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Laboratory Procedures for Identifying Parasitic Organisms and Their Ova 255

most frequently in areas with hosts suffering from poor on microscope slides and studied microscopically.
nutrition and low socioeconomic conditions. The host’s An indirect method of identification occurs when the
body has merely reached an equilibrium where the pop- patient’s immune response is used for determining the
ulation of parasites is somewhat controlled and do not presence and identification of the parasite(s) since infec-
cause overt reactions to the organisms. tions with multiple organisms sometimes occurs. When
a patient is infected, the level of antibodies against the
particular parasitic organism will greatly increase due to
IMMUNITY TO PARASITIC the patient’s body recognizing the organisms as foreign
INFECTIONS agents, and an immune response is elicited. Antibody
production is specific for each organism and tests have
In endemic areas of the world, some individuals show been developed for a few parasites. These tests are called
a tolerance or a resistance for becoming infected by indirect tests because the procedure determines whether
parasites with which the individual invariably comes in antibodies are present against a specific parasitic organ-
contact. Some immunity may be hereditary for some in- ism rather than determining the actual physical presence
dividuals. In others, some become asymptomatic (show of the parasite. In addition to microscopic methods for
no signs of infection) and harbor at least a small number direct identification, direct serological tests for some par-
of parasites indigenous to the area. These persons may asites (e.g., Giardia lamblia) have a test kit that provides
be carriers that are capable of transmitting the organisms antibodies that will react even against fragments of the
to others. In some areas, those who work in food services G. lamblia present in a stool sample. This method is
are required to be screened periodically for a variety of called a direct test, even though antibodies are a compo-
organisms, both for parasites and for other microorgan- nent of the test procedure, because the reaction is against
isms. It should be remembered from earlier chapters the organism itself and not a measure of the patient’s im-
that in many cases, certain animals are natural hosts for mune response.
specific parasites and may suffer no ill effects from the
infection, but are nevertheless able to pass the organism Preventing Infection or
to others, including humans.
Infestation by Parasites
Just as there is a cycle of infection, similar to the chain of
Natural and Acquired Immunity infection for other infective organisms such as bacteria

Antibodies may develop in the healthy individual against and viruses, a link in the chain must be broken to stop
parasites, and cell-mediated reactions, vital components the spread of a parasitic organism. In order to do this,
of an individual’s immune system, develop. This may knowledge of the organism’s life cycle is important. This
cause tissue damage in the infected regions of the body, includes both primary and intermediate hosts, method
but serves to hold the infective organisms at bay when of reproduction, and the routes by which transmission
a human or other animal host with good nutrition and of the infectious agent gains entry into the body, such
health becomes chronically infected. This group is much as vectors, or lifestyles of the victims. Various species of
less likely to develop the severe symptoms observed in parasites may be found in almost any body site or body
those with poor health. Tests for identifying certain fluid, from solid tissue such as muscle, feces, sputum,
parasites such as Giardia lamblia and Trichomonas aspirations from atria of the body, blood, and urine.
vaginalysis have been developed that use the immuno- Methods for preventing the transmission of parasites
logical system where the patient’s response to the infec- include:
tion can be utilized. ■ Personal sanitation
Most of the procedures for determining the pres-
■ Safe food and water supply
ence and identification of parasites use a direct method,
which means the parasites themselves are recovered ■ Health education
and are concentrated and sometimes stained to make ■ Determining infective form and preventing its
the identification easier. The treated samples are placed development