Compared to results from a decade and a half earlier, this study provides additional evidence that anti-HCV prevalence is declining among U.S. blood donors, most likely due to culling of seropositives as well as a birth cohort effect. In addition, we describe new associations of anti-HCV prevalence with increasing number of pregnancies among women, and with lower BMI among all donors. Finally, HCV RNA negative status was less likely to be observed among African-American and more likely to be observed among better educated and obese donors.
The decline in HCV prevalence in the current analysis compared to a similar study in 1992-1993[4
] is consistent with that seen in other studies of blood donors in the U.S. and Canada[5
]. Over this time, there has been little change in donor eligibility criteria, if any. We and others have previously proposed a birth cohort effect for HCV infection among North American blood donors as explaining declining prevalence, namely donors in the birth cohort at highest risk for HCV infection (those born from the late 1940's through the early 1960's) with the highest lifetime prevalence of injection drug use[10
] are progressively less represented among active blood donors[4
]. The current data showing lower age-specific prevalence in all age groups together with a shift in the maximum prevalence to a decade older donors compared with the 1992-1993 data supports the birth cohort hypothesis, as well as culling of seropositive donors since the introduction of HCV testing in the early 1990's. Similar indications of a birth cohort effect may be seen in U.S. and Japanese general population data[3
]. Confirmation of a birth cohort effect is important because it implies that HCV prevalence in the U.S. will likely continue to decline over upcoming decades. The generation born in the late 1940's through early 1960's will likely manifest the peak incidence of HCV disease outcomes of cirrhosis and hepatocellular carcinoma, with declining rates in subsequent generations.
Our finding of an association of HCV prevalence with increasing gravidity is novel and of potential public health concern if substantiated by other studies. In favor of the validity of this association is the apparent dose-effect with increasing number of pregnancies, and its persistence despite adjustment for potential confounding by age, previous transfusion history, race/ethnicity and education. In light of the weight of evidence against sexual intercourse as a major transmission route of HCV, we do not believe the pregnancy association is a surrogate for sexual transmission[13
]. Antenatal care and childbirth are accompanied by medical injections, instrumentation and a high proportion of caesarian sections that could be opportunities for iatrogenic transmission of HCV. Women in our study could also have been transfused during operations or anesthesia without remembering the event. The hypothesis of iatrogenic exposure is supported by the findings of a case control study of HCV infection among pregnant women in Karachi Pakistan, which found adjusted odds ratios of 1.25 for 3-4 pregnancies and 1.99 for 5 or more pregnancies. Those authors concluded that iatrogenic exposures during delivery could be the source of infection[14
We could find no published U.S. studies to suggest iatrogenic transmission in the obstetric setting, although a large outbreak of HCV infection has recently been attributed to unsafe injection practices at a Las Vegas endoscopy clinic[15
]. A review article counted 33 outbreaks in nonhospital health care settings in the past decade: 12 in outpatient clinics, 6 in hemodialysis centers, and 15 in long-term care facilities, resulting in 448 persons acquiring HBV or HCV infection[16
]. Thus the hypothesis of a low-level iatrogenic risk may be plausible, and ought to be examined by other studies. Finally, in this cross-sectional study, we cannot rule out residual confounding, perhaps by age or socioeconomic status, as contributing to the observed association.
Our analysis of HCV nucleic acid status found that presumptive clearance of HCV RNA was less likely in African-American subjects and more likely in those with higher educational status and obesity; no significant association was found between the sexes in the multivariable analysis. The association with African-American race/ethnicity is consistent with a number of reports in the literature, and may be due to genetic differences in HLA type or interleukin 18 promoter polymorphism[1
]. Polymorphism in the IL28b gene has recently been proposed as a major determinant of racial differences in both treatment response and spontaneous clearance of HCV viremia[19
]. The association of HCV clearance with higher educational status was not found in the previous cross-sectional analysis among blood donors[23
]. Better access to healthcare and antiviral treatment among higher socioeconomic status donors seems unlikely to explain this effect, since most donors were presumably unaware of their HCV status at the time of donation. Our lack of an association of female sex with HCV clearance adds to a divided literature on this topic. Several studies with and without adjustment for potential confounding factors have found that anti-HCV positive women are more likely to be RNA negative than their male counterparts[24
], and a prospective study of young injection drug users observed a higher incidence of clearance among females[28
]. However three other studies including subjects who were U.S. injection drug users, U.S. blood donors and Italian general population found no significant association of HCV clearance with sex[1
]. Because the latter studies generally showed a reduction in odds ratios from univariate to multivariable analysis, it is conceivable that observed associations with gender may be confounded by route of infection or risk group. In addition, the disappearance of anti-HCV which occurs slowly after resolution of viremia, could lead to differential underestimation of “ever infected” status in cross-sectional studies.
Another novel result of the study is that donors with higher BMI were less likely to have anti-HCV, and among antibody positives, less likely to test positive for HCV RNA. These findings were somewhat counterintuitive, as the literature implies that chronic infection with HCV may induce a “metabolic syndrome” including insulin intolerance and hepatic steatosis[31
]. Similarly, HCV patients who are also obese tend to have a higher risk of progressing to high-grade fibrosis[32
], perhaps due to increased inflammatory markers observed in patients who are both HCV positive and obese[33
]. Obesity is also a risk factor for hepatocellular carcinoma both with and without HCV infection[35
]. On the other hand, a study which specifically excluded obese subjects and those with hepatic steatosis found no effect of HCV on insulin resistance[36
]. Perhaps most similar to our own results, a recent large population-based study in Taiwan found that anti-HCV positive subjects tended to have lower levels of both cholesterol and triglycerides[37
]. Perhaps the key to reconciling these conflicting observations is to separate studies of generally healthy populations such as blood donors, where obesity may have a protective effect against HCV infection, from studies of patients with liver disease, where obesity and HCV may represent co-morbidities. One study suggested an underlying genetic explanation for these observations with its finding that LDL receptor polymorphisms may modulate immune response to HCV[38
]. Alternatively, an inflammatory state including macrophage infiltration of fat pads and increased production of IL-6 and TNF∀ has recently been linked to obesity and could contribute to the resolution of HCV infection[39
Finally, significantly higher prevalence of HCV clearance at one blood center was a puzzling result. One hypothesis is that this was due to differences in HCV genotype. Higher prevalences of the antibody positive/RNA negative state have been reported from countries with genotypes other than type 1[41
]. On the other hand younger injection drug users in San Francisco had a higher ratio of HCV genotypes 3 to 1 than older IDU, but their incidence of HCV clearance did not differ by genotype[28
]. Blood donors in the Southern U.S. had the highest incidence of new, antibody negative/RNA positive HCV infections but did not show more non-1 HCV genotypes[43
]. This suggests another possible explanation for higher proportions of HCV RNA negative status in Georgia. Most HCV RNA clearance occurs within the first year of infection[28
] and may be followed by disappearance of anti-HCV over several years[44
]. Therefore, a higher incidence of new HCV infections at our Georgia center during the time frame of the study could lead to oversampling of resolving infections that had lost RNA but not yet anti-HCV compared to a lower incidence center where most infections would be older, chronically RNA+.
Strengths of the current study include its large size and representation of generally healthy blood donors from several blood centers across the United States. All Anti-HCV and RNA testing was performed in parallel and with state-of-the-art assays under the rigorous quality control typical of blood center laboratories. Weaknesses include the cross-sectional design which does not allow true determination of cause and effect, and the possibility of residual confounding of reported associations by measured and unmeasured variables. Although blood center RNA performed on individual samples is generally more sensitive than assays used in the clinic[45
], there may be some misclassification of RNA status due to testing in pools of 16 or 24 instead of undiluted specimens, although this effect has been shown to be small[46
]. Finally, although we believe blood donors are more comparable to the general population than are hospital or clinic patients with HCV infection, conclusions derived from blood donors may not be directly comparable to the general population or to patients because of selection for better health.
In conclusion, this study provides additional evidence that HCV prevalence has declined substantially among blood donors since 1993, consistent with culling of seropositives and a birth cohort effect. We found a novel association between HCV prevalence and gravidity that will be important to replicate in other U.S. studies because of the public health implications regarding potential iatrogenic transmission in the obstetric setting. Finally, our intriguing results regarding an inverse association of obesity with both HCV infection and viremia may influence the direction of future research on the genetics and metabolic effects of chronic HCV infection, particularly among more healthy HCV carriers as opposed to those with pre-existing liver disease.