A new experimental study of SARS-CoV-2 in cats (Bosco-Lauth et al. 2020, available in pre-print on bioRxiv) re-inforces information from earlier studies and provides some important new insights. The standard disclaimer that the information is from a pre-print (non-peer-reviewed) source applies, but the science seems pretty sound.
For this study, seven adult cats and three adult dogs were studied.
Cats – Group 1
- One group of 3 cats was infected intra-nasally with SARS-CoV-2 and then tested for the virus on days 1-5, 7, 10 and 14 after infection. Blood was collected on days 7, 14, 21, 28, 35 and 42, looking for an antibody response. This group was then re-exposed to the virus on day 28, with samples for virus detection collected 1, 3, 5, 7 and 10 days after the 2nd exposure.
- None of the cats got sick, but cats shed the virus for up to 5 days after exposure, with peak shedding on day 3.
- Nasal viral levels (from nasal flush samples) were higher than oral viral levels. That’s important information for surveillance studies. Unfortunately it appears fecal samples weren’t tested. That’s a missed opportunity to learn about fecal shedding of the virus, which seems to be a potential issue worth exploring more.
- Infected cats developed detectable antibodies as early as day 7, with all cats reaching a threshold titre by day 14. Antibody titres stayed stable or increased moderately between days 28 and 42, after the second round of exposure. No viral shedding was noted after re-exposure.
Cats – Group 2
- Two cats were infected, then mixed with two unexposed cats 48 hours later.
- The two directly exposed cats shed virus like the group 1 cats. Interestingly, the other two cats they were mixed with started shedding virus within 24 hours of being housed with the infected cats, but had a more prolonged shedding period, with peak shedding occurring at 7 days post-exposure.
- All four cats developed an antibody response.
- Three dogs were exposed and tested as per the cats in group 1. None developed any signs of disease and viral shedding was not detected. Antibody response wasn’t evaluated.
The short duration of shedding by infected cats is encouraging (although a bit different from some natural infections that have been followed). However, the longer duration of shedding in the in-contact (vs directly infected) cats may be more relevant to a natural exposure situation. A shorter shedding period reduces the risk of passing the virus onto another animal or person. The relatively short shedding period also shows how active surveillance studies (like we’re doing, where we sample pets of infected people) can underestimate transmission because of the narrow window of time that infected animals will test positive. This shows again why antibody testing will be useful to characterize how much human-pet transmission has occurred.
The lack of viral shedding after re-exposure in the group 1 cats is also encouraging, as it suggests that previous infection provides some protection. There are many caveats, such as the very small sample size, experimental nature of infection and short interval between first infection and re-exposure, but it’s a start, and provides more hope that people may be resistant to reinfection (to at least some degree, and for at least a while) after infection.
The consistent production of antibodies and their presence over at least 42 days in these animals is also good news for future antibody surveillance studies to look back at previous infection. It’s also encouraging from an immunity standpoint since rapidly disappearing titres would suggest less protection from subsequent infection.
The lack of disease in this small group of cats is a bit different from other studies. We can’t say much about the occurrence of illness based on a small experimental study, since it seems like disease can occur in naturally exposed cats. This study shows that not all exposed cats get sick, as expected. Experimental studies can provide useful information but we have to be careful extrapolating too much to the real world situation, where animal health status, other risk factors and different types of exposure can occur.