This one’s as easy to write as the first version… we still have no clue.
Overall, the health risk to horses from SARS-CoV-2 is probably very low. If horses were getting sick, or at least very sick, we would have noticed by now. I’ve not had any indication that we’re seeing a disease impact in this species. We still don’t know if horses are getting infected from people or whether, if infected, they can spread it back to people, between horses or to other animals. That’s something we still need to figure out.
Why don’t we have much information about horses compared to other species?
- Food animals get studied because of concerns about food safety, food security and economics.
- Pets get studied because of our close contact with them (e.g. they live in our homes and sometimes even sleep in our beds).
- Wildlife get studied because we’re worried about this virus getting into wild animal populations that could then act as virus reservoirs and sources of new variants.
Horses are a mix of farm animals, companion animals and competition animals. There’s a relatively small pool of researchers who focus on infectious diseases in horses, and there’s limited research support. Experimental studies in horses are very expensive, because of their size, cost of housing, value of individual animals, and other factors. Field studies are more practical but require people to do the work (we have that) and interest from horse owners and other relevant parties to participate (that’s often missing).
There’s limited interest in (or downright opposition to) surveillance by some, because identifying potential problems leads to having to deal with those problems. If SARS-CoV-2 was identified as a significant issue in horses, that could mean restricting movements on farms with infected horses (which a lot of people want to avoid), as well as dealing with a lot of additional concerns from the general public (including both horse owners and non-horse owners). As a result, we run into barriers to testing in situations where exposure is plausible. I’ve had multiple situations where it would have been very informative to test horses, but where owners/trainers were wary of what would happen if there was a positive result. Since some people are more wary of the impact of a positive result than the impact of the virus itself, we’ve missed out on opportunities to figure out whether there’s any risk to (or from) horses.
So, what do we actually know about SARS-CoV-2 in horses?
One study from Italy tested 34 horses from 2 farms that were exposed to infected people. None of the horses tested positive. The timing of testing isn’t exactly clear, but the study suggests that horses were sampled fairly late after the onset of disease in people, and it’s quite plausible that short, transient infections could have been missed. In some other animal species, individuals tend to only shed the virus for a short period of time, so if sampling is delayed we may not find the virus (but we can still find antibodies after the fact). This was a good initial study to have in horses, but we need more studies where sampling is done closer to the time of onset of disease in people, and ideally with follow-up antibody testing in case we miss the potentially short window when an animal may be shedding the viral bits that can be detected by PCR.
Beyond that, we have older studies that looked at the composition of the ace2 receptor in different animal species. Ace2 is the structure that SARS-CoV-2 uses to attach to the body’s cells. If the virus can’t attach to cells, it can’t infect them. The structure of this receptor varies between species, and that accounts (in part) for differences in species susceptibility. One study ranked the likely susceptibility of horses to SARS-CoV-2 to be equivalent to cats (specifically domestic cats and lions, both of which we know can be infected) and camels (which we also know nothing about, beyond that they are a host for another zoonotic coronavirus that causes Middle Eastern Respiratory Syndrome (MERS-CoV)). We have to take predictive studies like these with a (big) grain of salt, because the real world situation hasn’t always mirrored what was predicted. These studies tell me that we should pay more attention to horses and see if there’s a problem, not that a problem is likely.
What should we do?
Some surveillance for SARS-CoV-2 in horses would be good. Testing horses that have been exposed to infected people would be interesting and let us know more about the potential interspecies spread of the virus. There have been outbreaks of COVID-19 in grooms in racing stables, a population that’s probably very high risk for infection and for working while sick, and of course they have close contact with their horses, so a situation like that would be a good place to start some equine surveillance.
However, the most important thing we need to do is stay away from any animal, including horses, if we have COVID-19. It’s better to prevent a problem than deal with it, and if we reduce the number of infected people that have contact with horses, we reduce any potential problems.