In this study of African-American farmers from the southeastern United States, we observed an inverse association between levels of p
′-DDE and IgG. We found no significant association between the concentration of p
′-DDE and IgA, the immunoglobulin class that is primarily responsible for protecting mucosal surfaces (e.g., the respiratory and gastrointestinal tract). Few other studies have focused on immunologic parameters and DDE or DDT exposure in humans. In contrast to our findings, Vine et al. (2001)
reported an increase in total lymphocytes and higher IgA levels, but not IgG levels, in relation to increasing p
′-DDE levels in a study of 302 adults residing around a pesticide dump site in North Carolina. In that study, exposure levels were lower (DDE median, 2 μg/L) compared with our study population (median, 7.7 μg/L). In the general population, the main source of p
′-DDE is diet (Gunderson 1995
), and diet undoubtedly contributed to exposure among the subjects in the present study of African-American farmers. In addition, subjects in our study who had used DDT on crops in the past had slightly higher serum p
′-DDE levels (data not shown). Although DDT was banned in 1972, the half-life is relatively long (> 5 years), and previous use likely accounts for the elevated levels in our subjects (Wolff 1999
). DDE is stored more tenaciously in humans than is DDT, and p
′-DDE levels increase in plasma after DDT intake has decreased (Smith 1991
). Besides diet, additional exposure in both our study and the study by Vine et al. (2001)
was through contaminated air; thus, the expected routes of inhalation and/or dermal absorption were similar. We do not know how differences in dose or other study characteristics contributed to the associations with different types of immunoglobulins observed in these two studies.
Higher levels of prenatal p
′-DDE were associated with an increased incidence of otitis media in a study of 171 Inuit infants (Dewailly et al. 2000
). Studies of individuals occupationally exposed to DDT also suggest that long-term exposure may lead to altered resistance to infectious diseases. Hermanowicz et al. (1982)
found a higher prevalence of infectious diseases in workers who had directly worked with DDT and lindane for 12–30 years compared with a control population of 1,000 individuals. Upper respiratory tract infections such as tonsillitis, bronchitis, and pharyngitis were the most frequently observed. These investigators also found deficits in neutrophil function, including decreased chemotaxis, phagocytic activity, and respiratory burst (Hermanowicz et al. 1982
). Similar associations between increased infectious disease and pesticide exposure were later reported for a larger cohort (Hermanowicz and Kossman 1984
); however, there were no significant differences in neutrophil function in the larger study.
The function of immunoglobulin is to inactivate or eliminate pathogenic organisms, and individuals with severely reduced levels of serum IgG due to primary immunodeficiencies suffer from recurring infections (Schur et al. 1970
). However, the clinical significance of modestly reduced serum IgG or IgG subclass levels remains controversial and is evidenced by the identification of asymptomatic individuals with abnormally low serum IgG subclass concentrations who do not have increased rates of infectious disease (Maguire et al. 2002
). At 4–6 months of age, neonates lose the protection of maternally derived IgGs, and at 7–12 months of age, IgG and IgM levels are approximately 50% of adult levels (Stiehm and Fundenberg 1966
). Infants of this age have been shown to be particularly susceptible to infections with encapsulated bacterial pathogens associated with upper respiratory tract infections. In the studies described above, the observed decrease in serum IgG levels with increasing p
′-DDE suggests the potential for increased susceptibility to pathogens whose clearance is IgG mediated, such as Haemophilus influenzae
and Streptococcus pneumoniae
(Maguire et al. 2002
). Our study was designed to examine immunologic parameters rather than clinical end points, but some medical history data were collected as part of the screening and enrollment process. Seventeen (12%) of the study participants reported a history of pneumonia. This prevalence did not vary by DDE level (14, 21, 10, and 15% in the lowest to highest DDE group, respectively). A larger study focusing on specific infectious diseases (and including validated medical history data) would be needed to examine the association between DDE and clinical outcomes.
Several studies have examined the relation between use of specific pesticides and presence of autoantibodies with inconsistent results (Colosio et al. 1993
; McConnachie and Zahalsky 1991
; Rosenberg et al. 1999
; Thrasher et al. 1993
). In the largest study of this type, self-reported use of some organochlorine pesticides (aldrin, chlordane, dieldrin, endrin, heptachlor, and lindane) was associated with increased prevalence of low-titer (1:40) antinuclear antibodies in a farming community (Rosenberg et al. 1999
). The authors noted that the association with diphenyl chlorines (DDT and methoxychlors) was not statistically significant, but data pertaining to the prevalence of this exposure and the magnitude and precision of the observed association were not given. Although we observed the highest prevalence of antinuclear antibodies in the highest category of p
′-DDE exposure in our study, this association was not statistically significant. Larger and expanded studies that are able to identify and quantify specific autoantibody levels are needed to more clearly examine this relationship.
Study participants were ambulatory members of the community, recruited through churches rather than through a hospital or clinic setting. When we excluded the six individuals who reported a history of kidney failure or dialysis or cancer, the results pertaining to the association between DDE and IgG levels were essentially unchanged (data not shown). Based on these self-reported data pertaining to medical conditions, we believe that major illnesses (cancer, malnutrition) were unlikely to be influencing the IgG levels seen in this study population.
A strength of this study is that the levels of p
′-DDE observed in study participants were higher than those reported by Vine et al. (2001)
in community residents and higher than in the general population (CDC 2003
). This difference in exposure level may reflect the influence of occupational exposure through farmwork, in addition to background exposure from the food supply and other environmental sources. This variability in exposure improves our ability to detect an effect of p
′-DDE that occurs with higher exposures, such as those that may occur in occupationally exposed populations. Although our study sample size was modest, our failure to confirm the association between p
′-DDE and IgA observed previously by Vine et al. (2001)
is unlikely to be due to limited statistical power because we saw no evidence of a trend that would have been strengthened with greater precision.