Parasitic Infection: Symptoms, Classification and Diagnosis

Parasitic Infection: Symptoms, Cause and Diagnosis


  • The term parasite came from the greek word “parasitos” where para means “along side of” and sitos means “food”. So the parasite is an organism that lies in or on and takes its nourishment from another organism.
  • The growth of a parasitic organism within the body is known as parasitic infection. Parasites usually enter the body through the mouth or skin.
  • Parasites that enter through the mouth are swallowed and can remain in the intestine and invade other organs of the body.
  • Parasites that enter through the skin bore directly through the skin or enter through the bites of infected insects. Some parasites enter through the soles of the feet.
  • When a person walks bare foot or through the skin when a person swims or bathes in water containing the parasites. Few parasites also spread through blood transfusions, through injections, or from a pregnant woman to her fetus.
  • Parasitic infections are more common in rural or developing areas than in industrialized areas. Such infections may occur in places with poor sanitation and unhygienic practices.


Taxonomically, parasites can be divided into three major groups:

The protozoan and helminthic infections differ in many important ways. Such as:


Protozoa are single-celled organisms that multiply by simple binary division. Protozoa can multiply in their human hosts and increase their number to cause overwhelming infection. With some exceptions, protozoan infections do not cause eosinophilia.


Helminths (worms) are multicellular organisms and have complex organ systems. Helminths can be further divided into Roundworms, Flatworms, Flukes. In contrast to protozoa, helminths do not multiply in humans but they can elicit eosinophilic responses when they migrate through tissue. Most of the helminths have complex life cycles which involve substantial time outside the human hosts.


Arthropods act as carriers or vectors for parasitic diseases and include insects and arachnids

There are a number of reasons to diagnose parasites accurately:

Treatment- Different parasites respond differently to different drugs.

  • Epidemiology -We need to know which individuals are infected, age differences, sex differences, etc.
  • Control – Diagnosis is the key to monitoring control programs and then evaluating the outcome.


Diagnostic methods can be divided into the following four types:

Traditionally Diagnosis

This method involves the detection of the presence of parasite or its larvae or eggs. A number of techniques are used for the separation of eggs and larvae from faeces.

  • Stained blood smears for blood borne parasites
  • Skin snips for Filaria
  • Muscle biopsy for Trichinella, etc

Disadvantages of traditional methods:

  • This method is not suitable for all the stages of parasites (eg. cystic stages of tapeworms)
  • It is not a sensitive and rapid method
  • It does not give the exact identification of the parasite
  • It requires skilled operators and results can take long time (xenodiagnosis—weeks)

Advantages of traditional methods:

  • Gold standard
  • Cheap

Biochemical Diagnosis

These methods are based on isoenzymic patterns. Isoenzymes are variants of the same enzyme and have same biochemical function, but have slightly different isoelectric points (because of mutations in non-essential parts of the molecule) and theretore migrate differently on gels. Different migration combined with enzymes specific stains allows the isoenzyme patterns to be visualized. Isoenzyme patterns are under genetic control and parasites with different banding patterns are genetically distinct. By this method we can distinguish between species and sub-species.

Advantages of isoenzyme diagnosis:

  • It is a simple technique
  • This is a simple technique in which many samples can be run at the same time.
  • Easy availability of large number of typing enzymes
  • Power to distinguish morphologically similar parasites
  • Disadvantages of isoenzyme diagnosis:
  • It requires very expensive equipment
  • Isoenzymes can be contamination by the host tissue
  • The enzymes are labile which can give false negative
  • Significant tissue is needed for analysis

Molecular Diagnosis

Molecular diagnosis is based on Southern Blotting or Polymerase Chain Reaction (PCR) technology. Both these methods are based on patterns which are unique to the specific parasites.

  • Southern blotting —DNA is extracted (cysts or eggs), cut by specific restriction enzymes and separated by gel-electrophoresis. The DNA probe will hybridize with the complementary sequence on the gel. Radioactive or fluorescent label is used.
  • Polymerase chain reaction — It provides rapid amplification of target DNA, so low levels of parasites can be detected. It has made possible the amplification of malaria sequences from whole blood and there is no need for isolation of the parasite.

Advantages of Molecular Diagnosis:

  • These methods are rapid and sensitive
  • They can detect very low levels of parasite
  • Small biopsy
  • Probes can be designed with flexibility
  • Specific — detect single parasite species.
  • Less specific — detect group of parasites.

Disadvantages of Molecular Diagnosis:

  • It does not distinguish between live and dead parasites
  • It requires parasite specific primers
  • It requires very expensive equipments and sophisticated laboratory facilities
  • Cross reactions with other micro-organisms can give false positives

Immunological Diagnosis

These methods are based on identifying specific antibodies in the blood of infected individuals. An early method was to inject a small amount of crude parasite extract under the skin and see if there was an inflammatory response.

  • Antibody diagnosis is commonly used in sleeping sickness disease.
  • Anti-trypanosomal IgM in the blood is detected by a card agglutination test called CATT (card agglutination test for trypanosomiasis). The reagent is basically freeze dried trypanosomes which are stained blue on a white plastic card. A drop of blood is added and if antibodies are present the trypanosomes will form clumps.
  • ELISA is used to diagnose Onchocerca.
  • Sandwich ELISA is also a method of detecting antigens, but is not suitable for mass screening.
  • Antigen capture assays (now—’dipstick format’) are suitable for self-diagnosis or mass screening.

Advantages of Immunological Diagnosis:

  • These assays can be developed into field-based tests for mass screening
  • These assays are highly sensitive and easy to perform

Disadvantages of Immunological Diagnosis:

  • These assays does not distinguish between present and past infections
  • They require high cost of development
  • Cross reactions can give false positives

Diagnosis of malaria using card test is an example of immunological methods. See Fig.

Parasitic Infection

A comparative analysis of some of the serologic and molecular tests for parasitic Infections is summarized in Table 1.

Parasitic Infection


Despite substantial investment and research, no vaccines are yet available for the prevention of parasitic infections. It is difficult to develop vaccines for parasitic infections because of the complex anatomy of parasites. We are close to develop a vaccine for malaria. Prevention is based on avoidance strategies.