It's the case of the unknown entity: You're examining a blood smear and are sure you've spotted a parasite. Then you pull
out every book on the shelf to help identify what you're seeing. I hope this article will help you feel more confident identifying
"the unknown" the next time you encounter it.
Smear prep and evaluation
Although identifying blood parasites from a smear can be difficult, certain steps can increase the reliability of your identification.
First, it's important to understand that blood parasites are only found in peripheral circulation during certain times of
the disease process. So infection does not always equal identification on a smear. Blood film evaluation is just one tool
within the diagnostic testing arsenal. Polymerase chain reaction (PCR) testing is the most reliable and accurate method of
diagnosis and should be used to confirm blood film results.
Gathering a thorough patient history is critical, especially when parasite infection is suspected. Animals that live outdoors,
go outdoors, travel, or visit dog parks or day care facilities carry a higher incidence of infection because of exposure to
vectors. Determining a pet owner's ectoparasite control program may further aid in ruling in or out possible exposure. Remember
that immunocompromised patients are at higher risk for infection.
When preparing a blood smear, make the smear directly from the patient's blood and not from an anticoagulated sample. EDTA
anticoagulant can increase the likelihood for stain precipitation as well as cause Mycoplasma species to fall off the erythrocytes. Prepare multiple slides and, if possible, include samples from both a capillary vein
and peripheral vein. A long read area on the smear is your best chance to find parasites, and a feathered edge is helpful
for trapping infected erythrocytes. Stain slides with both a modified Wright's stain such as Diff-Quik (Dade Behring) and
new methylene blue, making sure all stains are fresh and clean. Rinse slides gently and completely with distilled water, and
allow them to air dry thoroughly. Examine the slides on oil immersion. Be sure to leave some slides unstained for a reference
laboratory to evaluate.
Blood parasites can be found in most regions of the United States; however, knowing which hemoparasites are prevalent in your
area can aid in proper identification. Note that several blood parasites' names have changed. In an effort to help us keep
up-to-date, both the old and new names are referenced below.