Evaluation of surveillance strategies for bovine tuberculosis (Mycobacterium bovis) using an individual based epidemiological model

The Netherlands holds the bovine tuberculosis-free (BTB-free) status according to European Union standards, but in recent years small outbreaks of the infection have occurred. After the last outbreak in 1999 with 10 infected herds the question raised if the current surveillance system, visual inspection of carcasses at the slaughterhouse, is efficient enough to detect infected cattle in time and to maintain the official BTB-free status.

Through epidemiological modelling, the risk of a major outbreak is quantified, using one of six surveillance strategies. These are the currently used visual inspection of carcasses at the slaughterhouse (SL), the ELISA test on blood samples of carcasses at the slaughterhouse (ELISA-B), the γ-interferon test on blood samples of carcasses at the slaughterhouse (GAMMA-B), comparative tuberculination of the herd (CT), the combined method of single and comparative tuberculination of the herd (ST + CT) and the ELISA test on samples of bulk milk (ELISA-M). Test frequency of the last three methods was varied as well.

A stochastic individual based model (IBM) was developed to simulate a chain of infected herds, where each individual animal is followed in time. The model mimics the nation-wide situation after the introduction of one infected animal into one herd. BTB-transmission is simulated with an S-E₁-E₂-I state transition model. Output is time until detection of the infection, prevalence in the detected herd and the number of infected herds at the time of detection. For the assessment 500 simulations were used, representing 500 BTB-introductions. Model robustness to parameter values was analysed with Monte Carlo elasticity analysis, for which 1000 simulations were used.

Results of median time until detection and median number of infected farms at detection for SL (302 weeks and seven farms) were in agreement with estimates from an outbreak in the Netherlands in 1999. ELISA-B and GAMMA-B performed better than SL with a much lower median time until detection (189 and 97 weeks, respectively). The results for the tuberculination methods (ST + CT and CT) and ELISA-M depended heavily on the frequency in which the tests were performed. The tuberculination methods ST + CT and CT yield comparable results and detect the infection sooner than SL, also at the lowest tested frequency of once in 5 years. ELISA-M is comparable with SL at frequencies of once in 4 or 5 years, and this test works well at frequencies of once a year or higher. Our study results are used for an economical optimisation analysis of the six surveillance strategies.     

Stochastic efficiency analysis of bovine tuberculosis-surveillance programs in the Netherlands

We constructed a stochastic bio-economic model to determine the optimal cost-efficient surveillance program for bovine tuberculosis. The surveillance programs differed in combinations of one or more detection methods and/or sampling frequency. Stochastic input variables in the epidemiological module described the dynamics of infection and the probability of detection. By means of an efficiency frontier, the trade-off between the expected cost and the epidemiological risk parameter relating to the outbreak size was evaluated. The surveillance scheme based on visual inspection of lesions on carcasses at slaughter was optimal given the current prevalence of the disease in the Netherlands if the objective was to minimise the expected costs. However, the efficient set also included two other schemes: slaughterhouse inspection in combination with GAMMA-interferon testing of blood samples and slaughterhouse inspection in combination with two-stage tuberculin testing. The choice ultimately will depend on the risk attitude of the decision-maker; a more-stringent surveillance scheme will be enforced if the expected outbreak size is to be constrained. In future scenarios, ELISA testing of bulk-tank milk in combination with the current slaughterhouse inspection procedure would outperform the surveillance scheme of solely slaughterhouse inspection if ELISA testing of bulk-tank milk becomes feasible.     

Vaccination against Salmonella enteritidis in Dutch commercial layer flocks with a vaccine based on a live Salmonella gallinarum 9R strain: evaluation of efficacy, safety, and performance of serologic Salmonella tests

This study describes a field trial in which 80 commercial layer flocks, with an increased risk of Salmonella enteritidis (SE) infection and placed on farms with a certified Standardized Biosecurity Programme (SBP) or a request for a SBP certificate, were vaccinated with a vaccine based on a live attenuated Salmonella gallinarum (SG) 9R strain. An evaluation is presented of the efficacy of the vaccine against SE infections, the effect on the performance of serologic Salmonella tests, and the spread of the vaccine strain to the egg content. For the efficacy study, assessment of the flock level occurrence of SE infections in the vaccinated group of 80 flocks was compared with that of a nonvaccinated group of 1854 flocks hatched in the same period. This control group was examined according to the compulsory control programme in The Netherlands. An evaluation was done of the performance of serologic Salmonella tests and the spread of the vaccine strain to the inner egg content of five of the vaccinated flocks. Findings demonstrated the flock level occurrence of SE infections in the vaccinated group (2/80 = 2.5%) to be significantly (P = 0.01) lower than that of the nonvaccinated group (214/1854 = 11.5%). Vaccination resulted in 59.0% positive test results in lipopolysaccharide BD enzyme-linked immunosorbent assay (ELISA) for detecting antibodies against Salmonella serogroups B and D and 0% positive test results in the rapid plate agglutination test for detecting antibodies against S. pullorum (SP)/SG. The mean specificities of two blocking ELISAs (gm- and i-double antibody sandwich ELISAs) based on the flagellar antigen of SE and Salmonella typhimurium (ST) on the same sera were 99.6% and 96.1%, respectively. The vaccine strain could not be isolated from any of the 450 pools of 10 eggs. On the basis of these results, we concluded that vaccination with a vaccine based on an attenuated SG 9R strain contributes to the reduction of SE infections in commercial layer flocks. Furthermore, serologic monitoring of SE, ST, and SP/SG can still be carried out on flocks vaccinated with an attenuated SG 9R strain. Additionally, we found no indication of the spread of the vaccine strain to the egg content.     

Factors associated with the introduction of classical swine fever virus into pig herds in the central area of the 1997/98 epidemic in The Netherlands

A matched case-control study of 135 infected and 99 uninfected pig herds from the central area of the 1997 to 1998 epidemic of classical swine fever (CSF) in The Netherlands was undertaken to identify factors associated with the introduction of the virus. The herds were matched on the basis of herd type and the shortest geographical distance between pairs of herds. Data on management, hygienic measures, experiences during the depopulation of an infected nearest neighbour, and the frequency of contact with professionals and other agencies were collected by means of a questionnaire taken by personal interview. There were no significant differences between the infected and uninfected herds in the median total number of contacts per year with professionals and other agencies either with or without contact with the pigs. On the basis of a multivariable analysis, five variables were found to be significantly associated with an increased risk of infection: (1) the presence of commercial poultry on the premises; (2) visitors entering the pig units without wearing an overcoat or overalls and boots supplied by the farm; (3) the driver of the lorry transporting pigs for the Pig Welfare Disposal Scheme (PWDS) using his own boots instead of boots supplied by the farm; (4) herds of moderate size (500 to 1,000 animals) and very large herds (>7,000 animals) were at greater risk than small herds (<500 animals); and (5) an aerosol, produced during high-pressure cleaning of the electrocution equipment used to kill the pigs on a neighbouring infected herd less than 250 m away was carried by the wind on to the premises. Two variables were significantly associated with a decreased risk of CSFV-infection: (1) more than 30 years of experience in pig farming; and (2) additional cleaning of the lorries used to transport pigs for the PWDS before they were allowed on to the premises. In the opinion of the cooperating farmers, airborne transmission of the virus and its transmission during the depopulation of an infected neighbour were among the most important routes of infection.     

Flower initiation of chrysanthemum seedlings grown continuously in short days at four levels of irradiance

Chrysanthemum seedlings grown continuously under flower-inducing short-day conditions did not initiate flowers rapidly, but continued to produce leaves. A minimum leaf number seemed necessary before the seedlings responded to the relative length of day and night. The minimum leaf number varied with the initial size of the seedling, genotype and level of irradiance. When growing seedlings — or juvenile cuttings — for the purpose of selecting plants for vegetative propagation, the minimum leaf number required for flowering should be met before the start of the short-day period.     

Protection of chicks against salmonella infection induced by spray application of intestinal microflora in the hatchery

The efficacy of spray application of intestinal microflora of the adult bird in protection of broilers against salmonella infections was studied in 3 experiments under laboratory and field conditions. In chicks treated soon after hatching (in the hatchers at approximately 30 per cent hatch or in the chick delivery boxes in the hatchery) with crop-caecum homogenate or a mixture of aerobically and strict anaerobically cultured intestinal flora, very good results were achieved. A very obvious protective effect against a high infection dose (3 X 10(6) c.f.u.S. infantis bacteria per chick) and complete protection against natural infection with different salmonella types was induced. A significant improvement of growth rate was observed in broilers treated by spray in the hatchery and reared for 7 weeks under field conditions in an environment heavily contaminated with different salmonella types.     

Thorium 230 determination in polluted lake water — A study of interferences

The effect of contaminants, both known and unidentified, in interfering with the coprecipitation of ²³⁴Th tracer with BaSO₄ is described. The effect is illustrated by results obtained on water from Nero Lake, a uranium mine tailings dumping area. While most of the components of the water are known, attempts to duplicate the effect using synthetic water were less successful. Thus some strongly interfering substance in the water remains to be identified. A successful alternative method for Ra and Th determination is mentioned.     

Relationships between behavioral and meat quality characteristics of pigs raised under barren and enriched housing conditions.

In this study the effects of barren vs enriched housing conditions of pigs on their behavior during the lairage period (2-h holding period before slaughter), carcass characteristics, postmortem muscle metabolism, and meat quality were studied. The barren housing system was defined by common intensive housing conditions (i.e., with slatted floors and recommended space allowances), whereas the enriched environment incorporated extra space and straw for manipulation. Salivary cortisol concentrations were measured before transport and at the end of the lairage period. During the lairage period the percentage of time spent walking and fighting by the pigs was registered. Carcass characteristics such as weight, meat percentage, and backfat thickness were determined. At 5 min, 45 min, 4 h, and 24 h postmortem, pH, temperature, and lactate concentrations were determined in the longissimus lumborum (LL) and biceps femoris (BF) muscles. Capillarization of the muscle, mean muscle fiber area, and color and drip loss after 2 and 5 d of storage were determined for both muscle types. Pigs from the barren environment had a significantly higher increase in cortisol from farm to slaughter, but no differences in behavior were observed during the lairage period. Carcass characteristics did not differ between pigs from barren and those from enriched housing conditions. Postmortem lactate formation was significantly lower in LL muscles of enriched pigs at 4 and 24 h postmortem. Capillary density and mean muscle fiber area did not differ between the groups of pigs. The percentage of drip loss at 2 and 5 d after storage of LL muscle samples from enriched-housed pigs was significantly lower than that of the barren-housed pigs. Similar tendencies were found for the BF muscle from pigs kept in an enriched environment, but these were not statistically significant. The housing system did not affect meat color. It is concluded that on-farm improvement of animal welfare by environmental enrichment can also lead to beneficial economic effects after slaughter by improving the water-holding capacity of pork.