Canine parvovirus type 2 (CPV2, colloquioally parvo) is a contagious virus mainly affecting dogs. The disease is highly infectious and is spread from dog to dog by direct or indirect contact with their feces. It can be especially severe in puppies that are not protected by maternal antibodies or vaccination. It has two distinct presentations, a cardiac and intestinal form. The common signs of the intestinal form are severe vomiting and severe haemmorhagic (bloody) diarrhea. The cardiac form causes respiratory or cardiovascular failure in young puppies. Treatment often involves veterinary hospitalization. Vaccines can prevent this infection, but mortality can reach 91% in untreated cases.
There are two types of canine parvovirus called canine minute virus (CPV1) and CPV2. CPV2 causes the most serious disease and affects domesticated dogs and wild canids. There are variants of CPV2 called CPV-2a, CPV-2b and CPV-2c. Types 2a and 2b are distinct from the original CPV type 2 in terms of virulence and their ability to infect and cause disease in cats too. CPV-2c is a newly identified variant similar to 2b. The viral protein of 2c contains one amino acid different from CPV-2b but it is believed this could be significant. 2c strains have been identified in parts of Europe, the Americas and in Asia. Emergence of this strain has led to claims of ineffective vaccination of dogs, however studies have shown that the existing CPV vaccines still provide adequate levels of protection against CPV type 2c
CPV2 is a relatively new disease that appeared in the late 1970s. It was first recognized in 1978 and spread worldwide in one to two years. The virus is very similar to feline panleukopenia (also a parvovirus); they are 98% identical, differing only in two amino acids in the viral capsid protein VP2. It is also highly similar to mink enteritis, and the parvoviruses of raccoons and foxes.] The early belief was that the feline panleukopenia mutated into CPV2. Although this has not been proven, the strong similarity to feline panleukopenia makes this the most credible theory. However, it is possible that CPV2 is a mutant of an unidentified parvovirus (similar to feline parvovirus (FPV)) of some wild carnivore. A strain of CPV2b (strain FP84) has been shown to cause disease in a small percentage of domestic cats, although vaccination for FPV seems to be protective. CPV2, however, does not cause disease in cats and does so only mildly in mink and raccoons, and is a virus almost exclusively affecting canines.
Two more strains of canine parvovirus CPV2a and CPV2b were identified in 1979 and 1984 respectively. Most cases of canine parvovirus infection are believed to be caused by these two strains, which have replaced the original strain, and the present day virus is different from the one originally discovered although they are indistinguishable by most routine tests. A third type, CPV2c (a Glu-426 mutant), has been discovered in Italy, Vietnam, and Spain.
CPV2 is a non-enveloped single-stranded DNA virus. The name comes from the Latin parvus, meaning small, as the virus is only 20 to 26 nm in diameter. It has an icosahedral symmetry. The genome is about 5000 nucleotides long. CPV2 continues to evolve, and the success of new strains seems to depend on extending the range of hosts affected and improved binding to its receptor, the canine transferrin receptor. CPV2 has a high rate of evolution, possibly due to a rate of nucleotide substitution that is more like RNA viruses such as Influenzavirus A. In contrast, FPV seems to evolve only through random genetic drift.
CPV2 affects dogs, wolves, foxes, and other canids. CPV2a and CPV2b have been isolated from a small percentage of symptomatic cats and is more common than feline panleukopenia in big cats.
Previously it has been thought that the virus does not undergo cross species infection. However studies in Vietnam have shown that CPV2 can undergo minor antigenic shift and natural mutation to infect felids. Analyses of feline parvovirus (FPV) isolates in Vietnam and Taiwan revealed that more than 80% of the isolates were of the canine parvovirus type, rather than feline panleukopenia virus (FPLV).CPV2 may spread to cats easier than dogs and undergo faster rates of mutation within that species. The virus does not transmit to birds, or humans , but it can be spread by them when a bird comes in contact with feces and then the dog's environment, or when a cat goes to the groomers and returns with an exposed pet carrier.