Investigating the use of human COVID-19 rapid assays to detect antibody and antigen in domesticated dogs (Canis lupus familiaris) and cats (Felis catus)

This study evaluated the efficacy of human COVID-19 rapid antigen and antibody tests on domesticated dogs and cats, finding that while these kits exhibit variable sensitivity and specificity compared to gold-standard methods, they may still offer utility for field surveillance in the absence of species-specific assays.

The Gist

Imagine a world where a new, invisible storm (the SARS-CoV-2 virus) is sweeping through human neighborhoods. Scientists know this storm doesn't just stay inside houses; it can drift into the yards of our furry friends, too—our dogs and cats. But here's the problem: we didn't have any special "storm detectors" built specifically for pets.

So, a team of scientists asked a clever question: "Can we borrow the storm detectors we use for humans and use them on our pets instead?"

Here is the story of their experiment, explained simply.

The Mission: Borrowing Human Tools

Think of the human rapid tests (the ones you might have used at home during the pandemic) as universal flashlights. They are cheap, easy to use, and everywhere. But they were designed to shine a light on human viruses.

The scientists wanted to see if these same flashlights could shine a light on the virus inside a dog's nose or a cat's throat, and if they could find the "immune footprints" (antibodies) left behind in a dog's or cat's blood.

The Experiment: A Lab Detective Story

The team gathered samples from 60 pets (32 dogs and 28 cats) living in homes where the humans had recently been sick with COVID-19.

1. The Nose/Throat Test (Antigen Test): They took swabs from the pets' noses and mouths. Instead of using the special liquid the test kit came with (which is meant for human noses), they used the liquid the vets had already put the swabs in. They dropped this liquid into the human test kit to see if it would turn the little window red (positive) or stay blue (negative).
2. The Blood Test (Antibody Test): They took blood from the pets and used a different human test kit to see if the pets' bodies had built up defenses (antibodies) against the virus.

To make sure they were being fair, they compared their "borrowed" human tests against the Gold Standard—the super-accurate, expensive, high-tech lab tests (like a master detective with a magnifying glass).

The Results: Good, but Not Perfect

The results were a bit like trying to use a human-sized umbrella on a golden retriever. It fits, but it's a little awkward, and sometimes rain gets through.

• For Dogs:
  • Nose/Throat Test: The human test caught the virus in dogs about 75% of the time. It was pretty good at saying "No virus" when there wasn't one (80% accuracy).
  • Blood Test: It found the immune defenses in dogs about 70% of the time.
• For Cats:
  • Nose/Throat Test: This was trickier. The test only caught the virus in cats about 57% of the time. However, if it did say "Positive," it was almost certainly right (95% accuracy).
  • Blood Test: It found the immune defenses in cats only 50% of the time (half the time it missed them), but if it said "Positive," it was 100% correct.

The Takeaway: The tests worked, but they weren't perfect. They missed some infections (false negatives), especially in cats. However, when they did say a pet was sick, they were usually right.

Why Did They Miss Some Cases?

The scientists realized a few things went wrong with the "borrowed" tools:

1. The Recipe Was Wrong: Human tests are designed for fresh swabs dipped in a specific human-friendly liquid. These scientists used old samples that had been frozen and thawed, and they used a different liquid. It's like trying to bake a cake with a recipe meant for bread; it might work, but the texture will be off.
2. The "Faint Signal": Sometimes the virus or antibodies in the pets were just too weak to trigger the test's little red line. It's like trying to hear a whisper in a noisy room; the test just couldn't pick up the quiet signal.
3. Freezing Issues: The samples had been sitting in a freezer for months. Just like ice cream melts and loses its shape, freezing and thawing can sometimes damage the delicate virus particles or antibodies, making them harder to detect.

Why Does This Matter?

Even though the tests weren't perfect, the scientists say they are still useful tools, especially for the "One Health" approach.

Think of One Health as a neighborhood watch where humans, animals, and the environment are all neighbors. If a virus mutates (changes) in a cat or dog, it could potentially jump back to humans and cause a new, worse storm.

Since we don't have special, expensive "pet-only" tests yet, using these cheap, human rapid tests is like using a flashlight in the dark. It's not a perfect spotlight, but it's better than nothing. It helps veterinarians and public health officials keep an eye on the pets, catch outbreaks early, and stop the virus from spreading between species.

In short: We can use human COVID tests on pets to get a general idea of who is sick. It's not a perfect diagnosis, but it's a helpful, affordable first step to keep our furry friends and our families safe.

Technical Summary

1. Problem Statement

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) infects domestic dogs and cats, primarily through human-to-animal transmission. While gold-standard diagnostic methods like Real-Time Polymerase Chain Reaction (RT-PCR) for viral RNA and Plaque Reduction Neutralization Testing (PRNT) for antibodies are highly accurate, they are costly, require specialized laboratory infrastructure, and are inaccessible for widespread surveillance, particularly in low-resource settings.

Currently, no species-specific rapid diagnostic tests (RDTs) exist for SARS-CoV-2 in animals. There is a critical need to determine if commercially available, affordable human rapid antigen and antibody tests can be repurposed ("off-label") for surveillance in domestic pets to facilitate a "One Health" approach to monitoring zoonotic and reverse-zoonotic transmission.

2. Methodology

The study was conducted in a Biosafety Level 2 (BSL-2) facility between November 2021 and March 2022, utilizing archived biological samples collected from pets in COVID-19-positive households in Texas (2020–2021).

• Study Population:
  • Respiratory Samples: 60 animals (32 dogs, 28 cats) with nasal/oral swabs stored in Viral Transport Medium (VTM).
  • Serum Samples: 40 animals (20 dogs, 20 cats).
• Reference Standards (Gold Standards):
  • Antigen: RT-qPCR (molecular detection of viral RNA).
  • Antibody: PRNT90 (viral neutralization assay).
• Test Kits Evaluated:
  • Rapid Antigen Test (RAT): Abbott BinaxNOW® Covid-19 Antigen Card Test (Human use).
    • Protocol Modification: Instead of fresh swabs, 100 µL of VTM solution was applied directly to the device. The standard buffer was omitted.
  • Rapid Antibody Test: EcoTest® COVID-19 IgM/IgG Rapid Test Device (Human use).
    • Protocol Modification: 40–50 µL of animal serum was used in place of human serum.
• Experimental Design:
  • Samples were previously frozen and thawed (up to 6 months storage).
  • The investigator was blinded to the RT-qPCR and PRNT status of the samples.
  • Performance metrics (Sensitivity, Specificity, PPV, NPV) were calculated separately for dogs and cats.

3. Key Contributions

• Off-Label Validation: This is a comprehensive evaluation of human rapid diagnostic kits on canine and feline biological samples, addressing a gap in veterinary surveillance tools.
• Species-Specific Performance Data: The study provides distinct performance metrics for dogs versus cats, highlighting that these assays do not perform uniformly across species.
• Methodological Insight: It documents the limitations of using archived, frozen samples in VTM for rapid antigen testing, offering data on how sample handling affects test efficacy.
• One Health Framework: The study supports the integration of affordable, portable testing into veterinary and public health surveillance to monitor viral evolution and spillover risks.

4. Results

A. Rapid Antigen Testing (vs. RT-qPCR)

• Dogs:
  • Sensitivity: 75%
  • Specificity: 79%
  • PPV: 55% | NPV: 90%
• Cats:
  • Sensitivity: 57%
  • Specificity: 95%
  • PPV: 80% | NPV: 87%
• Observation: Cats showed higher specificity but lower sensitivity compared to dogs.

B. Rapid Antibody Testing (vs. PRNT)

• Dogs:
  • Sensitivity: 70%
  • Specificity: 60%
  • PPV: 64% | NPV: 67%
• Cats:
  • Sensitivity: 50%
  • Specificity: 100%
  • PPV: 100% | NPV: 40%
• Observation: All positive results were IgM positive (indicating recent infection), with no IgG detected. This aligns with the acute phase of infection in the owners at the time of sampling.

C. Comparative Analysis
The performance characteristics observed in animals were generally lower than or comparable to the average sensitivity (50.0–84.3%) and specificity (64.5–74.3%) reported for these same kits in human populations.

5. Significance and Discussion

• Utility for Surveillance: Despite imperfect sensitivity and specificity, these low-cost, portable tests offer a viable solution for field surveillance in veterinary clinics, shelters, and households where gold-standard lab testing is unavailable.
• Limitations Identified:
  • Sample Type: The use of frozen/thawed VTM samples rather than fresh swabs likely reduced antigen sensitivity.
  • Protocol Deviation: Using VTM directly without the kit's buffer and using animal serum instead of human serum may have altered test chemistry.
  • Subjectivity: Faint test bands (low viral load) made visual interpretation difficult.
• Biological Implications: The detection of IgM (rather than IgG) suggests the animals were sampled during acute infection. The higher specificity in cats compared to dogs remains unclear but may relate to species-specific biological differences or viral load dynamics.
• Future Directions: The authors advocate for the development of species-specific rapid assays. However, until then, human rapid tests can serve as a "first-line" screening tool to identify potential outbreaks, provided their limitations (particularly lower sensitivity) are understood.

Conclusion:
The study concludes that while human rapid SARS-CoV-2 assays are not perfect substitutes for gold-standard methods in animals, they possess sufficient utility for initial screening and surveillance in resource-limited settings. This approach supports the "One Health" initiative by enabling broader monitoring of SARS-CoV-2 at the human-animal interface to mitigate the risk of viral evolution and future pandemics.