Vida Pharmacal Chagas Registry Q4 2022 Summary Report
Happy and blessed New Year 2023. We have some exciting, useful updates this past quarter with our Chagas prevalence numbers. Cases are climbing! That means you’re testing and screening your patients, and it’s helping us get our head around true infection rates in our region.
One pattern that we’ve noticed over the past few years is low antibody titers on the T cruzi IFA at TVMDL, those below 1:80, and retested dogs sero-reverting to negative on a second test within a month or two of the initial positive Chagas screen. I’d like to take this opportunity to explain what and why we’re seeing this phenomenon.
IFA, or indirect immunofluorescent antibody test, has been the only commercially available antibody screening test in dogs for decades. Performed at TVMDL in College Station, this antibody test uses crude, crushed up epimastigotes from kissing bugs, fixed on a microscope slide, to detect antibodies in companion animals applied directly onto the slide. After washing the slide, a fluorescent tagged anti- antibody is then applied to the sample-slide mix and analyzed under a microscope using a special light. Any infected patient samples will fluoresce, indicating a positive result, and any negative samples will display zero fluorescence. Positive samples are diluted at various volumes, starting out at 1 part per 20 (1:20) and diluted all the way until the sample is either not fluorescing (negative) or if it gets to 1 part per 1280 (1:1280) when the test is terminated. Any fluorescence is indicative of antibodies against T cruzi, and considered an infected patient.
The advantage of this test is that it’s reasonably sensitive (90%) and specific (97%), and available to us. The disadvantages, however, are that it will miss certain antibodies made by our patients against 1) different strains of T cruzi and 2) antibodies against other parts of the parasite that are not on the slide in sufficient numbers. This results in a false negative results (patient actually infected but our test didn’t detect it). Another disadvantage is the high degree of cross reactivity to Leishmania species, and while we don’t have Leishmania in significant numbers in the US, this is a serious problem in Central and South America.
The newer technology, ELISA, uses subunit antigens that are present in ALL species of T cruzi, in high numbers, and have fewer cross reactions with Leishmania species. Sensitivity is 99-100% (chance of finding an infected patient) and specificity is 100%. This is a superior test to the IFA in many ways, and can detect lower levels of antibodies in infected animals. VRL laboratories in San Antonio is using this technology to detect Chagas antibodies, and available to veterinarians now.
Some pets have very low antibody levels on IFA (1:20-1:80) and the recommendations from TVMDL or some specialists may be to recheck this value in 30 days since it’s low. It’s important to know that ANY antibody detected on the IFA, no matter how “low”, is significant and means the patient is infected with T cruzi (if it hasn’t received treatment). Since the IFA has been modified in the past few years by increasing the threshold (lowering the ability of the test to detect antibody levels) of antibody detection, it takes a LOT more antibody to make that test light up and give us a positive. It would follow that when the pets antibodies dip below a certain point, even though they’re present, the test will falsely report negative. We see this clinically sometimes, and can be confusing to both the veterinarian and the owner.
Similar to low titers, patients can also sero-revert from positive to negative on the IFA without treatment. They have one of two processes going on: 1) they self-cured or 2) the antibodies dipped below the threshold for detection. While it’s possible to have self-cure in a dog, its rare, and shouldn’t be expected. The most likely scenario is the antibodies temporarily went below the tests ability to detect antibodies.
In either of these situations of low or conflicting results, ELISA represents a reasonable solution using a sensitive test. .
Please note, and this is VERY important, there are NO FALSE POSITIVES in Texas. We don’t have Leishmania, and there isn’t anything else that cross reacts with the ELISA antibody. You can rely on this test as a gold standard, and trust your results as a positive indicating infection and not just “exposure”.
If you have questions about testing or anything else concerning Chagas disease, please contact us, we’re happy to help.
Roy Madigan and The Vida team