Feline Calicivirus Virulent Systemic Disease: Clinical Epidemiology, Analysis of Viral Isolates and In Vitro Efficacy of Novel Antivirals in Australian Outbreaks

Abstract

Feline calicivirus (FCV) causes upper respiratory tract disease (URTD) and sporadic outbreaks of virulent systemic disease (FCV-VSD). The basis for the increased pathogenicity of FCV-VSD viruses is incompletely understood, and antivirals for FCV-VSD have yet to be developed. We investigated the clinicoepidemiology and viral features of three FCV-VSD outbreaks in Australia and evaluated the in vitro efficacy of nitazoxanide (NTZ), 2′-C-methylcytidine (2CMC) and NITD-008 against FCV-VSD viruses. Overall mortality among 23 cases of FCV-VSD was 39%. Metagenomic sequencing identified five genetically distinct FCV lineages within the three outbreaks, all seemingly evolving in situ in Australia. Notably, no mutations that clearly distinguished FCV-URTD from FCV-VSD phenotypes were identified. One FCV-URTD strain likely originated from a recombination event. Analysis of seven amino-acid residues from the hypervariable E region of the capsid in the cultured viruses did not support the contention that properties of these residues can reliably differentiate between the two pathotypes. On plaque reduction assays, dose–response inhibition of FCV-VSD was obtained with all antivirals at low micromolar concentrations; NTZ EC50, 0.4–0.6 µM, TI = 21; 2CMC EC50, 2.7–5.3 µM, TI > 18; NITD-008, 0.5 to 0.9 µM, TI > 111. Investigation of these antivirals for the treatment of FCV-VSD is warranted.

Calicivirus Infection in Cats

“…Based on this observation, it was suggested that FCV evolution is not associated with selective competition among different strains in the cat population [ 28 ]. No association has been demonstrated between FCV capsid gene sequences or antigenic characteristics and different disease manifestations [ 10 , 26 , 33 , 34 , 35 ].…”

“…Outbreaks of highly virulent and often lethal FCV infection in domestic cats have been described in the United States [ 3 , 8 , 57 , 67 , 105 ], China [ 106 , 107 ] and in several European countries: France [ 58 , 59 ], Germany [ 108 ], Italy [ 109 ], Switzerland (R.H.-L. personal communication), the UK [ 64 ], and Australia [ 35 ]. A single outbreak has also been described in exotic captive felids in the United States [ 110 ].…”

“…Phylogenetic analyses of VS-FCV and FCV from cats with other FCV-associated clinical signs or asymptomatic cats did not reveal clustering according to the different clinical presentations; thus, there was no clear association between disease severity and virus genetic background and no mutations could be identified that clearly distinguished different clinical presentations [ 35 ]. Therefore, RT-PCR or sequencing of FCV cannot be used to confirm or rule out virulent systemic disease per se; the clinical signs and the epidemic situation should also be considered before making such a diagnosis.…”

“…One study found that the capsid gene of a virulent FCV isolate was responsible for more rapid in vitro growth kinetics [ 24 ] and analysis of the amino acid properties of the E region of VP1 identified seven residue positions (438, 440, 448, 452 and 455 in the E3′ HVR; 465 in the E-conserved region; and 492 in the E5′ HVR; see also Figure 2 A) that were believed to be different between different pathotypes [ 135 ]. However, this assumption was not confirmed in a subsequent Australian study, where VS-FCV from one outbreak showed the typical mutations described for the VS-FCV pathotype, while VS-FCV from two other outbreaks had either the pattern characteristic for a respiratory pathotype or an intermediate configuration [ 35 ]. Thus, further studies are needed to further investigate this hypothesis and currently it should be emphasised that there are no diagnostic assays that specifically detect VS-FCV.…”

“…In vitro efficacy of novel antivirals was tested against VS-FCV from three Australian disease outbreaks and dose–response inhibition was obtained with several antivirals, including NTZ at low micromolar concentrations; further evaluation of the safety and efficacy of NTZ as a specific treatment option for FCV-VSD will be necessary [ 35 ].…”