In December 1994, a former El Paso, Texas resident with multiple sclerosis (MS) contacted the Texas Department of Health (TDH) to report an apparent "cluster" of MS cases among people who spent their childhoods in the Kern Place-Mission Hills area of El Paso. Early in the investigation, concerns were raised about the possible impact of air emissions from a local metals smelter, particularly the air emissions prior to the installation of pollution controls in the 1970s. Beginning operations in 1887, the ASARCO smelter in El Paso processed primarily lead, copper, cadmium, and zinc. ASARCO suspended operations at the smelter in 1999. Historically, the ASARCO facility's air emissions have been documented to contain high levels of metals such as iron, lead, arsenic, zinc, and cadmium. Through a cooperative agreement with the Agency for Toxic Substances and Disease Registry (ATSDR), TDH prepared a health consultation in 1996 which examined the available information about the MS cases and related environmental concerns. TDH concluded in the report that the reported number of MS cases in the neighborhood did appear to be high and noted that previous studies in the scientific literature had investigated the role of metals exposure in the development of the disease. It was not possible from the available information for El Paso, however, to determine if metals exposure was the cause of MS in this area or if there was a true excess of MS. The report recommended a study be conducted among persons who lived in the Kern Place"-"Mission Hills neighborhood and Smeltertown during the 1940s, 1950s, and 1960s to determine the number of people who had been diagnosed with MS. Smeltertown was a community of residences owned by ASARCO and located on the company's property. Each neighborhood was served by one local public elementary school, Mesita in the Kern Place"-"Mission Hills area and E.B. Jones in Smeltertown. TDH received a grant from ATSDR to examine the prevalence of MS in the group of children who lived in the two communities and attended the two public elementary schools. Because we could not reconstruct a list of children who lived in the neighborhoods, we used attendance at the schools as a proxy for living in the neighborhoods. The ATSDR grant also allowed TDH to investigate what historic environmental and biologic sampling data might still be available for use if future studies in the area were to be considered.
Students who attended Mesita or E.B. Jones Elementary schools from 1948 through 1970 were eligible to be included in the study. The former students were identified primarily through school records. Questionnaires were mailed to all former students for whom we could locate a mailing address. The questionnaire asked for demographic and medical information. For people who indicated they had been diagnosed with MS we asked for copies of their medical records. These medical records were reviewed by a board-certified neurologist to confirm the MS diagnosis. When the questionnaires were returned, we calculated prevalence estimates for the cohorts of former students from Mesita and E.B. Jones Elementary schools separately and for the combined student cohort of both schools. We also compared the actual number of MS cases we found among the former students with the number we would expect based on national estimates of MS prevalence. The comparison of the actual or observed number of MS cases to the expected number was done using a statistical procedure known as a standardized morbidity ratio (SMR).
A total of 5,272 students were identified as having attended Mesita and E.B. Jones Elementary schools from 1948 through 1970; 3,891 from Mesita and 1,384 from E.B. Jones. Several students reported attending both schools. We were able to identify current addresses for 33% (n=1,248) of former Mesita students and 32% (n=432) of former E.B. Jones students. Forty-four percent of the former Mesita students and 20% of the E.B. Jones students we contacted returned questionnaires. Fourteen cases of definite and probable MS were identified among former Mesita students. No cases were reported for the E.B. Jones cohort. Using the 14 cases of MS and dividing by the total combined cohort of 5,272 students, the crude MS prevalence estimate for the combined school cohort is 266 cases per 100,000 population (95% Confidence Interval or CI = 145 to 445 per 100,000). Using the 14 MS cases and dividing by the total Mesita cohort of 3,891, the crude MS prevalence for the Mesita cohort is 360 per 100,000 (95% CI = 197 to 603 per 100,000). The crude prevalence estimates for both the combined cohort and the Mesita cohort are higher than previously published U.S. prevalence estimates for MS which range from 87 to 160 per 100,000. Standardized morbidity ratios (SMRs) were also calculated for the combined Mesita and E.B. Jones cohorts and for the Mesita cohort separately. Using data from the 1989 to 1994 National Health Interview Survey (NHIS), the SMR for the combined cohort was 1.42 (95% CI = 0.78 to 2.40) indicating an elevated, but not statistically significant risk of MS among the combined cohort. For the Mesita cohort, the SMR was 1.93 (95% CI = 1.06 to 3.24) indicating a statistically significant twofold increased risk of MS.
This study was designed to (1) identify the number of people with MS in the school cohorts, (2) determine if the number of people with MS was more than we would expect based on national prevalence estimates, and (3) review historic environmental and biologic sampling data. We identified and confirmed 14 cases of MS among former Mesita students. No cases were reported among former E.B. Jones students. The number of people with MS among former Mesita students is twice as high as expected, based on national estimates. We were able to identify limited historic environmental and biological data for the study area. These results are reported in a separate addendum to the report. This study was not designed to investigate the specific cause or causes of MS and the results cannot tell us why there is an excess of MS among the former Mesita students.
Based on the findings of this study, we recommend four activities:
- Conduct an annual survey of the Mesita and E.B. Jones cohorts for a minimum of two additional years to determine if any additional people are diagnosed with MS. All former students of these schools are encouraged to participate in the surveys.
- Develop current MS prevalence estimates for Texas.
- Re-analyze the El Paso MS cluster data when Texas prevalence estimates become available.
- Conduct a national multi-site case control study to examine metals exposure as a possible etiologic risk factor (cause) for MS. Combining MS cases from multiple sites around the United States may provide the statistical power needed to study the association between metals exposure and MS. Participants in the El Paso study should be included in the multi-site study.