Also Known As: Parvovirus B19, B19 virus, Erythrovirus B19
The B19 virus, generally referred to as parvovirus B19 or sometimes erythrovirus B19, was the first (and until 2005 the only) known human virus in the family of parvoviruses, genus erythrovirus. B19 virus is most known for causing disease in the pediatric population; however, it can also affect adults. It is the classic cause of the childhood rash called fifth disease or erythema infectiosum, or "slapped cheek syndrome." 
The virus was discovered by chance in 1975 by Australian virologist Yvonne Cossart. It gained its name because it was discovered in well B19 of a large series of microtiter plates labelled in this way.[4
Erythroviruses belong to the Parvoviridae family of small DNA viruses. It is a non-enveloped, icosahedral virus that contains a single-stranded linear DNA genome. Approximately equal proportions of DNA of positive and negative sense are found in separate particles. At each end of the DNA molecule there are palindromic sequences which form "hairpin" loops. The hairpin at the 3' end serves as a primer for the DNA polymerase. It is classified as erythrovirus because of its capability to invade red blood cell precursors in the bone marrow. Three genotypes (with subtypes) have been recognised.
In humans the P antigen (also known as globoside) is the cellular receptor for parvovirus B19 virus that causes Erythema infectiosum (fifth disease) in children. This infection is sometimes complicated by severe aplastic anemia caused by lysis of early erythroid precursors.
The virus is primarily spread by infected respiratory droplets; blood-borne transmission, however, has been reported. The secondary attack risk for exposed household persons is about 50%, and about half of that for classroom contacts.
Symptoms begin some six days after exposure (between 4 and 28 days, with the average being 16 to 17 days) and last about a week. Infected patients with normal immune systems are contagious before becoming symptomatic, but probably not after then. Individuals with B19 IgG antibodies are generally considered immune to recurrent infection, but reinfection is possible in a minority of cases. About half of adults are B19-immune due to a past infection.
Parvovirus B19 causes an infection in humans only. Cat and dog parvoviruses do not infect humans. There is no vaccine available for human parvovirus B19, though attempts have been made to develop one.
Role in disease
Fifth disease or erythema infectiosum is only one of several expressions of Parvovirus B19. The associated bright red rash of the cheeks gives it the nickname "slapped cheek syndrome".Any age may be affected, although it is most common in children aged six to ten years. It is so named because it was the fifth most common cause of a pink-red infection associated rash to be described by physicians (many of the others, such as measles and rubella, are rare now thanks to vaccination) .
Once infected, patients usually develop the illness after an incubation period of four to fourteen days. The disease commences with high fever and malaise, when the virus is most abundant in the bloodstream, and patients are usually no longer infectious once the characteristic rash of this disease has appeared. The following symptoms are characteristic:
- A usual brief viral prodrome with fever, headache, nausea, diarrhea.
- As the fever breaks, a red rash of the cheeks forms, with relative pallor around the mouth ("slapped cheek rash"), sparing the nasolabial folds, forehead, and mouth.
- "Lace-like,(reticular)" red rash on trunk or extremities then follows the facial rash. Infection in adults usually only involves the reticular rash, with multiple joint pain predominating.
- Exacerbation of rash by sunlight, heat, stress.
Teenagers or young adults may develop the so-called "Papular Purpuric Gloves and Socks Syndrome".