HCV is spread primarily by contact with blood and blood products. Blood transfusions and the use of shared, unsterilized, or incompletely sterilized needles and syringes have been the most important causes of the spread of HCV in the United States. With the introduction of routine blood screening for antibody to HCV in 1990 and improvements in these tests in 1992, transfusion-related hepatitis C has virtually disappeared. At present, injection drug use is the most common risk factor for contracting the disease. However, many patients acquire hepatitis C without any known episode of exposure to blood or to drug use.
The major high-risk groups for hepatitis C are the following:
- People who had blood transfusions before 1992 and the availability of highly accurate anti-HCV screening.
- Patients who have frequent exposure to blood products, such as patients with chronic renal failure, hemophilia, or malignancies requiring chemotherapy.
- Health care workers who suffer needle-stick accidents.
- Injection drug users, including those who used drugs for a brief period only, long in the past.
- Persons with high-risk sexual behavior, multiple partners, and sexually transmitted diseases.
- Persons who use cocaine, particularly with intranasal administration, using shared equipment.
Sexual Transmission
Sexual transmission of hepatitis C between monogamous partners appears to be uncommon. Whether hepatitis C is spread by sexual contact is not conclusively proven and studies have been contradictory. Less than 5 percent of spouses of patients with chronic hepatitis C become infected. The risk of infection appears to correlate with the length of marriage and viral titers in the affected spouse; however, the studies reporting spouse transmission were confounded by other risk factors, such as other shared known sources of exposure, and came from areas of the world where hepatitis C is common in the general population.
Maternal-Infant Transmission
Maternal-infant transmission is also uncommon. In most studies, only 5 percent of infants born to infected women become infected. The disease in the newborn is usually mild and subclinical. The risk of maternal-infant spread correlates best with the viral titer in the mother. Breast-feeding has not been linked to HCV's spread.
Sporadic Transmission
Sporadic transmission, when the source of infection cannot be identified, occurs in about 10 percent of cases of acute hepatitis C and in 30 percent of cases of chronic hepatitis C. These cases are also referred to as sporadic or community-acquired infections. The sources are probably inapparent parenteral exposures, such as cuts, wounds, or medical injections. Some may be due to sexual transmissions.
 The Hepatitis C Virus
HCV is a small (50 nm in diameter), enveloped, single-stranded RNA virus of the family Flaviviridae. The genome of the virus mutates rapidly, and the resultant changes in the envelope protein may explain how it escapes clearance by the immune system. There are at least 6 major genotypes and more than 30 subtypes of HCV. The different genotypes have different geographic distributions. Genotypes 1a and 1b are the most common in the United States. Genotypes 2 and 3 are present in only 10 to 20 percent of patients. Overall there is little difference in the severity of disease or outcome of patients infected with different genotypes. However, patients with genotypes 2 and 3 are more likely to respond to alpha interferon treatment.
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