# Science Olympiad » Disease Detectives Event » National Event Exercises West Nile Virus

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### Exercise

Recommended time: 20 minutes.

Background: In late August 1999, an infectious disease specialist in a New York City hospital called the New York City Health Department to report 2 patients hospitalized with encephalitis (a clinical illness related to an inflammation of the brain tissues and frequently caused by viral infection). The health department contacted other hospitals in the area and identified 6 additional patients with similar illness. Public health surveillance conducted during August and September 1999 identified a total of 62 patients with encephalitis and positive tests for recent West Nile virus. Three of the 62 had only fever and headache and were not hospitalized.

1. Define Public Health Surveillance.

Background: Table 1 below shows the age and sex distributions of the 59 patients who were hospitalized and who had symptoms and positive lab tests for West Nile virus.

Table 1. Number of hospitalized patients with laboratory confirmed West Nile virus infection, New York City Department of Health, August 1 – September 31, 1999.

Age (yrs) No. Patients (%) Population at Risk Calculations
0-19 2 (3) 2,324,081
20-29 1 (2) 1,553,981
30-39 3 (5) 1,549,111
40-49 1 (2) 1,177,190
50-59 9 (15) 867,331
60-69 13 (22) 814,838
70-79 18 (31) 534,785
greater than or equal to 80 12 (20) 281,054
Sex (yrs) No. Patients (%) Population at Risk Calculations
Male 31 (53) 4,289,988
Female 28 (47) 4,812,383

Source: Nash D, Mostashari F, Fine A, et al. 1999 West Nile Outbreak Response Working Group. The outbreak of West Nile virus infection in the New York City area in 1999. NEJM 2001;344(24):1807-14.

2. Based on the data in Table 1, for which two age groups was the apparent risk of West Nile virus infection greatest? In the space in Table 1, show each calculation to support your answer and give all units. (Note: units must be given to receive credit.)

(a)

(b)

3. For which single age group was the apparent risk the least? In the space in Table 1, show a calculation to support your answer and give all units. (Note: units must be given to receive credit.)

4. Given that mosquitoes transmit this disease, give two explanations for the age distribution.

(a)

(b)

5. For which gender group is the risk of infection highest? In the table, show your calculations and give all units. (Note: units must be given to receive credit)

6. For the group you determine to be at greatest risk, give one explanation that might account for the increased risk.

Background: Next, the investigators collected data for various risk factors for more severe illness, using three measures: muscle weakness, encephalitis, or death. The results are shown below in Table 2.

Table 2. Risk Factors for West Nile virus infection, New York City Department of Health, August 1 – September 31, 1999.

Risk Factor Relative Risk (95% Confidence Interval)
Muscle Weakness Encaphalitis with Muscle Weakness Death
Known history of immunosuppression 1.6
(1.0 – 2.6)
1.4
(0.4 – 11.1)
2.1
(0.5 – 8.1)
Coronary artery disease 1.2
(0.5 – 2.6)
1.4
(0.7 – 2.6)
2.0
(0.6 – 6.6)
Hypertension 1.6
(0.9 – 2.5)
1.2
(0.6 – 2.3)
2.1
(0.3 – 12.2)
Diabetes mellitus 1.0
(0.5 – 1.9)
1.3
(0.6 – 2.7)
5.1
(1.5 – 17.3)

Source: Nash D, Mostashari F, Fine A. et al., op cit

7. From Table 2, the relative risk of death among people with diabetes mellitus appears to be substantially increased. Explain the quantitative meaning of this relative risk.