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EPIDEMIC.TXT
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1994-08-22
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EPIDEMIC--Paralytic Poisoning and Deaths in Felicidad
On July 30, 120 cases of sudden neurologic symptoms and 26
deaths occurred in the village of "San Felipe del Mar" in the
country "Felicidad." Paralytic shellfish poisoning, caused
by toxic dinoflagellates (the "Red Tide"), was suspected. On
request of the Felicidad Ministry of Health through the Pan
American Health Organization, two epidemiologists from the
Centers for Disease Control (CDC) traveled to Felicidad.
The CDC epidemiologists met with Ministry and local
officials, and they and Felicidadian epidemiologists carried
out several studies. A team of five local health
professionals reviewed hospital records and death
certificates and interviewed ill persons (cases) and
household controls or their relatives. The interviews were
conducted in Spanish, using a two-page typewritten
questionnaire.
The CDC investigators had brought a portable computer and
printer. These were set up in a motel room, and an Epi Info
questionnaire was constructed to enter the data. One
investigator read the data aloud while the other typed at the
keyboard, and 126 cases were entered in about 6 hours. They
then produced a listing of all variables in ANALYSIS, which
they checked against the questionnaires to find errors. By
the third day of investigation they had produced tables for a
descriptive study and a case-control study.
The file EPIDEMIC.REC contains the corrected records. The
variable names have been changed to English to facilitate
understanding by English-speaking readers. You can look at
and edit the file using the ENTER program.
The investigators' first step, after "cleaning" the data, was
to perform a descriptive study of the cases. The easy way to
perform this analysis for all variables is:
EPI>SELECT ILL = "Y" (This selects ill persons only)
EPI>FREQ * (Displays a frequency distribution for
each variable)
A useful scheme for analyzing epidemic information is to
divide items into Time, Place, and Person, or Time,
Geography, and Everything Else. Time is conveniently shown
on an epidemic curve, or graph of the course of the epidemic,
and you may wish to graph several types of frequencies from
the analysis of cases, including time of onset for cases who
survived and for those who died. Calculating incubation
period requires some mathematical gymnastics because the
hours are expressed as 1200, 1830, etc., with the first two
digits in hours and the third and fourth in minutes, with a
base of 60 per hour. The necessary calculations are shown in
the program called TIME.PGM, so
Place was explored initially, and it was found that all the
cases in San Felipe del Mar clustered in a few households in
the poorest part of town. Subsequent studies in San Felipe
del Mar were limited to these households; therefore, Place
does not figure prominently in the analysis.
Cases were identified and studied in several other
surrounding towns, but for the sake of simplicity, our
considerations will be limited to the cases in San Felipe del
Mar. The investigators also chose to focus their analysis on
these cases while they were still in the field.
Person usually includes all the aspects of demography such
as age, sex, and race, as well as exposure variables such as
food history, medications, other illnesses, and behavioral
risk factors. Many of these were explored with the help of a
case-control study.
Case-Control Study
For the case-control study, a more precise case definition
was used, namely "A person in San Felipe del Mar who
developed an acute illness on July 30 characterized by
headache and at least two of the following sensory symptoms:
(numbness of the lips, face, ears, fingers, toes, legs,
arms) and at least two of the following motor symptoms
(difficulty talking, difficulty walking, difficulty getting
up from a lying or sitting position, difficulty breathing,
dizziness)."
Programming ANALYSIS to identify the cases and controls by
this definition is somewhat complex, and the investigators
did not do so in the field. It is sometimes easier to make
such decisions manually and enter the result in an extra
variable like GROUP that has been included in the
questionnaire, or that can be merged with it. As an
illustration of techniques, however, we have included a
program to identify the cases and controls in the file
CASECON.PGM. To see the commands, print or display the file.
To use it with the file EPIDEM1.REC, run ANALYSIS, and give
the command RUN CASECON.PGM.
CASECON.PGM does the following:
*READs the file EPIDEM1.REC
*DEFINEs variables for GROUP, NEURO symptoms, and MOTOR symptoms
*SETS NEURO and MOTOR counts to zero
*Using IF statements, puts the count of NEURO and MOTOR
symptoms in these variables
*Sets GROUP to blank
*IF illness, onset on July 30, headache, two or more NEURO
symptoms and two or more MOTOR symptoms are all
present, GROUP is set to "CASE"
*For noncases who are not ILL, GROUP is set to "CONTROL"
*SELECT is used to limit processing to CASES and CONTROLS
*SET is used to ignore records with missing values
*A page HEADER provides title information
*Variables ANYCLAMS, ANYFISH, ANYSHRIMP, and ANYLOBSTER
are set to "+" if either the food or soup
made from the food (or both) were eaten
*A message is displayed on the screen to instruct the user
Alternatives to Programming
The investigators avoided many of the programming steps we
have included in the .PGM files by using the FREQ commands
combined with use of the delete feature in ENTER. To perform
an analysis, they deleted unwanted records temporarily and
then did the analysis. This can be done by using several
copies of the data file with different names. They then
combined the results from cases and controls, using STATCALC
to do the statistics. Although the method was laborious, it
was far less so than hand tabulation and also faster and more
accurate. Elegance in programming should always take second
place to getting accurate results in a field investigation.
General Results
The investigators concluded that consumption of clams and
clam soup was responsible for the epidemic and for the 26
deaths. Bioassay and chemical analysis of clams showed high
levels of saxitoxin, the chemical basis of the toxicity of
shellfish during periods of "red tide."
The investigating team made recommendations 1) for the
monitoring of clams for saxitoxin and 2) for health
education, since (despite the odds ratios) many residents did
not realize the danger of eating clams during periods of "red
tide." They also recommended a system for warning citizens
during danger periods and provided ideas for further
research, including exploration of the relationship of clam
beds to agricultural runoff and to a nearby shrimp farm.
A preliminary report--containing key tables, hand-drawn maps,
the questionnaire, a summary of methods, results of the
descriptive and case-control studies, a summary of cases
outside San Felipe del Mar, conclusions, and recommendations-
-was typed on the portable computer, discussed verbally with
CDC headquarters staff, and submitted to the Ministry of
Health and to the Pan American Health Organization before the
investigators left Felicidad, approximately 2 weeks after
they had arrived. Further analysis was conducted after their
return to Atlanta. For this purpose, the EXPORT program was
used to produce a SAS file for analysis on the CDC mainframe,
where logistic regression and other tests could be performed.