As I keep going through these updates, some are pretty easy since we’ve learned little since I originally wrote them last year. For cattle, that’s largely because the initial take home message was “they’re not susceptible to SARS-CoV-2, so mooove on.”
That’s not a completely accurate assessment since “susceptible” isn’t really yes/no distinction in many species when it comes to this virus. However, initial studies showed that cattle were so minimally susceptible to SARS-CoV-2 that there was no real concern about health risks to cattle, or about cattle being able to infect other cattle or other species.
The disclaimer I now must add for most of our experimental study results is that we don’t know it this is still true with current variants. Studies in cattle, as with most other experimental studies performed early in the pandemic, used the older / original SARS-CoV-2 strains. As we’ve moved through waves of different variants, particularly alpha to delta to omicron, this virus has changed a lot. For example, omicron is much more transmissible in humans, and behaves differently in multiple other ways too.
Might that impact susceptibility of different animal species? Maybe. We really don’t know (or at least I don’t).
Experimental studies with a containment level 3 virus like SARS-CoV-2 are a lot of work and expense, are only possible in a small number of facilities, and they require euthanizing experimental animals, so such studies are not common. It would be useful to know if omicron behaves any differently in cattle, but it’s hard to say whether we’ll see any more experimental studies to investigate this.
We can also learn from field studies, such as testing of cattle that have been in contact with infected people. However, there’s often a pronounced aversion to testing in situations like this, in large part due to a desire of some people not to know (because they would then potentially have to deal with the ramifications thereof). Our research group hasn’t tested any cattle, and I haven’t heard of any other field work being done in cattle.
So, we’re left with what we know from experimental studies involving strains of the virus that are basically no longer circulating. Cattle are probably still minimally susceptible, without relevant human or animal health risks, but it would be nice to be able to say something more specific than “probably”.
Here’s the recap of those experimental studies:
- One of the first experimental studies in cattle (Ulrich et al. 2020) supported the notion that cattle are pretty resistant to SARS-CoV-2. The researchers found viral RNA was transiently detected from nasal swabs of 2/6 cattle that were exposed to the virus, and those two animals mounted a very low antibody response. Viral RNA levels were also pretty low, based on the PCR results, suggesting there wasn’t enough being shed for the animals to be infectious to others, and no cattle that were co-housed with the inoculated cattle became infected. So, in two animals there was some degree of infection that stimulated the immune system to respond, but it was a minimal response.
- Another study (Falkenberg et al. 2021) infected colostrum-deprived calves (i.e. calves that didn’t get antibodies from their dam and are therefore more susceptible to some infections). After inoculation, the researchers only occasionally found detectable viral RNA by PCR in swabs from the calves, which suggests a very low viral burden. Other tested tissues didn’t show any evidence of infection. Infectious virus was not isolated from any calf, leading the authors to conclude that there was no replication of the virus in the animals.
- One other study (Bosco-Lauth et al. 2021) tried to infect 3 cattle with SARS-CoV-2. None shed infectious virus. One of the three had a positive PCR result after inoculation. When tissues were tested after euthanasia, live virus was isolated from the trachea of one calf but antibodies against the virus were not detected in any calf. They concluded cattle were “minimally permissive to infection”.
So, going back to our two key questions (and basing our answers on what we know from older SARS-CoV-2 strains):
1) Can cattle get sick from SARS-CoV-2?
- Probably not. Studies have been limited, but nothing suggests that cattle are high risk of getting infected and sick.
2) Can infected cattle shed SARS-CoV-2, posing a risk to other cattle or other animals (people being part of the “other animals” group)?
- That doesn’t look like a concern either. Even if the odd cow can get transiently and mildly infected, it’s very unlikely they would be able to pass the virus on to anyone else.
My conclusion in the first version of this review was “…let’s not ignore cattle completely, and still try to keep infected people away from them. But, we can probably relax when it comes to SARS-CoV-2 and this species.”
I’d say that’s probably still a fair assessment, but lack of information about current variants reduces confidence in this statement a bit.