Quantitative assessment of the likelihood of the introduction of classical swine fever virus into the Danish swine population

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Abstract

Classical swine fever virus (CSFV) is a major infectious-disease agent of livestock and causes production losses through increased morbidity and mortality, particularly of young pigs. We identified the pathways for introduction of CSFV into Denmark and assessed the annual probability of introduction (based on a US Department of Agriculture model).

We developed pathways based on material from scientific articles, reports from veterinary agencies and custom officers, and consultations with experts in the field.

Returning livestock trucks and legal meat imports were the most important pathways for CSFV introduction to Denmark from other EU states with predicted overall likelihood of one or more introductions of CSFV within a median of 130 years (46–280) provided mitigating steps, such as cleaning trucks, were maintained to a very high standard. The likelihood would increase dramatically if these activities were abandoned: one or more introductions within a median of 5.2 years (2–14). The predicted risks from live-animal imports and semen were extremely low given the very few imports of these products. The most important countries for Denmark's CSFV risk are Germany and the Netherlands, though this risk is again predicted to be dramatically reduced as long as mitigating activities are maintained. We predicted the risk from illegal movements of pork into Denmark to be low because little pork enters through this route and only a small fraction of this pork would be fed to pigs.

Introduction

The European Union (EU) is currently considered free of many of the trans-boundary animal infections considered by the World Organisation for Animal Health (OIE) to be of major economic and/or public-health importance including foot-and-mouth disease (FMD), African swine fever and classical swine fever (CSF). However, CSFV outbreaks in the Netherlands and United Kingdom in 2000 and the FMD outbreaks in several EU states in 2001 demonstrate that the EU is at continuous risk of introduction of infection. Clearly, when infection incidence is low or a country is infection-free, the most efficient method of maintaining this status is to prevent the introduction of the infection. For countries to minimise the risks of these infections, the countries must identify high-risk activities and potential pathways for infection introduction.

In order to facilitate trade and to prevent trade barriers, many countries have signed the Sanitary and Phytosanitary (SPS) agreement of the World Trade Organisation (WTO). To justify any trade barriers, a country trying to prevent imports are required to demonstrate that trade in particular animals or plants or their products is likely to put the country at an increased risk of infection introduction. To satisfy international regulations, these risks need to be assessed as part of a clear and transparent risk analysis as set out by the OIE.

CSFV has caused the most outbreaks of any of the former O.I.E. list-A diseases in Europe and was therefore considered the first priority for assessment. CSF is caused by a Pestivirus in the family Togaviridae. It is a highly contagious viral disease of domestic pigs and wild boar. In Europe it has been controlled for several decades through a stamping-out policy. Moreover, swill feeding is prohibited within the EU, although Germany and Austria were allowed swill feeding under certain conditions until October 2006.

Although 13 states have recorded CSFV outbreaks between 1996 and 2004 (Table 1), it is clear that several countries have on-going problems controlling CSFV. Over this same period, there have been repeated outbreaks of CSFV in pigs states bordering the EU such as Bosnia (257), Bulgaria (66) Romania (203) and Serbia (718). The repeated outbreaks in wild boar strongly suggest they are a reservoir (Artois et al., 2002).

CSFV can be transmitted from infected animals directly or indirectly; recognised routes of transmission include animal movements, movement of animal products such as carcases, human contact due to contaminated clothing, contaminated trucks due to faecal contamination and via genetic material such as semen (Elbers et al., 2001, Guerin and Pozzi, 2005, Ribbens et al., 2004). However, there is often considerable uncertainty about the probability of infection introduction associated with a particular mode of transmission. Moreover, illegal movements of animals and animal products are difficult to quantify (Wooldridge et al., 2006).

In general, the likelihood of introduction of CSFV will depend on (de Vos et al., 2003):

  • (1)

    the prevalence of infection in the country of origin;

  • (2)

    the trade contacts and movements of animals and people;

  • (3)

    the survival of the virus in the environment;

  • (4)

    the measures to reduce the risk taken by the importing country.

Despite its limited size, Denmark with ∼25 million slaughter pigs produced in 2004, is the largest exporter of pork in the world, with an annual export >1.8 million tonnes, accounting for ∼90% of the national production. The pork export makes up ∼6% of the total value of the Danish export—highlighting the importance of minimising infection introductions.

Our aim was to assess the likelihood of an introduction of CSFV-infected or -contaminated material infection–introduction pathways of importance to the swine population by quantifying the volume of trade and scale of movement of different products. We used simple probability models based on a United States Department of Agriculture model (Anon, 2000) to assess the relative importance of the different pathways and to give a quantitative assessment of the likelihood of introduction associated with each of the pathways considered.

Onward transmission post-introduction and therefore the likelihood of an outbreak was considered in a separate EpiLab project. The importance of wild boar for the introduction of CSFV was not dealt with because there are no free-range wild-boar populations in Denmark, and the wild-boar population in Northern Germany south of the Danish border is not infected with CSFV. Furthermore, the role of wild birds and rodents was not covered because this is primarily related to the spread and not to the introduction of CSFV into a country.

Section snippets

Data collection

We used the national research network of the International EpiLab in Denmark to meet this requirement.

The assessment incorporated a number of components including a literature review, discussions with experts in a wide range of fields and eliciting specific information regarding volumes of movements of animals, trucks and people, etc., from requests to the relevant Danish government agencies (and the Danish Veterinary and Food Administration in particular). We used this information to develop

Live animal movements

In 2004, Denmark imported four times more breeding pigs than in 2003 when 1207 breeding pigs were imported from Germany and the United Kingdom as well as a further 73 from North America. This compared to 2003 when only 254 pigs were imported (Bronsvoort et al., 2004) but none in 2004 were from France or Sweden which was previously had exported breeding pigs to Denmark. The swine industry currently operates a voluntary 30-day CSFV quarantine of animals follow import.

Our predictions are that

Conclusion

We predicted that the overall risk of CSFV entering Denmark if there were no mitigating activities would be at least one outbreak every 5.2 years (90% prediction interval: 2.0, 14 years) (Table 9). The predicted risk with mitigating activities in place (quarantine and cleaning trucks) would decrease to at least one outbreak every 139 years (46, 484 years).

Returning trucks and legal meat imports were predicted to represent the highest risk from other EU states, but we consider the risks to be

Acknowledgements

This research was funded as a project of the International EpiLab (93S-2465-Å02-01437). Mark Bronsvoort conducted the research as a guest scientist of the International EpiLab in Denmark.

In addition, we thank Karsten Flemin, Poul Tolstrup Christensen, Karsten Bruun Rasmussen, and Anne-Mette Olsen (Danish Meat) as well as Sten Mortensen (The Danish Veterinary and Food Administration) and Åse Uttenthal (Danish Institute for Food and Veterinary Research).

References (16)

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