Genotypes of erythrovirus B19, their geographical distribution & circulation in cases with various clinical manifestations

Introduction

Erythrovirus B19 (B19V) is a rather recently discovered virus, which has been associated with a spectrum of diseases, including erythema infectiosum in children (fifth disease), acute or chronic arthropathy in adults, transient aplastic crisis in patients with chronic haemolytic anaemia, persistent anaemia in immunodeficient/immunocompromised patients, and foetal hydrops in pregnant women1. Many genotypes of B19V (genotypes 1-3) are circulating; however, a comprehensive review on their geographic distribution and their association with different clinical manifestation is lacking. Therefore, the present review was written with a focus on geographic distribution and association of genotypes of B19V with different clinical manifestations.

Members of the family Parvoviridae are among the smallest known DNA-containing viruses that infect mammalian cells (Parvum ‘small’; Latin). The Parvoviridae family contains many viruses which are pathogenic to animals, and erythovirus B19 (B19V) is one of them. B19V is also one of the best-characterized members and is classified as a member of the Erythroparvovirus genus. Family Parvoviridae is divided into two subfamilies; Parvovirinae and Densovirinae. The Parvovirinae is further subdivided into eight genera, Protoparvovirus (previously known as Parvovirus), Dependoparvovirus, Erythroparvovirus, Bocaparvovirus, Amdoparvovirus, Aveparvovirus, Copiparvovirus and Tetraparvovirus. At least four different parvoviruses are known to infect humans: Parvovirus B19, human adeno-associated viruses (dependoparvoviruses), human bocaparvovirus (HBoV) and human Parv4 virus (a member of the newly created Tetraparvovirus genera)2.

B19V was discovered in 1975, in the serum samples of normal human individuals, during evaluation of assays for hepatitis B surface antigen using panels of serum samples3. Sample 19 in panel B (hence B19) gave a ‘false positive’ result in relatively insensitive counter immunoelectrophoresis assay. The precipitin line under electron microscopy showed 23 nm particles resembling parvoviruses3. Association of B19V infection with human diseases was first time made in 19814, and was subsequently identified in several experimental and seroepidemiologic studies56. B19V was identified as the causative agent of fifth disease (erythema infectiosum), common childhood exanthema and a polyarthralgia syndrome in adults78; transient aplastic crisis in patients with underlying haemolysis4; and spontaneous abortion9. Role of B19V in liver manifestations and hepatitis is also known10.

Although originally labelled as human parvovirus, the virus was officially recognized as a member of the family Parvoviridae in 1985, and the International Committee on Taxonomy of Viruses recommended the name B19V to avoid confusion with other viruses2. Parvovirus forms small icosahedral capsids of about 25 nm. The genome size of B19V is small, consisting of a single strand of DNA of approximately 5600 nucleotides, with identical 365 nucleotide long inverted terminal repeat sequences at each end11.

Genotypes of human erythrovirus

It was only about till three decades ago when researchers realized that genetic variations among parvoviruses exist. Since 2000, many B19V genotypic variants have been reported which vary extensively from the prototype B19V with respect to genomic sequence, exhibiting >13 per cent divergence versus the <2 per cent divergence in characteristic of previously characterized prototype B19V isolates12131415.

In the first effort of genotyping parvoviruses, the genome of 17 strains of the human parvovirus B19V was compared after restriction with eight endonucleases. All but four strains proved indistinguishable16. Following this study, another study from Japan used DNA fingerprinting to demonstrate that the strains of parvoviruses circulating during 1981 were different from the strains circulating during 1986-198717. Based on further studies and phylogenetic analysis, the B19V was classified into three distinct genotypes121314. Genotype 1 consists of all the prototype B19V isolates, A6, LaLi and their related isolates are included in genotype 2181920, and genotype 3 is composed of V9 and V9-related isolates2021.

Further, phylogenetic analyses revealed two subgroups within both genotypes 1 and 3. The analysis of 13 nearly full-length genotype 3 sequences from Ghana, Europe and Brazil identified two genetically distinct clusters, following which the classification of genotype 3 into two subtypes (3a and 3b) was made22. Rate of evolutionary change in strains of B19V genotype 3 (2 × 10⁻⁴ nucleotide substitutions per site per year) was similar to that of other B19V genotypes. The estimated divergence time between 3a and 3b was 525 years. Subtype 3a was predominant in Ghana22.

B19V genomes from Vietnam showed two major subgroups within genotype 1 (1A and 1B) with an estimated nucleotide difference of >5 per cent between each subgroup. The mean percentage of amino acid variation in NS1, VP1 and VP2 proteins, between both subgroups was >2 per cent23.

Reported nucleotide and amino acid changes among various genotypes are summarized in Table I. The most striking variation was observed within the promoter area (~20%). Within the NS1 gene, sequence divergence between genotypes 1, 2 and 3 was about 13 per cent at the nucleotide level. The two identical terminal repeats (ITRs) of approximately 365 nucleotides seen in B19V genotype 1 genome are imperfect palindromes and form hairpin loops. The terminal repeats of genotypes 2 and 3 have not been yet cloned and sequenced152425. The sequence of a human erythrovirus, termed V9, was markedly distinct (>11% nucleotide divergence) from that of B19V27. One V9-related strain (D91.1) with 5.3 per cent divergence from V9 and 13.8-14.2 per cent divergence to prototype B19V sequences was reported26. A6, a new atypical parvovirus sequence, exhibited 88 per cent similarity to prototype B19V and 92 per cent similarity to V9, compared to >98 per cent similarity between earlier reported B19V strains26. K71, a new B19V genotype, is carried in human skin and differs from prototype B19V by 10.8 and 8.6 per cent within protein-coding regions and non-coding region (covering nucleotides 189-435 of the promoter region) respectively, while divergence from V9 variant was 26.5 and 17.2 per cent within protein-coding regions and non-coding region (covering nucleotides 189-435 of the promoter region), respectively12. The amino acid sequence of A6 and V9 variants in NS1 protein diverges from that of the B19 V prototype-encoded counterpart by 6.2 and 6.1 per cent, respectively2425. Within the open reading frame encoding the VP1/2 proteins, genotypes 2 and 3 differ from the prototype B19V by 9 and 12 per cent, respectively, at the nucleotide level. However, at the amino acid level the difference is much less, 1.1 and 1.4 per cent. Genotypes 2 and 3 differ from genotype 1 by 4.4 and 6.6 per cent, respectively in the VP1 unique region (uVP1). uVP1 gene containing the reported phospholipase 2 activities (amino acids 130-195) is highly conserved, and variation is mostly seen in the N termini2425. The capsid protein sequence is conserved between the different genotypes, in spite of differences in the DNA sequences, as there is enough evidence in serologic and cross-neutralization reactions282930.

Table I Nucleotide and amino acid variance among different genotypes of B19V

Persistence of B19V in human tissues and distribution of B19V genotypes

Several studies have suggested that after primary infection, in both symptomatic and asymptomatic subjects313233 and in both immunocompromised and immunocompetent hosts34 the erythroviral genomic DNA is detectable in tissues. The genotype 1 replicates restrictively in the erythroid progenitors of human bone marrow producing high viral load viraemia1129. In contrast, high virus-load viraemia of genotypes 2 and 3 has been identified only occasionally13143035. Manning et al34 discussed the persistence of B19 in human tissues in immunocompetent hosts. A study by Norja et al36 done on a large number of human tissues including synovial, skin, tonsil and liver tissues and human serum samples concluded, “erythrovirus genome persistence in human tissues is ubiquitous and lifelong and represents an entity, named the Bioportfolio, which indicates that the newly discovered virus type 2 was actually ‘older’ in occurrence in Central and Northern Europe than the virus prototype and that the type 3 never attained wide circulation in the area during the 70 years observation period from the 1930s to the present day”. However, in north India, genotype 3 has been detected in cases of cardiomyopathy37 and solid tumors38. This analysis signifies that the distribution of genotypes is geographically restricted and distribution may change with time. Manning et al34 also maintained that B19V genotype 2 was more commonly seen in older study subjects, while B19V genotype 1 was commonly seen in younger subjects. Studies published from Germany, Italy and Finland, after 2007 demonstrated presence of all the three genotypes 1, 2 and 3, both in patients and asymptomatic controls. Genotype 2 was also demonstrated in blood and tissue biopsies3940414243. One study from South Africa detected genotype 2 from serum of a child providing evidence for its circulation44.

Only a single serotype of B19V is suggested as seen by 100 per cent cross-reactivity of antibodies among these three genotypes45.

Geographical distribution of circulating genotypes in various countries

Many studies have reported genotypes of B19V in human infections. Table II summarizes the studies with genotypic details of B19V. Studies are available from Europe, Asia, South America and Africa. Very limited or no data are available from North America and Australia. Table III shows the number and frequencies of total strains which have been genotyped from each geographical area. Only those continents from where more than three studies were available, are included in analysis.

Table II Prevalence of B19V genotypes as reported by different studies

Table III Frequencies of human erythrovirus B19 genotypes in different continents

Erythrovirus B19 genotypes and their associations with diseases

As shown in Table IV, prevalence of each genotype among various clinical groups varied significantly. Since genotype 1 was the most commonly reported genotype, its predominance in most of the clinical situations was also noticed except in individuals with cardiac manifestations, where genotype 2 was predominantly reported (Table IV). The site of virus detection (various tissues or blood) varied from study to study. All the three genotypes have been detected in different proportions from cases presenting with different clinical manifestations, for example, anaemia, aplastic crisis, erythema infectiosum, arthropathy and cardiomyopathy. All the three genotypes have been found in symptomatic as well as normal individuals (Table IV).

Table IV Frequency of human erythrovirus B19 genotypes in various clinical conditions

Conclusion

B19V is classified into three distinct genotypes 1, 2 and 3, differing from each other by 2-13 per cent. The classification of genotype 3 strains into two subtypes (3a and 3b) and genotype 1 into two subtypes (1A and 1B) has been made. Predominance of genotype 1 across all the continents is seen followed by genotypes 2 and 3. There are no disease-specific genotypes and association of genotypes with clinical manifestations has not yet been established. All the three genotypes have been found in symptomatic as well as asymptomatic individuals and have been reported from several countries across the world. The prevalence of genotype 2 in older populations and its absence from the current circulation in Northern Europe has been reported.