PHLT 8522 Week 4 Descriptive Summary Example

Who, Where and When, Not Yet Why: A Descriptive Summary of Eighteen Composite Histoplasmosis Cases in a Riverfront District by Time, Place and Person

Student Name

Walden University

PHLT 8522: Epidemiological Investigations

Instructor Name

Month Day, Year

Who, Where and When, Not Yet Why: A Descriptive Summary of Eighteen Composite Histoplasmosis Cases in a Riverfront District by Time, Place and Person

Purpose and Scope

This summary describes the cases identified through October 21 under the three-tier outbreak definition. It reports time, place and person and stops there. Hypotheses about the source belong to next week's memo, and the separation is deliberate: Goodman et al. (1990) describe descriptive epidemiology as the step that should shape hypotheses, not confirm one already held. Where the data suggest a pattern, the pattern is stated without an explanation.

Case Counts

Eighteen people meet the confirmed or probable tier of the outbreak definition, whose laboratory criteria follow the national surveillance standard (Centers for Disease Control and Prevention, 2017): 12 confirmed, most by a fourfold rise in complement fixation titer or an H band on immunodiffusion as convalescent sera returned, and 6 probable on antigen or a single elevated titer. Three more meet the suspect tier and are reported separately; each has been offered testing. Case finding added 11 cases to the first 7, mostly from outreach to patients with untested pneumonia.

Time

Onsets span about three weeks, rise to a peak in the last week of September and decline steadily, with no case since October 6. The curve has a single peak and no second wave. Its spread is wider than one incubation period would produce from a single moment of exposure, which suggests that exposure, if shared, lasted for days rather than hours. Published outbreaks report median minimum and maximum incubation periods of about 7 and 15 days (Benedict & Mody, 2016), so the last onset on October 4 to 6 places the end of any common exposure no later than about September 20 to 29.

Table 1

Onset of Illness Among Confirmed and Probable Cases, by Three-Day Period (Composite Figures)

Note. Composite figures; onset is the first day of fever or cough reported by the patient or chart.

Place

Each case's home, workplace and regular outdoor routes were plotted on a district map. Thirteen of the 18 had a home or workplace in the four blocks in the district's northeast quarter, between Front Street and Third Street. Three more worked at the same salvage and demolition company, which had crews at several sites in September. Two had neither home nor work in the district but reported walking or cycling the riverfront trail through it several times a week.

The map lists the district's landmarks, including two September demolition sites, a park where city crews trimmed trees and an athletic field under regrading to the north, without implying that any of them is linked to illness.

Table 2

Place of Home or Work, Confirmed and Probable Cases (Composite Figures)

Note. Composite figures; each case is counted once, in the first category that applies.

Person

Cases range from 22 to 74 years of age, with a median of 46; 12 are male. Six were hospitalized, two of whom take immunosuppressive medication; none died. Hospital stays ranged from two to nine days, and four of the six received antifungal treatment, a far higher share than the 6.7% treated across reported outbreaks (Benedict & Mody, 2016), as expected when only the sickest patients are counted. Fourteen had a chest radiograph or computed tomography with infiltrates, and five had enlarged mediastinal lymph nodes. Occupations include office work, retail, food service, printing, salvage and demolition, and two retirees. No case reported caving, keeping chickens or farm work in the preceding month, exposures commonly linked to histoplasmosis outbreaks (Benedict & Mody, 2016).

What the Description Suggests and What It Does Not

The pattern suggests a common source active for some days in September and concentrated in or near the district's northeast quarter. It does not identify a source. The concentration of homes and workplaces could reflect where the department looked hardest, since outreach began with patients from the district. The three company employees could share a work exposure unrelated to the residents' illness, since the company ran crews at several sites in September. Trail users could have been exposed anywhere along a route that runs several kilometers. Each of these possibilities will be addressed when hypotheses are stated and tested against controls, and the case count will continue to be updated weekly until two maximum incubation periods pass with no new onsets.

Exposure Histories Collected So Far

Initial interviews, conducted with a short form, asked each case about outdoor time, routes, work tasks and hobbies in the three weeks before onset. Fifteen cases reported spending time outdoors in the district on most weekdays in September, either walking between parking and work or eating lunch outside. The three company employees reported handling debris at company jobs, though the form did not ask which sites. Two retirees reported daily walks along the riverfront trail. Four cases reported gardening at home, all outside the district, and none reported indoor renovation. These answers are recorded as they were given and are not yet compared with anyone who stayed well, since a description of cases alone cannot show which activity, if any, matters. The fuller questionnaire for the analytic phase will ask about each candidate site by name and date.

Attack Rate

No attack rate can be calculated, because the population exposed is not yet defined. A rough denominator, the 2,300 residents of the northeast quarter's census blocks, would give a residential attack rate of about 0.3%, but that figure mixes people with very different outdoor time and should not be read as a risk. A meaningful rate will need a defined exposed group, such as the demolition crew or the staff of a single building, and the analytic phase may supply one.

References

Benedict, K., & Mody, R. K. (2016). Epidemiology of histoplasmosis outbreaks, United States, 1938-2013. Emerging Infectious Diseases, 22(3), 370-378. https://doi.org/10.3201/eid2203.151117

Centers for Disease Control and Prevention. (2017). Histoplasmosis 2017 case definition. National Notifiable Diseases Surveillance System. https://ndc.services.cdc.gov/case-definitions/histoplasmosis-2017/

Goodman, R. A., Buehler, J. W., & Koplan, J. P. (1990). The epidemiologic field investigation: Science and judgment in public health practice. American Journal of Epidemiology, 132(1), 9-16. https://doi.org/10.1093/oxfordjournals.aje.a115647