Implementing a probabilistic definition of freedom from infection to facilitate trade of livestock: putting theory into praxis for the example of bovine herpes virus-1.

International trade of livestock and livestock products poses a significant potential threat for spread of diseases, and importing countries therefore often require that imported animals and products are free from certain pathogens. However, absolute freedom from infection cannot be documented, since all test protocols are imperfect and can lead to false-negative results. It is possible instead to estimate the "probability of freedom from infection" and its opposite, the probability of infection despite having a negative test result. These probabilities can be estimated based on a pre-defined target prevalence, known surveillance efforts in the target population and known test characteristics of any pre-export test. Here, calculations are demonstrated using the example of bovine herpes virus-1 (BoHV-1). In a population that recently became free of BoHV-1 without using vaccination, the probability of being infected of an animal randomly selected for trade is 800 per 1 million and this probability is reduced to 64 (95% probability interval PI 6-161) per 1 million when this animal is tested negatively prior to export with a gB-ELISA. In a population that recently became free of BoHV-1 using vaccination, the probability of being infected of an animal randomly selected for trade is 200 per 1 million, and this probability can be reduced to 63 (95% PI 42-87) when this animal is tested negatively prior to export with a gE-ELISA. Similar estimations can be made on a herd level when assumptions are made about the herd size and the intensity of the surveillance efforts. Subsequently, the overall probability for an importing country of importing at least 1 infected animal can be assessed by taking into account the trade volume. Definition of the acceptable level of risk, including the probability of false-negative results to occur, is part of risk management. Internationally harmonized target prevalence levels for the declaration of freedom from infection from selected pathogens provide a significant contribution to the facilitation of international trade of livestock and livestock products by allowing exporting countries to design tailor-made output-based surveillance programs, while providing equivalent guarantees regarding the probability of freedom from infection of the population. Combining this with an approach to assess the overall probability of introducing at least 1 infected animal into an importing country during a defined time interval will help importing countries to achieve their desired level of acceptable risk and will help to assess the equivalence of animal health and food safety standards between trading partners.

Comparing the demonstration of freedom from Trichinella infection of domestic pigs by traditional and risk-based surveillance.

Traditionally, the routine artificial digestion test is applied to assess the presence of Trichinella larvae in pigs. However, this diagnostic method has a low sensitivity compared to serological tests. The results from artificial digestion tests in Switzerland were evaluated over a time period of 15 years to determine by when freedom from infection based on these data could be confirmed. Freedom was defined as a 95% probability that the prevalence of infection was below 0.0001%. Freedom was demonstrated after 12 years at the latest. A new risk-based surveillance approach was then developed based on serology. Risk-based surveillance was also assessed over 15 years, starting in 2010. It was shown that by using this design, the sample size could be reduced by at least a factor of 4 when compared with the traditional testing regimen, without lowering the level of confidence in the Trichinella-free status of the pig population.

A study to demonstrate freedom from Trichinella spp. in domestic pigs in Switzerland.

Trichinellosis is a food-borne zoonotic disease caused by the nematode Trichinella spp. Many omnivorous and carnivorous animal species can act as host for this parasite, including domestic pigs. To protect public health, it should be ensured that pork should not contain infective Trichinella larvae. Surveillance for Trichinella spp. can be done using direct (larval detection) and indirect (antibody detection) diagnostic techniques. The aim of this study was to demonstrate the absence of infection in Swiss domestic pigs. An ELISA was used as the initial screening test, and sera reacting in ELISA were further investigated using both a Western blot for serology and an artificial digestion test with 20 g of diaphragm tissue for larval detection. A total of 7412 adult pigs, 9973 finishing pigs and 2779 free-ranging pigs were tested. Samples from 17 (0.23%) adult pigs, 16 (0.16%) finishing pigs and nine (0.32%) free-ranging pigs were ELISA-positive, but all of these sera were subsequently negative by Western blot and by the artificial digestion method. Based on these findings, an absence of Trichinella infections in adult pigs (target prevalence 0.04%) and finishing pigs (target prevalence 0.03%) can be concluded. The results also demonstrated that the prevalence of Trichinella infections does not exceed 0.11% in free-ranging pigs, the group with the highest risk of exposure.

[Occurrence of Trichinella spp. in wild boar in Switzerland]. / Vorkommen von Trichinella spp. beim Wildschwein in der Schweiz.

Trichinellosis is a worldwide occurring zoonosis caused by the intracellular nematode Trichinella spp. One of the main infection sources in Europe is raw or undercooked meat from wild boar. Trichinella britovi is prevalent in wild carnivores in Switzerland, thus a possible inclusion of wild boar in this wildlife cycle cannot be excluded. In order to assess the prevalence of Trichinella infection in wild boar, we tested 1,458 animals with both parasitological and serological methods. In none of the animals Trichinella-larvae could be recovered by the artificial digestion method (prevalence of larvae: 0 %; 95 % CI 0.0 - 0.3). Antibodies in meat juice were detected in 57 animals using a standardized E/S-Ag-ELISA. However, in the confirmatory westernblot, only 3 animals remained seropositive (seroprevalence: 0.2 %; 95 % CI 0.07 %-0.60 %). The occurrence of wild boar positive for anti-Trichinella-antibodies indicates that meat inspection for Trichinella-larvae in this species is important to prevent human infections.

Validation of a Western Blot for the detection of anti-Trichinella spp. antibodies in domestic pigs.

Trichinellosis is a zoonotic disease in humans caused by Trichinella spp. According to international regulations and guidelines, serological surveillance can be used to demonstrate the absence of Trichinella spp. in a defined domestic pig population. Most enzyme-linked immunosorbent assay (ELISA) tests presently available do not yield 100% specificity, and therefore, a complementary test is needed to confirm the diagnosis of any initial ELISA seropositivity. The goal of the present study was to evaluate the sensitivity and specificity of a Western Blot assay based on somatic Trichinella spiralis muscle stage (L1) antigen using Bayesian modeling techniques. A total of 295 meat juice and serum samples from pigs negative for Trichinella larvae by artificial digestion, including 74 potentially cross-reactive sera of pigs with other nematode infections, and 93 meat juice samples from pigs infected with Trichinella larvae were included in the study. The diagnostic sensitivity and specificity of the Western Blot were ranged from 95.8% to 96.0% and from 99.5% to 99.6%, respectively. A sensitivity analysis showed that the model outcomes were hardly influenced by changes in the prior distributions, providing a high confidence in the outcomes of the models. This validation study demonstrated that the Western Blot is a suitable method to confirm samples that reacted positively in an initial ELISA.

Assessment of the prevalence of Trichinella spp. in red foxes and Eurasian lynxes from Switzerland.

Trichinella spp. larvae have not been detected in Swiss pigs, horses, or wild boar for many decades, whereas the parasite was repeatedly isolated from red foxes and Eurasian lynxes. Whenever the isolated larvae could be subjected to genotyping, T. britovi was found as infective agent. The present study was initiated to re-assess the epidemiological situation of Trichinella infection in Swiss carnivorous wildlife, namely in red foxes and lynxes. Tissue samples from 1,298 foxes were collected between 2006 and 2007, and those of 55 lynxes between 1999 and 2007. All samples were tested by a standard artificial digestion method and a multiplex-PCR to determine the species and/or genotypes of recovered larvae. Trichinella larvae were found in 21 foxes (1.6%) and 15 lynxes (27.3%), and T. britovi was identified as infecting species in all cases. The geographic distribution of positive foxes showed two main clusters: one in Central Switzerland and one in the West of the country, where also many lynxes were found to be positive. While the prevalence for Trichinella infection in foxes was not statistically correlated with sex or age class, the prevalence in lynx was significantly higher in males compared to females, and in adults compared to juveniles.

Clinical herd health, farm management and antimicrobial resistance in Campylobacter coli on finishing pig farms in Switzerland.

The world-wide increase of antimicrobial resistance in micro-organisms complicates medical treatment of infected humans. We did a risk-factor analysis for the prevalence of antimicrobial resistant Campylobacter coli on 64 Swiss pig finishing farms. Between May and November 2001, 20 faecal samples per farm were collected from the floor of pens holding finishing pigs shortly before slaughter. Samples were pooled and cultured for Campylobacter species. Isolated Campylobacter strains were tested for resistance against selected antimicrobials. Additionally, information on herd health and management aspects was available from another study. Because data quality on the history of antimicrobial use on the farms was poor, only non-antimicrobial risk factors could be analysed. Statistical analyses were performed for resistance against ciprofloxacin, erythromycin, streptomycin, tetracycline, and for multiple resistance, which was defined as resistance to three or more antimicrobials. Risk factors for these outcomes--corrected for dependency of samples at herd level--were analysed in five generalised estimation-equation models. Prevalence of antimicrobial resistance among Campylobacter isolates was ciprofloxacin 26.1%, erythromycin 19.2%, streptomycin 78.0%, tetracycline 9.4%, and multiple resistance 6.5%. Important risk factors contributing to the prevalence of resistant strains were shortened tails, lameness, skin lesions, feed without whey, and ad libitum feeding. Multiple resistance was more likely in farms which only partially used an all-in-all-out system (OR = 37), or a continuous-flow system (OR = 3) compared to a strict all-in-all-out animal-flow. Presence of lameness (OR = 25), ill-thrift (OR = 15), and scratches at the shoulder (OR = 5) in the herd also increased the odds for multiple resistance. This study showed that on finishing farms which maintained a good herd health status and optimal farm management, the prevalence of antimicrobial resistance was also more favourable.