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LABORATORY AIDS IN THE DIAGNOSIS OF
INFECTIOUS DISEASES
Respiratory Infections.
The bacterial infections of the respiratory tract in which laboratory
help is most useful and sometimes essential for diagnosis are sore
throat, acute and chronic chest infections, including lobar pneumonia,
tuberculosis and whooping cough. Of the virus diseases, influenza,
psittacosis and a typical pneumonia are the only ones that can be
readily diagnosed by laboratory.
Swabs from cases of sore throat are cultured for diphtheria bacilli,
hæmolytic streptococci and other pathogens; stained smears are
examined for the organisms of Vincent's angina. A provisional report
on any positive result is telephoned on the morning of the day after
receipt of the specimen. When a specimen is received late in the day,
diphtheria bacilli may not be found until the afternoon of the following
day. Rarely, the bacilli may grow so slowly that a provisional negative
report telephoned within 24 hours may have to be corrected at 48 hours
to a positive report.
Intestinal Infections.
The two main groups of specific infections causing diarrhœa with
or without vomiting in this country are bacillary dysentry and food-poisoning.
Bacillary dysentery due to Sonne's bacillus, which usually
causes a mild type of diarrhœa (often unaccompanied by blood or
mucus), is common and widespread. With selective media, dysentery
bacilli may be readily isolated from faeces during the acute and often
the convalescent stages of the disease.
Specimens should also be submitted from the other members of
the household when a case of dysentery has been diagnosed, since
symptomless excreters among intimate contacts are common. In such
circumstances a carefully taken rectal swab may be preferred, because
it ensures against "pot" contamination, although as a general rule a
sample of fæces is more satisfactory.
A provisional positive report can usually be made after 24 hours,
but the full examination of a specimen may take 2 to 3 days.
Food poisoning (excluding Botulism) may be defined as an acute
gastro-enteritis, typically chacterized by vomiting and diarrhæa, but
on occasion with these symptoms considerably modified. The illness
can generally be associated with the consumption, within a reasonably
short period of time, of a particular meal.
The bacteria most commonly responsible for food poisoning are
members of the Salmonella group (excluding the typhoid-paratyphoid
organisms), Staphylococcus aureus, and Cl. welchii.