Genetic evolution of Canine Coronaviruses

Canine coronavirus (CCoV), a member of the Al­phacoronavirus genus of the Coronaviridae family, is an envel­oped, positive-sense, single-strand RNA virus first described in Germany in 1971, and considered today enzootic worldwide. CCoV is responsible for mild gastroenteritis, though it may be more severe in young pups, or in combination with other pathogens. In the last two decades researchers have identified new CCoV genotypes/types. After preliminary observation of the variability of the M protein, two separate genetic clusters of CCoV, was observed and the new identified strain, Elmo/02, clearly indicates that a novel CCoV type CCoV type 1, closely re­lated to Feline Coronavirus type I (FCoV type 1), circulates among dogs. On the ba­sis of the genetic relatedness to Transmissible Gastroenteri­tis Virus (TGEV) of swine, CCoV type 2 was then classified into two subgenotypes, CCoV tipe 2a and CCoVtype 2b. Interestingly, an accessory gene, ORF3, unique to CCoV type 1, was also identified. In 2001, a novel CCoV, strain UWSMN, was identi­fied in Australia. Sequence analysis suggested that UWSMN-1 was generally divergent due to a gradual accumulation of mutations throughout its genome. Recently, a CCoV type 2a pantropic variant, strain CB/05, was identified and the virus was able to infect CCoV seropositive dogs posing question on the efficacy of CCoV vaccines currently employed. Respiratory Canine Coronavirus, CRCoV, represents another example of the evolution of dog corona­viruses. The virus showed a close relationship to the betacoronavirus in the polymerase and S genes, and was distantly related to enteric CCoVs. All these observations raise questions regarding the biology of CCoVs. Although nonsegmented RNA viruses generally exhibit undetectable recombination frequencies, the recombinations for the entire coronaviruses genome were calculated to be as high as 25%, probably due to the unique mechanism of coro­navirus synthesis, which involves discontinuous transcription and polymerase jumping. These events lead to the pro­liferation of new virus strains, serotypes and subtype, as hap­pened for SARS-CoV and for new CCoVs.