The analyses in this article are based on historical data collected by the “Early Indicators of Later Work Levels, Disease, and Death” project.7 The primary sample for this project consists of 35,570 white males mustered into the Union Army during the U.S. Civil War, who were chosen randomly from the military company books stored at the National Archives in Washington, D.C.8 The veterans are linked not only to recruitment, military, and medical records while in service, but also to pension and surgeon’s certificates data if they entered the pension system after being discharged. Because the data set provides detailed individual health information, we can determine how stressful their wartime experiences were, what specific diseases and injuries they suffered from over the life cycle, and what caused their deaths. With respect to the two main health outcomes considered, veterans’ height at enlistment was obtained from the recruitment records and the specific diseases that each veteran suffered from during the Civil War were provided by the military records.
The key ecological variable is the risk of contracting malarial fever (hereafter referred to as “malaria risk”) before enlistment. In order to control for the extent to which Union Army veterans were exposed to malaria risk in childhood and at young ages, I have searched for vital statistics showing county- or township-level malaria mortality or morbidity around 1850. However, I found that no reliable data are available for the period and that using malaria mortality data could be misleading. To overcome this limitation, I estimate county-level malaria risk, using epidemiological theories that say that malaria risk is determined primarily by environmental factors. The principal data for risk estimation are found in surgeons’ reports on the annual incidence of malarial fevers among soldiers at 143 U.S. forts in the mid-nineteenth century. I first estimate the correlates between malaria incidence rates at forts and environmental factors around those forts such as temperature, rainfall, and elevation. Then, using these correlates and county-level environmental factors, I estimate the malaria risk for U.S. counties that do not have reliable statistics.
The estimation results are depicted in Figure 1, which shows the malaria risk in mid-nineteenth century America. Risks were high in the Southern and Southeast regions and in the area along the Mississippi and Missouri Valleys and up through the Old Northwestern regions. I discuss the idea of risk estimation, detailed data source, estimation procedures and their results in Appendix 1. Some issues on measurement errors and selection biases in risk estimation are discussed in Appendix 2.
ESTIMATED MALARIA RISK OF U.S. COUNTIES IN THE 1850S
In short, this study investigates Union Army veterans’ height at enlistment and susceptibility to other infections during the Civil War, controlling for the estimated malaria risk of the counties where veterans lived in 1850.9 To obtain veterans’ residence information and socio-economic status in early life, I searched for those who are listed in the 1850 federal census records. In the next section, where I estimate the impact of malaria on height and nutritional status in childhood, I limit my analysis to the 1,067 U.S.-born veterans who were under age five in 1850.10 In the analysis estimating the effect of malaria on susceptibility to infections during the Civil War, I use all available 6,855 veterans who have a complete set of variables.
As all the Union Army veterans were enlisted from northern states, the sampling could under-represent people from certain malarial counties, a large number of which were located in the South. Table 1 presents the distribution of Union Army veterans and the general population by county of enlistment or residence in 1850 among five categories of malaria risk. Compared with the general population’s distribution among malarial areas, calculated from residence data of the 1850 Integrated Public Use Microdata Series (IPUMS) samples, the proportions of Union Army veterans who were enlisted or who lived in high malaria-risk counties (risk is 0.3 and over) prior to the Civil War were lower by approximately 12 percent. However, both samples are similarly distributed over the counties where malaria risk is below 0.3. Additionally, the distribution of Union Army veterans linked to the censuses is not significantly different from that of the entire Union Army sample, which is computed by malaria risk at place of enlistment. The samples used in the estimations of this article more often represent white male individuals in less malarial counties of the Northern States, but well represent the entire Union Army veterans in terms of malaria risk at county of residence. Consequently, this study’s results should be interpreted in light of these sampling considerations. But these selection problems are unlikely to seriously impair the results regarding the effect of exposure to malarial environments on lifetime health because the Union Army veterans lived in the counties covering all ranges of malaria risk.
DISTRIBUTION OF UA VETERANS AND GENERAL POPULATION BY MALARIA RISK OF COUNTY OF RESIDENCE
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