What a Toxoplasma antibody result can and cannot tell you

When a cat's blood test comes back with antibodies against Toxoplasma gondii, owners and clinic teams usually face three questions at once: Is the cat sick because of this parasite? Is it shedding infectious oocysts in the home? Is anyone in the household at risk? Answering them correctly starts with separating what an antibody test measures from what it cannot determine.

The short answer: an antibody titer documents an immune response to past or recent infection, not active clinical disease and not current oocyst shedding. An immunoglobulin G (IgG) result points to exposure at some point in the past. After infection, IgG usually becomes detectable within about 3 to 6 weeks (ELISA-based summaries put first detection at 3 to 4 weeks; the European ABCD guideline cites 4 to 6 weeks), peaks within a few weeks of first detection, and can stay positive for years because the parasite persists in tissue cysts.

Because those tissue cysts persist, a single high IgG titer, even a value such as 1:1000, in an otherwise healthy cat reflects prior infection and most likely immunity rather than active illness. Cornell's Feline Health Center describes such healthy cats as most likely immune and not excreting oocysts, and CAPC describes most seropositive cats as having already completed oocyst shedding and as unlikely sources of human infection.

Immunoglobulin M (IgM) is the recent-infection signal. In ELISA studies, about 80% of infected cats develop a positive IgM titer within 1 to 4 weeks, and it is usually negative again by around 16 weeks; one diagnostic laboratory summary notes that IgM generally does not persist past about 3 months. IgM has known interpretation traps, though. At the common 1:64 cutoff, roughly 2% to 5% of specific-pathogen-free cats, which have never met the parasite, can test falsely positive. About 20% of infected cats never mount a detectable IgM response, and IgM persisting beyond 4 months has been documented in cats coinfected with feline immunodeficiency virus (FIV) and in cats with ocular toxoplasmosis.

Neither antibody class proves clinical toxoplasmosis on its own. ABCD notes that T. gondii antibodies are common in both healthy and sick cats; in one US dataset, 31.6% of 12,628 clinically ill cats were seropositive. Seropositivity can therefore be a background finding rather than the cause of a cat's current signs. Just as important, CAPC states that serologic testing cannot accurately predict oocyst shedding status, that no serologic assay accurately indicates when a cat shed oocysts in the past, and that most cats shedding oocysts are seronegative at the time they shed.

IgG: the exposure result that stays for years

Reading an IgG titer well starts with how cats become infected. Cats and other felids are the only hosts in which Toxoplasma gondii completes its sexual cycle and produces oocysts. Cats are usually infected by eating tissue cysts in the muscle or brain of prey such as rodents and birds, or by being fed raw or undercooked meat.

After ingestion, the cyst wall is digested and the released parasites invade the cat's intestinal lining. Some go through the sexual cycle that is unique to cats and produce unsporulated oocysts, which leave the body in feces. Others spread beyond the gut to tissues such as muscle, lungs, liver, eyes and the central nervous system. As the cat's immune response controls this phase, the parasite settles into slow-growing tissue cysts that persist, which is why antibodies and latent infection can outlast the original exposure by years.

Circulating IgG is the long-term antibody response to that infection, and it does not appear immediately. A 2013 WSAVA congress review of ELISA serology describes positive IgG titers developing 3 to 4 weeks after infection and peaking 2 to 4 weeks after first detection; ABCD and the 2024 European laboratory study describe IgG becoming positive 4 to 6 weeks after infection, with the highest titers 2 to 3 weeks later. The practical range for owners is therefore roughly 3 to 6 weeks before IgG is reliably detectable.

Because the parasite persists in tissue cysts, IgG titers commonly remain positive for years. The WSAVA review states that a single high IgG titer, such as 1:1000, reflects only the presence of the organism in tissues and can persist for years, while a four-fold or greater rise between paired samples 2 to 4 weeks apart indicates recent or active infection. The European Advisory Board on Cat Diseases (ABCD) adds that very high or rising titers raise suspicion in a sick cat but that antibody presence alone does not prove clinical toxoplasmosis.

Population data show how ordinary a positive IgG is. A 2024 retrospective study of samples sent by veterinarians across Europe to a single commercial laboratory (LABOKLIN) between 2008 and 2022 found IgG antibodies in 37.3% of 37,882 cats tested serologically. The authors caution that these submissions were preselected by veterinarians and should not be read as Europe-wide prevalence, but the figure still shows that a positive IgG is a common finding. Treating every positive IgG as active disease, without paired testing and clinical context, would misclassify many cats.

IgM: the recent-infection signal with known traps

Immunoglobulin M (IgM) is the earliest antibody class produced after a first infection. The WSAVA review describes positive IgM titers in about 80% of cats 1 to 4 weeks after infection, usually turning negative by 16 weeks, and the 2024 Los Angeles County study notes that in healthy cats IgM is generally detected first, followed by seroconversion to IgG. A UK diagnostic laboratory summary similarly states that IgM generally does not persist past 3 months after infection.

Interpreting a feline Toxoplasma IgM titer still carries three documented traps:

• False positives near the cutoff: The 2024 Los Angeles County study notes that IgM results can be falsely positive in both IFA and ELISA in approximately 2% to 5% of specific-pathogen-free cats when read at the 1:64 dilution, which is why some field studies have used 1:128 as the cutoff. In that study, 9 of the 13 IgM-positive cats were healthy, IgG negative and exactly at the 1:64 low-end cutoff; the authors treated them as likely false positives, which lowered the IgM estimate from 5.8% to 1.8%. The WSAVA review also lists some healthy cats, FIV-infected cats and cats on glucocorticoids among those with a persistent IgM titer above 1:64 beyond the period of recent infection.

• Persistence in FIV coinfection and ocular disease: IgM titers persisting for more than 4 months have been documented in cats coinfected with FIV and in cats with ocular toxoplasmosis. Separately, the 2024 European laboratory study found that FIV-positive cats had higher odds of being IgM positive (odds ratio 2.650) and IgG positive (odds ratio 2.350). That association concerns positivity, not duration, but it is one more reason to know a cat's FIV status before reading an IgM result.

• Cats that never mount IgM, and reactivated infection: About 20% of infected cats do not develop a detectable IgM response. The WSAVA review also notes that in recrudescent (reactivated) infection there may be no rising IgG titer and IgM may stay negative. A negative IgM therefore does not exclude toxoplasmosis in a sick cat that was infected long ago.

Because of these confounders, a single positive IgM result should not be read as confirmation of active clinical toxoplasmosis. It is interpreted together with the IgG result, the cat's clinical signs, the assay's cutoff and, when the veterinarian wants it, a follow-up sample.

Reading the four IgG and IgM combinations

When both antibody classes are measured, the most informative step is reading them together. Each of the four possible patterns suggests something different about timing, and each has a clear limit on what it can establish. Not every laboratory offers both classes on the same order, so first confirm which antibodies were actually tested.

Among positive results, IgG positive with IgM negative is the pattern owners are most likely to see: in the 2024 European laboratory series, 37.3% of cats tested were IgG positive but only 4.1% were IgM positive. In a healthy cat, that pattern answers the exposure question and little more. In a sick cat, it keeps toxoplasmosis on the list without explaining the illness, so the workup continues for other causes that fit the signs, such as feline leukemia virus and feline immunodeficiency virus infection or, in cats with compatible effusions or neurologic signs, feline infectious peritonitis.

Titers are method-bound: why 1:64, 1:25, 1:50, IU/mL and NTU do not mix

A common misunderstanding is treating Toxoplasma titers as interchangeable numbers. A titer is the highest dilution of serum that still gives a positive reaction on a particular assay, so its meaning depends on the method, the reagents and the laboratory that ran it. Numbers from different methods or laboratories cannot be compared directly.

The named laboratories and studies below show how much the testing pathway, specimen rules and reporting scale can differ:

• Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL): TVMDL lists a Toxoplasma gondii IgG ELISA (test code 1597) as its default Toxoplasma test. The catalog asks for 1 mL of serum collected in a non-additive tube, such as a plain red-top or serum-separator tube, and states that excessively hemolyzed samples are not acceptable. The test runs in College Station on Mondays and Thursdays with a 1 to 2 day turnaround, and the listed price is $31.50. If the ELISA is positive, an IFA is performed at no additional cost at dilutions of 1:64, 1:128, 1:256, 1:512, 1:1024 and 1:2048 for canine or feline samples. No IgM test appears on that catalog page.

• Cornell University Animal Health Diagnostic Center (AHDC): Cornell's 2025 fee schedule lists three different in-house Toxoplasma methods: the FETOX Feline Toxoplasmosis ELISA ($24.50), the TOXOEL Toxoplasma gondii IgG ELISA ($23.00) and the TMAT Toxoplasma MAT ($22.00). Its referral list shows that a Toxoplasma IgG/IgM test (code RTOXOTN) is referred to the Colorado State University Diagnostic Lab. In other words, the IgM part of a feline workup may be run at a different institution from an in-house IgG assay.

• US commercial laboratories in a published study: The 2024 Los Angeles County study used results from IDEXX (IgM/IgG titers by IFA, and a toxoplasmosis split-titer ELISA) and Antech (an IgG and IgM 2-panel ELISA), noting that at least some ELISA samples had quality control performed at Colorado State University. For that study, a titer of 1:64 was considered positive on ELISA and 1:25 on IFA.

• A European commercial laboratory: The 2024 European study, from the LABOKLIN laboratory, used the MegaFLUO TOXOPLASMA IFAT with 1:50 or higher classed as positive until March 2022, then switched to VetLine ELISAs that report IgG in IU/mL (positive above 55 IU/mL) and IgM in NovaTec units (positive above 11 NTU). Even within one laboratory, the reporting scale changed partway through the study period.

The practical rule follows directly: when paired samples are used to look for a rising titer, both samples should go to the same laboratory and be run on the same assay. CAPC and ABCD both recommend antibody tests that report a titer rather than a simple positive or negative, because only a number can show whether a later sample has risen four-fold. Fees, schedules and referral routing above are dated snapshots from each institution and can change; confirm the current details with the laboratory your veterinarian uses.

Why a positive result is not proof of shedding

A common worry is that a seropositive cat is actively shedding infectious oocysts. The biology points the other way: oocyst shedding and antibody production happen at different times, and a positive antibody result usually means shedding is already over.

When an unexposed, immunologically naive cat ingests intermediate host tissues harboring bradyzoite cysts, the parasite initiates sexual replication within the feline intestinal mucosal enterocytes. This enteroepithelial cycle results in the excretion of millions of unsporulated oocysts in the feces. However, this excretion window is remarkably brief:

• Onset of shedding: Oocyst shedding begins approximately 3 to 10 days following initial ingestion of tissue cysts.

• Duration of shedding: Shedding continues for a brief window of roughly 10 to 14 days, and rarely persists beyond 3 weeks.

• Humoral seroconversion lag: In stark contrast, the feline systemic immune system requires 1 to 4 weeks to mount detectable IgM and 3 to 6 weeks to produce detectable IgG.

Because of this timing gap, by the time a cat has detectable IgG, the intestinal shedding phase has usually ended. Conversely, during the short window when a newly infected cat is shedding, it often has not yet produced detectable antibodies. CAPC states that most cats shedding oocysts are seronegative at the time of shedding, which is why a negative antibody result cannot be used to declare a cat is not shedding.

The 2024 European laboratory study illustrates the mismatch. IgG was positive in 37.3% of cats tested serologically, while fecal PCR was positive in 1.6% of 7,896 cats tested. Among 255 cats tested by both methods at the same time, 70 were IgG positive, and only one of those 70 (1.4%) was also fecal-PCR positive. The data come from a single laboratory's preselected submissions, but they are consistent with the guideline position that a positive antibody result mostly reflects past, completed shedding.

Oocyst shedding is often described as happening only once in a cat's lifetime, and ABCD states that natural re-shedding is very unlikely. That expectation is probabilistic rather than absolute. In a 13-cat experimental study cited by ABCD (Zulpo 2018), protection against re-excretion of oocysts after challenge was present in 90% of cats at 12 months, 25% at 24 months and 33.4% at 36 months after the first infection. Re-shedding in such experiments followed challenge with a different parasite strain, and ABCD notes that field studies find very few shedding cats, which indicates re-shedding is very unlikely under natural conditions.

Routine fecal flotation is not a useful way to judge an individual cat's infection status. CAPC notes that T. gondii oocysts (about 10 by 12 micrometers) look the same as those of Hammondia and Besnoitia, which can also appear in cat feces, and that because shedding lasts only about 1 to 3 weeks, most feces from infected cats contain no oocysts. CAPC therefore does not recommend fecal flotation for evaluating an individual cat, and Cornell likewise calls fecal oocyst detection an unreliable diagnostic method. A negative flotation does not show a cat is free of Toxoplasma.

How veterinarians actually confirm toxoplasmosis in a sick cat

Because antibodies are common in healthy cats, diagnosing clinical toxoplasmosis in a sick cat is a veterinarian-led process that combines several lines of evidence. Without finding the organism itself, the result is a presumptive diagnosis, not a definitive one.

The guideline and laboratory sources describe the presumptive diagnosis as resting on these elements together:

1. Compatible clinical signs: Signs described in the guidelines include fever, lethargy, weight loss, uveitis and other eye inflammation, breathing difficulty from pneumonia, neurologic signs such as behavior change, seizures or incoordination, jaundice from liver involvement, muscle pain or lameness, and enlarged lymph nodes. Most infections in cats cause no signs at all.

2. Serology that points to recent or active infection: A very high or rising IgM titer, or a four-fold or greater rise in IgG between paired samples taken 2 to 4 weeks apart and run at the same laboratory on the same method.

3. Exclusion of other causes: Other conditions that can produce the same signs need to be considered and, where possible, ruled out.

4. Response to treatment: Improvement once the attending veterinarian starts treatment. Cornell's Feline Health Center notes that if clinical improvement is not seen within two to three days, the diagnosis of toxoplasmosis may be questioned.

The diagnostic decision workflow is illustrated in the clinical pathway below:

flowchart TD
    A["Sick cat with compatible signs"] --> B["Veterinarian orders IgG and IgM with numeric titers"]
    B --> C{"First result pattern"}
    C -->|"Both negative"| D["Less likely, veterinarian weighs timing and other causes"]
    C -->|"IgG positive only"| E["Past exposure, cannot separate latent from reactivated infection"]
    C -->|"IgM positive"| F["Raises suspicion of recent infection, check cutoff and FIV status"]
    E --> G{"Paired sample 2-4 weeks later, same lab and method"}
    F --> G
    G -->|"At least 4-fold IgG rise"| H["Supports a presumptive diagnosis alongside signs, exclusion and treatment response"]
    G -->|"No 4-fold change"| I["Recent infection not shown, peak may have passed, keep other causes under review"]
    H --> J["Definitive diagnosis only by finding the organism in fluid or tissue"]

A presumptive diagnosis is as far as serology can go. Definitive diagnosis requires demonstrating the organism, usually tachyzoites, by cytology, histopathology, immunohistochemistry or cell culture in samples such as effusions, bronchoalveolar lavage (BAL) fluid, cerebrospinal fluid (CSF), aqueous humor or tissue. The WSAVA review notes that this approach is highly specific but has low sensitivity, so a negative search does not rule the disease out.

Polymerase chain reaction (PCR) testing answers narrower questions, and the right sample depends on the question being asked:

• Fecal PCR: A positive result indicates current oocyst shedding. This is the test for the shedding question, not antibody serology.

• Aqueous humor and CSF PCR: Useful support when eye or central nervous system disease is suspected, and generally preferred over blood PCR. Interpretation still needs clinical context, because the WSAVA review notes that parasite DNA can be amplified from blood, aqueous humor and CSF of some healthy cats with latent infection.

• Blood PCR: Parasites in the bloodstream are seldom detected, so a negative blood PCR does not exclude organ disease.

All treatment choices, including drug selection, dose and duration, remain with the attending veterinarian. This guide deliberately gives no doses: the drugs used for feline toxoplasmosis are chosen and monitored case by case.

The household question: pregnancy, immunosuppression and daily litter care

When a household includes someone who is pregnant or immunocompromised, a feline Toxoplasma report can understandably cause worry. The guideline evidence from Cornell's Feline Health Center, CAPC and ABCD is reassuring for most households, and it also explains which simple precautions matter.

ABCD summarizes the human evidence plainly: contact with cats is not a risk factor for T. gondii infection in people. ABCD identifies eating meat containing tissue cysts as the most common route of human infection, naming undercooked goat, mutton and pork, along with unwashed raw vegetables. CAPC adds that people can be infected by swallowing infective oocysts from contaminated soil, water or produce, and recommends gloves for gardening and washing fruit and vegetables.

A healthy, IgG-positive cat is an unlikely source of infection. CAPC describes most seropositive cats as having completed oocyst shedding and as unlikely to repeat shedding or to be a source of human infection. ABCD adds that stroking a cat will not spread the infection, that oocysts have not been found on the coat even of cats that are shedding, and that bites or scratches from an infected cat do not transmit it.

The same biology explains the practical precautions guidelines recommend:

• Oocysts need time to become infectious: Freshly shed oocysts are unsporulated and not yet infectious. CAPC states that they must develop for 1 to 5 days after being passed to become infectious, and need at least 24 hours.

• Daily litter removal: Removing feces from the litter box every day is designed to clear any oocysts before they can become infectious, which is why CAPC recommends it even though few cats are shedding at any given time.

• Who cleans the litter box: Where possible, a non-pregnant, immunocompetent household member should handle daily litter cleaning. A pregnant or immunocompromised person who must do it should use gloves and wash hands thoroughly afterward, and follow their own clinician's advice.

• Indoor living and diet: Keeping cats indoors to prevent hunting and feeding only commercially prepared diets, never raw or undercooked meat, reduces the chance that a susceptible cat becomes newly infected and sheds.

The antibody record: class, date, method, laboratory and the clinical question

Because a Toxoplasma result only makes sense in light of its timing, method and clinical context, a bare number should not travel alone in a cat's record. A note that reads only 'Toxoplasma positive' or 'titer 1:64' leaves the next veterinarian guessing which antibody was measured, on which scale, and why.

To ensure optimal clinical continuity, veterinary teams and proactive pet owners should maintain a standardized Toxoplasma Antibody Record that preserves the core technical parameters of the diagnostic submission:

When reviewing a laboratory report with your veterinary healthcare team, bring the following practical checklist of questions to ensure complete clinical clarity:

• Which antibody class was measured, IgG, IgM or both, and what was the exact numeric titer?

• Which laboratory and assay ran the test, and what is that assay's positive cutoff?

• Given my cat's examination and any signs, does this result mainly reflect past exposure, or does it raise suspicion of recent infection?

• Would a second sample in 2 to 4 weeks change the plan, and can it go to the same laboratory on the same method?

• Is my cat's FIV status known, and which other causes of the current signs should be checked at the same time?

Keeping antibody class, numeric titer, laboratory and method, cutoff, sample date, clinical question and any recheck plan together on one record lets the next veterinarian read the result correctly. This record is not proof of current oocyst shedding and not evidence of infection in any person in the household. Questions about shedding are answered by fecal PCR when a veterinarian thinks it matters, and questions about a person's own risk or testing belong with that person's physician or obstetrician.

Sources

The public sources below were used for the timing, laboratory and household statements in this guide. Laboratory fees, schedules and referral routing are dated snapshots and may change.

• Cornell Feline Health Center: Toxoplasmosis in Cats. Owner-facing overview of shedding timing (3 to 10 days onset, about 10 to 14 days), IgG and IgM meanings, fecal testing limits and treatment-response expectations.

• Companion Animal Parasite Council (CAPC): Toxoplasma gondii - CAPC Guidelines (Last updated Jul 1, 2025). Guideline on serology versus shedding, fecal flotation limits, fecal and fluid PCR, oocyst sporulation time and prevention.

• European Advisory Board on Cat Diseases (ABCD): ABCD Guideline for Toxoplasma gondii infection in cats (last updated 08/02/2024). European guideline on antibody interpretation, clinical signs, re-shedding data (Zulpo 2018) and human risk factors.

• Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL): Toxoplasma gondii IgG (ELISA) - TVMDL test catalog. Catalog listing for the IgG ELISA, specimen rules, schedule, fee and reflex IFA dilutions.

• Cornell University Animal Health Diagnostic Center (AHDC): Fees for 2025 - Cornell Animal Health Diagnostic Center. 2025 fee schedule listing FETOX, TOXOEL and TMAT Toxoplasma assays.

• Cornell University Animal Health Diagnostic Center (AHDC): Referral Laboratories and Tests - Cornell Animal Health Diagnostic Center. Referral list showing the Toxoplasma IgG/IgM test (RTOXOTN) sent to Colorado State University.

• Journal of Feline Medicine and Surgery (2024): Serological and molecular detection of Toxoplasma gondii in cats in Europe with evaluation of associated risk factors for pathogen contact/infection. Retrospective single-laboratory series of 45,523 cats (2008 to 2022) reporting IgG, IgM and fecal PCR results, named assays and cutoffs, and FIV associations.

• JFMS Open Reports (2024): Prevalence of Toxoplasma gondii IgM and IgG positive cats in Los Angeles County, California. Single-county study of 225 cats describing the commercial assays used, the 1:64 ELISA and 1:25 IFA cutoffs, and IgM cutoff sensitivity.

• WSAVA World Congress Proceedings (Barrs V., 2013): Feline Toxoplasmosis - WSAVA 2013 Congress proceedings. Clinician review of ELISA IgG and IgM kinetics, paired-sample interpretation, false-positive and false-negative IgM patterns, and definitive diagnosis.

• Batt Laboratories Ltd (UK): Toxoplasmosis: diagnostic tips - Batt Laboratory. Diagnostic laboratory summary of IgM persistence in FIV coinfection and ocular disease, shedding timing relative to IgG, presumptive-diagnosis criteria and PCR sample choice.