The virus itself is small and nonenveloped, making it one of the most robust viruses affecting animals. Because of its environmental persistence, both indirect and direct contact can efficiently spread the virus. When protected by organic matter, the virus may remain infectious for months.2 Seasonality of infection in the United States has been reported, with most cases occurring from July to September.3 This seasonality likely reflects the increase in susceptible populations with the advent of waning maternal immunity in puppies born in spring and summer.
CPV’s genome is single-stranded DNA of approximately 5 kilobases in length. The mutation rate of parvoviruses is actually closer to that of RNA viruses, most likely due to its single-stranded structure. Because of the virus’ mutability, variants of the original emergent virus have arisen, notably CPV-2a, 2b, and 2c.4 These variants differ from the original CPV-2 in only a few amino acids but have led to more efficient replication in the canine host and to reacquisition of the ability to replicate and cause disease in cats. In some areas of the world (eg, Vietnam, Taiwan), CPV-2 variants are more commonly isolated than feline panleukopenia virus.1,5,6 These variants may cause disease similar to panleukopenia.7
CPV-2, recognized as an enteric pathogen, is the agent of systemic infection and disease. On contact with the oral mucosa, the virus replicates in the local lymphatics, then spreads rapidly via the lymphatics and bloodstream, affecting cells systemically. The incubation period varies with immune status and virus dose and may range from 4 to 14 days.8 The virus targets cells that are rapidly dividing, as its propagation is completely dependent on replicating cells. In dogs, the virus targets hematopoietic bone marrow cells and divides lymphocytes in lymphoid tissue as well as intestinal crypt epithelia. This can lead to profound leukopenia, hemorrhagic diarrhea, lymphoid hypoplasia, and, in some cases, anemia. Concurrent disease or infection with other agents as well as stressors may worsen disease severity.8