Secular (Long-term) Trends

The implications of secular (or long-term) trends in disease are usually different from those of outbreaks or epidemics and often carry greater significance. The graph in Figure 3-1 from a CDC surveillance report on salmonellosis shows that the number of reported cases of salmonellosis in the United Sates has increased over time. The first question to ask is whether the trend can be explained by changes in disease detection, disease reporting, or both, as is frequently the case when an apparent outbreak of a disease is reported. The announcement of a real or suspected outbreak may increase suspicion among physicians practicing in the community and thus lead to increased diagnosis and increased reporting of diagnosed cases. Nevertheless, epidemiologists concluded that most of the observed increase in salmonellosis from 1955 to 1985 was real, because they noted increasing numbers of outbreaks and a continuation of the trend over an extended time. This was especially true for the East Coast, where a sharp increase in outbreaks caused by Salmonella enteritidis was noted beginning about 1977. A long-term increase in a disease in one U.S. region, particularly when it is related to a single serotype, is usually of greater public health significance than a localized outbreak because it suggests the existence of a more widespread problem.

Figure 3-1 Incidence rates of salmonellosis (excluding typhoid fever) in the United States, by year of report, 1955-1997.

(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:41, 1992; and Summary of notifiable diseases, United States, 1997. MMWR 46:18, 1998.)

Figure 3-2 shows the decline in the reported incidence and mortality from diphtheria in the United States. The data in this figure are presented in the form of a semilogarithmic graph, with a logarithmic scale used for the vertical y-axis and an arithmetic scale for the horizontal x-axis. The figure illustrates one advantage of using a logarithmic scale: The lines showing incidence and mortality trace an approximately parallel decline. On a logarithmic scale, this means that the decline in rates was proportional, so that the percentage of cases that resulted in death—the case fatality ratio—remained relatively constant at about 10% over the years shown. This relative constancy suggests that prevention of disease, rather than treatment of people who were ill, was responsible for the overall reduction in diphtheria mortality in the United States.

Figure 3-2 Incidence rates, mortality rates, and case fatality ratios for diphtheria in the United States, by year of report, 1920-1975.

(Data from Centers for Disease Control and Prevention: Diphtheria surveillance summary. Pub No (CDC) 78-8087, Atlanta, 1978, CDC.)

Seasonal Variation

Many infectious diseases show a strong seasonal variation, with periods of highest incidence usually depending on the route of spread. To determine the usual number of cases or rates of disease, epidemiologists must therefore incorporate any expected seasonal variation into their calculations.

Infectious diseases that are spread by the respiratory route, such as influenza, colds, measles, and varicella (chickenpox), have a much higher incidence in the winter and early spring in the Northern Hemisphere. Figure 3-3 shows the seasonal variation for varicella in the United States, by month, over a 6-year period. Notice the peaks after January and before summer of each year. Such a pattern is thought to occur during these months because people spend most of their time close together indoors, where the air changes slowly. The drying of mucous membranes, which occurs in winter because of low humidity and indoor heating, may also play a role in promoting respiratory infections. Since the introduction of varicella vaccine, this seasonal pattern has been largely eliminated.

Figure 3-3 Incidence rates of varicella (chickenpox) in the United States, by month of report, 1986-1992.

(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:53, 1992.)

Diseases that are spread by insect or arthropod vectors (e.g., viral encephalitis from mosquitoes) have a strong predilection for the summer or early autumn. Lyme disease, spread by Ixodes ticks, is usually acquired in the late spring or summer, a pattern explained by the seasonally related life cycle of the ticks and the outdoor activity of people wearing less protective clothing during warmer months.

Infectious diseases that are spread by the fecal-oral route are most common in the summer, partly because of the ability of the organisms to multiply more rapidly in food and water during warm weather. Figure 3-4 shows the summer seasonal pattern of waterborne outbreaks of gastrointestinal disease. The peak frequency of outbreaks attributable to drinking water occurs from May to August, whereas the peak for outbreaks attributable to recreational water (e.g., lakes, rivers, swimming pools) occurs from June to October.

Figure 3-4 Incidence of waterborne outbreaks of gastrointestinal disease in the United States, by month of report, 1991-1992.

(Data from Centers for Disease Control and Prevention: Surveillance for waterborne disease outbreaks, United States, 1991-1992. MMWR 42(SS-5):1, 1993.)

Figure 3-5 shows a late-summer peak for aseptic meningitis, which is usually caused by viral infection spread by the fecal-oral route or by insects. Figure 3-6 shows a pattern that is similar but has sharper and narrower peaks in late summer and early autumn. It describes a known arthropod-borne viral infection caused by California-serogroup viruses of the central nervous system.

Figure 3-5 Incidence rates of aseptic meningitis in the United States, by month of report, 1986-1992.

(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:20, 1992.)

Figure 3-6 Incidence of central nervous system infections caused by California-serogroup viruses in the United States, by month of report, 1981-1997.

(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:18, 1992; and Summary of notifiable diseases, United States, 1997. MMWR 46:20, 1998.)

Because the peaks of different disease patterns occur at different times, the CDC sometimes illustrates the incidence of diseases by using an “epidemiologic year.” In contrast to the calendar year, which runs from January 1 of one year to December 31 of the same year, the epidemiologic year for a given disease runs from the month of lowest incidence in one year to the same month in the next year. The advantage of using the epidemiologic year when plotting the incidence of a disease is that it puts the high-incidence months near the center of a graph and avoids having the high-incidence peak split between the two ends of the graph, as would occur with many respiratory diseases if they were graphed for a calendar year.

Other Types of Variation

Health problems can vary by the day of the week; Figure 3-7 shows that recreational drowning occurs more frequently on weekends than on weekdays, presumably because more people engage in water recreation on weekends.

Figure 3-7 Number of drownings at recreation facilities of U.S. Army Corps of Engineers, by day of week of report, 1986-1990.

(Data from Centers for Disease Control and Prevention: Drownings at U.S. Army Corps of Engineers recreation facilities, 1986-1990. MMWR 41:331, 1992.)

Surveillance for Bioterrorism

For at least a century, epidemiologists have worried about the use of biologic agents for military or terrorist purposes. The basic principles of disease surveillance are still valid in these domains, but there are special concerns worth mentioning. The most important need is for rapid detection of a problem. With regard to bioterrorism, special surveillance techniques are being developed to enable rapid detection of major increases in the most likely biologic agents³ (Box 3-1). Detection is made more difficult if the disease is scattered over a wide geographic area, as with the anthrax outbreak in the United States after terrorist attacks in late 2001.

Box 3-1 Diseases Considered Major Threats for Bioterrorism

A technique developed for more rapid detection of epidemics and possible bioterrorism is syndromic surveillance.³ The goal of this surveillance is to characterize “syndromes” that would be consistent with agents of particular concern and to prime the system to report any such syndromes quickly. Rather than trying to establish a specific diagnosis before sounding an alert, this approach might provide an early warning of a bioterrorism problem.