A multi-national European cross-sectional study of feline calicivirus epidemiology, diversity and vaccine cross-reactivity

Keywords

• Feline calicivirus;
• Vaccine;
• Neutralisation;
• Phylogeny;
• Epidemiology

1. Introduction

Feline calicivirus (FCV) is a common pathogen of cats causing oral and upper respiratory tract disease (URTD) [1]. It has a single-stranded, positive-sense RNA genome [2], the plasticity of which is important for antigenic evolution, viral persistence [3] ;  [4] recombination [5] ;  [6], and the sporadic outbreaks of highly virulent FCV strains causing severe disease [1]; [7] ;  [8]. Despite high levels of variability, FCV strains are generally considered to comprise one diverse genogroup with a radial phylogeny and little evidence for sub-species clustering [9]; [10]; [11]; [12]; [13] ;  [14].

This diverse genogroup is mirrored by a single diverse serotype; although individual strains are distinguishable antigenically, they generally show some cross-reactivity [15]; [16]; [17] ;  [18], allowing the development of several FCV vaccines based on different antigens [1]. Whilst vaccines reduce clinical signs, none are licensed to reduce virus shedding post-challenge and FCV infection remains highly prevalent in both vaccinated and unvaccinated populations [1]. Most live vaccines include FCV-F9 [19] ;  [20], whereas inactivated vaccines commonly include strains FCV-255, or a combination of FCV-431 and FCV-G1 [18]; [21] ;  [22]. These vaccine antigens are chosen based on their ability to induce broadly cross-reactive antisera against contemporary isolates circulating at the time of vaccine development [17]; [20] ;  [22]. The widespread use of such vaccines together with the high adaptability of FCV raises the theoretical possibility that vaccine resistant strains may evolve over time. Whilst some studies have supported this hypothesis [23]; [24] ;  [25], others have not [26].

Here we describe the antigenic and genetic relationships between FCV-F9 and a representative panel of currently circulating FCV strains, obtained from randomly selected veterinary practices across six European countries.

2. Materials and methods

2.1. Ethical statement

Ethical approval was from the Veterinary Research Ethics Committee, University of Liverpool. Informed consent was obtained from participating owners.

2.2. Recruitment

Samples were collected between October 2013 and May 2014 from cats attending veterinary practices in the UK, France, Italy, Netherlands, Sweden and Germany.

In the UK, three Unitary Authorities (UAs) were randomly chosen from each of the nine regions of England, as well as from Wales, Scotland and Northern Ireland. Geographically remote islands were also selected (Western Isles, Orkney, Shetland, Anglesey, Isle of Wight, Scilly Isles, Isle of Man, Channel Islands) based on convenience. From each of these 44 regions, a small animal practice was randomly selected from the Royal College of Veterinary Surgeons database. The remaining five countries were chosen based on convenience, divided into five regions based on official divisions and/or local geography, and a single practice randomly selected from each. If chosen practices declined to participate, a further practice was randomly selected. This process was repeated up to three times until a practice in each region agreed to take part.

2.3. Field isolates

There is much debate regarding the most appropriate FCV isolates to use for assessment of in vitro neutralisation. Several studies have used isolates obtained by convenience from diagnostic laboratories to represent pathogenic viruses [23]; [24] ;  [25]; lack of random sampling means such results may not be generalizable to the wider population [24]. Here we sample sick and healthy cats randomly to ensure our results are representative of the sampled population. The occasional description of non-pathogenic FCV strains [27] requires us to justify the inclusion of isolates from healthy animals. In this regard, it should be noted that FCV isolates from healthy cats can still be pathogenic: virulent FCV continues to be shed from cats recovered from acute disease [27], and seropositive cats previously exposed to vaccine or field virus may shed virus in absence of clinical signs when subsequently challenged with virulent virus [19]. Indeed, experimental challenge has confirmed that FCV from healthy cats can recreate typical disease [27].

In each practice, veterinary surgeons were asked to collect oropharyngeal swabs from the next 30 (UK) or 40 (other countries) cats presented at their surgery regardless of reason for presentation (diseased or healthy). Random recruitment of practices and random sampling of cats based on attendance at these practices were used to ensure results could be generalised to the sampled population, and is in contrast to an earlier study by the authors where sampling was by convenience [10].

Swabs were collected into virus transport medium, stored at −20 °C before shipping to the laboratory. The veterinary surgeon and owner were asked to complete a short questionnaire capturing demographic data, vaccination history and information about current respiratory disease, mouth ulcers and chronic gingivostomatitis (CGS).