calves investigated at postmortem in the S. dublin-infected herds were high-reactors, among which approx. 56% were culture positive. Surprisingly, 2 of the 14 animals found culture positive turned out to be culture positive for S. typhimurium only. In the S. typhimurium study, none of the 17 animals investigated at postmortem were salmonella culture positive. All sera from these animals were negative in the O:9 blocking ELISA, and no serum sample was positive in the S. dublin ELISA, alone. In conclusion, although serology based on the O-antigens appears to be useful to identify salmonella-infected herds, it seems to be insufficient for identification of persistently infected animals.

Mucosal immune responses in calves orally vaccinated with a live auxotrophic aroA Salmonella dublin strain

The mucosal immune responses were studied in two calves orally vaccinated with the live aroA mutant strain, Salmonella dublin SL5631, and in two non-vaccinated control calves. Intestinal secretions were collected through a permanent fistula in place during a 5-week period. Vaccinated calves responded with high IgM and IgA titres against the S. dublin lipopolysaccharide. Both IgA and the IgM titres appeared already after 3 days and IgM somewhat earlier than the IgA titres. Both antibody titres remained high for 2 weeks after the third and final vaccine dose. The calves also showed marked T-cell responses in lymphocytes collected from mesenterial lymph nodes and the spleen.

Veterinary vaccines

Vaccination of animals for the prevention of infectious diseases has been practised for a number of years with little change in product composition. Recent advances in molecular biology, pathogenesis and immunology have laid the groundwork for the development of a new generation of veterinary vaccines based on pure subunits as well as live vectored bacteria and viruses. Along with novel methods of antigen preparation, the use of new adjuvants and delivery systems will permit targeting of the appropriate immune response as well as offering flexibility in terms of vaccination protocols. These new technologies are also being applied to the development of vaccines to enhance animal productivity and to control reproduction.

Immune response of pigs to parenteral vaccination with an aromatic-dependent mutant of Salmonella typhimurium

Cellular and humoral immune responses to parenteral vaccination with an aromatic-defined (aroA) Salmonella typhimurium and to oral challenge with the S. typhimurium parent strain were examined in pigs. The effectiveness of aroA S. typhimurium vaccination for prevention of clinical disease following challenge was also evaluated. A split litter model was utilized and analysis of variance was by least squares. The statistical model accounted for the effects of vaccination and litter. Parenteral vaccination of pigs with the aroA mutant induced a significant O-polysaccharide (O-ps) specific lymphocyte blastogenic response as well as a significant antibody response to O-ps, lipopolysaccharide and killed bacteria. The aroA strain was avirulent in pigs, was not shed in the feces and significantly reduced the severity of diarrhea following oral challenge.

Proliferative and T-cell specific interleukin (IL-2/IL-4) production responses in spleen cells from mice vaccinated with aroA live attenuated Salmonella vaccines

T-cell responses were studied in mice immunized with the Salmonella typhimurium aroA SL3261 live attenuated vaccine strain. T-cell responses in the spleen, both in whole cell populations and in nylon wool non-adherent (T-cell enriched) cells, were studied in vitro as proliferation by incorporation of tritiated thymidine and production of T-cell specific cytokines [IL-2 (interleukin-2)/IL-4]. Stimulating antigens included whole Salmonella lysates and purified lipopolysaccharide (LPS), both untreated and after alkaline hydrolysis to prevent the non-specific mitogenic effect of LPS. Strong proliferative responses were obtained with untreated whole cell extract and LPS, which were decreased by polymyxin B (PB). Alkaline detoxification of the antigens decreased the proliferative response of nylon-wool non-adherent populations to LPS, but greatly increased their response to the Salmonella extract. Surprisingly, PB also reduced proliferation to detoxified LPS. Little or no IL-2/IL-4 production was seen in response to LPS or purified polysaccharide antigens, while there was a strong IL-2/IL-4 response to whole cell lysate, again markedly increasing after alkaline treatment. The results suggest that the T-cell response elicited by immunization with live Salmonella aroA vaccines in mice recognizes antigens other than LPS determinants, and that estimation of T-cell responses to Salmonella antigens by proliferation alone may yield misleading results.

Humoral and cell-mediated immunity in mice after immunization with live oral vaccines of Salmonella typhimurium: auxotrophic mutants with two attenuating markers

Persistence and clearance of the vaccinal strains, humoral and cell-mediated immune response and protective immunity were assessed in ICR mice, immunized intraperitoneally or intragastrically with double-marker auxotrophic mutants of Salmonella typhimurium strains 1771 and 3334 (his-pur-). Strain 3334 possesses in addition the antiepidemic Hst (high sensitivity to tensides) marker which confers decreased survival in the gut and environment. The results showed that the strains are not overattenuated. They revealed preserved multiplication capacity, established carrier state for 28-30 days and induced humoral and cell-mediated immunity as measured by passive haemolysis and microagglutination or footpad swelling tests, respectively. The immune response was determined by biological features of the strains. Strain 1771 induced synthesis of O- and H-antibodies 7-11 days earlier than strain 3334. Footpad reactions appeared 3-4 days after immunization and a week before the serum antibodies. Cell-mediated immunity showed clear correlation with the protection against challenge with a virulent strain. The present results suggest that the double-marker auxotrophic mutants of S. typhimurium might prove very useful in the development of a live vaccine in further studies on livestock.

Experimental oral Salmonella dublin infection in calves. A bacteriological and pathological study

Groups of calves (6-7, 12-14 and 24-28 weeks old) were orally infected with different numbers of the virulent Salmonella dublin strain SVA47. For the 6-7 weeks old calves the LD50-dose was estimated to be 1 x 10(7) bacteria. A dose of 10(9) bacteria was lethal within 24 hrs with the calves dying from septicemia and an acute necrotizing panenteritis. Calves 12-14 weeks old given 2 x 10(10) SVA47 bacteria succumbed to a progressive enteritis within one week. The 24-28 weeks old calves were resistant to an infective dose of 1 x 10(10) SVA47 bacteria. In the 6-7 and 12-14 weeks old calves SVA47 could be recovered from the entire intestinal tract, the liver and the spleen. In the oldest calves S. dublin SVA47 was recovered only from fecal specimens. However, the immunohistopathological examinations, using an S. dublin O-antigen-specific mouse monoclonal antibody and PAP-staining, showed the presence of S. dublin SVA47 in all tissues of the intestinal canal from calves of all ages and with a special affinity for the columnar enterocytes of the terminal jejunum and ileum, the follicle-associated epithelium over the Peyer's patches, and glandular tissues in the duodenum, tonsillar area and the lungs. Surviving calves responded with serum antibody titers against the O-antigenic lipopolysaccharide which appeared in the order IgM followed by IgA, IgG1 and IgG2.

Vaccination of chickens with a Salmonella enteritidis aroA live oral Salmonella vaccine

A mouse-virulent strain of Salmonella enteritidis, Se795 (LD50 less than 10 organisms for mice), was non-virulent for 12-day-old chickens given 10(6) cfu intravenously; the organisms were cleared from liver and spleen by day 14 as measured by direct plating and by day 21 by enrichment. An Se795aroA mutant, CU58, was also cleared from liver and spleen by day 14 after intravenous inoculation of 10(7) cfu. Day-old chicks vaccinated orally with either one dose of 10(9) CU58 at 1 day of age, 10(7) at 1 and 14 days, or 10(5) at 1 and 7 days followed by 10(9) at 14 and 21 days of age, were challenged orally with a nalidixic acid resistant variant of the virulent phage type 4 S. enteritidis strain 109. All vaccinated groups showed a reduction in faecal shedding of the challenge. Chickens given four doses of CU58 showed a significant reduction of cfu in liver, spleen and faeces following intravenous challenge with virulent strain 109. Intramuscular vaccination with 10(9) cfu of Aro strain CU58 at 1 day of age gave no protection against oral challenge with virulent strain 109. Serum antibody production to LPS (ELISA) was minimal in all vaccinated birds. The results indicate that oral vaccination with Aro- S. enteritidis can confer protection to day old chicks against virulent S. enteritidis.

Oral Salmonella: malaria circumsporozoite recombinants induce specific CD8+ cytotoxic T cells

Oral immunization with an attenuated Salmonella typhimurium recombinant containing the full-length Plasmodium berghei circumsporozoite (CS) gene induces protective immunity against P. berghei sporozoite challenge in the absence of antibody. We found that this immunity was mediated through the induction of specific CD8+ T cells since in vivo elimination of CD8+ cells abrogated protection. In vitro studies revealed that this Salmonella-P. berghei CS recombinant induced class I-restricted CD8+ cytotoxic T cells that are directed against the P. berghei CS peptide epitope spanning amino acids 242-253. This is the same peptide that previously was identified as the target of cytotoxic T lymphocytes (CTL) induced by sporozoite immunization. Salmonella-P. falciparum CS recombinants were constructed that contained either the full-length CS gene or a repeatless gene consisting of CS flanking sequences. Both of these vaccines were able to induce CD8+ CTL directed against P. falciparum CS peptide 371-390, which is identical to the target of CTL induced by sporozoites and vaccinia CS recombinants. These results directly demonstrate the ability of an intracellular bacteria such as Salmonella to induce class I-restricted CD8+ CTL and illustrate the importance of CD8+ CTL in immunity to malaria.

Efficacy of one or two doses of Ty21a Salmonella typhi vaccine in enteric-coated capsules in a controlled field trial. Chilean Typhoid Committee

Typhoid fever remains an important public health problem in many areas of the world and an effective, non-reactogenic vaccine would be useful to control this disease. An attenuated Salmonella typhi strain (Ty21a), which has shown promise in previous trials, was evaluated in a controlled field trial in Santiago, Chile. In this trial, 82,543 schoolchildren were randomly assigned to receive one or two doses of Ty21a vaccine in enteric-coated capsules or placebo. The enteric-coated vaccine formulation was well tolerated and practical for mass oral immunization. In the first two years of surveillance, 213 cases of bacteriologically-confirmed typhoid fever were found in schoolchildren participating in the trial; annual rates in the placebo group were 139 and 227 per 100,000. Vaccine efficacy in the first two years after vaccination was 59% for two doses and 29% for one dose; no efficacy was found 3-5 years after vaccination. These results indicate that it will be necessary to identify a vaccine formulation and schedule for Ty21a S. typhi that is practical and provides high level protection for greater than 2 years.

Adjuvants

In summary, HIV vaccine studies described have generally not been designed to measure the effect of the adjuvant or to make comparisons between adjuvants. In only one study was a head-to-head comparison made between HIV antigen alone and antigen formulated with different adjuvants. We hope that future experiments with HIV/SIV vaccine candidates will be designed to determine the relative potency and safety of different adjuvants. Unfortunately, such experiments tend to be tedious and expensive. The design of these studies will need to address a number of variables which influence the response to the vaccine, including route and schedule of immunization, genotype and species of the vaccinated subject, and intrinsic characteristics of the antigen. In addition, the immunologic endpoints should include measurement of both B and T cell function. The carrier/adjuvant/antigen formulation should be hand-tailored and then standardized so that it is manufactured reproducibly without producing different biological effects between lots, and the vaccine formulation should be stable on storage and shipping. Finally, we obviously need to identify and test the protective antigen or antigens. The best adjuvant will never correct the choice of the wrong epitope.

Selective delivery of antigens by recombinant bacteria

The means to attenuate Salmonella and to endow such avirulent strains with the ability to express colonization and virulence antigens from other pathogens has achieved considerable progress during the past several years. One can therefore begin to design and construct strains with specificity to a given animal host and to express in a defined way specific colonization and virulence antigens in a manner to stimulate long-lasting immunity to the Salmonella and to the pathogen supplying the genetic information for the colonization and virulence antigens. Since most pathogens colonize on or invade through mucosal surfaces, the use of recombinant bivalent Salmonella vaccine strains to stimulate a mucosal immune response would induce the development of a first line of defense against a diversity of pathogens. Mucosal immunity should therefore reduce contagious spread of many pathogens since the dose to overcome the mucosal immune barrier would be increased to result in a diminished likelihood of infection. The fact that the recombinant Salmonella vaccine strains also induce humoral and cellular immune responses justifies their use for induction of long-lasting immunity. Although considerable progress has been made in targeting antigens to the GALT by use of avirulent Salmonella, a similar strategy for delivery of antigens to the BALT has yet to be discovered and developed. In addition to constituting a system for induction of immunity against a diversity of pathogens, the recombinant avirulent Salmonella system should provide a means to explore parameters of the mucosal immune response. This would include investigation of the location and duration of memory, the age dependence of induction of mucosal immunity, and the means for the possible induction of oral tolerance with regard to either the mucosal or humoral response to an antigen expressed by the recombinant Salmonella. It is also possible to contemplate using the avirulent Salmonella to target expression of various modulators of the immune system such as interleukin-2 and interferon-gamma to the GALT and thus further enhance the immune response. Lastly, one can introduce into avirulent Salmonella strains genes for putative colonization antigens in order to investigate whether induction of an immune response against the putative colonization antigen does or does not interfere with infection. This system, therefore, permits another means to analyze the relative importance of various bacterial surface attributes in conferring pathogenicity to the microbe.

Infection and immunity to Vibrio cholerae, Salmonella typhimurium and Escherichia coli in a rabbit model

Cholera disease can be induced in the rabbit by duodenal inoculation (DI) of Vibrio cholerae organisms after ligation of the cecum (C) (DIC model). When ligation of the cecum is omitted, no disease symptoms develop. In contrast, the animals are primed which becomes apparent as vibriocidal protection upon challenge with V. cholerae in the DIC model. This protection coincides with high anti-O antigen IgA levels in the bile. The O antigen was shown to be the protective antigen and it must be presented by live organisms. A non-enterotoxigenic mutant of V. cholerae induced protective immunity in the rabbit but was reported to cause mild diarrhea in human volunteers. Looking for alternatives, we applied cholera toxin, known as a mucosal adjuvant, together with killed V. cholerae cells to rabbits. Unfortunately, the minimum adjuvant dose was equal to the minimum toxic dose. A Salmonella typhimurium strain expressing also the V. cholerae O antigen induced systemic rather than local immunity which was not protective. Several Escherichia coli strains were able to elicit a local immune response, but the animal to animal differences were considerable. Therefore, V. cholerae itself was thought to be the most appropriate carrier organism. Some non-enterotoxigenic and auxotrophic mutants of V. cholerae were able to prime and did not show any undesired side-effects in the DIC model. Therefore, further attenuation of non-toxigenic V. cholerae strains by means of stable deletions in nutritional genes seems to be the most promising way to obtain acceptable vaccine candidates.

Oral Salmonella typhimurium vaccine expressing circumsporozoite protein protects against malaria

Immunization with radiation-attenuated malaria sporozoites induces potent cellular immune responses, but the target antigens are unknown and have not previously been elicited by subunit vaccines prepared from the circumsporozoite (CS) protein. A method is described here for inducing protective cell-mediated immunity to sporozoites by immunization with attenuated Salmonella typhimurium transformed with the Plasmodium berghei CS gene. These transformants constitutively express CS antigens and, when used to immunize mice orally, colonize the liver, induce antigen-specific cell-mediated immunity, and protect mice against sporozoite challenge in the absence of antisporozoite antibodies. These data indicate that the CS protein contains T cell epitopes capable of inducing protective cell-mediated immunity, and emphasize the importance of proper antigen presentation in generating this response. Analogous, orally administered vaccines against human malaria might be feasible.

The way ahead for vaccines and vaccination: symposium summary

In this summary, several biological and non-biological constraints to rapid progress in the molecular vaccine era are highlighted such as genetically-based unresponsiveness, induction of inappropriate immune responses, sustaining protective immunity in the vaccines problems in demonstrating efficacy, and ensuring availability and usage of this class of biologicals throughout the world. Some approaches to increasing immunogenicity are discussed with emphasis on composite antigens. Finally, the eight broad strategies for constructing vaccines outlined by Chanock in 1984 are re-examined.

Safety, infectivity, immunogenicity, and in vivo stability of two attenuated auxotrophic mutant strains of Salmonella typhi, 541Ty and 543Ty, as live oral vaccines in humans

Two Salmonella typhi mutants, 541Ty (Vi+) and 543Ty (Vi-), auxotrophic for p-aminobenzoate and adenine, were evaluated as live oral vaccines. 33 volunteers ingested single doses of 10(8), 10(9), or 10(10) vaccine organisms, while four others received two 2 X 10(9) organism doses 4 d apart. No adverse reactions were observed. Vaccine was recovered from coprocultures of 29 of 37 vaccinees (78%) and from duodenal string cultures of two; repeated blood cultures were negative. The humoral antibody response to S. typhi O, H, Vi, and lysate antigens in serum and intestinal fluid was meager. In contrast, all vaccinees manifested cell-mediated immune responses. After vaccination, 69% of vaccinees overall and 89% of recipients of doses greater than or equal to 10(9) responded to S. typhi particulate or purified O polysaccharide antigens in lymphocyte replication studies but not to antigens of other Salmonella or Escherichia coli. All individuals, postvaccination, demonstrated a significant plasma-dependent mononuclear cell inhibition of wild S. typhi.

Role of outer membrane proteins in immunity against murine salmonellosis--1. Antibody response to crude outer membrane proteins of Salmonella typhimurium

The humoral immune response to crude outer membrane proteins (comp) of S. typhimurium in mice has been characterized. Maximal and quicker antibody response was observed when 50 micrograms of comp was injected intraperitoneally. The comp of smooth C5 strain of S. typhimurium evoked antibody response to both lipopolysaccharide (LPS) and proteins. Absorption of these sera with LPS coated erythrocytes eliminated the antibodies to LPS completely, while the antibody level to protein was left unaltered. The comp from rough mutant (lacking O-specific chain of LPS) of S. typhimurium elicited antibodies to proteins but not to LPS. These results indicate the concomitant production of antibodies to Salmonella outer membrane proteins also. The significance of such antibodies in protection and diagnosis has been discussed.

Salmonella typhimurium infection in calves: protection and survival of virulent challenge bacteria after immunization with live or inactivated vaccines

Salmonella typhimurium SL1479, an auxotrophic mutant strain having a complete block in the aromatic biosynthetic pathway and therefore requiring p-aminobenzoic acid and 2,3-dihydroxybenzoate not available in mammalian tissues, was given orally in a dose of 10(10) live bacteria to 4- to 5-week-old calves. Only a mild transient fever response was seen. Strain SL1479 was unable to colonize and persist in the calves for more than 2 weeks. In a vaccination experiment, groups of six calves were (i) orally vaccinated with the live S. typhimurium SL1479 strain, (ii) subcutaneously vaccinated with a heat-inactivated S. typhimurium SVA1232 strain with aluminum hydroxide as adjuvant, or (iii) not vaccinated, to serve as controls. Calves were orally challenged with the live, calf-virulent S. typhimurium SVA44 strain: either 10(6) bacteria (equivalent to 100 25% lethal doses [LD25S]) or 10(9) bacteria (100,000 LD25S doses). The live oral vaccine gave significantly better protection than the heat-vaccinated, subcutaneously injected vaccine since (i) only control calves and calves given the killed vaccine developed profuse diarrhea, (ii) clinically, the mild fever responses seen after challenge in calves given the live vaccine were significantly lower (P less than 0.0005), (iii) autopsies performed 21 days after the challenge infection revealed normal findings in calves given the live vaccine, whereas calves given the killed vaccine showed signs of acute enteritis and chronic salmonellosis, (iv) all 12 calves given either 100 X or 100,000 X the LD25 survived the 21-day observation period; the mean survival time in nonvaccinated calves was 8.0 days; in calves given heat-inactivated vaccine and 100 X the LD25 it was 21.0 days, and in calves given 100,000 X the LD25 it was 11.5 days, (v) the fecal bacterial counts of the challenge S. typhimurium SVA44 strain were significantly lower (P less than 0.0005) in both groups given the live vaccine, and (vi) upon autopsy followed by culture, the qualitative recovery of the challenge strain from the alimentary canals and tissues of calves given the live vaccine was significantly lower (P less than 0.005).