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Dr Richard Hunt |
HUMAN PARVOVIRUSES |
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Parvoviruses are small, naked icosahedral DNA viruses. Human parvovirus causes a relatively mild disease (human parvovirus disease, erythrema infectiosum or fifth disease) which is associated with a mild rash in many cases. In adults arthralgia may occur. Infection frequently occurs in childhood and confers lifelong immunity. However, in people with chronic anemia this virus can be a problem because it replicates in red blood cell precursors and temporarily interferes with red blood cell production. This can be serious in a person with short lived red blood cells - one can get a temporary aplastic crisis. The virus can also cause problems in patients with immune deficiencies, and on rare occasions it can cause problems for the fetus during pregnancy. Humans are infected by two types of parvovirus
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VIRUS Parvoviruses are the smallest (18 to 26nm) of the DNA viruses and have a single strand, negative or positive sense DNA genome of about 5500 nucleotides with terminal repeats. Each virus particle contains a single strand of DNA which can be of either sense. The genome codes for three structural proteins and one large non-structural protein plus several smaller proteins. There is one promotor in the DNA and thus one primary transcript. This is is spliced by host cell enzymes to form mRNAs for the various proteins. There is no membrane. The round nucleocapsids have icosahedral symmetry with 60 capsomeres per virion. There are surface projections with distinct spikes. Parvoviruses usually need conditions typical of late mitosis for replication that are found in rapidly proliferating cells, hence their infection of red blood cell precursors. In volunteers infected by the virus, there is a drop in blood hemoglobin because of this. In laboratory experiments, parvoviruses have been found to inhibit erythroid colony formation. FIFTH DISEASE (ERYTHEMA INFECTIOSUM) This is a disease usually seen in children and is usually mild. There may be a feeling of malaise, headache and mild fever or the child may appear to have a cold. A few days later a rash breaks out but this resolves in a week or so. This rash, which sometimes itches, may be on the face (slapped cheek rash) or on the trunk and limbs (lace-like rash). Often the face rash spreads to the rest of the body (upper and lower extremities). The rash may reappear under certain conditions such as temperature extremes and bright light. Adults who have not acquired immunity from an infection earlier in life can get fifth disease. In adults there may be the typical rash or swollen joints (arthropathy) or both. The joints that are most often affected are knees, hands and wrists. This can last two weeks or even a few months. Fifth disease can be spread from person to person during the infectious period which occurs before the rash develops. Once the rash has appeared, the patient is usually no longer infectious. Since an early symptom of infection is "cold-like" symptoms, spread is via nasal ands oral secretions. The incubation time for fifth disease is from one to three weeks but is usually about two weeks. One infection usually leads to life-long immunity. Epidemiology In the United Kingdom, 61% of adult have been found to be seropositive for parvovirus Most hemophiliacs are seropositive because of viral contamination of clotting factor. Fifth disease occurs in all months of the year but is most common in spring and winter. The risk of secondary infection is around 50%. Often outbreaks of fifth disease and aplastic anemia occur simultaneously. The disease is caused by an infection with the parvovirus B19. In many cases (about 20%) no symptoms of the infection are apparent. Infection results in long term immunity. Diagnosis The typical rash is the most common means of diagnosis but antibody test are also available. The presence of IgM indicates a recent infection Treatment Normally only the mild symptoms require treatment (e.g. aspirin). In severe anemia, blood transfusion may be required. Immunocompromised patients who cannot control the virus may need immune globulin treatment Complications The disease is usually mild with no long term complications but in certain persons serious problems may develop. People with chronic anemia or sickle cell disease can develop a severe anemia (which often occurs without the usual rash). This resolves as the immune system controls the virus. People who are immunocompromised may suffer from a chronic anemia as a result of B19 infection. In certain patients a very severe disease may result (such as in AIDS patients, transplant recipients etc). Arthritis in adults may also be associated with parvovirus infections. |
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Pregnancy If a pregnant women is exposed to B19, there are usually no complications as most expectant mothers are immune as a result of a B19 virus infection in childhood. Indeed, even if the mother has no acquired immunity, problems usually do not arise for either mother or child, even though the virus can cross the placenta. In under 5% of infected mothers (infected in the first half of pregnancy), however, major complications occur. These result in sever anemia in the fetus and possible miscarriage. Aborted fetuses are often hydropic with liver damage and iron accumulation. The liver contains more erythroid precursor cells than usual and thse have inclusion bodies in the nucleus that are eosinophilic. The fetus dies of anemia and heart failure. Unlike a rubella infection, no neurological symptoms result from B19 infection during pregnancy.
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