Longitudinal studies were set up in Leogane neighborhoods to monitor antifilarial immune responses associated with exposure to LF and development of patent filarial infection. The median period of follow-up for the children in this study was 4.7 years, with the first sample collected at a median age of 1.4 years. Ninety-six (67.6%) of the one hundred and forty-two children were from the Bino neighborhood of Leogane, and boys (57.7%) outnumbered girls (42.3%) in the study population (). Median age at time of first sample was 0.9 years in Cada, 1.4 years in Bino, and 2.2 years in “other” locales. The difference was significant at p
0.006. There was no difference, however, in the median number of years of follow up between children from different areas (p
Characteristics of the Study Population.
Children were monitored periodically by nocturnal blood exam for microfilaremia and by microscopic examination of stool samples for intestinal parasites. Early infection and re-infection with intestinal helminths was a common occurrence throughout the study. The prevalence of Trichuris
, hookworm and Strongyloides
infection in children under the age of 5 is shown in . By the age of 3, more than 59% and 30% of children were infected with Trichuris
, respectively. Over the period of follow-up, hookworm prevalence increased dramatically in the community 
Prevalence of intestinal helminths.
Nocturnal blood smears were prepared for microfilaremia assessment during each sampling period. The cumulative prevalence of microfilaremia in the children was 23.2% by the end of the study (); this prevalence should be considered a minimum estimate because of the small volume of blood (20 µl) examined. Serum samples were assessed for circulating filarial antigen using the Og4C3 ELISA to determine when children first became antigen positive; cumulative antigen prevalence was 56.3%. All microfilaria-positive children were also antigen-positive. The mean age at which children acquired W. bancrofti infection as assessed by microfilaremia and antigenemia was 6.3 and 4.3 years, respectively ().
Antibody responses to Bm14, Bm33, and WSP antigens were measured using a multiplex assay platform. A novel antigen, Wb123, was measured using LIPS technology. Positive antibody responses for both techniques were defined based on cutoff values determined from nonendemic control samples. Representative plots from two children are shown in . Increases in antifilarial antibody to Wb123, Bm33 and Bm14 were noted in conjunction with (e.g., panel A) or prior to the detection of filarial antigen (panel B). Antibody to WSP was not detected in most children. Children were treated with DEC when microfilariae were detected and as previously reported 
, treatment often led to declines in levels of antibody against all the filarial antigens as seen in , panel B.
Representative antibody profiles.
Age prevalence curves showing the profiles of circulating filarial antigen, microfilaria, and the antibody responses to Bm14, Bm33, Wb123, and WSP filarial antigens with age are shown in . Responses to Bm14, Bm33, and Wb123 increased markedly between one and three years of age. Bm33 was the first antibody response to be detected in children with a mean age of incidence occurring at 2.8 years, followed by Bm14 (3.4 years), Wb123 (3.7 years), and WSP (4.3 years) ().
Age prevalence of microfilaremia, antigenemia and antifilarial antibody responses.
The longitudinal nature of the study provided an opportunity to analyze serocoversion rates. Over the course of the study, 55.6, 47.8, and 61.9% of children seroconverted to Bm14, Bm33, and Wb123, respectively. Only 23.9% of children developed responses to WSP and these responses were often transient in nature and unrelated to changes in antibody to the other filarial antigens. The rate of seroconversion was measured using total person years and was highest for Bm33, with 49.9 seroconversions per 100 person-years, followed by Bm14, Wb123, and WSP with seroconversion rates of 34.7, 31.0, and 5.5 per 100 person-years, respectively ().
Seroconversion rates by antigen.
The correlation of children's infection status and antibody status at the end of the study is represented in . All of the children who were microfilaria-positive at the end of the study had antibody responses to Bm14 and Bm33 antigens, and 96.7% of these children were found to have a Wb123 response. Of the 80 children determined to be antigen-positive, 100, 100, and 97.5% had responses to Bm14, Bm33, and Wb123, respectively. Of children who were considered uninfected by antigen tests (Og4C3-negative) and microscopy (microfilaria-negative), 88.7, 100, and 83.9% had filarial antibody responses to Bm14, Bm33, and Wb123, respectively. The antibody prevalence was not significantly different by infection status for any of the filarial antigens.
Antibody response by infection status.
Quantitative analyses of the antibody responses of antigen-positive and antigen-negative children are shown in in and , respectively. Antigen-positive children had levels of anti-Bm33 and anti-Bm14 antibody that were at or near the maximum level of the assay at the serum dilution tested across all ages; however, Wb123 responses were lower than the peak responses. Among antigen-negative children, anti-Bm33 responses increased with age, reaching maximal values by age 5. Similar increases in antibody levels with age were noted for Bm14 and Wb123, but antibody levels did not reach assay maximums for either antigen. A Cox proportional hazards model was generated to analyze factors influencing antibody responsiveness, including gender, child's infection status, maternal infection status, community of residence, and study year. The analysis for Bm14 is shown in . Bm14 responses were influenced by gender with females responding at an earlier age than males, but not by antigen status, maternal infection status, or community of residence. Children who were sampled during the early study period of 1990–1995 were significantly more likely to develop a Bm14 antibody response than children sampled during the later study period (1996–1999) (p
0.0036). A similar result was seen for Wb123 and WSP, but not Bm33 (p
Quantitative changes in antibody among antigen-positive children.
Quantitative changes in antibody among antigen-negative children.
Cox proportional hazards model for initial Bm14 antibody response.