Parvoviruses



Parvoviruses


James D. Cherry



Parvoviruses infect and cause disease in a great variety of insects and animals. The human parvovirus B19 was discovered serendipitously in 1974, and it was found to be associated with human disease in the early 1980s. B19 virus is the cause of erythema infectiosum (fifth disease) and transient red blood cell aplasia (aplastic crisis), as well as other less common clinical manifestations.

Parvovirus B19 belongs to the family Parvoviridae and the subfamily Parvovirinae, of which three genera have been identified: Parvovirus, Dependovirus, and Erythrovirus. Although B19 virus now is classified as the only member of the genus Erythrovirus and not a Parvovirus, contemporary literature still refers to it as human parvovirus B19.

Parvovirus B19 is a small (23 nm), single-stranded DNA, nonenveloped virus. It has been propagated in suspension cultures of human erythroid bone marrow and in primary fetal liver cells.


EPIDEMIOLOGY

Most epidemiologic information comes from studies of erythema infectiosum outbreaks. These outbreaks are most prevalent in the winter and spring and last for 3 to 6 months. The disease has a cyclic pattern similar to that of rubella. Peak periods of disease occur approximately every 6 years, with increased activity lasting an average of 3 years.

The case-to-case interval of erythema infectiosum usually is between 4 and 14 days, and the attack rate is high. In school-related outbreaks, attack rates of 25% are common. The highest attack rates are in children 5 to 14 years old; secondary cases occur in preschool children, teachers, and parents. In the home, secondary cases occur more commonly in mothers than in fathers.

Antibody studies indicate the following prevalence data: in children younger than 5 years old, 2% to 9%; in children and adolescents 5 to 18 years old, 15% to 35%; and in adults older than 18 years, 40% to 60%. The prevalence of antibody suggests that many infections are either asymptomatic or symptomatic but are unrecognized as typical B19 viral infections.

The mode of spread is by droplet via the respiratory tract.


PATHOGENESIS

After infection has occurred via the respiratory tract, viremia occurs in which 1010 or 1011 viral particles per milliliter of blood may be found. Associated with viremia (approximately 7 to 10 days after infection), a profound reticulocytopenia occurs. In vitro studies indicate that early erythrocyte precursor cells are susceptible to B19 virus infection. The blood group P antigen is the cellular receptor for B19 virus, and persons who lack this antigen are resistant to infection. Neutropenia, lymphopenia, and a decrease in the platelet count occur in conjunction with the reticulocytopenia. During the second week of infection, hemoglobin values decrease slightly.

In erythema infectiosum, the exanthem occurs approximately 17 to 18 days after infection develops, when virus no longer can be detected in throat swabs or blood specimens. At the time of rash, virus-specific IgM antibody is present, suggesting that the exanthem may be immune-mediated. However, the virus also has been identified in the rash by skin biopsy. In immunocompromised patients who fail to produce effective neutralizing antibodies, persistent infections occur.


CLINICAL MANIFESTATIONS AND COMPLICATIONS

Human parvovirus B19 can cause various manifestations that may depend on the immune status of the host (Table 192.1).


Erythema Infectiosum

Although erythema infectiosum is recognized classically by its distinct rash, studies in volunteers suggest a biphasic illness. Approximately 1 week after infection occurs, a nonspecific febrile illness with headache, chills, malaise, and myalgia develops. These symptoms last 2 to 3 days, followed by an asymptomatic interlude of approximately 7 days, and then the exanthematous phase of the illness begins.

The exanthem occurs in three stages. The first stage is the appearance of a fiery red rash on the cheeks (“slapped-cheek” appearance) and a relative circumoral pallor. The facial appearance is suggestive of scarlet fever, an allergic reaction, or collagen vascular disease. The facial exanthem may be accentuated when the affected person moves from outdoors to a warm room. The second stage follows the onset of facial involvement by 1 to 4 days, as an erythematous maculopapular rash on the trunk and extremities. Initially, this rash is discrete, but soon it takes on a characteristic lacy or reticular pattern. The third stage of the exanthem is characterized by changes in the intensity of the rash, with periodic evanescence and recrudescence. The duration of the third stage is highly variable;
fluctuations are related to environmental factors such as exposure to sunlight and temperature. The rash often is pruritic, especially in adults, and generally is more prominent on the extensor surfaces. Occasionally, slight desquamation is noted in some patients.

Only gold members can continue reading. Log In or Register to continue

Stay updated, free articles. Join our Telegram channel

Jul 24, 2016 | Posted by in ORTHOPEDIC | Comments Off on Parvoviruses

Full access? Get Clinical Tree

Get Clinical Tree app for offline access