Parvovirus

Parvovirus, commonly abbreviated to parvo, is a genus of the Parvoviridae family linear, non-segmented single stranded DNA viruses with an average genome size of 5 kbp. Parvoviruses are some of the smallest viruses found in nature (hence the name, from Latin parvus meaning small).

Many types of mammalian species have a strain of parvovirus associated with them. Parvoviruses tend to be specific about the taxon of animal they will infect, but this is a somewhat flexible characteristic. Thus, all strains of canine parvovirus will affect dogs, wolves, and foxes, but only some of them will infect cats.

No members of the genus Parvovirus are currently known to infect humans, but humans can be infected by viruses from three other genera from the Family Parvoviridae. These are the Dependoviruses (e.g. Adeno-Associated Virus), the Erythroviruses (e.g. Parvovirus B19) and the Bocaviruses.

Structure
The viral capsid of parvovirus is made up of 3 proteins known as VP1, VP2 and VP3 that form an icosahedral structure that is resistant to pH, solvents and temperature up to 50°C.

Inside the capsid is a single stranded DNA genome. At the 5’ and 3’ ends of this genome are palindromic sequences of approximately 115 nucleotides that form hairpins and are essential for viral genome replication.

Parvovirus Replication
In order to enter host cells parvoviruses bind to a cell surface receptor. Once in the host cell the virus' DNA genome is translocated to the nucleus where transcription of the genes encoding the non-structural proteins into mRNA occurs. The mRNAs are transported out of the nucleus into the cytoplasm where the host ribosomes translate them into viral proteins. Next the CAP (capsid) proteins are transcribed and translated in the same way as the non-structural proteins. The replication of the viral genome can then occur. The process by which the parvovirus genome is replicated is poorly understood, although host DNA polymerase is needed for replication. Once the genome has replicated it is packaged inside the viral capsid within the cytoplasm.

Parvoviruses do not have an envelope and so are only released when the cell undergoes lysis.

In order for viral replication to take place the infected cells must be non-quiescent cells (i.e. must be actively mitotic). This is because the virus relies heavily on the host cell's replication machinery and therefore require the cell to pass through S phase. Unlike polyomaviruses, parvoviruses are unable to turn on DNA synthesis in host cells.

Diseases
Parvoviruses can cause disease in some animals. Because the viruses require actively dividing cells in order to replicate, the type of tissue infected varies with the age of the animal. The gastrointestinal tract and lymphatic system can be affected at any age, leading to vomiting, diarrhea and immunosuppression, but cerebellar hypoplasia is only seen in cats that were infected in the womb or at less than two weeks of age, and disease of the myocardium is seen in puppies infected between the ages of three and eight weeks.

Canine parvovirus is a particularly deadly disease among young puppies, causing gastrointestinal tract damage and dehydration as well as a cardiac syndrome in very young pups. It is spread by contact with an infected dog's feces. Symptoms include lethargy, severe diarrhea, fever, vomiting, loss of appetite, and dehydration. Mouse parvovirus 1, however, causes no symptoms but can contaminate immunology experiments in biological research laboratories. Porcine parvovirus causes a reproductive disease in swine known as SMEDI, which stands for stillbirth, mummification, embryonic death, and infertility. Feline panleukopenia is common in kittens and causes fever, low white blood cell count, diarrhea, and death. Infection of the cat fetus and kittens less than two weeks old causes cerebellar hypoplasia. Mink enteritis virus is similar in effect to feline panleukopenia, except that it does not cause cerebellar hypoplasia. A different parvovirus causes Aleutian disease in minks and other mustelids, characterized by lymphadenopathy, splenomegaly, glomerulonephritis, anemia, and death. The most accurate diagnosis of parvovirus is by ELISA. Dogs and cats can be vaccinated against parvovirus.

Parvovirus B19, which causes fifth disease in humans, is a member of the Erythrovirus genus of Parvoviridae rather than Parvovirus.