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Serology Blood Testing for Parasites

Even if a parasite isn’t in your bloodstream, your body’s immune response can reveal that you’ve been exposed to a parasite. This form of testing is called serology, and it works by finding parasite antibodies in your blood. These newer and more sensitive blood tests can help identify a parasite anywhere in your body, but they are far from perfect.

Parasite Serology Quick Facts:

  • Serology detects parasites by looking for your body’s immune response to parasites
  • Intestinal, urine and blood parasites can be detected using serology
  • Serology doesn’t reliably distinguish past infections from current infections unless the infection is tested for immediately after exposure.

What can blood tests do in parasite testing?

Blood tests for parasites offer 3 options:

  1. You can look directly at a blood smear for parasites in the blood sample.
  2. You can check the blood to see if your immune system is responding to the parasite. This latter type of testing is called “serology”.
  3. You can have a blood test looking for blood parasite DNA vis PCR (this test is very difficult to find)

Only a few parasites will show up in your bloodstream. These can be seen directly in a blood smear sample. Almost all of these parasites are tropical diseases (like malaria) and all happen only if the parasite has been contracted through an insect bite (usually through wood ticks or mosquitos).

Parasites in your gut or other body areas rarely have any part of their lifecycle in your bloodstream, so they can’t be detected by a blood smear. Instead, we look at the serum (the liquid part of blood) to see if the immune system has recognized these infections.

The way the immune system responds to infection is through making antibodies. These are proteins that tag the parasite, allowing immune cells to kill them. If you’ve ever had a specific parasite infection, you would have antibodies against it. That specific antibody type stays with you (sometimes for life) to indicate you’ve had the infection before. But if you’ve never had a parasite exposure, you would test negative for those antibodies.

The Limitations of Serology

The problem with serology is that much of the time, it cannot tell you if you are infected now or have had the infection in the past. Technically, there is one way you could tell a recent from a past infection, but you would have to catch your parasite infection early – within a couple of weeks or months of getting infected.

How would this work? There are two major antibody types made when you catch an infectious disease: IgM and IgG antibodies. IgM antibodies are made first (often within 1 to 2 weeks) and help in the early stages of an infection. IgG antibodies are latecomers to the fight but have more staying power.

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If a laboratory can tell the difference between antibody types (not all labs can do this), the presence of IgM antibodies would probably indicate you still have the infection. Having IgG antibodies means you have had the infection before but will not say when you’ve had it. If you have both antibody types in your serum sample, you may be recovering from a relatively recent infection.

So while an antibody (serology) test can tell you very specifically the type of parasite you have, it rarely helps in deciding what to do about your symptoms now. That positive antibody test only indicates your immune system was once active in fighting the parasite.

Microscope-based O&P tests remain the gold standard in helping you find out what is causing your symptoms in the present time. Parasitologists can find the the parasites, or the parasite eggs (ova) in your stool, blood or urine.

Best Parasites to Test for With Serology

  • Toxoplasma gondii: If a pregnant woman has recently been exposed to T. gondii, then there is a risk of serious birth defects. That’s why a T. gondii serology test is commonly given to pregnant women.
  • Toxocara: Serology is one of the best ways to screen for this neglected parasite infection, which can cause blindness (especially in children).

Frequently Asked Questions:

Can A Serology Blood Test Detect An Intestinal Parasite?

Amazingly, yes! If your serology test is looking for immune markers for an intestinal parasite, like Schistosoma, then it could be detected. But if your test doesn’t look for those exact immune markers, then you couldn’t check if you were ever exposed to that parasite. I am not aware of a good place to purchase a blood test for intestinal parasites.

Where Can I Buy A Serology Parasite Test?

Unfortunately, this test is not very useful in many clinical situations. So they are not easy to purchase, even with your doctor’s help. Probably the most common serology parasite test is for toxoplasma gondii.

Is Serology The Best Parasite Test?

No, it is not. There are many flaws with the test, and it is not widely used clinically. Even if you had a positive test result for this test, it may only prove that you were exposed to that parasite…it may not reliably show that you’re currently infected by that parasite.

References:

Ndao M. (2009). Diagnosis of parasitic diseases: old and new approaches. Interdisciplinary perspectives on infectious diseases, 2009, 278246. https://doi.org/10.1155/2009/278246. Ricciardi A, Ndao M. Diagnosis of Parasitic Infections: What’s Going On? Journal of Biomolecular Screening. 2015;20(1):6-21. doi:10.1177/1087057114548065.

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