Blood parasites on a blood smear in cats and dogs
This line means that a stained blood smear was searched under the microscope for organisms living in or on blood cells. Finding one is a strong clue, but the smear is insensitive: many infections are missed, so “not seen” does not rule an infection out. The vet often confirms or excludes a specific agent with PCR or an antibody test.
What it shows
What can be recognised on a smear:
- Haemotropic mycoplasmas (haemoplasmas) — small blue cocci, rings or rods on the edges or across the faces of red cells.
- Babesia — inside red cells. Large species such as B. canis appear singly or in groups; B. gibsoni is much smaller and forms rings with a dark nucleus on one side. A related parasite, Theileria, is told apart from Babesia by specific PCR.
- Cytauxzoon felis (cats) — tiny ring or safety-pin shaped bodies in red cells; rarely, large macrophages filled with dividing forms of the parasite (schizonts) at the thin edge of the smear.
- Ehrlichia and Anaplasma — clusters called morulae: in granulocytes (A. phagocytophilum, E. ewingii), in monocytes or lymphocytes (E. canis) or in platelets (A. platys).
- Hepatozoon — parasites inside white cells.
- Microfilariae — larval worms, for example of heartworm (Dirofilaria immitis).
Higher than reference — what it may be a sign of
For a smear, “higher than reference” means organisms were seen. What the vet weighs:
- Active infection — organisms in red cells together with anaemia point to a cause of red cell destruction; morulae together with low platelets or fever point to tick-borne disease.
- Carriage without disease — subclinical Cytauxzoon infections are recognised, so seeing the organism does not always mean it is causing the illness. Haemoplasma carriers also exist.
- A look-alike — fine stain precipitate from old stain can mimic organisms on red cells and is often mistaken for haemoplasmas. Water in the fixative creates shiny spots taken for inclusions, and a platelet lying on top of a red cell can be mistaken for a red cell parasite.
Cytauxzoon schizonts in a blood smear are considered proof of that infection.
Lower than reference
“Not seen” is the usual result, but the smear misses a lot:
- Morulae are difficult to find even in the acute stage and are found only during acute infection.
- Haemoplasma numbers in blood rise and fall, and organisms fall off red cells in samples older than 24 hours.
- Babesia is readily missed on a quick look, and in chronic cases parasites in blood are very hard to detect — PCR is needed.
- Cytauxzoon is easily missed when parasite numbers are low, as in early infection.
- Microfilariae are absent in some heartworm-infected dogs, so a heartworm antigen test is a separate question.
Look at together with
- Haemoplasma PCR and Babesia tests — when anaemia raises the question of red cell parasites.
- Ehrlichia and Anaplasma antibodies or PCR — morulae are rare, so these tests do most of the work.
- Heartworm antigen test — microfilariae and adult-worm antigen are different findings.
- Hematocrit, reticulocytes, platelets — the blood count that gives the finding its context.
Units and conversion
There are no units: the result is a description. The form usually says “not seen” or names what was seen (for example, “Babesia-like organisms” or “microfilariae”), sometimes with a rough amount such as few or many. In the app the result is stored as text, as written on the form.
Reference range
The reference range is on your lab’s form. For this screen the expected result is that no parasites are seen. A positive smear is a clue that the vet usually confirms with a specific test; a negative smear, given how often organisms are missed, is read together with the history, the blood count and other tests.
Cats and dogs
In cats the smear may show Mycoplasma haemofelis; infected cats can become carriers without signs and develop milder disease again when stressed. Cytauxzoon felis is spread mainly by the Lone Star tick, with bobcats as the reservoir; it is common in Missouri and Oklahoma and found in other south-eastern US states, and disease is typically sudden. Babesia felis is rare and very hard to see. Fewer than 20% of cats with mature heartworm infection have microfilariae in blood, and only for a short time.
In dogs large Babesia such as B. canis and B. vogeli, and the small B. gibsoni can be seen in red cells. With Hepatozoon canis parasite numbers in blood are often high, while H. americanum is rarely found in blood and is diagnosed mainly by muscle biopsy. A carrier dog with M. haemocanis can develop anaemia after its spleen is removed. Heartworm microfilariae have to be told apart from those of Acanthocheilonema reconditum.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University College of Veterinary Medicine). Infectious agents in blood smears.
- eClinPath (Cornell University College of Veterinary Medicine). Mycoplasma.
- eClinPath (Cornell University College of Veterinary Medicine). Babesia/Theileria.
- eClinPath (Cornell University College of Veterinary Medicine). Cytauxzoon.
- eClinPath (Cornell University College of Veterinary Medicine). Infection mimics.
- Companion Animal Parasite Council (CAPC). Ehrlichia spp. and Anaplasma spp. guidelines.
- Companion Animal Parasite Council (CAPC). Heartworm guidelines.
- Merck Veterinary Manual, professional version. Old World Hepatozoonosis and American Canine Hepatozoonosis.
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.