Page 51 - parasitology for medical and clinical laboratoryprofessionals

Epidemiology and Conditions Leading to Parasitic Infestations 31

should not be allowed to prepare food for others until all external and internal structures that fibers do not have.
symptoms are gone. Remember that parasites are trans- Only experience will enable the laboratory worker to
mitted most often under dirty conditions, and some may quickly discount suspect elements that are not parasites
even be present in the lungs and could be transmitted such as vegetable cells and fibers.
through sputum during certain stages of their life cycles, For the limited number of parasites that have had
so are easily spread. serological tests developed for determining the presence
of antigens that were developed only relatively recently,
these tests may be a valuable adjunct to confirm the
OTHER CONSIDERATIONS FOR findings as parasites when suspicious objects are seen
EFFECTIVE IDENTIFICATION AND microscopically. The antigen tests have been developed
TREATMENT FOR PARASITES for several common parasites including tests for Giar-
dia, cryptosporidium, and E. histolytica. The antigen
Is it necessary to have a fresh stool sample? Yes, the tests detect protein structures on the parasite, and they
fresher the better, as the morphology of the parasites can detect the presence of fragments of the parasite in
and even the eggs may be deteriorated in unpreserved a stool sample if no intact parasites can be seen. An ad-
stool, which would destroy the identifying characteris- vantage of this is allowing detection of a specific parasite
tics of the parasite and make the infection more difficult even if it is not definitely seen in the microscopic O&P
to detect. In addition, trophozoites are more easily vi- examination.
sualized in fresh samples. Sometimes, if a specimen is Blood antibody tests may also be ordered to deter-
impossible to examine while it is fresh, a small amount mine if a patient has had previous exposure to a parasite,
of formalin-ether (diluted formaldehyde) can be added because specific antibodies may persist throughout life
to preserve the organisms for a period of time. The for those with previous infections. These tests may in-
sample should be viewed microscopically if possible to dicate a past or a chronic infection but are not used to
determine the presence of parasites before preserving detect a current infection. Sometimes it is necessary to
the sample for transport to another location for iden- obtain a biopsy of the small intestines, where a small
tification. There are a number of other preservatives, amount of tissue is examined for parasitic infestation. Re-
including polyvinyl alcohol (PVA), which contains the member that reinfection may occur by repeat exposure or
fixative mercuric chloride. PVA has been considered the by an incomplete treatment for parasitic infections. It is
“gold standard” for the fixation of ova and parasites in quite common to become reinfected with a repeat expo-
the preparation of permanently stained smears of stool sure, particularly in cases where a family member has an
specimens. However, mercuric chloride is potentially asymptomatic parasitic infection, such as giardiasis (due
hazardous to laboratory personnel and may also present to Giardia lamblia infection), and continues to shed the
disposal problems. organisms and reinfects others until everyone is treated.
It is common for the inexperienced student or A person infected by a parasite should not take an
laboratory worker to mistakenly identify normal cells antidiarrhea medicine unless prescribed by the treating
and other components of the stool as parasites. Small physician. This is due to the fact that diarrhea is one of
parasitic worms that are thought to be present in a stool the ways in which the body helps to rid itself of the infec-
sample may be undigested food (cellulose) fibers from tion. If the normal activity (peristalsis) of the intestines
vegetables that were recently ingested. Most of the com- slows down and prevents the fecal material along with
mon parasites are much too small to be seen with the parasites from leaving the body by taking anti-diarrhea
naked eye and the fact that solid material from the stool medication, the amount of time may be prolonged for
is present compounds the problem of discovery and a person who is ill and may make the infection become
identification. However, parasites have characteristic more severe.