Scientific Opinion on “The probability of transmission of Porcine Reproductive and Respiratory Syndrome virus (PRRSv) to naive pigs via fresh meat” - By the European Food Safety Authority - The EFSA Journal (2005) 239, 1-85. The EU Commission requested the European Food Safety Authority (EFSA) to assess the risk of transmitting PRRS via fresh pig meat to naive pig populations, which may be exposed to the virus via illigal feeding of catering waste or by other contact with animal waste.

Summary

Porcine Reproductive and Respiratory Syndrome (PRRS) is a viral disease that is characterised by high abortion rates and mortality in pre-weaned pigs and respiratory disease in fattening pigs.

In the European Union, PRRS is not notifiable or subject to other harmonised disease control measures. However, several third countries, which are free of the disease, apply safeguard measures to protect their herds. International agreements demand that such safeguard measures should be proportionate to the risk involved. However, available data on the prevalence of the virus, its tissue distribution during the different phases of the disease, its degradation during meat maturation, its survival in the environment and other factors influencing the release of the virus are not sufficiently consolidated and evaluated to allow the Commission to draw a clear picture of the risk of virus release.

The Commission therefore asked the European Food Safety Authority (EFSA) to assess the risk of transmitting PRRS via fresh pig meat to naive pig populations, which may be exposed to the virus via (illegal) feeding of catering waste (swill) or by other contact with animal waste.

The mandate was accepted by the Panel on Animal Health and Welfare (AHAW) at the Plenary Meeting, on 14 September 2004. It was decided to establish a Working Group of AHAW experts chaired by one Panel member. The risk assessment addressing the risk questions specified by the Commission follows the methodology for RA which can be summarised as hazard identification, release assessment, exposure assessment, consequence assessment and overall risk estimation.

PRRSv replicates only in a limited number of cell types. Permissive cells include pulmonary macrophages and cells of the monocyte/macrophage lineage in lungs, tonsils, lymph nodes and spleen whereas muscle cells are not considered permissive. In the early stages of infection virus can be found in high titres in most organs and blood and may persist for extended periods in the lungs and tonsils.

Based on the age-related pattern of incidence of PRRSv-infection in endemically infected populations and the tissue distribution of the virus during viraemia it is considered likely, that a small fraction of slaughter pigs will be replicating PRRSv at the time of slaughter. The lack of clear clinical or pathological signs of replication in slaughter-age pigs precludes detection by ante mortem and/or post mortem inspection.

Systematic use of live vaccines in young and growing pigs may lower the likelihood of viral presence in animals at the time of slaughter but it is considered that under practical conditions vaccination will only marginally limit the overall risk of transmission of the virus via fresh meat. Vaccine viruses may be present in slaughtered pigs and thus could present a hazard to naive populations.

The available data on reduction of infectivity during maturation, chilling, and freezing of fresh meat are somewhat variable but may on average be expected to lead to 2 log10 reduction of infectivity. Treatments such as de-boning or removal of lymph nodes will not be able to remove infectivity from carcasses.

The rate of inactivation of PRRSv is highly temperature-dependant, therefore the probability of survival of the virus in catering waste (swill) will depend on both ambient temperature and time before swill is actually being fed to pigs. The minimal oral infectious dose is not known but the observation that meat containing PRRSv detectable by PCR but below the detection limit for virus isolation in vitro suggests that the minimal infectious dose may be moderate or low. These factors, together with enforcement of controls prohibiting the use of non-heat treated swill for pig feed will determine the overall risk of exposure of pigs to PRRS-virus via possible, illegal feeding of swill. These factors will in every case be dependant on local conditions and are therefore not readily amenable to a generalised quantitative risk assessment.

The direct consequences associated with the introduction of PRRS are related to the production losses in the individual herd and the number of herds infected. Indirect consequences may include possible measures for prevention and control and restrictions of trade in live pigs, semen and pig meat.

Historically, pig meat from PRRSv-infected countries has been imported into PRRSvfree countries in Europe and New Zealand over the past decade without any evidence of dissemination of PRRSv. In most of these countries strict measures for treatment and disposal of animal waste were in place and this probably contributed significantly to the absence of transmission of PRRSv by that route. Thus, there is to date no documented field evidence to support or quantify the overall risk of importing PRRSvinfected meat.

Table of Contents

Tables and Figures Summary
Terms of Reference
2.1. Background
2.2. Mandate
2.3. Approach
2.4. General background information on PRRS
Hazard Identification
3.1. Description of the disease
3.2. Description of PRRS Virus

1. 3.2.1. Physical and chemical characteristics
   

3.3. Description of PRRS epidemiology

1. 3.3.1. Geographical distribution
   3.3.2. Herd prevalence
   3.3.2.1. Monitoring data
   3.3.3. Within herd prevalence
   3.3.4. Transmission of PRRS
   3.3.5. Prevention and control
   

3.4. Pathogenesis and Disease Description

1. 3.4.1. Clinical signs and gross pathology
   3.4.1.1. Weaners, growers and finishing pigs
   3.4.1.2. Breeding herds
   3.4.2. Pathogenesis
   3.4.3. Immunity
   3.4.4. Specific virus degradation during meat maturation, chilling, freezing etc
   

3.5. Immunity and Vaccination

1. 3.5.1. Replication of vaccine virus
   

3.6. Diagnostics

1. 3.6.1. Introduction
   3.6.2. Interpretation of diagnostic sensitivity/specificity
   3.6.3. How to collect and transport samples for detection of PRRS
   3.6.4. Assays to detect PRRSv
   3.6.5. Assays to detect of serum antibodies against PRRSv (serology)
   3.6.6. Differentiation between PRRSv isolates
   3.6.7. Summary of diagnostic tests
   

Risk Pathways
Risk Assessment for Risk Question 1: What is the probability of viable PRRS virus in meat?
5.1. Description of pathway and associated probabilities leading to residual PRRS virus in pig meat
Risk Assessment for Risk Question 2: What is the probability that viable PRRS virus in pig meat reaches naive pigs?
6.1. Description of pathway and associated probabilities leading to exposure of PRRS virus in pig meat to naive pigs
6.2. Conclusion on the probability of infecting a pig with PRRSv by oral exposure to infected pig meat
Consequence assessment
7.1. Direct consequences – in the pig herd
7.2. Indirect consequences - trade
Overall risk estimation
Conclusions
9.1. Epidemiology
9.2. Pathogenesis
9.3. Virus stability
9.4. Diagnostic tests
9.5. Hazard identification
9.6. Release assessment
9.7. Exposure assessment
9.8. Consequence assessment
Recommendations
References.
Acknowledgements.
AHAW Scientific Panel Members
Tables and Figures

Further Information

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Source: European Food Safety Authority (EFSA) - August 2005