The virus

Parvovirus B19 is a small DNA virus, which belongs to the genera erythrovirus in the family Parvoviridae; it is the only known human parvovirus. Many other mammalian species including dogs have parvoviruses, but they don't cause infection in humans.

The virus replicates in the erythroid precursor cells, which it infects by attachment to one of the blood group antigens (P antigen) expressed at the surface of the cells which act as a receptor for the virus.

Epidemiology

Route of spread

The virus replicates initially in the respiratory mucosa and is spread by droplet (rather than aerosol) transmission by the respiratory route. Attack rate is about 50% in susceptible household contacts, but much lower in the community setting.

There is a short period of viraemia before a rash appears; therefore occasional transmissions by blood transfusion have been recorded where a donor has donated in the prodromal period.

Prevalence

Parvoviruses have worldwide prevalence. Parvovirus is a childhood infection with the prevalence of antibody rising with age; about 50% of young adults show evidence of previous infection.

Incubation period

Is about 12–18 days.

Infectious period

The highest infectivity period is in the prodromal phase, which is 2 days before the rash appears, but patients are not infectious after the rash appears.

At-risk groups

Pregnant women, immunocompromised patients and those with haemolytic anaemia.