Page 47 - parasitology for medical and clinical laboratoryprofessionals

Epidemiology and Conditions Leading to Parasitic Infestations 27

Direct testing for the actual infective organisms invasion occurs, rising to several times the normal per-
by testing specimens against known antibodies com- centage of less than the normal range of 1 to 3 percent of
mercially available are gaining acceptance. The use of the white blood cells. An elevated eosinophil count does
antibodies is similar to that of indirect testing with the not aid in identifying the particular organism with which
exception that the patient’s antibodies are not used but the victim is infected. It merely shows that there may be a
are obtained from a commercial source. This procedure tissue invasion by a parasite. Specific antibodies against
is still considered direct testing because the antigen, a some of the more common pathogens are used to iden-
part of the actual organism, is being tested for. These tify components of the parasite, and measuring specific
fragments that are comprised of parts of the organism antibodies the host has developed is another method for
may be present in stool specimens and will yield a posi- specifically identifying an infecting parasite. Secretory
tive result when using this methodology. Three of the IgA, an immunoglobulin of which antibodies are com-
parasitic organisms mentioned previously have screen- prised, may also be measured to determine the ability of
ing tests that measure the presence of antigens from the the intestinal tract to fight infection in local areas of the
parasites by using antibodies formed against the organ- body, such as the intestinal tract.
ism. These three are Giardia lamblia, Cryptosporidium, All persons who become infected with a bacterial,
and Entamoeba histolytica antigen tests. Commercially viral, or parasitic pathogen should exhibit an immune re-
prepared antibodies against the organisms are used to action by forming antibodies that are specific against the
test for antigens from parasites which might have disin- infective agent. Parasitic infections also cause the forma-
tegrated in the intestine. tion of specific antibodies, and although the techniques
for testing have evolved fairly recently, the antibodies
formed against the particular parasite in some cases may
INDIRECT TESTING FOR be used as an indirect manner of identification of the
PARASITES parasite. An indirect test measures the body’s response
against the causative organism, and does not require
Other recent developments provide an immunological direct evidence of the organism itself where it may be
profile of the blood that can be used to indicate some viewed microscopically. Testing for antibodies against
but not all parasites that may infect humans. This type of parasites is not as well developed as those for viruses.
test is quite effective unless the immune system has been
greatly diminished by some underlying medical condition,
as merely having a parasite infection serves to wreak havoc IDENTIFICATION OF PARASITES
with the immune system. When a blood test is used, it is AND THEIR OVA IN BODY FLUIDS
possible to determine the presence of antibodies formed AND WASTES
against parasitic organisms to confirm the presence of par-
asitic pathogens such as Entamoeba; Strongyloides; blood, This is an area in which laboratory workers perform the
liver, and lung flukes; heartworm found most often in dogs; vast majority of the diagnoses of parasite infectations. It
malaria; Toxoplasma; microfilaria; and Trichinella, as well is important to bear in mind that parasites range from
as others. It is necessary to collect a sample of blood at es- unicellular organisms such as amoebae to large “worms”
tablished intervals, such as every 4 to 6 hours for 72 hours of several feet in length. Therefore, microscopic features
around the clock, because some parasites only come out of of one-celled organisms and identification of the eggs of
the tissue in which they spend most of their time during larger parasite forms may be required to properly iden-
the middle of the night. A shortcoming of the procedure tify a parasite. A number of characteristic morphological
that determines the presence of antibodies to the various structures associated with various species of parasites
parasites is that it would be impossible to determine if the enable definitive identification of the most common va-
antibodies were due to a past or a present exposure. rieties of parasites routinely found in infected persons
Indirect testing is related to procedures where the of the developed world. Observation of certain mor-
evidence of an immune reaction against a parasitic infec- phological characteristics may be valuable in identifying
tion exists rather than actually finding the organism it- the genus and sometimes the species of some parasitic
self. The eosinophil count (for a type of white blood cell organisms known as protozoa (one-celled organisms) as
already mentioned) is most likely increased when tissue provided in the following chart (Table 2-2). This table