Diagnosis of Parvoviruses B19 Infection
The differential diagnosis of erythema infectiosum includes all diseases where a maculopapular rash may be present eg. rubella, enteroviruses, arboviruses, streptococcal infection, allergy. Rubella causes the greatest problems as the two viruses may circulate together. Therefore a definitive diagnosis can only be made by serological tests. Aplastic crisis in a patient may be diagnosed by finding a reduction in Hb of >= 2g and a reticulocyte count of less than 0.2%. Although B19 infection is the commonest cause of aplastic crisis, bacterial infections ( eg. pneumococcal septicaemia ) or marrow suppressive drugs such as chloramphenicol can cause the same condition.
Detection of Virus;- The diagnosis of aplastic crisis may be made early in the disease by the detection of the virus. This may be accomplished simply and rapidly by countercurrent immunoelectrophoresis of the patient's serum against parvovirus Ab. A +ve result can be confirmed by direct or immune EM. The virus will be detected in 30% of the cases where a specimen is taken within 24 hrs of the onset of symptoms. A more sensitive method such as RIA or DNA-DNA hybridization, or PCR may increase this detection rate to more than 60%. The diagnosis of B19 in a fetus depends on the detection of the virus. As maternal infection will have occurred some weeks earlier and both maternal and fetal sera will probably be B19 IgM -ve. ( In contrast to rubella where the infection would have occurred months earlier so that maternal IgM is no longer present and so that the presence of IgM in the fetus would be diagnostic of congenital infection ). However there is often a persistent viraemia in the fetus. Thus the diagnosis can be made by the detection of the virus in fetal blood samples or autopsy material where DNA-DNA hybridization and PCR can be carried out.
Antibody Detection;- Counter-immunoelectrophoresis
using B19 virus as antigen, detects antibody of all classes.
Class specific Ab may be detected by ELISA or RIA. Current tests
employ the principle of "antibody capture" whereby the
class specific patient antibody is bound to a solid phase coated
with antibody to IgM or IgG. Diagnosis of B19 infection can be
made by the detection of either (i) B19-specific IgM or (ii) a
rising titre of B19 specific IgG. B19 specific IgM may be
detected in such tests up to 3 months after the onset of
symptoms. B19 IgG lasts longer but this antibody is not
detectable lifelong following infection. therefore caution must
be exercised in interpreting the results of such tests in a
screening situation ; patients with no detectable IgG may
nevertheless have experienced previous B19 infection.
F. Treatment and Prevention
The only specific treatment for B19 infection is the administration of HNIG in cases of persistent B19 infection in the immunocompromised. Controlled trials have not been done but these cases are rare and HNIG is worth giving where there is persistent viraemia. Symptomatic therapy for erythema infectiosum is rarely necessary. Cases of aplastic crisis require transfusion of erythocytes until a satisfactory Hb level is obtained. Consideration should be made to giving HNIG to susceptible patients with chronic haemolytic anaemias who requires short-term protection e.g. if they are in the same ward as a patient having a aplastic crisis. In any case they should be isolated from patients with B19 induced aplastic crisis.
If B19 infection occurs during pregnancy, there is no cause for alarm. The pregnancy should be allowed to proceed and carefully monitored. At delivery examination of the cord blood for B19 IgM will reveal whether the virus has crossed the placenta and infected the virus. The child should be carefully followed up for several to look out for any delayed sequelae.