All the research carried out so far has clearly demonstrated that Canine parvovirus-2 (CPV-2) [3]is an antigenic derivative of Feline panleucopenia virus (FPV) which developed around 1970. Indeed the amino acid homology of CPV-2 and FPV is more than 98%. The diagram below shows the probable evolution of CPV-2 over time:
CPV-2 was initially spread among various different species of wild carnivores (mink, raccoon, fox, etc.). Around 1977-78 the virus first appeared in dogs. Following an initial pandemic, during which numerous dogs died, in the 1980s the original CPV-2 began to disappear. In the meantime the antigenic profile of this virus underwent new mutations generating two other variants that were called CPV-2a and CPV-2b. In the following years and up to the present these variants have spread through the canine population, to varying degrees, in the whole world.
In 2001 a group of researchers in Bari (Italy) isolated a new variant of CPV-2 with a mutation of an amino acid in position 426 of the major capsid protein (VP2). The original CPV-2 strain and the CPV-2a variant have an asparagine in this position, which is considered strategic from an immunological point of view, while the CPV-2b variant has an aspartic acid. The new variant has a glutamic acid at position 426 and has been named Glu-426 or, better, CPV-2c. Observations in the field have revealed that the CPV-2c variant is pathogenic also for dogs from 7 months to 1 year old. A case of disease caused by the CPV-2c variant in a 12-year old dog has also been reported. In the last few years CPV-2c has spread not only in different areas of Italy, but also in Africa, Asia and America.
BIOLOGICAL DIFFERENCES BETWEEN CPV-2 AND THE NEW ANTIGENIC VARIANTS
Research carried out on the CPV-2a and 2b variants has revealed that these have a greater pathogenic strength than the original, now no longer circulating, strain for the following reasons:
- the period of incubation, at 4 or 6 days, is shorter;
- the haemorrhagic enteritis is more severe;
- the mortality rate is higher;
- the haemagglutinin titre in the faeces is greater.
The most important biological feature of the 2a and 2b variants is, however, their capacity to infect cats: about 5% of cases of parvovirosis in the cat are caused by CPV-2a or CPV-2b. Isolates of CPV-2a and 2b from cats in Japan, Germany and the United States have been reported.
The new CPV-2c variant has been shown to have a similar pathogenic profile and it has recently also been found in the feline species.