Paralytic Illness in Ababo

PART I 

It is the early 1990s. The World Health Organization is planning a program for the global eradication of polio by the year 2000. Likura, a fictitious nation in south-central Africa, may become one of the countries selected to test the effectiveness of WHO's polio eradication strategies. Unfortunately, little is known about polio in Likura. The Minister of Health therefore assigned the task of assessing the polio situation to a Ministry worker who has recently returned from an epidemiology course in Atlanta, and who is about to become the District Health Officer in the Ababo District. The Ababo District is a relatively poor, rural district with a single hospital and several health centers. The Ababo District has attempted to conduct surveillance on polio cases and deaths over the past five years. The hospital, health centers, and all health workers are supposed to report such cases to the District Health Officer. 

Question 1 

Define incidence and write the formula to calculate the incidence rate (1 point). 

Incidence can be defined as the rate of occurrence or frequency of a disease. It is a measurement of cases in a given population at a given time period (Hinton, 2014). 

Incidence rate = Number of cases / population at risk in a given time period. 

Question 2 

Define prevalence and write the formula to calculate the prevalence rate (1 point). 

Prevalence is the proportion of a population which has or had a certain thing such as illness or condition in a given time. 

Prevalence rate = new and preexisting cases / total population in the same time period. 

PART II 

To characterize the incidence of polio over time, the new District Health Officer tabulated the routinely collected surveillance records for the past five years. In Ababo, the operational surveillance case definition for polio is acute onset of flaccid paralysis plus fever. The data are shown in Table 1. The most recent census was conducted in 1986, when the population of the Ababo District was determined to be 360,000 persons. The population in Ababo is assumed to be growing at a constant rate of 3.8% per year. 

Polio Morbidity and Mortality, Ababo District, 1986-1990 

Year	# New Cases	# Deaths	Mid-Year Population	Incidence Rate Per 100,000	Mortality Rate Per 100,000
1986	54	5	360,000	15	1.39
1987	56	7	373,680	15	1.87
1988	50	6	387,880	13	1.55
1989	68	8	402,619	17	1.99
1990	74	10	417,919	18	2.39

Question 3 

Complete Table 1 by calculating the annual midyear population estimates, polio incidence rates, and disease-specific mortality rates for each of the past five years (7 points). 

Seasonal Distribution of Polio 

Month	1989	1990
January	5	7
February	19	16
March	4	8
April	9	13
May	4	8
June	4	5
July	2	3
August	0	2
September	1	1
October	2	1
November	4	4
December	7	5

Question 4 

Describe the seasonal occurrence of polio in Ababo. (Note that Ababo is in the Southern Hemisphere.) (1 point). 

The chart shown above shows that the start of a new year between December and April marks an increase in the number of incidences of Polio. The colder months between July and October have a less prevalence of Polio. 

Age Distribution of Polio Cases, Ababo District Hospital, 1989 and 1990 

Age in Years	Number of Cases per Age
< 1 = 0	34
1	50
2	25
3	27
4	7
5	2
6	3
7	2
8	0
	N = Total # of Cases = X

Question 5 

What are the total number of cases for 1989 and 1990? (1 points) 

The total number of cases is 150 

Question 6 

Determine the median age of cases (5 points) 

Median = middle value (after the data have been ranked), Steps to find median: 

Arrange the data in increasing or decreasing order (already done) 

Find the middle rank with the following formula: middle rank = (n + 1) / 2 

Median value = (150 + 1)/2 = 75.5 

The value at 75.5 is Age 1. 

Question 7 

Determine the mean age of cases (5 points) 

Steps to find the mean: 

Multiple each age by the number of cases and total them 

0 X 34 = 0 

1 X 50 = 50 

Etc until all are multiplied and added together 

Take the sum from step one and divide by the total number of cases 

Mean = SUM/Total number of cases = 251/150= 1.67 

Mean = 1.67 

Sex and Ethnic Distribution of Polio Cases, Ababo District Hospital, 1989 and 1990 

	Male	Female
Zanu	73	53
Hanzu	12	2
Other	8	2

Question 8 

What is the ratio of male to female cases? (1 point). 

Ratio = Total male / total female case = 93/57 = 1.63:1 which is close to 2:1 

Question 9 

Review the ethnic distribution of cases. Can you conclude, based on these results, that being a member of the Zanu tribe is a risk factor for polio? Why or why not? (3 points). 

Based on the results, being a member of the Zanu tribe appears riskier to obtain polio compared relative to other tribes. However, the conclusion cannot be valid because of the lack of information about other tribes. 

References 

Hinton, P. R. (2014).  Statistics explained . Routledge.