 New York State), the District of Columbia, and 5 U.S. territories compile surveillance data from their reporting sites and voluntarily transmit disease incidence data to CDC through the National Electronic Telecommunications System for Surveillance.
Conditions associated with critical biologic agents that were nationally notifiable, reported to NNDSS, and included in this study were anthrax, botulism, brucellosis, cholera, plague, tularemia, and selected viral encephalitides. Botulism is reported as two distinct conditions: foodborne botulism and other or unspecified forms of botulism, including wound botulism. All of the study conditions, except tularemia and selected viral encephalitides, were designated as nationally notifiable throughout the study period. Other than tularemia, only cases reported for those diseases designated as nationally notifiable and from states in which the disease was legally reportable were analyzed. Although tularemia was deleted from the nationally notifiable disease list in 1995 because of decreasing incidence, the disease remained reportable in most states, and the annual number of cases reported to NNDSS remained stable in subsequent years; therefore, tularemia incidence data for the entire study period were included in the analysis.
Incidence rates were calculated for the demographic and geographic descriptors of sex, age (grouped as <1 year, 1–4, 5–14, 15–24, 25–39, 40–64, and >65 years), racial category (American Indian or Alaska Native, Asian or Pacific Islander, black, white, and other), Hispanic ethnicity, and state of residence. Seasonal incidence (spring, summer, fall, and winter) was examined on the basis of data reported with one of three types of dates: onset date, date of diagnosis, or date of laboratory result.
Average annual age-, sex-, race-, ethnicity-, and state-specific disease incidence rates for the period 1992–1999 were estimated by averaging the total annual number of case counts by subcategory, and dividing by the study’s mid-year (1995) U.S. population. State-specific annual incidence rates were calculated by using postcensus estimates for July 1, 1992, through July 1, 1998, and population projections for 1999 from the U.S. Bureau of the Census. Incidence rates were calculated per 1 million population because of the small number of cases reported to NNDSS during the study period. Rates were not calculated for extremely rare conditions (anthrax and western equine encephalitis) or for conditions for which data were not collected in all years in the study period (eastern equine encephalitis). Data from U.S. territories were excluded in the analysis.
To provide an example of how historical disease incidence data may be used to assess the likelihood of a reported incident case in the future, we estimated the probability that a given reported case would have the distribution of age, sex, race, ethnicity, geographic residence, and season occurrence using the following formula: P(case) = P(age) x P(sex) x P(race) x P(ethnicity) x P(geographic residence) x P(season). The probability is derived from the NNDSS surveillance data and is calculated under the assumption that these demographic and geographic variables are independent.
Disease reports for seven conditions caused by critical biologic agents were available for analysis by using NNDSS data for 1992 through 1999 (Table 1). The number of reported cases and incidence rates of the diseases examined in this study, excluding botulism and eastern equine encephalitis, declined or remained stable in the United States during the study period. Tularemia and brucellosis were the most frequently reported diseases (111 and 102 cases/year on average, respectively, yielding the highest estimated incidence rates of 42.1 and 38.7 cases/1 million persons/year, respectively). The least commonly reported diseases were anthrax, with only one case reported in 1992, and western equine encephalitis, with one case reported in 1999.
In general, sex-specific incidence rates were higher among male patients than among female patients for most stu