Bovine salmonellosis

Salmonella Phage CKT1 Effectively Controls the Vertical Transmission of Salmonella Pullorum in Adult Broiler Breeders

Phage therapy is widely being reconsidered as an alternative to antibiotics for the treatment of multidrug-resistant bacterial infections, including salmonellosis caused by Salmonella. As facultative intracellular parasites, Salmonella could spread by vertical transmission and pose a great threat to both human and animal health; however, whether phage treatment might provide an optional strategy for controlling bacterial vertical infection remains unknown. Herein, we explored the effect of phage therapy on controlling the vertical transmission of Salmonella enterica serovar Gallinarum biovar Pullorum (S. Pullorum), a poultry pathogen that causes economic losses worldwide due to high mortality and morbidity. A Salmonella phage CKT1 with lysis ability against several S. enterica serovars was isolated and showed that it could inhibit the proliferation of S. Pullorum in vitro efficiently. We then evaluated the effect of phage CKT1 on controlling the vertical transmission of S. Pullorum in an adult broiler breeder model. The results demonstrated that phage CKT1 significantly alleviated hepatic injury and decreased bacterial load in the liver, spleen, heart, ovary, and oviduct of hens, implying that phage CKT1 played an active role in the elimination of Salmonella colonization in adult chickens. Additionally, phage CKT1 enabled a reduction in the Salmonella-specific IgG level in the serum of infected chickens. More importantly, the decrease in the S. Pullorum load on eggshells and in liquid whole eggs revealed that phage CKT1 effectively controlled the vertical transmission of S. Pullorum from hens to laid eggs, indicating the potential ability of phages to control bacterial vertical transmission.

Whole-Genome Investigation of Salmonella Dublin Considering Mountain Pastures as Reservoirs in Southern Bavaria, Germany

Worldwide, Salmonella Dublin (S. Dublin) is responsible for clinical disease in cattle and also in humans. In Southern Bavaria, Germany, the serovar was identified as a causative agent for 54 animal disease outbreaks in herds between 2017 and 2021. Most of these emerged from cattle herds (n = 50). Two occurred in pig farms and two in bovine herds other than cattle. Genomic analysis of 88 S. Dublin strains isolated during these animal disease outbreaks revealed 7 clusters with 3 different MLST-based sequence types and 16 subordinate cgMLST-based complex types. Antimicrobial susceptibility investigation revealed one resistant and three intermediate strains. Furthermore, only a few genes coding for bacterial virulence were found among the isolates. Genome analysis enables pathogen identification and antimicrobial susceptibility, serotyping, phylogeny, and follow-up traceback analysis. Mountain pastures turned out to be the most likely locations for transmission between cattle of different herd origins, as indicated by epidemiological data and genomic traceback analyses. In this context, S. Dublin shedding was also detected in asymptomatic herding dogs. Due to the high prevalence of S. Dublin in Upper Bavaria over the years, we suggest referring to this administrative region as "endemic". Consequently, cattle should be screened for salmonellosis before and after mountain pasturing.

BLOOD AND ACID-BASE CHANGES IN CAILVES EICPERIMENTALLY INFECTED WITH SALMONELLA TYPHIMPRIUM

The experimental design of this study was included (12) Friesian calves aged between 3-5 weeks. Calves were divided into two groups. Group I consisted of (8) calves were infected experimentally with 1.5 x 10“ of Salmonella typhimurium and often that treatment with chloramphenicol and electrolytes fluid were takes place. Group II consisted of (4) calves were infected with organisms but without treatment. The clinical findings of the disease were characterized by two forms which includes the septicemic and enteric forms. Haematological changes revealed a variable number of the total and differential leukocytic count, increased PCV, fibnnogen and decreased of the total plasma protein. . The changes in the acid-based balance indicate the development of metabolic acidosis.

Control and Eradication Programs for Six Cattle Diseases in the Netherlands

Within the European Union, infectious cattle diseases are categorized in the Animal Health Law. No strict EU regulations exist for control, evidence of disease freedom, and surveillance of diseases listed other than categories A and B. Consequently, EU member states follow their own varying strategies for disease control. The aim of this study was to provide an overview of the control and eradication programs (CPs) for six cattle diseases in the Netherlands between 2009 and 2019 and to highlight characteristics specific to the Dutch situation. All of these diseases were listed as C,D or E in the New Animal Health Law. In the Netherlands, CPs are in place for six endemic cattle diseases: bovine viral diarrhea, infectious bovine rhinotracheitis, salmonellosis, paratuberculosis, leptospirosis, and neosporosis. These CPs have been tailored to the specific situation in the Netherlands: a country with a high cattle density, a high rate of animal movements, a strong dependence on export of dairy products, and a high-quality data-infrastructure. The latter specifically applies to the dairy sector, which is the leading cattle sector in the Netherlands. When a herd enters a CP, generally the within-herd prevalence of infection is estimated in an initial assessment. The outcome creates awareness of the infection status of a herd and also provides an indication of the costs and time to achieve the preferred herd status. Subsequently, the herd enrolls in the control phase of the CP to, if present, eliminate the infection from a herd and a surveillance phase to substantiate the free or low prevalence status over time. The high-quality data infrastructure that results in complete and centrally registered census data on cattle movements provides the opportunity to design CPs while minimizing administrative efforts for the farmer. In the CPs, mostly routinely collected samples are used for surveillance. Where possible, requests for proof of the herd status are sent automatically. Automated detection of risk factors for introduction of new animals originating from a herd without the preferred herd status i.e., free or unsuspected, is in place using centrally registered data. The presented overview may inspire countries that want to develop cost-effective CPs for endemic diseases that are not (yet) regulated at EU level.

Whole-genome sequencing reveals resistome of highly drug-resistant retail meat and human Salmonella Dublin

Non-typhoidal Salmonella (NTS) are a significant source of foodborne illness worldwide, with disease symptoms most often presenting as self-limiting gastroenteritis; however, occasionally the infection spreads and becomes invasive, frequently requiring anti-microbial treatment. The cattle-adapted Dublin serovar of NTS has commonly been associated with invasive illness and anti-microbial resistance (AMR). Here, the enhanced resolution conferred by whole-genome sequencing was utilized to elucidate and compare the resistome and genetic relatedness of 14 multidrug-resistant (MDR) and one pan-susceptible S. Dublin, isolated primarily in Pennsylvania, from fresh retail meat (one isolate) and humans (14 isolates). Twelve different genetic AMR determinants, including both acquired and chromosomal, were identified. Furthermore, comparative plasmid analysis indicated that AMR was primarily conferred by a putative IncA/C2 plasmid. A single pan-susceptible S. Dublin isolate, collected from the same timeframe and geographical region as the MDR isolates, did not carry an IncA/C2 replicon sequence within its genome. Moreover, the pan-susceptible isolate was genetically distinct from its MDR counterparts, as it was separated by ≥267 single nucleotide polymorphisms (SNPs), whereas there was a ≤38 SNP distance between the MDR isolates. Collectively, this data set advances our understanding of the genetic basis of the highly drug-resistant nature of S. Dublin, a serovar with significant public health implications.

Model of Persistent Salmonella Infection: Salmonella enterica Serovar Pullorum Modulates the Immune Response of the Chicken from a Th17-Type Response towards a Th2-Type Response

Salmonella enterica infection affects a wide range of animals and humans, and a small number of serovars cause typhoid-like infections, one characteristic of which is persistent infection in convalescents. Avian-specific S. enterica serovar Pullorum produces systemic disease in young chickens, which is followed by a carrier state in convalescent birds, leading to infection of the ovary at sexual maturity and vertical transmission. However, the immunological basis of persistent infection remains unclear. S. enterica serovar Enteritidis is taxonomically closely related but does not show this characteristic. Differences in the immune responses between S Pullorum and S Enteritidis were compared by using Salmonella-infected chicken monocyte-derived macrophages (chMDMs) and CD4⁺ T lymphocytes that had been cocultured with infected chMDMs or chicken splenocytes in vitro and also in 2-day-old chickens in vivo In comparison with S Enteritidis, S Pullorum-infected chMDMs showed reduced mRNA expression levels of interleukin-12α (IL-12α) and IL-18 and stimulated the proliferation of Th2 lymphocytes, with reduced expression of gamma interferon (IFN-γ) and IL-17 and increased expression levels of IL-4 and IL-13 There was little evidence of clonal anergy or immune suppression induced by S Pullorum in vitro. S Pullorum also increased the levels of expression of IL-4 and decreased the levels of IFN-γ in the spleen and cecal tonsil of infected birds. This suggests that S Pullorum is able to modulate host immunity from a dominant IFN-γ-producing Th17 response toward a Th2 response, which may promote persistent infection in chickens. S Pullorum in chickens is presented as a good model of the typhoid group to study persistent infection.

Epidemiology of Salmonella enterica Serotype Dublin Infections among Humans, United States, 1968-2013

Infection incidence and antimicrobial drug resistance are increasing.

Salmonella enterica serotype Dublin is a cattle-adapted bacterium that typically causes bloodstream infections in humans. To summarize demographic, clinical, and antimicrobial drug resistance characteristics of human infections with this organism in the United States, we analyzed data for 1968–2013 from 5 US surveillance systems. During this period, the incidence rate for infection with Salmonella Dublin increased more than that for infection with other Salmonella. Data from 1 system (FoodNet) showed that a higher percentage of persons with Salmonella Dublin infection were hospitalized and died during 2005−2013 (78% hospitalized, 4.2% died) than during 1996–2004 (68% hospitalized, 2.7% died). Susceptibility data showed that a higher percentage of isolates were resistant to >7 classes of antimicrobial drugs during 2005–2013 (50.8%) than during 1996–2004 (2.4%).

Simultaneous Detection of Multiple Salmonella Serovars from Milk and Chicken Meat by Real-Time PCR Using Unique Genomic Target Regions

A novel genomic and plasmid target-based PCR platform was developed for the detection of Salmonella serovars Heidelberg, Dublin, Hadar, Kentucky, and Enteritidis. Unique genome loci were obtained through extensive genome mining of protein databases and comparative genomic analysis of these serovars. Assays targeting Salmonella serovars Hadar, Heidelberg, Kentucky, and Dublin had 100% specificity and sensitivity, whereas those for Salmonella Enteritidis had 97% specificity and 88% sensitivity. The limits of detection for Salmonella serovars Heidelberg, Kentucky, Hadar, Enteritidis, and Dublin were 12, 9, 40, 13, and 5,280 CFU, respectively. A sensitivity assay was also performed by using milk artificially inoculated with pooled Salmonella serovars, yielding a detection limit of 1 to10 CFU/25 mL of milk samples after enrichment. The minimum DNA detected using the multiplexed TaqMan assay was 75.8 fg (1.53 × 101 genomic equivalents [GE]) for Salmonella Heidelberg, 140.8 fg (2.8 × 101 GE) for Salmonella Enteritidis, and 3.48 pg (6.96 × 102 GE) for Salmonella Dublin. PCR efficiencies were 89.8% for Salmonella Heidelberg, 94.5% for Salmonella Enteritidis, and 75.5% for Salmonella Dublin. Four types of 30 pasteurized milk samples were tested negative by culture techniques and with a genus-specific Salmonella invA gene PCR assay. Among 30 chicken samples similarly tested, 12 (40%) were positive by both culture and the invA PCR. Testing of these 12 samples with the serovar-specific PCR assay detected single and mixed contamination with Salmonella Kentucky, Salmonella Enteritidis, and Salmonella Heidelberg. Five unique primers were designed and tested by multiplex conventional PCR in conjunction with the use of the multiplex TaqMan assay with three of the primers. The diagnostic assays developed in this study could be used as tools for routine detection of these five Salmonella serovars and for epidemiological investigations of foodborne disease outbreaks.

Effect of exposure to Neospora caninum, Salmonella, and Leptospira interrogans serovar Hardjo on the economic performance of Irish dairy herds

The objective of the current study was to quantify the effects of exposure to Salmonella, Neospora caninum, and Leptospira interrogans serovar Hardjo (L. hardjo) on dairy farm profitability and to simulate the effect of vaccination for Salmonella and L. hardjo on dairy farm profitability. The production effects associated with exposure to each of these pathogens in study herds were defined under 3 categories: (1) milk production effects, (2) reproduction effects (including culling), and (3) mortality effects. The production effects associated with exposure to Salmonella, N. caninum, and L. hardjo were incorporated into the Moorepark Dairy Systems Model. In the analysis, herds negative for exposure to Salmonella, N. caninum, and L. hardjo were assumed baseline herds, with all results presented relative to this base. In simulations examining the effect of vaccination for Salmonella and L. hardjo on farm profitability, vaccinated herds (vaccination costs included) were considered as baseline herds and results were presented relative to this base. Total annual profits in unvaccinated herds were reduced by €77.31, €94.71, and €112.11 per cow at milk prices of €0.24, €0.29, and €0.34/L, respectively, as a result of exposure to Salmonella. In the current study, herds positive for exposure to Salmonella recorded a 316-kg reduction in milk yield, whereas no association was detected between exposure to N. caninum or L. hardjo and milk production. Exposure to both N. caninum and L. hardjo was associated with compromised reproductive performance. Herds positive for exposure to N. caninum and Salmonella had greater rates of adult cow mortality and calf mortality, respectively. Vaccination for both Salmonella and L. hardjo was associated with improved performance in study herds. Exposure to N. caninum resulted in a reduction in annual farm profits of €11.55, €12, and €12.44 per cow at each milk price, whereas exposure to L. hardjo resulted in a reduction in annual farm profits of €13.83, €13.78, and €13.72 per cow at each milk price. Herds that tested positive for exposure to Salmonella and L. hardjo were compared with herds vaccinated for the respective pathogens. Herds vaccinated for Salmonella generated €67.09, €84.48, and €101.89 per cow more profit at each milk price compared with herds positive for exposure. Similarly, herds vaccinated for L. hardjo generated €9.74, €9.69, and €9.63 per cow more profit compared with unvaccinated exposed herds. However, herds that tested negative for exposure to Salmonella and L. hardjo generated additional profits of €10.22 and €4.09 per cow, respectively, compared with vaccinated baseline herds.

Only one of the two type VI secretion systems encoded in the Salmonella enterica serotype Dublin genome is involved in colonization of the avian and murine hosts

The type VI secretion system (T6SS) is a virulence factor for many Gram-negative bacteria. Salmonella genus harbors five phylogenetically distinct T6SS loci encoded in Salmonella Pathogenicity Islands (SPIs) SPI-6, SPI-19, SPI-20, SPI-21 and SPI-22, which are differentially distributed among serotypes. The T6SSs encoded in SPI-6 and SPI-19 contribute to pathogenesis of serotypes Typhimurium and Gallinarum in mice and chickens, respectively. Salmonella Dublin is a pathogen restricted to cattle where it causes a systemic disease. Also, it can colonize other hosts such as chickens and mice, which can act as reservoirs of this serotype. Salmonella Dublin harbors the genes for both T6SS_SPI-6 and T6SS_SPI-19. This study has determined the contribution of T6SS_SPI-6 and T6SS_SPI-19 to host-colonization by Salmonella Dublin using avian and murine models of infection. Competitive index experiments showed that, a mutant strain lacking both T6SSs (∆T6SS_SPI-6/∆T6SS_SPI-19) presents a strong colonization defect in cecum of chickens, similar to the defect observed for the ∆T6SS_SPI-6 mutant, suggesting that this serotype requires a functional T6SS_SPI-6 for efficient colonization of the avian gastrointestinal tract. Colonization of mice was also defective, although to a lesser extent than in chickens. In contrast, the T6SS_SPI-19 was not necessary for colonization of either chickens or mice. Transfer of T6SS_SPI-6, but not T6SS_SPI-19, restored the ability of the double mutant to colonize both animal hosts. Our data indicate that Salmonella Dublin requires only the T6SS_SPI-6 for efficient colonization of mice and chickens, and that the T6SS_SPI-6 and T6SS_SPI-19 are not functionally redundant.

Human Salmonella clinical isolates distinct from those of animal origin

The global trend toward intensive livestock production has led to significant public health risks and industry-associated losses due to an increased incidence of disease and contamination of livestock-derived food products. A potential factor contributing to these health concerns is the prospect that selective pressure within a particular host may give rise to bacterial strain variants that exhibit enhanced fitness in the present host relative to that in the parental host from which the strain was derived. Here, we assessed 184 Salmonella enterica human and animal clinical isolates for their virulence capacities in mice and for the presence of the Salmonella virulence plasmid encoding the SpvB actin cytotoxin required for systemic survival and Pef fimbriae, implicated in adherence to the murine intestinal epithelium. All (21 of 21) serovar Typhimurium clinical isolates derived from animals were virulent in mice, whereas many (16 of 41) serovar Typhimurium isolates derived from human salmonellosis patients lacked this capacity. Additionally, many (10 of 29) serovar Typhimurium isolates derived from gastroenteritis patients did not possess the Salmonella virulence plasmid, in contrast to all animal and human bacteremia isolates tested. Lastly, among serovar Typhimurium isolates that harbored the Salmonella virulence plasmid, 6 of 31 derived from human salmonellosis patients were avirulent in mice, which is in contrast to the virulent phenotype exhibited by all the animal isolates examined. These studies suggest that Salmonella isolates derived from human salmonellosis patients are distinct from those of animal origin. The characterization of these bacterial strain variants may provide insight into their relative pathogenicities as well as into the development of treatment and prophylactic strategies for salmonellosis.

Transmission of Salmonella in dairy herds quantified in the endemic situation

Salmonella is a cause of concern in the cattle industry, because it is a zoonosis causing severe invasive infections in humans and because it causes economic and welfare losses in infected herds. In general, cattle in the Netherlands are infected with two types; Salmonella Dublin and Salmonella Typhimurium. Both types cause clinical signs but S. Dublin outbreaks are more prevalent and clinical signs are more severe than S. Typhimurium outbreaks. Our knowledge of the transmission of Salmonella within herds is still limited, while this is an essential component for modelling the success of intervention strategies to control Salmonella. The aim of our study was to estimate the basic reproduction ratio (R(0)), the number of secondary cases produced from each primary case in a totally susceptible population, for S. Dublin and S. Typhimurium in dairy herds. Serological data were obtained from eight farms with a clinical outbreak of Salmonella, two with an outbreak of S. Dublin and 6 of S. Typhimurium. R(0) was estimated from the serological data of the herds that were in an endemic state of the infection. R(0) across herds was estimated to be 2.5 (95% CI 1.7-9.8) and 1.3 (95% CI 1.1-1.7) for S. Dublin and S. Typhimurium, respectively. The between herd variation was significant and fairly large. The results of the sensitivity analysis showed that the R(0) estimate was not sensitive for changes in the latent, infectious or seropositive periods. The R(0) estimates indicated that the infection would not spread very extensively in susceptible populations under management systems similar to the ones in the study herds.

Investigation into the seasonal salmonellosis in lactating dairy cattle

Sporadic salmonellosis has been reported in mature lactating dairy cattle in the southwestern United States and is an intriguing problem in that Salmonella can be cultured from faecal samples of these cattle throughout the year. However, it is pathogenic only during late summer/early autumn and in certain years. We sampled apparently healthy (n=10) and diarrhoeic (n=10) cattle during an outbreak on a 2000 head dairy in 2003. The following year, monthly faecal (from the same 30 head), total mixed ration, water, and pen soil samples were collected for Salmonella culture. No serogroup, serotype, genetic, or antimicrobial susceptibility differences were observed in comparison of isolates from healthy and sick cattle. During year 2 of the study, Salmonella was routinely cultured (although highly variable from month to month) from the cattle and the environment, although no outbreak of salmonellosis was observed.

Assessment of producer conducted antemortem inspection of market pigs in Australia

OBJECTIVE

A field trial was conducted to evaluate the effectiveness of on-farm antemortem inspection performed by Australian pig producers in terms of proficiency in detecting unhealthy animals (suspects), the impact on food safety and animal welfare outcomes.

ANIMALS

A total of 64 trial groups of market pigs (n= 10,703) were dispatched from 20 herds to three abattoirs in three states.

PROCEDURE

All producers and abattoir antemortem inspectors were trained in antemortem inspection prior to the trial using a standardised set of antemortem criteria. Study 1 consisted of three comparisons: on-farm versus abattoir antemortem inspections on the same pigs, transport effects in suspect and normal pigs, and meat rejection and carcase disposition in suspect and normal pigs. Study 2 compared Salmonella spp status of suspect and normal pigs. Studies 3 and 4 were conducted to assist the interpretation of the results of on-farm versus abattoir inspection. Antemortem inspection results of three producers and one abattoir antemortem inspector were compared with those of a reference inspector (Gold Standard).

RESULTS

Of the pigs examined, 2.12% (95% CI: 1.84%, 2.42%) were classified as suspect at on-farm antemortem inspection compared to only 0.14% at abattoir antemortem inspection. Forty one percent of pigs classified as suspect on-farm had locomotor problems (arthritis, lameness and foot abscess). Compared to normal pigs, suspect pigs had a relative risk of 67.7 (P < 0.0001) of suffering transport injuries. Meat rejection amounted to the equivalent of 9.68% of suspect carcases, compared to the equivalent of 0.35% of normal carcases. The isolation of Salmonella species from caecal content from 20.9% of 67 suspect pigs and 18.8% of 133 control pigs (two matched healthy pigs from the same batch for each suspect pig) were not significantly different. When compared with the reference inspector on-farm, three producers classified abnormalities in 2,495 pigs with high specificity (0.98), acceptable sensitivity (0.82) and kappa (0.57).

CONCLUSIONS

With training, producer sensitivity in antemortem inspection may be high and more proficient than abattoir antemortem inspection. Use of producer inspections may have benefits for animal welfare and chain efficiency, but not food safety.

Use of IgG avidity ELISA to differentiate acute from persistent infection with Salmonella Dublin in cattle

AIMS

To investigate whether an immunoglobulin (Ig)G avidity ELISA can be used to differentiate between acute and persistent infection with Salmonella (S.) Dublin in cattle. To determine whether the IgG isotype, IgG(1) and IgG(2) responses in acute and persistent infections differ.

METHODS AND RESULTS

Animals were selected from two herds with long-term infection (years) and two herds recently infected (<3 months). Forty-seven animals were categorized into groups based on the persistence of their antibody level in milk. Based on titre from two serial dilutions the avidity index (AI) was calculated for IgG (IgG-AI), IgG(1) (IgG(1)-AI) and IgG(2) (IgG(2)-AI). The mean IgG-AI for suspected carrier animals with either persistently high (group 1) or persistently high to medium high (group 2) antibody levels was significantly (P = 0.003) higher (32.1% and 38.4%) than for acutely infected animals (21.7% and 22.3%). The probability of being a suspect carrier was associated with IgG-AI, antibody level in the sample and age. However, the effect of age could be the result of a biased sample selection. Specificities and sensitivities were calculated at a range of cut-off values for IgG-AI and IgG(1)-AI. Overall, IgG(2)-AI was high compared with IgG(1)-AI, and there was no difference in IgG(2)-AI between infection groups. There was no difference in the ratio IgG(2):IgG(1) for acute and persistent infection groups.

CONCLUSIONS

Assuming that a persistently high antibody response is indicative of persistent infection with S. Dublin in cattle, it can be concluded that the IgG-AI can aid in differentiating between acute and long-term infection on herd level. However, for the test to be useful as an alternative tool to repeated sampling over time for detection of persistently infected carriers during control strategies in cattle herds, the test needs to be optimized and studied further in a larger sample of well-characterized infections in cattle. The affinity of IgG(2) is higher than IgG(1) early in the S. Dublin infection. There appears to be no difference in the IgG(2)-AI between the acute and chronic infection stages.

SIGNIFICANCE AND IMPACT OF THE STUDY

For decades the strategies for detection of persistently infected cattle in S. Dublin infected herds have involved repeated bacteriological culture of faecal samples or repeated antibody measurements over several months. Both methods are time consuming and costly, leaving a new method for detection of carrier animals based on a single sampling highly desirable. This study illustrates a tool, IgG-AI, which may prove useful, although more validation of the method is required before it is used in practice.

Isolation of Escherichia coli O157:H7 from the gall bladder of inoculated and naturally-infected cattle

To determine if Escherichia coli O157:H7 is capable of residing in the gall bladder of cattle, inoculation studies were conducted with O157:H7 strain 86-24 in weaned Holstein calves. Strain 86-24 was isolated from the gall bladders of five calves 36 days after inoculation. Two other calves contained the inoculation strain in the distal colon but the organism was absent in their gall bladders. A second trial in which the calves were euthanized 15 days after inoculation found strain 86-24 in six of seven inoculated calves but only in colon and/or rumen samples. In a third trial that inoculated eight calves with a four-strain cocktail of O157:H7 strains, the gall bladders from all eight animals were positive 9 days after inoculation. The colon and rumen samples from these calves were also positive. E. coli O157:H7 isolates recovered from bile samples and subtyped by pulsed field gel electrophoresis found that three of the four inoculation strains were present in one or more of the calves. Thus, residence in the gall bladder is not restricted to a single strain. Additional evidence of the ability to localize in the gall bladder of cattle was provided by testing the bile from 150 gall bladders (five collection dates, 30 samples each) obtained at an abbatoir and the isolation of E. coli O157:H7 from four samples (2.7%). This study establishes that E. coli O157:H7 can reside transiently or permanently at a low level in the gall bladder of cattle.

Assessment of producer conducted antemortem inspection of market pigs in Australia

OBJECTIVE

A field trial was conducted to evaluate the effectiveness of on-farm antemortem inspection performed by Australian pig producers in terms of proficiency in detecting unhealthy animals (suspects), the impact on food safety and animal welfare outcomes.

ANIMALS

A total of 64 trial groups of market pigs (n = 10,703) were dispatched from 20 herds to three abattoirs in three states.

PROCEDURE

All producers and abattoir antemortem inspectors were trained in antemortem inspection prior to the trial, using a standardised set of antemortem criteria. Study 1 consisted of three comparisons: on-farm versus abattoir antemortem inspections on the same pigs, transport effects in suspect and normal pigs, and meat rejection and carcase disposition in suspect and normal pigs. Study 2 compared Salmonella spp. status of suspect and normal pigs. Studies 3 and 4 were conducted to assist the interpretation of the results of on-farm versus abattoir inspection. Antemortem inspection results of three producers and one abattoir antemortem inspector were compared with those of a reference inspector (Gold Standard).

RESULTS

Of the pigs examined, 2.12% (95% CI: [1.84%, 2.42%]) were classified as suspect at on-farm antemortem inspection compared to only 0.14% at abattoir antemortem inspection. Forty one percent of pigs classified as suspect on-farm had locomotor problems (arthritis, lameness and foot abscess). Compared to normal pigs, suspect pigs had a relative risk of 67.7 (P < 0.0001) of suffering transport injuries. Meat rejection amounted to the equivalent of 9.68% of suspect carcases, compared to the equivalent of 0.35% of normal carcases. The isolation of Salmonella species from caecal content from 20.9% of 67 suspect pigs and 18.8% of 133 control pigs (two matched healthy pigs from the same batch for each suspect pig) were not significantly different. When compared with the reference inspector on-farm, three producers classified abnormalities in 2495 pigs with high specificity (0.98), acceptable sensitivity (0.82) and kappa (0.57).

CONCLUSIONS

With training, producer sensitivity in antemortem inspection may be high and more proficient than abattoir antemortem inspection. Use of producer inspections may have benefits for animal welfare and chain efficiency, but not food safety.

Salmonella Epidemiology and Pathogenesis in Food-Producing Animals

This review reviews the pathogenesis of different phases of Salmonella infections. The nature of Salmonella infections in several domesticated animal species is described to highlight differences in the epidemiology and pathogenesis of salmonellosis in different hosts. The biology of Salmonella serovar host specificity is discussed in the context of our current understanding of the molecular basis of pathogenesis and the potential impact of different virulence determinants on Salmonella natural history. The ability to colonize the intestine, as evidenced by the shedding of relatively large numbers of bacteria in the feces over a long period, is shared unequally by Salmonella serovars. Studies probing the molecular basis of Salmonella intestinal colonization have been carried out by screening random transposon mutant banks of serovar Typhimurium in a range of avian and mammalian species. It is becoming increasingly clear that Salmonella pathogenicity island 2 (SPI2) is a major virulence factor during infection of food-producing animals, including cattle and poultry. The prevalence of Salmonella serovars in domestic fowl varies in different countries and with time. Although chickens are the natural hosts of serovars Gallinarum and Pullorum, natural outbreaks caused by these serovars in turkeys, guinea fowl, and other avian species have been described. There are two possible explanations to account for the apparent host specificity of certain Salmonella serovars. Environmental factors may increase exposure of particular animal species to certain serovars. Alternatively, there are genetic differences between these serovars, which allow them to survive and/or grow in specific niches only found within ruminants or pigs.

Infection of the reproductive tract and eggs with Salmonella enterica serovar pullorum in the chicken is associated with suppression of cellular immunity at sexual maturity

Salmonella enterica serovar Pullorum causes persistent infections in laying hens. Splenic macrophages are the main site of persistence. At sexual maturity, numbers of bacteria increase and spread to the reproductive tract, which may result in vertical transmission to eggs or chicks. In this study we demonstrate that both male and female chickens may develop a carrier state following infection but that the increases in bacterial numbers and spread to the reproductive tract are phenomena restricted to hens, indicating that such changes are likely to be related to the onset of egg laying. The immunological responses during the carrier state and through the onset of laying in hens were determined. These indicate that chickens produce both humoral and T-cell responses to infection, but at the onset of laying both the T-cell response to Salmonella and nonspecific responses to mitogenic stimulation fall sharply in both infected and noninfected birds. The fall in T-cell responsiveness coincided with the increase in numbers of Salmonella serovar Pullorum and its spread to the reproductive tract. Three weeks after the onset of egg laying, T-cell responsiveness began to increase and bacterial numbers declined. Specific antibody levels changed little at the onset of laying but increased following the rise in bacterial numbers in a manner reminiscent of a secondary antibody response to rechallenge. These findings indicate that a nonspecific suppression of cellular responses occurs at the onset of laying and plays a major role the ability of Salmonella serovar Pullorum to infect the reproductive tract, leading to transmission to eggs. The loss of T-cell activity at the point of laying also has implications for Salmonella enterica serovar Enteritidis infection and transmission to eggs, along with its control by vaccination offering a "window of opportunity" in which infection may occur.

The prevalence of Salmonella spp. in bovine faecal, rumen and carcass samples at a commercial abattoir

AIMS

To determine the prevalence, serotype and antibiotic resistance profile of Salmonella isolates in cattle and on carcasses at a commercial Irish abattoir.

METHODS AND RESULTS

Faecal, rumen and carcass samples were collected from a beef abattoir over a 12-month period and examined for the presence of Salmonella spp. Isolates were serotyped, phage typed (when serotype was found to be S. Typhimurium) and tested for susceptibility to a panel of antibiotics. Salmonella was isolated from 2% of faecal, 2% of rumen and 7.6% of carcass samples. Salmonella was most frequently isolated from samples taken during the period August to October. S. Dublin was isolated from 72% of positive samples. S. Agona and S. Typhimurium definitive type (DT)104 were each isolated from 14% of positive samples. All S. Typhimurium DT104 isolates were resistant to ampicillin, chloramphenicol, streptomycin, sulphafurazole and tetracycline (ACSSuT). On occasion, from a single animal, the same serotype was isolated from more than one sample (i.e. faeces and rumen; faeces and carcass; rumen and carcass; faeces, rumen and carcass).

CONCLUSIONS

Salmonella is present in cattle at slaughter and on beef carcasses at an Irish abattoir, with a higher frequency of occurrence during the period August to October. Most isolates from the study are not commonly associated with human clinical infection, with the exception of S. Typhimurium DT104 (R-type ACSSuT).

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides epidemiological data that is necessary for the understanding of beef as a source of human Salmonella infection.

Diversity in antimicrobial resistance and other characteristics among Salmonella typhimurium DT104 isolates

Multiresistant Salmonella enterica subspecies enterica serovar Typhimurium definitive type 104 (S. Typhimurium DT104 or DT104) bacteria are important pathogens in animals and humans. DT104 isolates are often called pentaresistant strains that spread clonally. The purpose of this study was to determine phenotypic, genotypic, and epidemiologic characteristics of 175 S. Typhimurium DT104 strains isolated from food-producing animals in Canada. More than 90% of the isolates were resistant to ampicillin (Amp), chloramphenicol (Chl), florfenicol (Flo), sulfisoxazole (Sul), and tetracycline (Tet), 53% of the isolates were additionally resistant to spectinomycin (Spc) and streptomycin (Str), and 28% to kanamycin (Kan) and neomycin (Neo). Sixty-one percent of the strains harbored a single 60-MDa plasmid, 21% contained 60- and 2.0-MDa plasmids, and 4% had 60, 4.6- and 2.0-MDa plasmids. Resistance to Kan and Neo was encoded by the aminoglycoside aphA-1 gene on 2.0-MDa plasmids, whereas resistance to trimethoprim (Tmp) and Sul was encoded by the dhfrIb gene on 4.6-MDa plasmids. Polymerase chain reactions (PCR) showed the presence of integrons with the ant (3")-Ia aminoglycoside adenyltransferase and the bla(PSE-1) beta-lactamase gene cassettes, and the presence of the flost gene in all but one strain resistant to Spc and Str, Amp, and Chl and Flo, respectively. DT104 isolates from cattle at six feedlots represented a separate clone; they were sensitive to Str and Spc and lacked the ant (3")-Ia gene. Pulsed-field gel electrophoresis (PFGE) using Bln I, Spe I, and Xba I resulted in 15, 12, and 8 PFGE patterns, respectively. In summary, we observed considerable diversity in phenotypic, genotypic, and epidemiological characteristics among the DT104 isolates.

Salmonella enterica serovar Pullorum persists in splenic macrophages and in the reproductive tract during persistent, disease-free carriage in chickens

Salmonella enterica serovar Pullorum is worldwide a poultry pathogen of considerable economic importance, particularly in those countries with a developing poultry industry. In addition to the characteristic high mortality rates among young chicks, one of the features of Salmonella serovar Pullorum infection is that it persists for long periods in convalescent chicks in the absence of clinical disease. This can lead to colonization of the reproductive tract of chickens and at sexual maturity can result in infected progeny through transovarian transmission to eggs. The sites of Salmonella serovar Pullorum persistence in convalescent birds are not known, and the mechanisms of persistence are not understood. Here we show that Salmonella serovar Pullorum can persist in both the spleen and the reproductive tract for over 40 weeks following experimental infection in chickens. During the period of sexual maturity, Salmonella serovar Pullorum colonized both the ovary and the oviduct of hens and led to 6% of laid eggs being infected by Salmonella serovar Pullorum. The colonization of several different sites of the reproductive tract suggests that Salmonella serovar Pullorum may employ more than one mechanism of egg infection. Persistence occurred despite a strong humoral response, suggesting an intracellular site of infection. By use of a Salmonella serovar Pullorum strain containing a plasmid stably expressing green fluorescent protein, we demonstrated that the main site of carriage in the spleen is within macrophages. This raises interesting questions about the biology of Salmonella serovar Pullorum, including why there is an increase in bacterial numbers when birds become sexually mature and in particular how Salmonella serovar Pullorum avoids clearance by macrophages and whether it modulates the immune system in other ways.

Prevalence of Salmonella spp in cull (market) dairy cows at slaughter

OBJECTIVE

To determine the prevalence of Salmonella spp in the cecal-colon contents of cull (market) dairy cows at slaughter because of potential public health ramifications.

DESIGN

Survey study.

SAMPLE POPULATION

Cecal-colon contents collected from 5,087 cull (market) dairy cows at slaughter at 5 slaughter establishments across the United States.

PROCEDURE

During 2 periods of the year, winter (January and February) and summer (July through September), 5 cull (market) cow slaughter establishments in the United States--west (WE), southeast (SEE), central (CE), north central (NCE), and south central (SCE)--establishments were visited, and cecal-colon contents of cull dairy cows were obtained at the time of slaughter. Samples were examined by microbiologic culture at a single laboratory for Salmonella spp.

RESULTS

Salmonella spp were detected in 23.1% of cecal-colon content samples from cull dairy cows across the 5 slaughter establishments. The highest site prevalence (54.5%) was detected at the WE during the summer period, whereas the lowest was found at the CE during the summer (4.3%) and at the NCE during the winter (4.5%). Considerable variation in the daily prevalence of Salmonella spp was found, particularly at the WE and the SCE. Salmonella spp were isolated from 93% of cecal-colon contents collected on a summer day at the WE.

CONCLUSIONS AND CLINICAL RELEVANCE

Results strongly suggest that there is a high prevalence of Salmonella spp in cull dairy cows at slaughter, which could burden Hazard Analysis Critical Control Point programs implemented in slaughter establishments. Procedures to reduce Salmonella load at the dairy farm and during transport to slaughter could reduce the risk of spread during the slaughter process.

Risk factors for clinical salmonellosis in Virginia, USA cattle herds

Veterinarians working in several Virginia, USA counties noticed an increase in clinical salmonellosis in cattle herds during 1994. We conducted a case-control study to identify potential risk factors for salmonellosis in cattle herds in this region. The potential impact of exposure to poultry-related factors was of particular interest because of the close proximity of poultry and cattle operations in this part of the state. Information was collected from 23 case farms and 23 control farms matched by herd veterinarian, type of enterprise (beef vs. dairy), and location. Faecal and environmental samples were collected during herd visits for bacteriologic culture. Salmonella was isolated from 4.7% of 531 faecal, feed, water, and environmental samples collected. Salmonella Typhimurium was the serotype found most frequently in clinical cases in participating herds and from samples collected during study visits. Case herds had a higher percentage of study samples positive for Salmonella. Potential risk factors were screened using methods for pair-matched data and then analyzed in individual conditional logistic-regression models to estimate odds ratios with 95% profile likelihood confidence intervals. Significant factors included: the number of mature cows in the herd (OR=1.01; CI 1.00, 1.03), percent change in the number of mature cows during 1994 (OR=0.68; CI 0.11, 0.96), having calves usually born in a building rather than outdoors (OR=0.17; CI 0.01, 0.98), poultry manure spread on bordering property (OR=4.00; CI 1.00, 26.50), signs of rodents in cattle-housing or feed-storage areas (OR=2.75; CI 0.94, 9.92), and contact of wild geese with cattle or feed (OR=4.5; 1.16, 29.51).

An occurrence of Salmonella typhimurium infection in sika deer (Cervus nippon)

Seven sika deer (Cervus nippon) in a herd of 30 deer in a park died. Upon examination of three dead deer, Salmonella Typhimurium was isolated from the organs and intestinal contents. Histopathological examination revealed catarrhal enteritis and focal necroses in the liver. Immunohistochemically, Salmonella antigen of O4 was detected in the enteric lesions. The case was diagnosed as S. Typhimurium infection in the sika deer. Because of the importance of Salmonella in public health, fecal and soil samples were continuously collected from the paddock. However, no Salmonella was isolated from any samples collected after medication of the deer and thorough disinfection of the immediate environment.

Salmonella enterica serotype Dublin infection: an emerging infectious disease for the northeastern United States

Salmonella enterica subspecies enterica serotype Dublin (S. enterica Dublin) emerged for the first time in New York, Pennsylvania, and Ohio in 1988. Since that time this host-adapted serotype has spread throughout the veal- and dairy beef-raising operations in the region; very few dairy farms have experienced clinical S. enterica Dublin infections. This study details the epidemiology of the outbreaks in cattle. During the period 1988 through 1995, nine New York and four Pennsylvania counties have been affected; 13 different locations were involved in New York, and 10 were involved in Pennsylvania. The morbidity and mortality and seasonal distribution of outbreaks, which totaled 35, is described. The antimicrobial susceptibility pattern of isolates revealed that many of the strains were resistant to a number of commonly used drugs. Clinical case details and pathology information are provided, with a caution to clinicians and microbiologists presented with suspect animals, i.e., most cases occurred in older calves, which is atypical for salmonellosis for this region (calves were 8 or more weeks old) and presented as pneumonia and septicemia rather than the primarily diarrheal syndrome that is more typically recognized for the region. The epidemiology of cases is analyzed through cluster analysis of bacterial isolates and their fatty acid methyl ester profiles; at least six clones appeared in the region during the study period. Results of the epidemiology analysis are used to support a hypothesis regarding the source of S. enterica Dublin for the region and its manner of dissemination.

Passive immunisation of neonatal lambs via colostrum and milk of ewes previously immunised with live attenuated Salmonella typhimurium protects neonatal lambs from experimental salmonellosis

Lambs sucking non-immunised ewes or ewes immunised 4-5 weeks before lambing with live attenuated, aromatic-dependent (aroA) Salmonella typhimurium (strain CS 332) were challenged orally at either 2, 4 or 7 days of age with virulent S. typhimurium (strain CS 94) at doses ranging from 10(9) to 10(13) colony forming units. No lambs displayed signs of clinical salmonellosis and all survived challenge but those sucking immunised ewes had organisms of the challenge strain in their faeces for much shorter periods of time than lambs of the control ewes. High titres of specific antibodies were measured in colostrum and milk of immunised ewes in comparison with very low titres measured in samples from control ewes; these differences were reflected by the titres of antibodies in the sera of corresponding lambs. At 2 days after lambing, the major antibody isotype in the colostrum of immunised ewes and sera of their lambs was IgM whereas at 7 days IgG1 was the predominant isotype. While it was clear that vaccination of pregnant ewes with the live attenuated vaccination conferred protection against experimentally-induced salmonellosis in their lambs, considerable protection was observed in control lambs in spite of there being very low titres of antibodies in the mammary secretion of their dams. The latter observation could be related to the presence of contain non-antibody potent bactericidal factors previously described in colostrum and milk.

Management and nutritional factors associated with the detection of Salmonella sp. from cattle fecal specimens from feedlot operations in the United States

In a convenience sample of 100 feedlot operations (included in the United States Department of Agriculture: Animal and Plant Health Inspection Service 1994 Cattle on Feed Evaluation), up to 25 cattle fecal samples were collected and tested for the presence of Salmonella from each of two pens (the pen which contained the most-recent arrivals, and the pen with cattle that had been on feed the longest). One or more Salmonella spp. were recovered from 38 (38.0%) of the 100 feedlots, 52 (26.0%) of the 200 pens and 273 (5.5%) of the 4977 fecal samples collected. Multivariable logistic regression indicated that feeding tallow and feeding whole cottonseed or cottonseed hulls within seven days prior to fecal sample collection was associated with an increased risk of finding Salmonella in a pen. Variables not found to be significantly associated with the detection of Salmonella in a pen included region, operation size, use of sprinklers, time on feed, type of cattle in the pen, number and concentration of cattle in a pen, feeding probiotics, and various other feeds.

Salmonella Dublin infection in Queensland dairy cattle

OBJECTIVE

To investigate the presence of Salmonella Dublin in Queensland cattle.

DESIGN

An epidemiological study using diagnostic laboratory information and farm records.

PROCEDURE

Outbreaks of gastroenteritis or pneumonia in calves, and abortions and enteritis in cows were routinely investigated for the presence of salmonellae. Where S Dublin was isolated, attempts were made to gather further epidemiological information.

RESULTS

Prior to 1983 only two outbreaks of S Dublin have been recorded in Queensland dairy cattle. In 1983 S Dublin abortions were diagnosed in dairy heifers introduced from southern Australia to south-east Queensland. Sampling indicated that at least 10% of the 500 introduced heifers were faecal excretors of S Dublin. On 3 of the 7 farms from which S Dublin was recorded, infection spread to other cattle that were in contact. From February 1985 to February 1996, 29 outbreaks of S Dublin in cattle occurred on 29 farms (28 in south east Queensland and 1 in north Queensland). Calves were primarily affected. Continuing outbreaks were confirmed on only 4 of these 29 farms. On 15 farms S Dublin infections were associated with the purchase of infected calves or cows, while another farm adjoined 2 previously infected farms. No source of S Dublin was evident for the other 13 farms, where histories were often inadequate.

CONCLUSION

There has been a marked increase in S Dublin outbreaks in Queensland dairy cattle since 1983. Introduction of S Dublin carrier and aborting dairy heifers from southern Australia, where S Dublin is not uncommon, was associated with the initial outbreaks.

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.

Detection of Salmonella enteritidis in equine feces using the polymerase chain reaction and genus-specific oligonucleotide primers

Salmonella was identified in feces from horses, using the polymerase chain reaction (PCR) and genus-specific oligonucleotide primers. Feces from healthy horses were determined to be culture negative and PCR negative for Salmonella. Fecal samples were inoculated with known numbers of colony-forming units (CFU) of S. enteritidis. The fecal samples were enriched overnight in tetrathionate broth, and then DNA was extracted and amplified by PCR using genus-specific primers. Sensitivity of the assay extended to 10 degrees CFU Salmonella enteritidis/g feces; sensitivity of microbiologic culture with enrichment extended to 10 degrees CFU Salmonella enteritidis/g feces. Feces that were not inoculated with S. enteritidis were negative by the PCR. Detection of salmonellae in feces was possible using the PCR within 24 hours from the time of submission of samples. Because samples were enriched, isolates were available for determining antibiograms and serologic grouping or typing.

Live oral avirulent Salmonella vaccines

Infection of animals and humans with Salmonella is a consequence of oral consumption of food or fluids contaminated with Salmonella. Once in the intestine, Salmonella usually attach to, invade, and proliferate in enterocytes or the cells of the gut associated lymphoid tissue (GALT). The latter route of infection can lead to disease or to an asymptomatic carrier state or stimulate the induction of mucosal, systemic and cellular immune responses. Infection of animals with virulent invasive Salmonella can result in suppression of the immune responses which in turn can facilitate the establishment of a carrier state. It is possible to attenuate Salmonella by introducing mutations that (i) confer auxotrophy, (ii) interfere with sugar metabolism and LPS biosynthesis or (iii) affect some global means of regulating genes needed for the full display of virulence. Oral immunization of animals such as mice and chickens with avirulent Salmonella strains usually is not associated with suppression but rather with stimulation of mucosal, systemic and cellular immune responses. Vaccination by injection of killed vaccines or bacterins does not lead to the induction of either mucosal or cellular immune responses, and humoral immunity may be relatively short lived. Thus, killed vaccines are inferior to orally administered live avirulent Salmonella vaccines which induce a long-lasting protective immunity. In this manuscript we discuss desirable attributes of a safe, efficacious live attenuated Salmonella vaccine, describe attenuated Salmonella mutants so far isolated and their properties and present information on the evaluation of a live attenuated Salmonella oral vaccine for poultry.(ABSTRACT TRUNCATED AT 250 WORDS)

Genus-specific detection of salmonellae using the polymerase chain reaction (PCR)

Oligonucleotide primers for the polymerase chain reaction (PCR) that enable genus-specific detection of members of the genus Salmonella were developed. The primers amplify a 496-bp genetic sequence of members of the genus Salmonella. Amplification of DNA extracted from all other genera of the family Enterobacteriaceae and various other gram-positive aerobic and anaerobic bacteria yielded negative results. Applications of the PCR using these genus-specific primers are discussed.

Stress induction of the virulence proteins (SpvA, -B, and -C) from native plasmid pSDL2 of Salmonella dublin

The virulence region of the wild-type plasmid pSDL2 contained in Salmonella dublin is highly conserved among plasmids from several nontyphoid Salmonella serotypes and is essential for the development of systemic infection in BALB/c mice. Polyclonal antibodies against three proteins (SpvA, -B, and -C) expressed from a 4.1-kb EcoRI subclone of the plasmid virulence region were generated. These antibodies were used to detect expression of the Spv proteins when S. dublin was grown in vitro under stress-inducing conditions, such as nutrient deprivation and increased temperature, that the bacteria may encounter during the course of infection within the host. Glucose starvation resulted in expression of all three proteins shortly after the lag phase. When the bacteria were grown to the late-log phase without glucose, heat shock strongly induced expression of SpvA but not SpvB or SpvC. The addition of 0.2% glucose to the medium resulted in loss of expression of the proteins until the late-log to stationary phase. Iron limitation or lowered pH induced expression of the proteins during exponential growth even in the presence of glucose. Insertion mutations into the positive regulator gene spvR upstream from spvABC and insertions into spvA and spvC resulted in loss of expression of SpvA, -B, and -C, suggesting a complex regulation of expression. These studies define a variety of environmental conditions that induce expression of the Spv virulence proteins from the wild-type plasmid pSDL2 in S. dublin in vitro.

Plasmid profile analysis, phage typing, and antibiotic sensitivity of Salmonella dublin from clinical isolates in the United States

One hundred clinical isolates of Salmonella choleraesuis subsp. choleraesuis serovar dublin (Salmonella dublin) were examined for phage sensitivity, antibiotic resistance patterns, and plasmid content. Computer analysis of the lysis patterns observed by using 27 typing phages divided the S. dublin isolates into 26 groups. One lytic pattern (Designated pattern 16) contained 52% of the isolates examined whereas 16 isolates had unique patterns, and nine patterns had fewer than ten members. Although 14 antibiotic resistance patterns were observed among the 100 isolates, 79% of the isolates grouped in three major patterns. Seven plasmid groups were identified and designated A-G based on the large plasmids found in the isolates. Of the 100 isolates, 28 contained the plasmid profile of Group A, 28 were Group B, 7 were Group C, 34 were Group D, and 1 isolate each was observed in Groups E, F, and G. The strong association between antibiotic resistance pattern and plasmid type suggest that the drug resistance genes are plasmid borne.

Persistent experimental Salmonella dublin intramammary infection in dairy cows

Experimental intramammary infections were induced in five post-parturient Holstein cows by inoculation of low numbers (5000 colony forming units) of virulent Salmonella dublin via the teat canal of mammary gland quarters. Rectal temperature, pulse and respiratory rates, milk yield, and milk quality as assessed by the California Mastitis Test (CMT) and somatic cell counts (SCC) were recorded every 12 hours at milking. Bacteriologic cultures of foremilk quarter samples and feces were obtained daily, as were complete blood counts. ELISA titers for IgG and IgM recognizing S. dublin lipopolysaccharide (LPS) were obtained weekly on serum and quarter milk samples. All cows excreted S. dublin intermittently from infected quarters, but no changes were detected in rectal temperature, appearance of the mammary gland or secretions, CBC, milk yield, and pulse and respiratory rates. Somatic cell counts were modestly increased in infected quarters as compared with uninfected quarters (P = .015, paired t test); however, CMT scores after infection remained low, and were not significantly different from pre-infection scores (P greater than .10, sign test). After infection, administration of dexamethasone resulted in signs of clinical mastitis and increased excretion of S. dublin from mammary quarters (P = .0004, paired t test). One cow had necrotizing mastitis and S. dublin septicemia and was euthanatized. In the four surviving cows, clinical improvement was observed after systemic gentamicin therapy and intramammary infusion with polymyxin B, but all cows continued to excrete S. dublin intermittently from one or more quarters and occasionally from feces for the remaining period of observation. All infected cows demonstrated a rise in IgG and IgM ELISA titers recognizing S. dublin LPS in serum and milk. At necropsy (13-25 weeks postinfection), S. dublin was recovered only from the mammary tissue or supramammary lymph nodes in three of four cows. In one cow, mammary gland and lymph-node samples were negative for S. dublin despite positive milk cultures. In all cows, histopathologic examination revealed multifocal areas of chronic active mastitis. These lesions were similar to histopathologic findings from mammary gland carriers with naturally acquired S. dublin infection.

Generation of aromatic-dependent Salmonella havana and evaluation of its immunogenic potential in mice and sheep

The generation of aromatic-dependent (aro-) Salmonella havana (Group G2, 01, 13, 23) from a smooth wild-type parent strain by transduction with phage P1 is reported. Mice immunized with this live aro- S. havana strain (CS234) by the intraperitoneal (i.p.) route were protected against challenge with wild-type S. havana, whereas those immunized by the oral route were not. Mice immunized with two doses of formalin-killed aro- S. havana by the i.p. route were also unprotected, in spite of high antibody titers. However, only those mice immunized with live aro- S. havana by the i.p. route developed significant delayed-type hypersensitivity. Following i.p. inoculation in mice, the aro- S. havana strain CS234 was detected in the liver, spleen and mesenteric lymph nodes on day 9 but not on day 15 post-inoculation (p.i.). On the other hand, when mice were inoculated with the parent wild-type strain (CS4) or the aro- derivative strain CS234 by the oral route, the organisms were recovered from the mesenteric lymph nodes and intestine only on day 3 but not on day 6 post-inoculation. In sheep inoculated with the aro- strain CS234 in the gastroc muscle, organisms were recovered from the muscle, and popliteal and medial iliac lymph nodes for up to 21 but not 28 days p.i. However, no mutant organisms were recovered from liver, spleen, mesenteric lymph nodes or faeces. In orally-inoculated sheep, the mutant organisms were recovered from the mesenteric lymph nodes, rumen, intestinal contents, and faeces up to 14-21 days post-inoculation but not at 28 days. When sheep immunised with the aro- S. havana strain CS234 by the intramuscular or oral route were challenged with the parent wild-type S. havana strain CS4 by the oral route, the latter strain was detectable in the mesenteric lymph nodes and faeces of immune sheep up to 14 days post-challenge in contrast with the non-immune sheep, where the challenge strain was detectable even at 28 days post-challenge. Only sheep immunized by the intramuscular route developed high antibody levels and delayed-type hypersensitivity.