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Research article2009Peer reviewedOpen access

Animal health safety of fresh meat derived from pigs vaccinated against classic swine fever

Algers, Bo; Blokhuis, Harry; Bötner, A; Broom, DM; Costa, P; Domingo, M; Greiner, M; Hartung, J; Koenen, F; Müller-Graf, C; Raj, M; Morton, DB; Osterhaus, A; Pfeiffer, DU; Roberts, R; Sanaa, M; Salman, M; Sharp, JM; Vannier, P; Wierup, Martin

Abstract

Classical swine fever (CSF) is a disease that has been causing major socio-economic damages in the EU during the last decades. Although considerable progress has been made in the eradication and prevention of the disease, the threat for an epidemic still exists. The measures to control and eradicate CSF are laid down in Community legislation (Council Directive 2001/89/EC and Commission Decision 2002/106/EC) and are based on stamping-out when CSF is confirmed on pig holdings. Emergency vaccination with 'conventional' live attenuated vaccine or marker vaccine can be used as an additional tool to control and eradicate the disease. In order to support and to improve the control and eradication measures as regards CSF in domestic pigs, EFSA was requested by the Commission to provide scientific advice on the safety of fresh meat (freedom from field virus) derived from vaccinated pigs. Specifically, two terms of reference were given: a) what is the risk that wild-type CSF virus is present in fresh meat obtained from pigs vaccinated in an emergency situation during an outbreak?; and b) what are the sampling schemes and testing procedures needed to detect field virus in fresh meat derived from such vaccinated pigs? Two types of vaccines are currently authorised in the EU for CSF: - A modified live vaccine (MLV, C-strain), which is safe and efficacious, inducing early protection; - An E2 subunit vaccine (E2subV, marker), safe, allowing serological DIVA, but with lower efficacy than MLV. The usefulness of emergency vaccination in order to limit disease spread and avoid excessive culling and economic damages remains to be evaluated. Very limited data related to the presence of CSFV in fresh meat have been generated after the implementation of a non-vaccination strategy. Therefore, a simulation modelling approach was developed to assess the risk of emergency vaccination on the safety of meat compared to the current control of CSF in domestic pigs without vaccination. CSFV is relatively stable in fresh meat and resists the maturation process in meat. Depending on the strain virulence, the age and breed, infected pigs die rapidly or may recover or can develop chronic infections. Viraemia is variable in duration and degree but it is always linked to the spread of virus to other body tissues including skeletal muscles (meat). The risk scenario includes two events that must happen before an infected animal is slaughtered and infected meat released: an infected herd has to escape clinical diagnosis before lifting of restrictions and during the final screening such a herd is not detected due to sample selection or false negative laboratory tests. Three scenarios were evaluated through the model: 1. Stamping-out of CSF detected as infected herds, standstill and pre-emptive culling. 2. Stamping-out of CSF detected as infected herds, standstill and emergency vaccination assuming a rapid protection. 3. Same as 2 but assuming a slower protection and DIVA property. It was concluded from the model that none of the strategies could reduce the risk to absolute zero. Lifting of restrictions can only occur after a certain time span (longer than the viraemic period of infected animals) after the last outbreak, thus the number of virus-positive animals in vaccinated infected herds at lifting will be very small. The model indicated that there is a lower risk of virus in fresh meat after emergency vaccination around outbreaks compared to the conventional strategy including pre-emptive culling, provided that control measures are adjusted to the applied vaccine and test systems. Adjustments may for instance include size of vaccination area, sampling schemes and timing of lift-up of restrictions. Any non-compliance of control measures will increase the risk of infectious animals remaining after lift-up. Chronically infected pigs are a potential hazard for fresh meat. However, in the risk assessment it was assumed that chronically infected pigs will be identified either by clinical signs and/or laboratory tests. Clinical diagnosis is unreliable and laboratory confirmation is needed. Highly sensitive and specific diagnostic assays are available to diagnose CSF. Agent detection tests are more suitable during early stages of infection. A positive rRT-PCR diagnosis indicates that an animal has been infected with the wild type or MLV virus but it is not necessarly still infectious. Antibody detection tests are mainly suitable for monitoring and surveillance purposes, but not for early diagnosis of suspect cases. Concerning the efficiency of the monitoring scheme, it is directly related to the diagnostic systems applied (organ sample, sensitivity, specificity) and to the number of samples taken. In case of selecting samples for rRT-PCR based on fever measurement in vaccinated animals, the sample size should be corrected. In terms of overall efficiency, rRT-PCR for virus detection and ELISA-systems for antibody investigation are the tests of choice. Vaccinated animals, that at an appropriate lift-up time are tested rRT-PCR negative for the wild type virus, can be considered as “zero risk” animals. If only a few animals become infected in a herd, which is possible especially when vaccination is applied, sampling and testing only a proportion of the animals may result in not detecting such an outbreak

Keywords

classical swine fever; meat; rRT-PCR; surveillance; control; monitoring; emergency vaccination; vaccination-to-live; standstill; pre-emptive culling; lifting of restrictions; modelling

Published in

EFSA Journal
2009, article number: 933
Publisher: EFSA