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Real use case: Two male neutered Felis catus individuals live in a household where up to three humans live of which all were infected with SARS-CoV2 at the same time, proven by antigen and PCR tests. All three humans were clearly symptomatic. Funny sidenote: All three humans most likely contracted the infection at the same event, however two were diagnosed with omicron and one with delta by sequentiation.

After a week and with the symptoms of the humans already declined one of the cats developed symptoms with similarities to the humans in the following temporal order:

  • sneezing
  • coughing
  • repeated choking while feeding
  • sneezing with sputtering blood from the nostrils

After onset of symptoms and three days later an antigen lateral flow test designed for detection of SARS-CoV2 in humans was applied to the symptomatic cat. Sample was taken from the throat. Test kit was from brand Longsee. Both tests yielded a clear positive result. The cat was treated symptomatically and has recovered.

The results were reported to different veterinaries including the district veterinary office. All stated that the test result is void and most likely false positive, because tests designed for use on humans can't be used on other animals.

I want to discuss, which possible causes might lead to a false positive in this case.

I think it is very unlikely that we have a false positive here, my thoughts are

  • To my knowledge lateral flow tests for SARS-CoV2 detect either a spike protein or a capsid protein, so they react only on presence of parts of the virus.
  • The lateral flow tests are designed to be very specific to that virus. There are lots of corona viruses able to infect humans and provoke symptoms of a typical cold. So the tests should not react to other corona viruses.
  • There are corona viruses specific to other mammals (e.g. FCoV), however many of them have strong similarities to human corona viruses, so it is very likely that the tests are specific enough to rule out false positive

What aspects did I miss? Which effects you can think of preventing fairly good specificity of such tests when used in cats?

Maybe this question includes another question: What is the nature of false positives in lateral flow antigen tests for SARS-CoV2?

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I had a look around the literature, and it seems that the tests are slightly less sensitive for some of the variants of concern on non-human saliva samples than they are for human1, but in general can be used to test for SARS-CoV-2 infection in animals. The linked study investigated bats, cats, ferrets, sheep, goats, cattle and pigs. RAT (Rapid Antigen Test, AKA LFA for Lateral Flow Assay) style tests have also been used successfully to test for SARS-CoV-2 in lions in Spain2

The tests largely detect nucleoprotein, as this is the most abundant protein in the virion.

to quote conclusion of the first linked article:

In summary, saliva of the species tested has no impact on the functionality and performance of SARS-CoV-2 RATs. Therefore, the use of RATs can be recommended as a point-of-care surveillance tool for SARS-CoV-2 infections in these species. However, the tests should be checked beforehand for their suitability to equally detect VOCs B.1.1.7 and B.1.351.

The only thing I would be concerned about is a false positive from the person sampling the animal contaminating the test.

However, I've also had a look around and the CDC and USDA do not currently recommend testing animals (admittedly, this is for farm-animals, not pets) unless all other causes of illness have been ruled out, and in consultation with state authorities, and it looks like the only authorized tests in the USDA regulated diagnostic labs are the PCR tests. Based on this, it is likely that your vet is being cautious and is following the guidelines published by your state/federal authorities on testing for SARS-CoV-2 in animals - which would mean a PCR test is needed for confirmation of symptomatic cases.

  1. Hagag, I. T., Weber, S., Sadeghi, B., Groschup, M. H., & Keller, M. (2021). Impact of animal saliva on the performance of rapid antigen tests for detection of SARS-CoV-2 (wildtype and variants B.1.1.7 and B.1.351). Veterinary microbiology, 262, 109243. https://doi.org/10.1016/j.vetmic.2021.109243

  2. Fernández-Bellon, H., Rodon, J., Fernández-Bastit, L., Almagro, V., Padilla-Solé, P., Lorca-Oró, C., Valle, R., Roca, N., Grazioli, S., Trogu, T., Bensaid, A., Carrillo, J., Izquierdo-Useros, N., Blanco, J., Parera, M., Noguera-Julián, M., Clotet, B., Moreno, A., Segalés, J., & Vergara-Alert, J. (2021). Monitoring Natural SARS-CoV-2 Infection in Lions (Panthera leo) at the Barcelona Zoo: Viral Dynamics and Host Responses. Viruses, 13(9), 1683. https://doi.org/10.3390/v13091683

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  • $\begingroup$ Can you please define RAT? $\endgroup$
    – MattDMo
    Mar 30 at 14:39
  • $\begingroup$ @MattDMo - will do: Rapid Antigen Test. AKA LFA (Lateral Flow Assay). $\endgroup$
    – bob1
    Mar 30 at 19:05

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