HCV is transmitted by parenteral contact with blood or blood products. Recognized routes of infection include transfusion of blood or blood products, injection drug use, needlestick or other forms of contaminated injury among healthcare workers, maternal-infant transmission and sexual spread. In most developed countries post-transfusion HCV has been virtually eliminated by screening of donated blood, but chronic infection remains prevalent as demonstrated in studies of volunteer blood donors.
Injection drug use appears to be the most common remaining risk factor for HCV infection.
Maternal-infant transmission may occur if a mother is sero-positive for HCV RNA. This occurs in approximately 5% of infants, although the risk appears to be increased by the presence of co-infection with the human immunodeficiecy virus (HIV) in the mother, presumably because the serum levels of HCV RNA are higher in mothers with HIV co-infection.
Although occupational exposure to HCV may occur, the risk of acquiring HCV though needlestick injury appears very slight. Sexual transmission of HCV has been well documented to occur but the exact frequency of this occurrence is debated. Studies from the Centers for Disease Control and Prevention (CDC) have shown an increased risk of HCV infection in those individuals with multiple sexual partners or partners known to have HCV infection.
However, studies of stable monogamous couples have shown very little evidence of sexual spread of HCV. Furthermore, the risk of HCV infection among men having sex with men is only about 5%, lower than what might be expected if HCV was readily spread by sexual contact.
Recent reports from the CDC have shown a dramatic decrease in the number of new cases of HCV infection in the USA from nearly 300 000 in the early 1990s to only 25 000 in 2001. Most of this decrease seems to have occurred among injection drug users and can be attributed to various public health measures.
Although the exact incidence and prevalence of HCV infection are not known in the USA accurate estimates of these figures have been possible by the use of sampling methods. Information on HCV incidence is derived from the Sentinel Counties Study while information on the prevalence of HCV is derived from the National Health and Nutrition Evaluation Survey (N-HANES) which is based on a random sample of the adult, non-institutionalized population of the USA.
Based on these studies, it is estimated that 1.8% of the population is seropositive for anti-HCV, of whom about 75% have HCV RNA detectable in serum. Thus there are approximately 2.7 million HCV-infected individuals in the USA, a figure that is perhaps underestimated because individuals in institutions, such as prisons and nursing homes, were not sampled.
Given this frequency of HCV in the USA, the impact of this condition is not surprising. In addition to 8000-10 000 deaths each year attributed to HCV, it leads to more than one-third of liver transplants and is associated with expenditures of more than $1 billion annually.