B19V.
B19V infection is common worldwide, showing regional epidemiological differences, with generally over one-half of the adult population having been exposed. The prevalence of B19V-specific antibodies in the population is age dependent, increasing from 2 to 20% in children <5 years old to 15 to 40% in children 5 to 18 years old and to 40 to 80% in the adult population, depending on both the assays used and the population (
214–225). Seroprevalence, however, is much lower in some isolated areas, such as the Rodriguez Islands and among some Brazilian tribes, with adult seroprevalences of only 2 to 10% (
226,
227). Prevalence can also be much higher, such as the 85% seroprevalence reported for 9-year-olds in Papua New Guinea (
228). The typical age when an individual contracts B19V infection is 5 to 15 years, but susceptible adults may also be infected. Infection induces an immune response, which confers lifelong protection against reinfections. Neutralizing IgG is formed about 2 weeks after infection and is very effective in eradicating the virus from the bloodstream.
B19V is transmitted mainly by the respiratory route, but prodromal symptoms are fever, malaise, headache, and myalgia rather than respiratory symptoms (
229). It is currently unknown how B19V overcomes the airway epithelium barrier to eventually reach bone marrow for infection. The virus can also be transmitted via blood or pooled-blood products, from a pregnant mother to her fetus, and possibly even from tattooing (
230). Higher seroprevalences than those among controls have been detected among patients receiving blood products and women having experienced abortions but not in people with tattoos (
218,
231,
232).
Droplet transmission was evident after intranasal inoculation of volunteers, as B19V was shown to be able to infect subjects and cause disease (
229,
233). Furthermore, during the prodrome, viral DNA can be detected in the upper airways (
229,
234–236). Detectable DNA also coincides with a transient high-titer viremia of >10
10 vgc/ml, which rapidly declines to a low level that can persist for many months or even years (
236–242). The viral load in the acute phase, however, does not correlate with disease severity (
229). In patients with different chronic pathological backgrounds, B19V DNA has also been detected at a low frequency in the lower respiratory tract (
243).
Due to the relative ease of spread of the virus, outbreaks of B19V-induced childhood rash (erythema infectiosum) are most common in schools and day care centers, affecting up to one-half of schoolchildren and one-fifth of susceptible staff (
244–246). B19V outbreaks occur mostly in the winter and spring, with major epidemics occurring every few years. The high-risk period for spread is early in the acute phase of infection, before rash or arthralgia appears, when the viral loads are at their highest. A convalescent child, even with recurring episodes of rash, is no longer infectious and may attend school. In patients with an underlying hemolytic disorder who suffer from B19V-induced aplastic crisis, titers as high as 10
14 vgc/ml can be observed (
146). In contrast to erythema infectiosum patients, these patients are at the time of disease extremely contagious, so to hinder nosocomial spread, aplastic crisis patients should be isolated. Among both hospital staff and patients, the risk of nosocomial spread of the disease, acquired from close contact or environmental surfaces, is quite high, with reported attack rates of 50% (
247–249). Control measures such as handwashing, closure of the ward, utilization of B19V-immune staff, and B19V education likely are crucial to contain transmission. To avoid contagion, standard and droplet precautions and isolation should be implemented (
250).
The timing of viremia before rash symptoms, high viral load, persistence, and resistance of this nonenveloped virus to most virus inactivation procedures used in the manufacturing of blood products create a risk of transmission through blood or blood products such as plasma, blood cells, and clotting factors (
251–264) as well as through bone marrow and solid-organ transplantations (
265–270). Comparisons of subjects with and those without blood transfusions have revealed a significantly higher seropositivity rate in individuals who have received blood transfusions (
218,
231). Even if symptomatic transfusion-transmitted B19V infections are generally rare (
259,
271), among eight patients with transfusion-transmitted B19V infection, five became ill with anemia, pure red blood cell aplasia (PRCA), or pancytopenia, all of whom had an underlying hematological disorder, whereas recipients without such disorders exhibited only moderate symptoms (
264). Among solid-organ transplant recipients, most seronegative pediatric kidney transplant recipients of B19V DNA-positive organs became infected within 1 month (with four exhibiting anemia) (
265). Patients at high risk of severe complications due to B19V infection from contaminated blood products are immunocompromised individuals (AIDS patients, patients with congenital immunodeficiencies, transplant recipients, and other immunosuppressed patients), individuals who are hematopoietically deficient, and pregnant women. In 2004, the U.S. Food and Drug Administration (FDA) implemented the regulation that B19V DNA levels in plasma pools used for manufacturing of blood products must not exceed 10
4 IU/ml (see
http://www.fda.gov/BiologicsBloodVaccines/GuidanceComplianceRegulatoryInformation/Guidances/default.htm ). Similar regulations apply in Europe (European Pharmacopoeia Commission, Council of Europe, European Directorate for the Quality of Medicines). This DNA limit is intended to ensure the safety of blood products (
259,
261). Most probably, the existence of neutralizing antibodies in the donor is protective for the recipient if the viral load is low and the recipient has no underlying hemolytic diseases (
238).
B19V can also be transmitted from an infected pregnant mother to the fetus. Although the normal outcome of intrauterine infection is delivery of a healthy baby, miscarriage and fetal death can also result if the mother is infected before her 20th week of pregnancy (
272–275) (see “Erythema infectiosum,” below). The rate of transmission from mother to fetus has been estimated to be 25 to 50%, and the incidence of fetal loss in B19V-infected mothers has been estimated to be 1.7 to 12.5% (
276–280). Seroprevalence has been shown to be higher in pregnant women who have experienced abortion than in those who have not (
232). The risk of B19V infection during pregnancy is greatest among susceptible day care workers and schoolteachers (
281), but as the risk is also high for pregnant women who are not in these professions, it has been debated whether a policy of excluding women from high-risk workplaces should be recommended (
246,
277,
281–284). The increased risks of B19V infection among day care employees compared with those of socioeconomically similar health care professionals was recently estimated by using proportional-hazard regression. The relative risks were estimated to be 2.63 (95% confidence interval [CI], 1.27 to 5.46) among all women and, eliminating the effect of a woman's own children, 5.59 (95% CI, 1.40 to 22.4) among nulliparous subjects (
281).
HBoV.
The other human-pathogenic parvovirus, HBoV1, is most likely also transmitted by the respiratory route; it causes respiratory illness, and it can be detected in very high loads in the airways during the acute phase, after which it may persist at low viral loads for months (
10,
285–292). The general lack of the other HBoVs, HBoV2 to -4, in airway samples and their presence in feces suggest that these viruses are transmitted by the fecal-oral route (
11–13,
293–301).
The HBoVs have not been shown to be transmitted by blood products or vertically (
302), nor have they been shown even to be present in blood products (
146,
303,
304), with the exception of one recent study from China (
305). The most common method of detection of bocavirus infections is PCR, by which HBoV1 has been found globally and throughout the year in about 2 to 20% of airway samples, mainly from children aged 6 months to 5 years with upper or lower respiratory tract illness (
306–310). In adults and the elderly, detection is infrequent (
310–314). In stool, the most prevalent bocavirus is HBoV2 followed by HBoV1, HBoV3, and HBoV4. Besides airway and stool samples, HBoVs have also been detected worldwide by PCR in serum, tonsils, saliva, urine, gut, and cerebrospinal fluid (CSF) (
28,
291,
315–327) as well as in river and sewage water (
328–331).
Like B19V, HBoVs also cause systemic infections leading to viremia and an immune response (
9,
28,
322,
323,
332–334). However, viremias seem to be more rare and short-lived and/or of lower titers in infections by the enteric HBoVs than in infections by HBoV1 (
334,
335). Likewise, the corresponding IgG responses are generally weaker and more prone to waning (
335,
336). In a follow-up study of children from birth to adolescence, the median age for HBoV1 infection was 2 years, whereas the median age for both HBoV2 and -3 infections was slightly lower (
335). HBoV1 is the most common HBoV in the population, with a seroprevalence of 80% in 6-year-olds, while the seroprevalences of the enteric HBoVs for the same age group are 50% for HBoV2 and 10% for HBoV3 (
335). HBoV4 is too rare to make any conclusions regarding transmission or seroprevalence. Due to serological cross-reactivity leading to overestimation and the immunological phenomenon of “original antigenic sin” (
337) leading to underestimation (see “HBoV Laboratory Diagnosis,” below), the true frequency of exposure to these closely related viruses is difficult to determine (
334,
335,
338).
In a study of saliva samples from 87 infants monitored from birth to 18 months, 76% had a primary HBoV1 infection. Based on the detection of single-nucleotide polymorphisms (SNPs), 12 of these infants had HBoV1 DNA demonstrating multiple variants over time, suggesting reinfection with different HBoV1 strains (
291). However, high mutation rates of a persistent virus or contamination from other infants was not excluded, and secondary infections were not confirmed by, e.g., detected increases in IgG levels (
336). Moreover, another follow-up study based on identical virus sequences suggested that prolonged DNA positivity could be the result of reactivation of a latent virus (
292); if reactivations were to occur, the rate of HBoV1 detection would be expected to be high in elderly individuals, which is not the case (
314).