Hepatitis C virus (HCV) is a member of the Flaviviridae family, genus Hepacivirus. HCV was discovered in 1989 by Choo et al. by screening the blood of patients suffering from viral hepatitis of unknown ethiology [Choo et al., 1989]. Virus RNA encodes for a polyprotein of approx. 3000 amino acids, which is processed into 3 structural (core protein (C) and two envelope proteins (E1 and E2)) and 7 nonstructural (NS) proteins (p7, NS2, NS3, NS4a, NS4b, NS5a and NS5b).
Incubation period of hepatitis C is 14-180 (average:45) days. After this period, symptoms could arise. They include fever, fatigue, loss of appetite, nausea, vomiting, abdominal pain, joint pain, jaundice. Yet, most (70-80%) people infected with HCV are asymptomatic.
Roughly 20% of infected patients clear the virus spontaneously; the rest develop chronic infection. HCV is a leading cause of chronic hepatitis, which progresses into cirrhosis in 5-20% of cases over a period of 20-30 years.
HCV is spread mainly through blood-to-blood contact. Therefore, in developed countries virus infects primarily persons who have injected illicit drugs and recipients of blood transfusions before introduction of regular blood screening for HCV. In developing countries many HCV infections occur in the health-care institutions as a result of unhygienic injections and various surgical manipulations such as tattooing or circumcision [Unitaid, 2015]. Of other routes of transmission, the most important are sexual and vertical, from mother to fetus. Sexual transmission is regarded as a minor risk factor. Virus transmission from HCV-infected mother to unborn child is possible, with rates of transmission of around several percent.
HCV is divided into six major genotypes that can be further divided into several subtypes from A to L. The amino acid sequences of the major HCV genotypes differ approximately 30% from each other. The genotypes 1, 2 and 3 are found throughout the world whereas the distribution of the other genotypes is much more restricted. The immunity after cleared infection does not result in reliable protection against reinfections.
The overall worldwide prevalence of HCV is approximately 3%. The highest HCV prevalence figures (up to 10–20%) are found in Egypt. The prevalence of HCV infection varies remarkably and, for instance, in different European countries it ranges from 0,1% to 4%.
Adaptive immune responses are typically delayed during acute HCV infection. HCV RNA can be detected 1–3 weeks following infection, but neither HCV-specific T-cells nor HCV-specific antibodies are observed until 1–2 months after infection. The titre of IgG antibodies during the acute phase is relatively low in comparison with other virus infections in the majority of patients, gradually increasing during transformation to chronicity. In patients with resolved infection the titers of IgG after cure are low and often not detectable.
The IgM response in acute HCV infection also does not follow the classical pattern when IgM antibodies precede IgG response. Firstly, it was shown that HCV-specific IgM is more readily detected in chronically than in acutely infected patients (80% and 50% respectively); besides, the IgM titers under chronic infections are higher. Secondly, HCV-specific IgM and IgG are both almost simultaneously detected in acute infection. In individuals recovered from the infection no anti-HCV IgM antibodies are detectable.
A number of diagnostically relevant antigenic epitopes have been found within the C region, E2, NS3, NS4A/B and NS5 proteins, while E1, NS2 and NS5B are less immunogenic. In one study on chronic HCV patients, the following data on prevalence of antibodies were obtained: E2 - 98%, core-97%, NS3-88%, NS5A-68%, and NS4-48% [Chen et al., 1999]. These data were similar to those observed by other investigators [Nakatsuji et al., 1992; Kuo et al., 1989; Sällberg et al., 1992; Lok et al., 1993; Chen et al., 1999]. Antibody titers were highest for core protein while titers for other proteins were considerably lower.
Antibody response against different HCV proteins is temporarily regulated. After infection, relatively early in the acute phase anticore antibodies are produced whereas significant levels of anti-E2 and anti-NS antibodies are detected only during the chronic phase. In recovering patients, anti-core antibodies persist longer than anti-NS antibodies, which often disappear.
Detection of HCV is performed mostly by determination of virus RNA or HCV elicited antibodies in blood. Testing for HCV includes primary screening, confirmatory testing, genotyping, fibrosis staging, determination of prognostic markers, treatment monitoring. Serological testing is useful in following main situations:
• For screening of groups at risk: persons who inject drugs; recipients of blood transfusions or donated organs before the introduction of systematic screening for HCV; pregnant women and children born to infected mothers; HIV-infected persons; patients with symptoms of liver disease; regular sexual partners of someone with HCV-by IgG.
• For determination of prognostic markers-by IgM
• For monitoring of treatment-by IgM [Unitaid, 2015].
Third generation IgG ELISA, which utilizes recombinant epitopes from HCV core, NS3, NS4, and NS5A proteins as antigens, is now a dominant screening method, and the specificity and sensitivity of ELISA assays is greater than 99%. False-negatives are more likely to occur in the presence of HCV-HIV coinfection; therefore, for persons who are immunocompromised, alternative method testing for HCV RNA can be considered. False-positive results are possible in the case of previous successfully resolved infection. For this reason, all persons with positive results from anti-HCV screening assays should have a confirmatory HCV test in order to determine whether positive serology tests are due to the presence of active infection.
As high levels of HCV RNA in serum correlate with anti-HCV IgM antibodies, the level of IgM is a marker of active viremia. Since a significant relationship was found between a sustained treatment response and lower viremic levels before therapy, the level of IgM before therapy could be used as a valuable prognostic marker, minimizing the need for HCV RNA tests. In another study it was demonstrated that in the cohort of patients recovered from HCV, IgM was absent at 6th month after therapy withdrawal, while in the group in which the treatment failed, IgM was still detectable. Hence, these data demonstrate the possible use of IgM ELISA also for monitoring of treatment success.