Classical swine fever virus marker vaccine strain CP7_E2alf: Shedding and dissemination studies in boars

Highlights

• Shedding and distribution in boar was assessed for CSFV vaccine strain CP7_E2alf.

• Primary replication in lymphatic organs was confirmed.

• Complete lack of CP7_E2alf detection was shown in genitals and accessory sex glands.

• No shedding in urine, faeces, or saliva occurred.

• The study further substantiates safety of marker vaccine strain CP7_E2alf.

Abstract

Over the last decade, pestivirus chimaera CP7_E2alf has proven to be a most promising marker vaccine candidate against classical swine fever (CSF). To provide further background data for the risk assessment towards licensing and release, especially on presence of the vaccine chimaera in faeces, urine, and organs of the male reproductive tract, supplementary studies were carried out under controlled laboratory conditions. In detail, the shedding and dissemination pattern of Suvaxyn^® CSF Marker (“CP7_E2alf”) was assessed in 12 adult boars after single intramuscular vaccination with a tenfold vaccine dose. Four and seven days post vaccination, six animals were subjected to necropsy and triplicate samples were obtained from reproductive and lymphatic organs as well as urine, faeces, blood, and several additional organs and matrices. The sampling days were chosen based on pre-existing data that indicated the highest probability of virus detection. Upon vaccination, neither local nor systemic adverse effects were observed in the experimental animals. It was confirmed that primary replication is restricted to the lymphatic tissues and especially the tonsil. While viral genome was detectable in several samples from lymphatic tissues at four and seven days post vaccination, infectious virus was only demonstrated at four days post vaccination in one tonsil sample and one parotid lymphnode. Sporadic detection at a very low level occurred in some replicates of liver, lung, bone marrow, and salivary gland samples. In contrast, viral genome was not detected in any sample from reproductive organs and accessory sex glands, in faeces, urine, or bile.

The presented data on the dissemination of the vaccine virus CP7_E2alf in adult boars are supplementing existing safety and efficacy studies and indicate that the use of the vaccine is also safe in reproductive boars.

Introduction

Classical swine fever (CSF) remains one of the major threats for profitable pig production worldwide, and due to its impact, it is notifiable to the OIE [1]. In most industrialized pig production systems, prophylactic vaccination against the viral disease is banned and outbreaks are controlled through strict and legal binding hygiene measures (see e.g. European Union Council Directive 2001/89/EC and Commission Decision 2002/106/EC). In densely populated livestock areas and in larger outbreak scenarios, this policy can lead to tremendous numbers of animals that have to be culled in restriction zones. To avoid this, emergency vaccination could be implemented as laid down in the above-mentioned legislation. However, the option of emergency vaccination is hampered by limitations in the available subunit marker vaccines, and trade restrictions that are imposed on conventionally vaccinated animals [2]. To overcome these problems, research activities were directed towards the development of new generations of marker vaccines [3]. Over the last decade, several approaches have been followed and tested, among them chimeric pestiviruses such as “CP7_E2alf” [4], [5], [6], [7], [8].

After its first description as possible marker vaccine candidate against CSF in 2004 [4], “CP7_E2alf” was evaluated in the framework of two EU funded research projects and beyond. The chimaera with a cytopathogenic BVDV “CP7” backbone and the glycoprotein E2 of CSFV “Alfort 187” has proven safety for target and non-target species as well as against different challenge virus strains. Furthermore, genetic and serological DIVA concepts were established [9], [10], [11].

Given the fact that the vaccine is a genetically engineered virus, assessment of all safety aspects is crucial for field applications and the evaluation towards licensing. In this respect, one of the outstanding issues was the question whether vaccine virus could be found in semen of vaccinated boars. Moreover, absence of shedding in faeces and urine still had to be demonstrated. These issues are going far beyond the standard requirements of the European Pharmacopoeia and were thus only done upon completion of the routine studies.

Here, a trial was conducted under controlled laboratory conditions with 12 boars.

As substitute for semen, all genitals and accessory sex glands were tested for vaccine virus. In addition, lymphatic and parenchymatous organs as well as a range of additional sample matrices including saliva, bile, and bone marrow, were screened for vaccine virus in order to provide additional data on distribution and primary replication sites.