Environmental Engineering Reference
In-Depth Information
accumulated data, to select the suspicious foods for laboratory examination and
set aside the remaining food in protected sterile containers under refrigeration
at a temperature of less than 40
◦
F(4
◦
C) for possible future use. Laboratory
procedures should be followed for collection, preservation, and shipment of all
specimens and samples.
Samples of water should be collected directly from the source, storage tanks,
high and low points of the distribution system at times of high and low pressure,
kitchens, and taps near drinking fountains for chemical and bacterial examina-
tions. It should be remembered that the time elapsing before symptoms appear
is variable and depends on the causative agent and size of dose, the resis-
tance of individuals, and the amount and kind of food or drink consumed. For
example, an explosive outbreak with a very short incubation period of a few
minutes to less than an hour would suggest a chemical poisoning. Antimony,
arsenic, cadmium, cyanide, mercury, sodium fluoride, sodium nitrate, or perhaps
shellfish poisoning, favism, fish poisoning, and zinc poisoning are possibilities.
An explosive outbreak with an incubation period of several hours would sug-
gest botulism or fish, mushroom, potato, rhubarb-leaf, shellfish, chemical, or
staphylococcus food poisoning. An incubation period of 6 to 24 hours would
suggest botulism, mushroom poisoning, rhubarb poisoning, salmonella infection,
or streptococcus food poisoning. An incubation period of one to five days would
suggest ascariasis, botulism, diphtheria, amebic dysentery, bacillary dysentery,
leptospirosis, paratyphoid fever, salmonella infection, scarlet fever, streptococ-
cal sore throat, or trichinosis. For other diseases with more extended incubation
periods, refer to Table 1.4. The laboratory examinations might be biologic, tox-
icologic, microscopic, or chemical, depending on the symptoms and incubation
period.
The CDC
138
classifies outbreaks of unknown etiology into four subgroups by
incubation period of the illnesses: less than 1 hour (probable chemical poisoning),
1 to 7 hours (probable
Staphylococcus
food poisoning), 8 to 14 hours (probable
C. perfringens
food poisoning), and more than 14 hours (other infectious or toxic
agents).
The sanitary and medical surveys may involve the swimming pool or bathing
beach. In that case, samples should be collected at the peak and toward the end
of the bathing period for examinations.
Laboratory analyses for water samples should include the standard plate count
(heterotrophic plate count), in addition to the test for coliform bacteria, since large
bacterial populations may suppress the growth of coliform organisms. Where
large volumes of water are needed, use 2- to 5-gallon sterile containers and store
at 41
◦
F(5
◦
C). Sampling for recovery of viruses and
Giardia or Entamoeba
cysts
may require special on-site filters and equipment.
139
It is customary to notify the laboratory in advance that an outbreak has
occurred and that samples and specimens will be delivered as soon as possible.
All should be carefully identified, dated, sealed, and refrigerated. A preliminary
report with the samples and specimens, including the probable cause, number
