The range of influence between cattle herds is of importance for the local spread of Salmonella Dublin in Denmark

The objective of the study was to estimate the range of influence between cattle herds with positive Salmonella Dublin herd status. Herd status was a binary outcome of high/low antibody levels to Salmonella Dublin in bulk-tank milk and blood samples collected from all cattle herds in Denmark for surveillance purposes. Two methods were used. Initially, a spatial generalised linear mixed model was developed with an exponential correlation function to estimate the range of influence simultaneously with the effect of potential risk factors. An iteratively reweighted generalised least squares procedure was used as a second method for verifying the range of influence estimates. With this iterative procedure, deviance residuals were calculated based on a generalised linear model and the range of influence was estimated based on the residuals using an exponential semivariogram. The range of influence was estimated for six different regions in Denmark using both methods. The analyses were performed on data collected during 1 year after initiation of the Salmonella Dublin surveillance program providing herd classifications for the 4th year-quarter of 2003 and 2 years later for the 4th year-quarter of 2005. The prevalence of dairy herds with a positive Salmonella Dublin herd classification status in this period had decreased from 22.1 to 17.0%. In non-dairy herds, the prevalence was nearly unchanged during the same period (3.4 and 3.7% in 4th quarter of 2003 and 2005, respectively). For all cattle herds, the range of influence was 2.3–6.4 km in 2003 and 1.5–8.3 km in 2005. There seemed to be no association between the range of influence and the density of herds in the different regions.     

Factors associated with variation in bulk-tank-milk Salmonella Dublin ELISA ODC% in dairy herds

Our objective was to determine factors that contribute to variation in bulk-tank-milk Salmonella Dublin enzyme-linked immunosorbent assays (ELISA) corrected optic-density measurements (ODC%) in dairy herds. We constructed hierarchical mixed models with repeated bulk-tank-milk ELISA ODC% in 31 Danish dairy herds. Four models included different combinations of explanatory factors, and we compared how well these models described the variation in the data. Herd was included as a random effect nested within Salmonella status and barn type.

Detection of Salmonella Dublin or Salmonella Typhimurium by bacteriological culture of individual faecal samples or of slurry samples was associated with higher bulk-tank-milk ELISA ODC%, as was apparent Salmonella prevalence, the mean ELISA ODC% or mean-yield-corrected ELISA ODC% in milk samples collected from all individual cows. However, combinations of risk factors that included number or prevalence of cows with a very high ELISA ODC% provided better models, indicating that the effect of the cow-level explanatory variables on the bulk-tank-milk ELISA ODC% was related to the activity of the infection in the herd. Barn type (loose housing or tie stalls) was not associated with the variation in bulk-tank-milk ELISA ODC% in these models, which might be useful in planning of surveillance programs and intervention strategies.     

Towards incorporating spatial risk analysis for Salmonella sero-positivity into the Danish swine surveillance programme

An increased incidence of pork-related human salmonellosis in Denmark led to the development of a national control programme for Salmonella in Danish swine herds in 1993. The aim of the programme has been met and now the issue of cost-effectiveness is receiving greater attention. An appropriate way to address this is to bring a risk-based focus to the programme.

We describe a practical approach to risk-based surveillance through spatial risk assessment using serological and questionnaire data from 2280 herds in 1995. A mixed effects logistic regression model was fitted and both first- and second-order spatial properties of the random effects were investigated. We identified wet-feeding (OR: 0.64; 95% CI: 0.54–0.75) and SPF health status (OR: 0.65; 95% CI: 0.52–0.81) as protective factors for Salmonella sero-positivity. Purchasing feed (OR: 1.81; 95% CI: 1.61–2.04) was a risk factor. The west of the study area generally, and the north of Jutland in particular, experienced the greatest disease risk after controlling for the covariates. There was some evidence for spatial dependency between farms at distances of 6 km (95% CI: 2–35 km) on the Jutland peninsula.

We conclude that when farm location details are analysed in conjunction with routinely recorded surveillance information (such as that collected by the Danish swine Salmonella control programme) and targeted industry surveys (such as those conducted by slaughterhouse co-operatives), our knowledge of the behaviour of disease in animal populations is enhanced and this provides a more informed framework for designing efficient, risk-based surveillance strategies.     

Associations between the proportion of Salmonella seropositive slaughter pigs and the presence of herd level risk factors for introduction and transmission of Salmonella in 34 Danish organic, outdoor (non-organic) and indoor finishing-pig farms

This paper evaluates the association between herd level risk factors for introduction and transmission of Salmonella in farms with three different production systems: organic, outdoor (non-organic) and indoor finishing-pig farms, and the presence of seropositive animals in the herds. Potential risk factors for Salmonella in the three pig production systems were identified through a literature review, and management information as well as serological data were collected in 34 pig farms: 11 organic farms, 12 outdoor farms, and 11 indoor farms. There were no general differences in the proportion of Salmonella seropositive animals in the organic, outdoor, and indoor pig farms. Correspondence analysis showed that the occurrence of seropositive animals in the herds was mostly associated to the risk of introducing Salmonella in the herds by purchasing and transporting growing pigs. No associations between herd risk factors for transmission and survival of Salmonella and seropositive animals in the herds were found.     

Cattle transfers between herds under paratuberculosis surveillance in The Netherlands are not random

The rate and structure of cattle transfers between 206 Dutch cattle herds with a ‘Mycobacterium avium subsp. paratuberculosis (Map)-free’ status by November 2002, were analyzed over a 3-year period (November 1999–November 2002). Of the 206 ‘Map-free’ herds, 184 were closed herds during the period studied. In total, 280 cattle had been introduced into 22 herds at an average rate of 0.33 animals per year per 100 cattle present in the 206 herds. Assuming a random herd-contact structure, the observed rate of cattle transfers between certified ‘Map-free’ herds was sufficiently low to relax the surveillance scheme to biennial herd examinations by pooled fecal culture of all cattle ≥2 years of age.

The cattle transfers were not randomly distributed over the herds. Forty-four of the 280 cattle originated from 12 other ‘Map-free’ herds. The other 236 cattle did not originate from a ‘Map-free’ herd and were introduced into a herd before it obtained the ‘Map-free’ status. No cattle were introduced into any of the ‘Map-free’ herds from which cattle were transferred to other ‘Map-free’ herds. Thus, continued propagation of the infection by cattle transfers was impossible in the group of herds studied during the study period. Therefore the surveillance scheme may be further relaxed, and may be differentiated regarding the risk herds pose to other herds.     

Estimating the number of undetected multi-resistant Salmonella Typhimurium DT104 infected pig herds in Denmark

In Denmark, the detection of multi-resistant Salmonella Typhimurium DT104 (MRDT104)-infected pig herds relies on the national Salmonella surveillance programme at the farm and slaughterhouse levels of production. With the surveillance sampling protocol and the diagnostic methods currently used, some herds might remain undetected. The number of undetected Danish pig herds infected with MRDT104 in the period 1 August 2001–31 July 2002 was estimated and compared with the number of culture-confirmed detected herds. A flow chart was constructed to illustrate where infected herds will go undetected in the surveillance system and Monte Carlo simulation was used to model the actual number of pig herds infected with MRDT1104. We estimated that 52 (90% CI [28, 178]) finisher herds were infected with MRDT104 compared to 23 (44%) detected. Among sow herds with production of weaners or growers, we estimated that 38 (90% CI [23, 74]) were infected with MRDT104 compared to 7 (18%) actually detected. Among breeder and multiplier herds, we estimated that five (90% CI [3, 8]) herds were infected with MRDT104 compared to three (60%) detected. In total, we estimated that 102 pig herds were infected with MRDT104 from 1 August 2001 till 31 July 2002 (90% CI [63, 228]). In comparison, 33 (32%) infected herds were detected in this period. The predicted proportion of undetected herds varied considerably with herd type. We infer that the proportion of detected MRDT104 infected herds depended on the intensity of the combined serological and bacteriological testing.     

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).     

Prevalence of multidrug resistant (MDR) Salmonella in bovine dairy herds in western France

As a part of our effort in quantitative risk analysis of food-borne diseases, we carried out an epidemiologic study to estimate the prevalence of multidrug resistant (MDR) Salmonella in dairy herds situated in western France. The study population consisted of 489 farms in the region and manure or slurry was sampled from these operations and tested for the Salmonella spp. All strains isolated during the study were serotyped and tested for their antimicrobial susceptibility. Salmonella spp. was isolated from 8.1% (95% confidence interval (CI 95%): 4.5–13.3%) of the sampled herds. The herd prevalence of MDR Salmonella among the sampled herds was 1.9% (CI 95%: 0.5–5.4%). Spatial statistics were used to check for sampling representativeness and to determine if infected herds were clustered spatially.     

Nation-wide Salmonella enterica surveillance and control in Danish slaughter swine herds

A nation-wide Salmonella enterica surveillance and control programme was initiated in Danish finishing herds over the first quarter of 1995. In Denmark, all swine for slaughter are identifiable by a unique herd code. For each herd code, and depending on the herd's annual kill, random samples ranging from four to more than 60 swine are obtained quarterly at the abattoir. A meat sample from each pig is frozen, and meat juice (harvested after thawing) is examined for specific antibodies against S. enterica using an indirect enzyme-linked immunosorbent assay (ELISA). The ELISA combines several S. enterica O-antigens, and allows detection of antibody response after a variety of different S. enterica serovar infections. Results are transferred to a central database, which each month (based on meat-juice tests obtained in the previous 13 weeks) assigns all herds into three S. enterica infection levels: Level 1, in which the S. enterica prevalence is deemed low and acceptable; Level 2, where there is a moderate prevalence of S. enterica seroreacl.ors (from > 50% in the smallest to > 10% in the largest herds); Level 3, in which S. enterica seroreactor prevalence is clearly unsatisfactory ( > 50% for most herd sizes). Irrespective of Salmonella level, all herds receive a monthly update on the current results of the S. enterica test results. If a herd is categorized in Level 2 or 3, it must receive an advisory visit by a practising veterinarian and a local swine extension specialist, and certain management hygiene precautions must be taken. If a herd is categorized in Level 3, the finishers from the herd must additionally be slaughtered under special hygiene precautions. This is supervised by the veterinary authorities.

During 1995, 604000 samples were tested for S. enterica, corresponding to 3.0% of the total kill. In December 1995, 15522 herds (representing > 90% of the national production) were categorized into one of the three levels: 14551 herds (93.7%) in Level 1; 610 herds (3.9%) in Level 2; 361 herds (2.3%) in Level 3. The proportion of serologically positive meat-juice samples collected during 1995 ranged from a mean of 2.9% in smaller herds (101–200 swine slaughtered per year) to 6.1% in relatively large herds (more than 5000 swine slaughtered per year).     

Salmonella in Belgian laying hens: an identification of risk factors

Since the 1980s, the prevalence of Salmonella in Belgian poultry layers and broilers has greatly fluctuated with a rise observed in 2003 and a significant decrease in 2005. In order to alleviate the risk at egg consumer level, it is crucial to understand the factors which influence the contamination and the spread of Salmonella in laying hens. To study such determinants we explored the Belgian data from the 2005 baseline study on the prevalence of Salmonella in laying flocks of Gallus gallus in the European Union. The response variables corresponded to presence or absence of Salmonella from dust and faecal samples taken from the environment of a Belgian layer flock. The explanatory variables included: region of Belgium, sampling time (month the flock was sampled), production type (cage or barn and free range), Salmonella vaccination status, flock age and flock size. Analyses of these data were performed using a bivariate logistic regression model assuming independence between the two responses and bivariate generalized estimating equations model, which incorporates the correlation between the two responses on the same flock. The main risk factor that was identified was rearing flocks in cages compared to barns and free-range systems. The results also showed a significant higher risk for Salmonella for a 1 week increase in flocks’ age as well as with a unit increase in the size of the flock.     

Herd-level risk factors for Salmonella enterica subsp. enterica in U.S. market pigs

Midwest U.S. herds (n = 63) were studied to identify risk factors for harboring Salmonella enterica among slaughter-weight pigs. Samples collected on farms (feces) and at slaughter (distal colonic content, cecal content and ileocolic lymph nodes) were cultured using conventional means. Approximately 15 pigs were studied per herd, for a total of 3754 samples. The proportion of pigs positive in one or more samples was calculated for each herd. Herd characteristics were described by a combination of interview and written survey. Logistic regression was used to detect relationships between the detection of Salmonella and potential herd-level risk factors. The mean individual pig prevalence was 5% for feces, 4% for distal colonic content, 15% for ileocolic lymph nodes, and 17% for cecal contents. One or more Salmonella isolates were detected in at least one sample type in every herd. The five most common serovars were S. Agona, S. Derby, S. Schwarzengrund, S. Typhimurium and S. Senftenberg, with 25 additional serovars detected. Salmonella prevalence estimates were positively correlated among all samples except distal colonic content and ileocolic lymph nodes. Pigs with culture positive fecal samples were at increased odds of being detected positive for each of the slaughter-collected samples examined, namely distal colonic content (OR = 30.5), ileocolic lymph nodes (OR = 12.9) and cecal content (OR = 23.2). Herds with positive fecal sample(s) had increased odds of having positive cecal content (OR > 1.5), distal colonic content (OR = 15.3) and ileocolic lymph nodes (OR = 12.7). Pigs from herds with at least some bowl drinkers had eight-fold higher odds of testing Salmonella positive than did pigs from herds with only nipple drinkers. Pigs from herds with only dry feeders had five-fold higher odds of testing Salmonella positive when compared with pigs from herds with combinations of wet/dry style feeders. Interventions at these two points should be considered when designing growing pig facilities to reduce Salmonella shedding.     

Simulation model estimates of test accuracy and predictive values for the Danish Salmonella surveillance program in dairy herds

The Danish government and cattle industry instituted a Salmonella surveillance program in October 2002 to help reduce Salmonella enterica subsp. enterica serotype Dublin (S. Dublin) infections. All dairy herds are tested by measuring antibodies in bulk tank milk at 3-month intervals. The program is based on a well-established ELISA, but the overall test program accuracy and misclassification was not previously investigated. We developed a model to simulate repeated bulk tank milk antibody measurements for dairy herds conditional on true infection status. The distributions of bulk tank milk antibody measurements for infected and noninfected herds were determined from field study data. Herd infection was defined as having either >or=1 Salmonella culture-positive fecal sample or >or=5% within-herd prevalence based on antibody measurements in serum or milk from individual animals. No distinction was made between Dublin and other Salmonella serotypes which cross-react in the ELISA. The simulation model was used to estimate the accuracy of herd classification for true herd-level prevalence values ranging from 0.02 to 0.5. Test program sensitivity was 0.95 across the range of prevalence values evaluated. Specificity was inversely related to prevalence and ranged from 0.83 to 0.98. For a true herd-level infection prevalence of 15%, the estimate for specificity (Sp) was 0.96. Also at the 15% herd-level prevalence, approximately 99% of herds classified as negative in the program would be truly noninfected and 80% of herds classified as positive would be infected. The predictive values were consistent with the primary goal of the surveillance program which was to have confidence that herds classified negative would be free of Salmonella infection.     

Prevalence of antimicrobial resistance among Salmonella on midwest and northeast USA dairy farms

The objective of this study was to determine the prevalence of antimicrobial resistance among Salmonella isolated from dairy herds in New York, Minnesota, Michigan, and Wisconsin, USA. Serogroup and antimicrobial susceptibility characteristics were determined for Salmonella from cattle and environmental samples collected during August 2000–October 2001 as part of a longitudinal study where 129 herds were visited at 2-month intervals. Salmonella isolates were tested (using a broth microdilution method) for susceptibility to amoxicillin/clavulanic acid, ampicillin, ceftiofur, ceftriaxone, cephalothin, chloramphenicol, ciprofloxacin, gentamicin, kanamycin, nalidixic acid, streptomycin, sulfamethoxazole, tetracycline, and trimethoprim/sulfamethoxazole. Of the 1506 isolates tested for minimum inhibitory concentrations to these 14 antimicrobial agents, 81.2% were pan-susceptible and for most herds (81.6%) the predominant antimicrobial resistance pattern was pan-susceptible. At least 1 Salmonella isolate resistant to 5 or more antimicrobial agents was found on 23.6% of herds. This resistance phenotype was most common among serogroups B and E1 and among samples from calves and farmer-designated sick cows. Resistant samples most frequently exhibited resistance to tetracycline, streptomycin, and/or ampicillin. No samples were resistant to ceftriaxone (though 13 were in the intermediate range), and very few samples were resistant to ciprofloxacin (n = 1), nalidixic acid (n = 5), or trimethoprim/sulfamethoxazole (n = 7).     

Review of pathogenesis and diagnostic methods of immediate relevance for epidemiology and control of Salmonella Dublin in cattle

Salmonella enterica subsp. enterica serovar Dublin (S. Dublin) receives increasing attention in cattle production. It is host-adapted to cattle, and leads to unacceptable levels of morbidity, mortality and production losses in both newly and persistently infected herds. Cattle health promoting institutions in several countries are currently constructing active surveillance programmes or voluntary certification programmes, and encourage control and eradication of S. Dublin infected cattle herds. There is a need to understand the underlying pathogenesis of the infection at both animal and herd level to design successful programmes. Furthermore, knowledge about and access to diagnostic tests for use in practice including information about test accuracy and interpretation of available diagnostic test methods are requested.     

Effect of previous antimicrobial treatment on fecal shedding of Salmonella enterica subsp. enterica serogroup B in New York dairy herds with recent clinical salmonellosis

The association of herd- and sample-level factors with the isolation of Salmonella group B from cattle fecal samples was analyzed. Study farms were 65 dairy herds with a recent history of laboratory-confirmed clinical salmonella infections. Herds were visited once per month for three months to collect data and samples for bacteriological culture. Herd size varied widely from 34 to 3700 total cattle on the farm (median=370). Salmonella serogroup B was isolated from 270 of 2726 samples tested. The predominant serotypes identified were S. Typhimurium and S. Typhimurium var. Copenhagen. Logistic regression was used to analyze the relationship between potential risk factors and isolating Salmonella serogroup B. The only herd-level factor which was significantly associated with fecal shedding was total herd size (hundreds of cattle OR=1.09; 95% confidence interval (CI): 1.05, 1.14). The probability of a positive sample decreased substantially for longer intervals between the initial clinical case and sampling (interval in months OR=0.5; 95% CI: 0.3, 0.6). The presence of diarrhea increased the risk of shedding (OR=2.1; 95% CI: 1.4, 3.0). The effect of recent treatment with antimicrobial agents depended on age group. For heifers and cows, recent antimicrobial treatment increased the probability of isolating Salmonella (heifers OR=11.8; 95% CI: 2.9, 48.8; cows OR=4.1; 95% CI: 2.0, 8.4), but this effect was not statistically significant for calves before weaning. Among animals without recent antimicrobial treatment, preweaned calves were more likely to have positive samples than cows (OR=3.5; 95% CI: 1.8, 6.9; heifers OR=4.7; 95% CI: 2.3, 9.6).     

Potential infection-control benefit for Ireland from pre-movement testing of cattle for tuberculosis

Pre-movement testing for bovine tuberculosis (BTB) was compulsory in Ireland until 1996. We determined the proportion of herd restrictions (losing BTB-free status) attributable to the recent introduction of an infected bovid; described events between restoration of BTB-free status (de-restriction) and the next herd-level test for BTB; estimated the proportion of undetected infected cattle present at de-restriction; identified high-risk movements between herds (movements most likely to involve infected cattle); and determined the potential yield of infected cattle discovered (or herds that would not lose their BTB-free status) by pre-movement testing, relative to the numbers of cattle and herds tested. We used national data for all 6252 herds with a new BTB restriction in the 12 months from 1 April 2003 and 3947 herds declared BTB-free in the 12 months from 1 October 2001. We identified higher-risk animals from our logistic generalized estimating-equation models. We attributed 6-7% of current herd restrictions to the recent introduction of an infected animal. There were considerable changes to herd structure between de-restriction and the next full-herd test, and infection was detected in 10% of herds at the first assessment (full-herd test or abattoir surveillance) following de-restriction. Following movement from a de-restricted herd, the odds of an animal being positive at the next test increased with increasing time in the source herd prior to movement, increasing time between de-restriction and the next full-herd test and increasing severity of the source herd restriction. The odds decreased with increasing size of the source herd. We estimated that 15.9 destination-herd restrictions per year could be prevented for every 10,000 cattle tested pre-movement and that 3.3 destination-herd restrictions per year could be prevented for every 100 source herds tested pre-movement. The yield per pre-movement test can be increased by focusing on high-risk movements; however, this would result in a substantial decrease in the total number of potential restrictions identified.     

Prevalence of a Salmonella subspecies diarizonae in Norwegian sheep herds

We studied the prevalence of Salmonella enterica subspecies diarizonae serovar 61:k:1,5,(7) (S. IIIb 61:k:1,5,(7)) in 133 Norwegian sheep herds from throughout the country. We used a nested case-control study to identify major risk factors. Two pooled samples (one from lambs and one from adult sheep) were collected from each herd from (10 different) abattoirs. The regional prevalence varied from 0 to 45%, and was more prevalent in adults than in lambs. Sixteen of the 133 herds tested positive to S. IIIb 61:k:1,5,(7), and these positive herds were included in the questionnaire-based nested case-control part of the study. The controls were stratified according to abattoir. Above-median herd size, having white crossbreed sheep, and higher annual replacement percentage were the most evident risk factors.     

Risk factors for Brucella spp. infection In dairy cattle farms in Asmara, State of Eritrea

A cross-sectional study was conducted to identify risk factors for herd infection by Brucella spp. in dairy cattle in the suburbs of Asmara, Eritrea. Data were collected from 64 herds, randomly selected from a total of 99 herds with a minimum herd size of 9 cows. A questionnaire was used to gather data on management, hygiene and herd structure. Serum samples collected from all pregnant heifers, cows and bulls, were screened for Brucella infection by the Rose Bengal test (RBT), and all RBT-positive sera re-tested with the complement-fixation test (CFT) for confirmation. A seropositive herd was defined as one in which at least one animal tested positive in the CFT. There were 23 (36%) positive herds among the 64 studied. Both multiple logistic and multiple betabinomial regression modeling were used to analyze the data. Mixed-breed herds, compared to single (exotic)-breed herds, were found to be independently associated with increased herd seroprevalence (OR=5.2, 95% confidence interval 1.4–18.7) in the multiple logistic model with the herd infection status as the dependent variable. The importance of this variable was supported by the multiple betabinomial regression model (OR=3.3, 1.4–7.6) with animal-level prevalence within herd as the outcome variable. Both models also revealed the presence of a negative association between seropositivity and cattle stocking density.     

Impact of changes in cattle movement regulations on the risks of bovine tuberculosis for Scottish farms

Legislation requiring the pre- and post-movement testing of cattle imported to Scotland from regions with high bovine tuberculosis (bTB) incidence was phased in between September 2005 and May 2006 as part of efforts to maintain Officially Tuberculosis Free (OTF) status. In this analysis, we used centralized cattle movement records to investigate the influence of the legislative change on import movement patterns and the movement-based risk factors associated with new bTB herd breakdowns identified through routine testing or slaughter surveillance. The immediate reduction in the number of import movements from high incidence regions of England and Wales into Scotland suggests that pre- and post-movement testing legislation has had a strong deterrent effect on cattle import trade. Combined with the direct benefits of a more stringent testing regime, this likely explains the observed decrease in the odds of imported cattle subsequently being identified as reactors in herd breakdowns detected through routine surveillance compared to Scottish cattle. However, at the farm-level, herds that recently imported cattle from high incidence regions were still at increased risk of experiencing bTB breakdowns, which highlights the delay between the introduction of disease control measures and detectable changes in incidence. With the relative infrequency of routine herd tests and the insidious nature of clinical signs, past import movements were likely still important in determining the present farm-level risk for bTB breakdown. However, the possibility of low-level transmission between Scottish cattle herds cannot be ruled out given the known issues with test sensitivity, changes in import animal demographics, and the potential for on-farm transmission. Findings from this analysis emphasize the importance of considering how farmer behavioural change in response to policy interventions may influence disease transmission dynamics.     

Culling decisions of dairy farmers during a 3-year Salmonella control study

Salmonella enterica subsp. enterica-serotypes lead to periodically increased morbidity and mortality in cattle herds. The bacteria can also lead to serious infections in humans. Consequently, Denmark has started a surveillance and control programme in 2002. The programme focuses on Salmonella Dublin which is the most prevalent and most persistent serotype in the Danish cattle population. A field study in 10 dairy herds with persistent Salmonella infections was carried out over three years to gain experience with control procedures including risk assessment, targeted control actions and test-and-cull procedures. From autumn 2003 until end of 2006 quarterly milk quality control samples from all lactating cows and biannual blood samples from all young stock above the age of three months were tested using an indirect antibody ELISA. The most recent and previous test results were used to categorise all animals into risk groups. These risk groups and all individual ELISA-results were communicated to the farmers as colour-coded lists four to six times per year. Farmers were advised to manage the risk of Salmonella transmission from cattle with repeatedly high ELISA results (flagged as "red") or cows with at least one recent moderately high ELISA result (flagged as "yellow") on the lists. Risk management included, e.g. culling or separation of the cows at calving. We analysed culling decisions using two models. For heifers a hierarchical multivariable logistic model with herd as random effect evaluated if animals with red and yellow flags had higher probability of being slaughtered or sold before first calving than animals without any risk flags. For adult cows a semi-parametric proportional hazard survival model was used to test the effect of number of red and yellow flags on hazards of culling at different time points and interactions with prevalence in the herd while accounting for parity, stage of lactation, milk yield, somatic cell count and the hierarchical structure of the data with animals clustered at herd level. This study illustrates how investigation of culling decisions made by herd managers when they have access to test-status of individual animals and overall apparent prevalence during control of an infection can lead to useful new knowledge. Overall herd managers were more likely to cull cattle with increasing number of yellow and red flags than animals with no flags. However, cattle were more likely to be culled with yellow and red flags during times with low or medium high within-herd seroprevalence than at times with high seroprevalence. These results are valuable knowledge for modelling and planning of control strategies and for making recommendations to farmers about control options.