Ehrlichia tests (E. canis antibody and PCR) in dogs and cats
Ehrlichiae are tiny tick-borne bacteria that live inside white blood cells. Two tests look for them in different ways: an antibody test shows that the dog’s immune system has met the bacteria, and a PCR looks for their genetic material in the blood. The two answer different questions, so they are often read together and sometimes repeated.
What it shows
The main species in dogs is Ehrlichia canis, carried mainly by the brown dog tick (Rhipicephalus sanguineus). A form can show two lines:
- E. canis antibody (IFA) — the lab measures antibodies in serum by immunofluorescence and reports a titer. In-clinic rapid tests look for the same kind of antibodies but give only positive or negative. Antibodies take time to appear: seroconversion may take 1–4 weeks after exposure, and a dog can fall ill before it.
- Ehrlichia PCR — looks for the bacteria’s DNA in blood. It does not need the immune response to develop, so it can find infection earlier, and it is the test that tells the species apart.
Looking for the bacteria in a blood smear is possible, but they are rarely found there.
Higher than reference — what it may be a sign of
- Antibodies positive — the dog has met Ehrlichia. That is not the same as current disease: titers can persist for months or years, even after signs have gone. Antibodies alone, with no signs of illness, are not regarded as a reason for therapy. Antibodies also cross-react: a positive result may come from a related Ehrlichia species or from Anaplasma.
- Rising titer — to tell a fresh infection from an old one, the test is repeated 2–4 weeks later; a fourfold or greater rise fits active ehrlichiosis. Rapid tests cannot be used for this, since they give no number.
- PCR detected — generally confirms infection and shows which species it is. False positives caused by related organisms have been described. After therapy, DNA may stay detectable for a short time.
Lower than reference
- Antibodies negative — does not rule out an acute infection: in the first days and weeks antibodies may not have formed yet. In regions where the infection occurs, a repeat test 3–4 weeks later is recommended.
- PCR not detected — also does not exclude infection. The number of bacteria in the blood may be below the test’s limit, for example after antibiotics; substances in the sample can also block the test.
Whether and when to repeat a test is the vet’s decision.
Look at together with
- Platelets — a low platelet count is common in E. canis infection and can be steady or come and go.
- Haematocrit — a mild anemia without a strong marrow response is typical.
- Globulins — raised globulins are the most common biochemistry finding.
- Anaplasma and other tick-borne infections — share ticks and cross-react in antibody tests, so the vet may test for them together.
Units and conversion
Both results are qualitative and have no unit. The IFA result is a titer written as 1:N — the highest dilution of serum at which antibodies are still seen, so 1:640 means more antibodies than 1:80. The lab sets its own positive cut-off and prints it on the form. Rapid tests are reported as positive or negative, PCR as detected or not detected. In the app these results are stored as text.
Reference range
The reference range is on your lab’s form: for PCR it is “not detected”, for the antibody test “negative” or a titer below the lab’s cut-off. A positive result is read together with the dog’s signs, platelet count and, if needed, a repeat test.
Cats and dogs
In cats, Ehrlichia infection is much less studied: little is known about how common it is or how it shows itself. An E. canis-like organism has been described in cats, but clinical signs are not well described, and the tests above are mainly used in dogs.
In dogs, acute E. canis disease starts 1–3 weeks after infection; a chronic form can affect the bone marrow. Some Ehrlichia species also infect people, but through tick bites, not from the dog; questions about human risk are for a doctor. Dogs can serve as a sign that infected ticks are present in an area.
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
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.