Viraemic Pigs Research Articles

Viraemic pigs entering the food chain are the most likely source of hepatitis E virus (HEV) in pork meat: Modelling the fate of HEV during slaughtering of pigs

Hepatitis E Virus (HEV) is an emerging foodborne pathogen and consumption of raw or undercooked pork products has been associated with increased risk of human infection. This work represents the first attempt to evaluate the risk of HEV being present on pig carcasses and in meat at the end of the slaughtering process considering the steps of bleeding, scalding, dehairing, singeing, polishing, evisceration and trimming. Based on available knowledge on the epidemiology and biology of HEV, the risk pathways leading to the presence of HEV on carcasses as a consequence of (i) faecal contamination of the skin from environment and contacts with contaminated faeces during transport and lairage, (ii) contact with viraemic blood at bleeding and (iii) faecal/bile cross-contamination during evisceration were assessed qualitatively. The pathway through which HEV could be present in meat of viraemic pigs, as conveyed by residual blood in muscular tissue after bleeding was instead modelled quantitatively. Of the three risk pathways evaluated qualitatively, only the occurrence of HEV on carcasses as a consequence of accidental rupture of the gut or gallbladder at evisceration was found to be non-negligible, but with a very low likelihood of occurrence. The quantitative output for the expected amount of HEV in meat of viraemic pigs shows minimum and maximum values of 0.10 and 1.1 × 104 genome copies (gc)/g respectively with 4.8 × 102 and 5.3 × 103 gc/g at 95th and 99th percentile of the cumulative distribution. These results are consistent with the existing evidence that levels of HEV RNA in meat samples are usually low even in the presence of high viral loads in livers of the same animals. Results of the sensitivity analysis confirm highly viraemic pigs entering the slaughter line as those posing the greater risk for consumers. Our study suggests that prevention of HEV infection through consumption of pork meat at pre-harvest/harvest stages should focus on reducing the flow of highly viraemic pigs into the food chain.

Quantitative risk assessment of hepatitis E virus: Modelling the occurrence of viraemic pigs and the presence of the virus in organs of food safety interest

Hepatitis E virus (HEV) is a zoonotic pathogen with consumption of pork and derived products identified in different countries as a risk factor for human exposure to HEV. Great efforts have been made to understand the dynamics of virus transmission within domestic swine populations through modelling. However, from a food safety prospective, it is critical to integrate the parameters involved in the transmission dynamics with those governing the actual presence of HEV in the bloodstream, the liver, gallbladder or faeces. To date, several aspects related to the pathogenesis of the disease are still unknown or characterized by significant levels of uncertainty, making this conjunction challenging. We used published serological data obtained from pigs in a farrow-to-finish farm to implement an Immune-Susceptible-Infected-Recovered (MSIR) model reproducing the on-farm dynamics that lead to the occurrence of viraemic pigs at slaughter. Expert opinion on the length of time infectious HEV can be detected in liver, gallbladder/bile and faeces after recovery from viraemic status were used to inform a stochastic model aimed at estimating the expected proportion of viraemic pigs, pigs with infectious HEV in liver, gallbladder/bile and faeces entering the slaughterhouse. To simulate the potential effect of on-farm mitigation strategies, we estimated the changes in outcomes of interest as a function of variations in the baseline transmission parameters. The model predicted a proportion of viraemic pigs entering the slaughterhouse of 13.8% while the proportions of, and ranged from 13.8% to 94.4%, 13.8% to 94.7% and from 25.3% to 30.8% respectively, due to the uncertainty surrounding the experts’ opinions. Variations in MSIR model’s parameters alert of the need to carefully consider the application of mitigation strategies aimed at delaying the decay of maternal immunity or the peak of the within herd transmission. When the rate of decay of maternal immunity and the transmission rate were decreased between 80% and 5% and 40% and 5% from the baseline values respectively, adverse effects on were observed. The model highlights the relevance of specific aspects in the pathogenesis of the disease from a food safety prospective and it was developed to be easily reproducible and updatable as soon as accurate data becomes available. As presented, the model can be directly connected to existing or future pig-related models to estimate the significance of the identified parameters on the risk of human exposure to HEV through consumption of pork products.

Probability of Exporting Infected Carcasses from Vaccinated Pigs Following a Foot‐and‐Mouth Disease Epidemic

Emergency vaccination is an effective control strategy for foot-and-mouth disease (FMD) epidemics in densely populated livestock areas, but results in a six-month waiting period before exports can be resumed, incurring severe economic consequences for pig exporting countries. In the European Union, a one-month waiting period has been discussed based on negative test results in a final screening. The objective of this study was to analyze the risk of exporting FMD-infected pig carcasses from a vaccinated area: (1) directly after final screening and (2) after a six-month waiting period. A risk model has been developed to estimate the probability that a processed carcass was derived from an FMD-infected pig (P(carc)). Key variables were herd prevalence (P(H)), within-herd prevalence (P(A)), and the probability of detection at slaughter (P(SL)). P(H) and P(A) were estimated using Bayesian inference under the assumption that, despite all negative test results, > or =1 infected pigs were present. Model calculations indicated that P(carc) was on average 2.0 x 10(-5) directly after final screening, and 1.7 x 10(-5) after a six-month waiting period. Therefore, the additional waiting time did not substantially reduce P(carc). The estimated values were worst-case scenarios because only viraemic pigs pose a risk for disease transmission, while seropositive pigs do not. The risk of exporting FMD via pig carcasses from a vaccinated area can further be reduced by heat treatment of pork and/or by excluding high-risk pork products from export.

Variation in resistance to the Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) in Pietrain and Miniature pigs

Porcine reproductive and respiratory syndrome (PRRS) is one of the economically most important diseases of swine. Viraemia and the prolonged persistence of the virus are among the most critical factors. Virus replication and severity of disease vary with virus isolates, and there is rising evidence for a genetic component of the host susceptibility. Dissecting the genetic basis of resistance/susceptibility to PRRS virus (PRRSV) might lead to improved knowledge on the molecular mechanisms of PRRS and the establishment of genetic markers for future disease control. The aim of this study was to establish a porcine model with emphasized genetic differences in PRRSV susceptibility. Seven 'Wiesenauer Miniature' pigs (MI), a local German breed and eight commercial Pietrain (PI) pigs were challenged with 10(5) TCID(50) of an attenuated PRRSV strain (Ingelvac PRRSV MLV). Clinical status, viraemia and seroconversion of the pigs were compared. No clinical signs were observed during the experiment. Viraemia peaked on day 6 p.i., with 100% of viraemic pigs in PI and on day 12 p.i with 87% of viraemic MI. Viraemia lasted for up to 35 days in MI and for at least 72 days in PI. This surprising result was confirmed by a second study with another four MI. MI and PI showed maximum virus titres of 10(2.5) TCID(50)/ml of serum and 10(4.5) TCID(50)/ml, respectively, indicating a virus replication in MI of approximately 3.3% that of PI over the complete period. MI were more efficient in antibody production. With such pronounced breed differences, the model is of high relevance for the genetic dissection of PRRS pathogenesis and susceptibility.

Genetic diversity of Porcine reproductive and respiratory syndrome virus (PRRSV) in selected herds in a pig-dense region of North-Western Germany

The Porcine reproductive and respiratory syndrome virus (PRRSV) can persist for several weeks in infected pigs, and readily spreads from viraemic pigs to other pigs as well as to other herds. In regions with dense pig populations such as the North-Western part of Germany the virus is endemic. The aim of this study was to analyse the genetic diversity of PRRSV found in 18 herds in this region, which had pigs with respiratory or reproductive failures, and where PRRSV had been demonstrated by RT-PCR. Phylogenetic trees were calculated using the complete nucleotide sequences of the ORF5. Of the 65 samples sequenced, five PRRSV from four herds were of the North American (NA; Type 2) genotype, and 60 of the European (EU; Type 1) genotype. To ascertain if the field PRRSV varied with time and to monitor the health condition of the herds they were revisited 2 years later. Although only two herds still reported clinical signs, PRRSV was found by RT-PCR in 10 of the 18 herds. Phylogenetic analysis showed that of the 23 PRRSV sequenced, 15 were of the European (EU) genotype. The EU genotype isolates from both samplings could be assigned to one of 12 clusters. There was no indication for the existence of herd specific clusters. ORF5 sequence identities between PRRSV from one herd in one cluster were either 100%, or had single base exchanges. These data indicate that the mutation rates for the European field isolates are similar to that found for the NA genotype vaccine strain used in Germany.

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection in wild boars

The Porcine Reproductive and Respiratory Syndrome (PRRS) is one of the economically most important swine diseases worldwide. The virus can be spread by viraemic and persistent infected pigs. Spread and infection are advanced in areas with high herd and population densities. As wild boars have been found seropositive for PRRSV in Germany, France, and the USA, exchange between wild and domestic pig populations may exist. However, comprehensive information on PRRSV infection in wild boars is presently not available. The aim of the current study was thus, to systematically study PRRSV infection in wild boars to provide information on spatiotemporal, host and viral effects. The study was based on 531 wild boars from 52 hunts in Germany (2004–2007). PRRSV infection was determined and strains (US/EU) were classified by PCR. A total of 15.9% of the wild boars were PRRSV-positive (US: 14.2%; EU: 6.2%), with remarkable effects of state (US: 5.1–46.2%; EU: 0–17.6%), season (0–36.5%) and tissue (lungs: 89%; tonsils: 11%). Prevalences did neither correlate with age or weight, nor with density of production units, domestic pigs or wild boars. Open reading frame (ORF) 1-sequences within EU- and US-strains did not differ among wild boar samples. Homologies between EU-samples/Lelystad-virus and US-samples/PRRSV-MLV virus were 99.3 and 97%, respectively. This is the first comprehensive evidence of PRRSV infection in wild boars. We conclude that there is only a weak relation between wild boar and domestic PRRSV infection.

Effect of orally administered Lactobacillus casei on porcine reproductive and respiratory syndrome (PRRS) virus vaccination in pigs

The purpose of this study was to determine whether intranasal/oral administration of probiotics can assist vaccination efficacy against an important swine pathogen, porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV). A controlled challenge trial was performed employing: (a) pigs vaccinated against PRRS and treated with a Lactobacillus casei, (b) pigs vaccinated against PRRS only, (c) pigs treated with L. casei only, and (d) pigs neither vaccinated against PRRS nor treated with L. casei. All pigs were challenged intranasally with a wild PRRSV strain. There was no difference in clinical signs or rectal temperature among the four groups. However, pigs that received L. casei gained significantly more weight than pigs that did not. Vaccinated pigs did not gain more weight than nonvaccinated pigs. Vaccinated groups had significantly fewer viraemic pigs on days post-challenge 4, 11 and 17 than nonvaccinated groups of pigs. There was no effect of probiotic on prevalence or duration of viraemia. Among viraemic pigs, there was no significant difference in mean log base 10 titer of PRRS virus among groups. These results suggest that orally administered L. casei does not affect immune response in such a way as to affect PRRS viraemia or nasal shedding. However, it still appears to provide significant benefit when administered during vaccination as indicated by the higher bodyweight gain following PRRS virus infection.

Correlation between positivity for immunoglobulin A antibodies and viraemia of swine hepatitis E virus observed among farm pigs in Japan

To evaluate the usefulness of detection of antibodies to hepatitis E virus (HEV) to screen for viraemic pigs, serum samples obtained from 1425 1-6-month-old pigs in Japan were tested for swine HEV RNA and IgG, IgM and IgA classes of anti-HEV antibody. Fifty-five (5 %) of the 1071 2-5-month-old pigs were positive for swine HEV RNA, but none of 218 1-month-old pigs or 136 6-month-old pigs had detectable HEV RNA. The prevalence of anti-HEV IgG among the viraemic pigs (67 %, 37/55) was similar to that among the non-viraemic pigs (55 %, 757/1370) and the prevalence of anti-HEV IgM among the viraemic pigs and non-viraemic pigs was 7 and 3 %, respectively. However, anti-HEV IgA was detected significantly more frequently among viraemic pigs than among non-viraemic pigs (55 vs 10 %, P<0.0001). These results suggest that anti-HEV IgA is more useful than anti-HEV IgM to screen for viraemic pigs.

Porcine reproductive and respiratory syndrome: temperature and pH stability of Lelystad virus and its survival in tissue specimens from viraemic pigs

We investigated the growth of Lelystad virus (LV) in porcine alveolar macrophages, the thermal and pH stability of the virus in cell culture medium, and its survival in tissue specimens from viraemic pigs. Lelystad virus grew to titres of 10 6 TCID 50/ml, which were found at 40 h after virus inoculation when the macrophage cultures showed a cytopathic effect of approximately 40%. In culture medium at pH 7.5, LV was stable for prolonged periods of storage at −70°C and −20°C. At higher temperatures the half-life of LV was 140 h at 4°C, 20 h at 21°C, 3 h at 37°C and 6 min at 56°C. The half-life of LV, both at 4°C and 37°C, changed considerably when the pH of the medium was varied. At 4°C and pH 6.25 a maximum half-life of 50 h and at 37°C and at pH 6.0 a maximum half-life of 6.5 h was observed. However, increasing or decreasing the pH of the medium rapidly decreased the half-life of LV at both temperatures. Although, LV proved to be more stable at pH 6.00 than at pH 7.5, it did not replicate at pH 6.0. We also tested various tissue specimens from viraemic pigs for the presence of LV. The virus was detected in tonsils, lymph nodes, lungs, serum, and sporadically, albeit at low titres, in muscle tissue. The titre of virus in muscle tissue and organs was only minimally affected by storage for up to 48 h at 4°C.

Experimental infection of the argasid tick, Ornithodoros moubata porcinus, with African swine fever virus.

The argasid tick,Ornithodoros moubata porcinus, is a natural reservoir and vector of African swine fever virus. The stability of ASFV in dead ticks was first determined and then laboratory-reared, nymphal ticks were infected by feeding on viraemic domestic pigs; “Uganda” strain virus multiplied and persisted over periods of 13–15 months in 70–75% of ticks. Infected tampans regularly transmitted ASF to domestic pigs, even when fed singly; in only about two-thirds of cases was virus excreted in the coxal fluid. “Tengani” strain virus, however, produced persistent infection in but 5% of ticks, following the ingestion of approximately 104 to 105 HAD50. The minimum infective dose of 3 strains of ASFV for ticks collected in Uganda, was investigated by the capillary-feeding technique. Some ticks were infected by 100.9 to 101.9 HAD50 of “Uganda” virus and virtually all by doses ⪖ 104.0 HAD50; a recent warthog isolate in a dose of 105.0 HAD50 caused persistent infection in 60% of ticks, whilst 104.0 HAD50 of a tick isolate produced only 40% persistent infections. Some Uganda ticks exhibited virus proliferation of at least 3–4 log10 units and transmitted ASFV to pigs during natural feeding; virus was sometimes excreted in the coxal fluid. These results were discussed in relation to the maintenance of ASFV and the probable sites of localisation in the tick; ASFV has the essential characteristics of a tick-borne arbovirus.