Epidemiologic Inference in Outbreak Investigation

An Obscure Disease in an Island

The island is close to the equator and few miles off the coast of the nearest continent. The climate is tropical; January-June is dry and July-December is rainy. The total surface area of the island is 1,863 square miles and has a population of slightly over 1 million persons. Important local industries include oil, agriculture and fishing.

Patients determined to have the disease were interviewed and the date of onset, age, sex, school-attended, and food histories were recorded. Laboratory and hospital records of persons hospitalized with suspect disease from various parts of the island were reviewed. Patients with positive cultures and/or a clinical presentation were considered to have this disease.

Public water supplies exist in main urban areas and are maintained with varying degrees of sanitary discipline and maintenance of chlorination by the public authorities. Around the periphery of each of the cities and throughout the rural areas, there are no public water supplies as such. Water is drawn from a wide variety of largely contaminated sources. There are good road communications among all of the cities in the island and a brisk commerce is carried out. A wide variety of foods are distributed from the main city (Capital) to other cities and villages throughout the whole island.

See data below in Tables 1-3 and Figures 1-5.

Questions:

1. Examine available data in tables and figures. Write for each Table and Figure (8 items in total) few lines (five full sentences at most) to describe the data and what these data suggest.

2. what is the Hypothesis and express what type of an outbreak (e.g., vector borne, occupational exposure, waterborne, airborne, etc.) occurred and what how the disease spread in this island Table 1

Verified Cases in Tropical Island

February to May by Date of Onset and Residence According to Major City

Date of Onset	City 1	City 2	City 3	Other Areas1	Total
February		2			2	4
March	2 - 4
	5 - 7
	8 - 10
	11 - 13
	14 - 16
	17 - 19
	20 - 22			1		1
	23 - 25
	26 - 28	1				1
	29 - 31
April	1 - 3	1		1	1	3
	4 - 6	2	2		2	6
	7 - 9	6	1	1	5	13
	10 - 12	7	3	1	2	13
	13 - 15	4	4	6	6	20
	16 - 18	4	5	6	4	19
	19 - 21	3	5	5	5	18
	22 - 24	1	2	6	2	11
	25 - 27		1	5	3	9
	28 - 30
May	1 - 3	1	1	4		6
	4 - 6	1				1
	7 - 9			1		1
	10 - 12		1	1		2
	13 - 15
	16 - 18
Total Feb - May	33	25	38	32	128

Page BreakTable 2

Distribution by Administrative Area

Administrative Area	Major City	Population Est. 1967	Number of Cases	Attack Rate per 100,000
Area 1	City 1	454,500	58	12.8
Area 2	City 2	207,450	25	11.1
Area 3	City 3	47,000	38	79.5
Area 4	T/V	107,650	7	6.5
Area 5	T/V	129,300	-	-
Area 6	T/V	26,500	-	-
Area 7	T/V	36,850	-	-
Total		1,010,050	128	12.5

Table 3

Attack Rates by Age and Gender

	Male	Female	Both Sexes
Ill	Total	Attack Rate (per 100,000)	Ill	Total	Attack Rate (per 100,000)	Ill	Total	Attack Rate (per 100,000)
0 - 4	1	75,150	1.3	3	73,150	4.1	4	148,300	2.7
5 - 10	18	77,550	23.2	26	74,650	34.8	44	152,200	28.9
10 - 14	18	65,800	27.4	40	65,250	61.3	58	131,050	44.3
15 - 19	4	52,900	7.6	5	52,300	9.6	9	105,200	8.6
20 - 29	1	76,600	1.3	1	79,450	1.3	2	156,050	1.3
30 - 39	1	55,400	1.8	5	54,150	9.2	6	109,550	5.5
40 - 49	0	43,400	0	3	45,300	6.6	3	89,250	3.4
50 - 59	0	35,750	0	0	33,900	0	0	69,650	0
60 +	0	27,060	0	1	32,250	3.1	1	59,300	1.7
Unknown	0						1
Total	43			84			128

Fig. 5 60 ACE SPECIFIC ATTACK RATES So Tropical Island 40 @ Males CASES PER 100, 00- X- ---X Females EX- --- - -- ---X- 10 -X_ _ -X S 10 15 20 25 30 35 40 45 so 55 60 65 70 AGG IN YEARS 4 /12 City a HumDER OF CASES MAR DATE OF PRI ONSET ( There 2. Joy potteds during April mme May) 4/7 City 2 NUMBER OF CASES MAR ( 4 7 10 13 16 19 24 25 x8 1 4 7 10 13 16 19 APRIL MAY DATE OF ONSET 4/21 city 3 . . MundoR Of CASES FEB MAR | 4 7 ( 13 16 19 22 25 28 1 4 7 10 15 16 19 APRIL MAY Mo Median Ase DATE OF ONSET (omitting Fab . Man. cases\fD9.9 09.U 70 GO 50 40 31) 1 . Cumulative Probabilt Fig .. 3 City 2 0.05 0.1 01.2 0.5 2 60 90 99.5 59.0 19.9\fig1 : Frequencyof Cares in 22 Tropical Island 21 19 18 17 16 15 Cases 14 13 12 - - 722- 825 828-10 1- 114, 127- 1310 - 1413- 15/6-16 17 is 18 '0427-9 10-12 - 13-15 5 167 12 16 21 24 27 30 Apal MAY THE