Blood-Borne Viruses: Hepatitis A to G

TRANSFUSION SAFETY

The safety of blood transfusions depends on the incidence of certain viruses in the general population and also in the blood donors who are part of this population and their prevalence, in other words, how many donors and recipients have been infected with a virus and, as often happens, have developed immunity. For immune donors, fresh exposure to a virus is of no importance. An example of this is hepatitis B virus (HBV), in which, if anti-HB levels are high enough, fresh exposure cannot give rise to an infection severe enough to produce renewed clinical symptoms.

In addition, safety also depends on the length of the viremic phase after infection. In the case of hepatitis A virus (HAV), the viremic phase normally does not last longer than 2 weeks. Therefore, the chance of a donation of blood being made during the viremic phase is small.[1] However, if the viremic phase lasts for several months or if the chronic phase lasts years, as can happen with hepatitis C virus (HCV),[2] then there is a good chance that the pathogen can be transmitted via preserved blood or its products.

HAV is a typical example of a water-borne pathogen. If measures for handling sewage and drinking water are beyond reproach, exposure cannot take place and the virus will not be transmitted from person to person. Therefore, there is no risk of transmission associated with donating blood. However, if someone has visited a foreign country where the water treatment is less than satisfactory, the virus can be brought back home and transmitted to a very limited number of donors by transfusion. However, if the viremic plasma donation is processed to obtain a blood product without any adequate inactivation or removal procedures being carried out, then a large number of recipients can become infected, depending on the extent of distribution of the product.[3] Where the standard of water treatment is high and a self-limiting disease such as HAV is involved, the endemic disease is not propagated any further, except possibly for a few persons who have come into direct contact with the infected subject.

HCV is rarely transmitted by sexual contact[4]; therefore, human immunodeficiency virus (HIV) should be regarded as an example. Although HIV is transmitted via blood, the driving force for its spread is sexual contact. If a potential donor is infected with HIV and then gives blood, there is a 100% chance that the recipient will become infected with HIV. HIV is always a chronic infection, and during every phase of the infection or disease, there are at least intracellular virus particles circulating in the blood. If the recipient is not told about the infection and has a number of sexual partners, then after a period of time he or she will have infected them and the mature endemic disease will reach epidemic proportions. From the pool of HIV-infected sexual partners, and this can include the third generation, for example, an individual can donate blood again and the cycle of infection begins again.[5] Subsequently, no immunity can be developed against HIV, and this virus can no longer be eliminated from the general and donor population.

Consequently, with regard to safety, it is always advantageous as far as blood donation is concerned, if an attempt is made to keep the incidence of an infectious pathogen low in the population.[6] [7]

Donor Selection and Testing

Based on hematological and health grounds, persons are only considered as blood donors if they are over 15 years of age and below 65. As far as infections are concerned, particular attention is given to excluding the infectious diseases that are transmitted in childhood but not those that are mainly transmitted by sexual contact or close contact between adults. However, the only relevant pathogens are those that exhibit a viremic phase during their afebrile period (see later). In Germany, donating blood is a voluntary activity in which both men and women participate. Monitoring markers of HBV and HCV in blood has shown that women are less frequently exposed and infected. This is insufficient reason for preferring women over men as far as blood donations are concerned.

In order to exclude the particular danger of infection from tropical diseases there is a quarantine period of 12 months for those who have been in the tropics. This measure may be the reason why hepatitis E virus (HEV) infections have not yet been transmitted via blood. Other important pathogens that ought to be excluded by the ``tropical block'' include the plasmodial and trypanosomal diseases, malaria and sleeping sickness, and Chagas disease, as well as microfilarial transmissions and other viruses, particularly those transmitted by mosquitoes such as yellow fever or dengue fever.[8]

When selecting blood donors, histories are taken of the infectious diseases that the candidates have had in the past and their temperature is also taken to exclude acute infection, which results in an elevated temperature. Monitoring surrogate markers and specific tests for known viruses are also included in the selection procedure.

SURROGATE MARKERS In Germany, the measurement of alanine transferase (ALT) (glutamate pyruvate transaminase [GPT]) was introduced in the 1960s and is still performed. The measurement of ALT was suitable for identifying acute, and sometimes chronic, infections with hepatitis viruses because most forms of hepatitis are characterized by a reduction in liver cells and an associated increase in transaminases during the acute phase.9 However, some forms of hepatitis are anicteric, and there may be only small changes in hepatocytes so that many hepatitis infections cannot be detected by an increase in ALT. Thus, ALT measurement can be regarded as a supplementary test that is of limited significance as far as individual donors are concerned. In the United States, testing for anti-hepatitis B core (HBc) has been introduced as a surrogate marker for HBV and HCV. Because of the low sensitivity for HBV and HCV, which can be improved on by other tests, anti-HBc testing has not been generally introduced. In Austria, an attempt has been made to identify acute infections earlier by measuring neopterin instead of using specific tests. This procedure has not met with any success as far as blood donors are concerned.

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