Toxoplasma tests (IgG, IgM, PCR) in cats and dogs
Toxoplasma gondii is a single-celled parasite. Cats are its only definitive host: after a first infection they shed its oocysts — tiny egg-like forms of the parasite — in faeces for a short time. Dogs and many other animals can be infected too. Antibody tests show contact with the parasite and roughly how recent it was; PCR looks for the parasite itself. Many healthy cats have antibodies, so a positive result is not a diagnosis of illness.
What it shows
A form can show three lines:
- IgG — antibodies that appear a few weeks after infection and can stay raised for years. They show an infection that the body has met and brought under control.
- IgM — antibodies that appear sooner than IgG and generally do not persist beyond 3 months. They point to a recent infection, though they are occasionally found in healthy cats.
- PCR — looks for the parasite’s DNA. The parasite is rarely found in blood, so PCR is done on fluid or tissue from the affected area: cerebrospinal fluid, eye fluid (aqueous humour), effusions, lavage fluid or biopsies.
The antibody lines may name the method: IFA (immunofluorescence) or ELISA; agglutination tests are also used.
Higher than reference — what it may be a sign of
- IgG positive — the animal has been infected at some point. High IgG in a healthy cat is read as past infection. An antibody-positive cat is extremely unlikely to shed oocysts, even after re-exposure or immunosuppression. Antibodies are common in healthy and sick cats alike, so they do not prove a current illness.
- IgM positive — fits recent or active infection. In Merck’s general section, IgM titres above 1:256 are consistent with recent infection; in cats, high IgM in serum or cerebrospinal fluid supports the diagnosis.
- Rising titre — very high or rising IgG or IgM raises the suspicion of clinical toxoplasmosis. IgG is compared in paired samples, taken during illness and later in recovery, looking for at least a fourfold change.
- PCR detected — the parasite’s DNA is in the sample; it is read together with the signs and antibodies.
Lower than reference
A negative antibody result in a healthy cat suggests it has not met the parasite and can still become infected. Very early after infection antibodies may not have appeared yet. A negative PCR does not exclude infection: the parasite may simply not be in that sample. Faecal tests are not a reliable way to diagnose toxoplasmosis: oocysts look like those of related parasites, flotation needs over 1000 oocysts per gram of faeces to see them, and cats are often no longer shedding when they show signs.
Look at together with
- Signs and the organ involved — eyes, nervous system, muscles; the sample for PCR is chosen accordingly.
- FIV — cats with FIV are extremely susceptible to acute generalised toxoplasmosis.
- In dogs — tests for Neospora caninum, a related parasite from which toxoplasmosis has to be told apart.
Units and conversion
Results have no unit. Antibodies are reported as positive or negative or as a titre — the highest dilution of serum, written 1:N, at which antibodies are still detected; the larger N, the more antibody. PCR is reported 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, where the lab states from which titre it calls a result positive. Results are read as a pattern: IgG alone in a healthy animal points to past infection, IgM or a rising titre to recent or active infection. A definitive diagnosis requires finding the parasite in tissue under the microscope, so the vet combines signs, antibodies and, where possible, PCR.
Cats and dogs
Cats usually shed oocysts only once in a lifetime. Shedding begins three to 10 days after eating infected tissue and lasts around 10 to 14 days. Eye and nervous-system disease can appear in immunocompromised and elderly cats. What a cat’s result means for people in the household is a question for their own doctor; the chance of exposure from a cat at home is relatively small.
Dogs can be infected but do not shed oocysts, because only felids are definitive hosts. The parasite can occasionally cause inflammation of the brain and spinal cord in puppies. It has to be told apart from Neospora caninum, a common cause of such inflammation in dogs. As a general rule, toxoplasmosis is diagnosed by finding the organism in tissue or a fourfold rise of IgG in paired sera.
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.