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What blood work shows parasites?

Blood tests are an important diagnostic tool that can help detect the presence of parasitic infections. When a person is infected with a parasite, there are often clues that show up in routine blood work. Being aware of what to look for can help healthcare providers diagnose parasitic diseases early and begin appropriate treatment.

Complete Blood Count

One of the most common blood tests ordered is a complete blood count (CBC). This test measures different components of the blood, including red blood cells, white blood cells, hemoglobin, and platelets. Changes from normal levels in a CBC can provide clues about the presence of parasites.

Some key things that may indicate a parasitic infection on a CBC include:

  • Low red blood cell count (anemia) – This can result from blood loss caused by intestinal parasites, interference with red blood cell production, or increased red blood cell destruction. Anemia is commonly seen with hookworm infections.
  • Eosinophilia – This is an increased number of a specific type of white blood cell called eosinophils. Elevated eosinophils are a very common sign of parasitic infections. They are often seen with tissue-invasive parasites like schistosomiasis and toxoplasmosis.
  • Thrombocytopenia – This refers to a low platelet count and can occur in some parasitic diseases like malaria and babesiosis when the parasites infect the bone marrow.

Stool Ova and Parasites Test

When intestinal parasites are suspected, a stool ova and parasites (O&P) exam will often be ordered. This involves checking a stool sample under the microscope for the presence of parasites or eggs.

Some examples of what might be found on O&P testing include:

  • Protozoan parasites like Giardia lamblia or Cryptosporidium
  • Helminth worms or their eggs (roundworms, tapeworms, flukes)
  • Cysts containing parasitic organisms like Entamoeba histolytica or Blastocystis hominis

The O&P exam allows for direct visualization of parasites in the intestinal tract. Multiple stool specimens are often needed, as shedding of organisms can be intermittent.

Antibody Tests

When parasites are located in tissues or the bloodstream, they can trigger the production of antibodies. These are proteins made by the immune system to target foreign invaders. Detecting these antibodies through serological blood tests can provide evidence of parasitic infection.

Some examples of parasites that can be diagnosed using antibody detection include:

  • Toxoplasmosis – Caused by Toxoplasma gondii, a parasite that spreads via cat feces. IgM and IgG antibodies are used to distinguish between acute and past infection.
  • Echinococcosis – Infection by tapeworms of the genus Echinococcus. Antibodies confirm exposure and can indicate active cystic disease.
  • Chagas disease – Caused by Trypanosoma cruzi parasites transmitted by insects. A positive IgM signal is evidence of acute infection.
  • Strongyloidiasis – Infection by Strongyloides stercoralis roundworms that migrate through tissues. ELISA testing detects antibodies.

In some cases, both antigen and antibody testing is used. Detecting the presence of parasitic proteins and the host immune response provides definitive diagnosis.

Molecular Testing

Newer molecular techniques like polymerase chain reaction (PCR) are also increasingly used to diagnose parasitic infections. This involves detecting parasitic DNA in blood or tissue samples.

Molecular testing is very sensitive, specific, and can often identify specific species of parasites. Examples of parasites detected by PCR testing include:

  • Plasmodium species that cause malaria
  • Leishmania species that cause leishmaniasis
  • Trypanosoma cruzi that causes Chagas disease
  • Toxoplasma gondii

PCR is useful for parasites that are difficult to grow in culture or visualize by microscopy. It also allows diagnosis at early stages of infection before antibodies develop.

Imaging Tests

Medical imaging like x-rays, CT scans, MRIs, and ultrasounds are sometimes performed when parasitic infections cause abnormalities in organs or tissues.

Examples of imaging findings that may indicate parasitic disease include:

  • Cysts in the brain or other organs due to cysticercosis or echinococcosis
  • Calcified lesions showing past infection with parasites like Toxoplasma
  • Ultrasound images of enlarged lymph nodes or spleen enlargement
  • Dilated bile ducts caused by liver flukes
  • Swollen intestines due to cryptosporidiosis or giardiasis

Imaging cannot directly detect parasites, but is useful for revealing organ damage and discriminating between differential diagnoses.


Parasitic diseases can often be detected through abnormalities that show up on routine blood work like complete blood counts and stool O&P exams. More specialized antibody detection, molecular tests, and medical imaging are also used to diagnose certain parasitic infections affecting tissues or specific organs. Being aware of the spectrum of findings across these tests is key for clinicians to recognize and accurately diagnose parasitic diseases.