Based on the crude frequencies with which risk factors were reported, injection drug use was the risk factor most commonly reported by hepatitis C/NANB patients (Table 3). Many of these persons also reported more than one potential source of infection. Of those reporting contact with another person with hepatitis C/NANB, 25% also reported injection drug use and 5% reported employment in a medical or a dental field. Of those reporting multiple sex partners, 35% also reported injection drug use.

The behaviors commonly associated with hepatitis A that were reported by persons with hepatitis C/NANB to have occurred within 6 weeks of illness are generally not applicable to the transmission of hepatitis C/NANB (Table 3). Since transmission of NANB hepatitis by the fecal-oral route has not been demonstrated in this country, reporting an association with a foodborne or a waterborne outbreak represents misclassification of the source.

As with hepatitis B, potential exposures associated with dental work, surgery, acupuncture, tattooing, and other percutaneous procedures are not judged to be probable sources of sporadic infection.⁽¹²⁾ Hepatitis C/NANB patients with no known source of infection reported these exposures at rates no different from those of the general population.

Based on assignment to mutually exclusive categories, persons with hepatitis C/NANB reported injection drug use most frequently, accounting for 23% of cases during 1993 (Table 6). Blood transfusion accounted for 2% of cases, declining from 6% in 1990; contact with another infected person accounted for 5%, and health-care employment for 4%. Of those patients reporting health-care employment, the percentage reporting frequent (several times weekly) blood contact dropped over 1990 to 1993. Fifty-seven percent of patients employed in health-care reported frequent blood contact in 1990. By 1993, the percentage dropped to 17%. Patients classified as having multiple (2 or more) sex partners as their most likely source of infection accounted for 7% of the patients with hepatitis C/NANB; in a case-control study, this risk factor was associated with acquiring disease.⁽¹²⁾ Overall, 58% of persons reported no known source for their infection. This percentage varied by age, with > 70% of persons younger than 15 years old or 40 years old and older reporting no known source for their infection, compared with 50% for persons 15-39 years of age. Among persons less than 15, 13.6% had a history of blood transfusion.

Among persons 15 to 39 years of age, injection drug use was reported by 28% of all cases during 1993, unchanged from 1992 (Table 6). Ten percent reported multiple sex partners, 7% reported contact with another infected person, 4% reported health-care employment, and 1% reported blood transfusions. Of reported contacts with another infected person, an average of 59% were sexual contacts, 16% were household nonsexual contacts, and 25% were other (unspecified) types of contact. In prior years, persons 40 years old and older reported a history of blood transfusion most frequently among their risk factors (in 1990, 16%), but this percentage declined substantially to 4% by 1993. Injection drug use is now the most frequent risk factor for this age-group (Table 6).

Because total numbers of cases of hepatitis C/NANB have declined, trends in the distribution of risk factors are more accurately reflected by trends in the absolute numbers of cases attributed to each factor. In the core states, hepatitis C/NANB cases attributable to drug use have declined rapidly since 1988, showing a more than 62% decrease (Figure 5). A similar decrease of over 50% was seen in the Sentinel Counties Study.⁽¹⁴⁾

The numbers of hepatitis C/NANB cases attributable to blood transfusions have decreased even more dramatically, dropping by 94% from 1985 to 1993. The significant decline in transfusion-associated cases, which began in the mid-1980s, resulted from a series of events: changes in the blood donor population caused by self-exclusion of high-risk donors, as part of efforts to prevent HIV infection;^(15,16) the introduction of screening blood donors for alanine aminotranferase and anti-HBc as surrogate markers for hepatitis C/NANB in 1986 and 1987; and use of first- and second-generation anti-HCV markers for screening donors in 1990 to the present.

Jaundice was reported as a clinical symptom in 67% of reported hepatitis C/NANB patients in 1993 (Table 6). Hospitalization and case-fatality rates were higher in hepatitis C/NANB patients than in patients with hepatitis A or B. Those 40 years old and older experienced the highest rates.

The majority of NANB hepatitis cases in this country are caused by the hepatitis C virus;⁽¹⁴⁾ the remainder are probably due mostly to other bloodborne hepatitis agents. Outbreaks of hepatitis E, an enterically transmitted form of hepatitis NANB, have been reported in rural Mexican villages (17), as well as in Asia and North and West Africa,⁽¹⁸⁾ but no outbreaks have been reported in this country.⁽¹⁹⁾ In the United States and other countries where hepatitis E outbreaks have not been documented to occur, rare hepatitis E cases have been reported, primarily among travelers returning from HEVendemic regions.⁽²⁰⁾ No secondary transmission to family members or other persons in association with these cases has been reported. In the United States, hepatitis E cases have been reported with no history of travel to HEVendemic areas; however, the mode of HEV transmission for these cases has not been determined.