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Background: Influenza surveillance in Indiana is limited as influenza is not a reportable disease. Therefore precise information on the total number and distribution of cases is unknown. The Centers for Disease Control and Prevention (CDC) and the Indiana State Department of Health (ISDH) monitor influenza activity with the assistance of sentinel physicians who report to CDC the number of cases of "influenza-like" illness seen in their practices weekly and by the culturing of influenza viruses from throat swabs of patients presenting with influenza-like illnesses. Other physicians report influenza and/or obtain cultures when outbreaks are identified in special populations such as a nursing home or schools.
Each week during "flu season" five physicians from around the state provide CDC with number of patients with "influenza-like" illness they have seen that week ISDH cooperates with 32 physicians from around the state who submit throat swabs to the ISDH Laboratory for culturing. While this method of surveillance does not supply sufficient information to draw conclusions about the incidence of influenza in Indiana, it allows ISDH and CDC to recognize influenza activity and to participate in worldwide laboratory-based surveillance of viral genetic drift and genetic shift.
Outbreaks: The first outbreak was reported the last week of November. Approximately 421 cases from 670 students and 20 staff at the Atterbury Job Corps Center in Johnson County were reported to ISDH. Influenza A(H3N2) was isolated from two students. During December three outbreaks were identified from Marshall county. These outbreaks occurred in a school and in two nursing homes. Another outbreak was reported from a nursing home in Howard County in the last week of December. All of these outbreaks were due to influenza A(H3N2).
Summary: Sentinel physicians reporting to CDC the number of patients seeking care for influenza-like illness started increasing in the 42nd week of 1996 (week ending Oct 19, 1996) and increased sharply in week 47 (ending November 23, 1996), reaching peak activity in the first week of January 1997 (Figure 1). At that time 9.3% of patients seen by the participating physicians were seeking care for an influenza-like illness. By February 15, 1997 influenza activity at the 5 medical practices contributing to the CDC Sentinel Physicians Influenza Surveillance Program was back to a sporadic level. Of the patients seeking care for influenza-like illness the majority (over 60%) were between ages 5 and 44; age groups 1-4 and 45-64 were next in the percent of patients, each representing 15.6% of patients with influenza-like illness. See Fig. 2 for a graphic presentation of patient age distribution. During the period from first week of October 1996 to the last week of April 1997, sentinel physicians saw 16,870 patients of whom 263 (1.56%) were identified as having an influenza-like illness.
From the week ending November 2, 1996 to the week ending May 3, 1997 251 specimens were submitted for virus isolation. Influenza isolates were cultured from 75 (29.9%) of the specimens with influenza A (H3N2) being the most frequently isolated virus (84% of all isolates). Influenza A (H3N2) was the predominant isolate (96.8%) through the week ending February 15, 1997. Thereafter Influenza B was the predominate isolate (69.2%). A similar trend was reported by CDC for the United States, with influenza A viruses predominating from October to February; from February to early April, influenza B predominated (88% of isolates). Figure 2 illustrates the number of virus isolates by type and week of isolation.
The 1995-96 "flu season" provides a slightly different picture of influenza virus activity than that in 1996-97. During the 1995-96 season influenza A(H1N1) was the most frequently isolated virus, 47 of 71 isolates (67%), followed by influenza A(H3N2), 17 isolates (24%). Influenza A (H3N2) (10 isolates) and influenza B (Beijing-like) (5 isolates) was more frequently isolated after Jan 13, 1996 with only 6 of the influenza A(H1N1) isolates identified after that date. Figure 3 illustrates the number of isolations by week and by type of the 1995-96 season. It should be noted that influenza A(H1N1) was not isolated in Indiana during the 1996-97 "flu season." CDC reported that influenza A(H1N1) was not isolated in the US during the 1996-97 season but has been sporadically isolated in other parts of the world.
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