AlbertoAscherio, Harvard School of Public Health, 665 Huntington Ave, Boston, MA 02115aascheri@hsph.harvard.edu
Marc G. Weisskopf, Eilis J O'Reilly, Marjie L McCullough, Eugenia E Calle, Merit Cudckowicz, Michael J Thun
Submitted March 01, 2005
We do not believe that the association between military service and
increased ALS mortality that we observed in our study [1] can be
attributed to an artifact of self-selection of healthy volunteers into the
study as proposed by Horner et al. This is because the age-specific death rates from
ALS were similar in our cohort to those in the U.S. [7] (Table).
Furthermore, readers may be confused that the heading of Horner's table refers to ALS mortality rates, but the data shown from McGuire et
al [3] refer to ALS incidence rates, as indicated in the footnote. Any
comparison between the incidence rates of ALS among men in Washington
State [3], and the death rates from ALS in either our study [1] or for the
U.S. general population [7] should be interpreted taking into account that
the mortality rates reflect the incidence of the disease at younger ages
and that there is some under-reporting of ALS in death certificates.
Furthermore, the decline in ALS incidence in the oldest age group, as
discussed by McGuire et al may be due to under ascertainment in older
people due to greater difficulty in diagnosing the disease and shorter
survival of elderly patients.
Horner et al also wrote that those who qualify for the military are
a “highly selected population”, and suggest that this selection may
explain the higher ALS mortality rate among the military as compared with
those who did not serve. Since men who served in the military (over two
thirds of men in our cohort) are likely to be healthier than those who did
not serve, this statement is at odds with the hypothesis of Horner et
al that selection of healthy volunteers in the study resulted in lower
ALS rates. Concerning the lack of increased risk among men who served in
the Marine Corps, the number of men in this category is too small for any
meaningful inference, as we have stated in our paper and is further
indicated by the wide confidence intervals.
We agree with Horner et al that the investigation of
ALS among Gulf War veterans is important and further research is needed.
1. Weisskopf, PhD, E. J. O’Reilly, MSc, M. L. McCullough, ScD,
E. E. Calle, PhD, M. J. Thun, MD, M. Cudkowicz, MD and A. Ascherio, MD.
Neurology 2005; 64: 32-37
3. McGuire V, Longstreth WT, Jr., Koepsell TD, van Belle G.
Incidence of amyotrophic lateral sclerosis in three counties in western
Washington state. Neurology 1996; 47:571-3.
6. Weisskopf MG, McCullough ML, Calle EE, Thun MJ, Cudkowicz M,
Ascherio A. Prospective study of cigarette smoking and amyotrophic lateral
sclerosis. Am J Epidemiol 2004; 160:26-33.
7. National Center for Health Statistics. Compressed mortality file
for 1989-1998 on CDC Wonder on-line database
(http://wonder.cdc.gov/mortSQL.html).
We do not believe that the association between military service and increased ALS mortality that we observed in our study [1] can be attributed to an artifact of self-selection of healthy volunteers into the study as proposed by Horner et al. This is because the age-specific death rates from ALS were similar in our cohort to those in the U.S. [7] (Table).
Furthermore, readers may be confused that the heading of Horner's table refers to ALS mortality rates, but the data shown from McGuire et al [3] refer to ALS incidence rates, as indicated in the footnote. Any comparison between the incidence rates of ALS among men in Washington State [3], and the death rates from ALS in either our study [1] or for the U.S. general population [7] should be interpreted taking into account that the mortality rates reflect the incidence of the disease at younger ages and that there is some under-reporting of ALS in death certificates.
Furthermore, the decline in ALS incidence in the oldest age group, as discussed by McGuire et al may be due to under ascertainment in older people due to greater difficulty in diagnosing the disease and shorter survival of elderly patients.
Horner et al also wrote that those who qualify for the military are a “highly selected population”, and suggest that this selection may explain the higher ALS mortality rate among the military as compared with those who did not serve. Since men who served in the military (over two thirds of men in our cohort) are likely to be healthier than those who did not serve, this statement is at odds with the hypothesis of Horner et al that selection of healthy volunteers in the study resulted in lower ALS rates. Concerning the lack of increased risk among men who served in the Marine Corps, the number of men in this category is too small for any meaningful inference, as we have stated in our paper and is further indicated by the wide confidence intervals.
We agree with Horner et al that the investigation of ALS among Gulf War veterans is important and further research is needed.
Table
References
1. Weisskopf, PhD, E. J. O’Reilly, MSc, M. L. McCullough, ScD, E. E. Calle, PhD, M. J. Thun, MD, M. Cudkowicz, MD and A. Ascherio, MD. Neurology 2005; 64: 32-37
3. McGuire V, Longstreth WT, Jr., Koepsell TD, van Belle G. Incidence of amyotrophic lateral sclerosis in three counties in western Washington state. Neurology 1996; 47:571-3.
6. Weisskopf MG, McCullough ML, Calle EE, Thun MJ, Cudkowicz M, Ascherio A. Prospective study of cigarette smoking and amyotrophic lateral sclerosis. Am J Epidemiol 2004; 160:26-33.
7. National Center for Health Statistics. Compressed mortality file for 1989-1998 on CDC Wonder on-line database (http://wonder.cdc.gov/mortSQL.html).