Canine coronaviruses: Epidemiology, evolution and pathobiology

Abstract

Coronaviruses (CoVs; order Nidovirales, family Coronaviridae) are viruses exceptionally prone to genetic evolution through the continual accumulation of mutations and by homologous recombination between related members. CoVs are organised into three antigenic groups of which group 1 is subdivided in subgroups 1a and 1b, the former including highly related viruses such as feline coronaviruses (FCoVs), canine coronaviruses (CCoVs), transmissible gastroenteritis virus of swine (TGEV) and its derivative porcine respiratory coronavirus (PRCoV). The scope of this thesis was to investigate the epidemiological, evolutionary and pathobiological features of CCoVs, which are paradigmatic of coronaviruses’ evolution and complexity. To date, two different CCoV genotypes are known, CCoV types I (CCoV-I) and II (CCoV-II), that share up to 96% of genetic identity, being highly divergent only in the spike (S) protein gene. In addition, CCoV type I displays a novel open reading frame (ORF), ORF3, coding for a putative glycosylated protein which is likely secreted from the infected cells. By using molecular methods (real-time RT-PCR assays), the two CCoV genotypes were commonly detected in the feces of dogs with diarrhea, causing frequently simultaneous infections of the same dogs. Both CCoV genotypes have been associated with mild, self-limiting enteritis in pups. In 2005, a highly virulent variant of CCoV type II (strain CB/05) was reported in Italy which caused systemic disease with fatal outcome in pups. A CCoV-II strain was detected at high titers in the internal organs of the dead puppies by real-time RT-PCR. Experimental infection of seronegative pups with the viral isolate reproduced the disease with occurrence of severe clinical signs, including pyrexia, anorexia, depression, vomiting, diarrhea and leukopenia. The evolution of CCoVs and TGEV is strictly interconnected. In fact, it has been postulated that TGEV originated from CCoV-II through a cross-species transmission, which is supported by the high genetic relatedness between the two viruses and by the presence of ORF3 remnants in CCoV-II and TGEV genomes. Novel CCoV-II strains were isolated that originated from a double recombination event with TGEV occurring in the 5’ end of the S-protein gene. Accordingly, genotype II has been further divided into two subtypes, CCoV-IIa and CCoV-IIb, including extant and TGEV-like CCoVs, respectively. Future investigations should provide new insights into the molecular mechanisms responsible for the change in pathobiology of CCoV and into the pathogenic and immunological aspects of CCoV-induced systemic disease. At the same time, constant epidemiological surveillance will help a timely identification of additional CoV strains with novel genetic and biological properties. The lessons still to be learned will be valuable to predict and counter changes in coronavirus pathogenicity in general and hence, they will be of both human clinical and veterinary importance.