This one’s easy to answer: we have no clue.
There’s been almost no investigation or research regarding this virus in horses. Horses often get left out in situations like this because they’re livestock, but not (typically) food animals, and investigation of livestock tends to focus on food animal species. Horses are often more akin to companion animals, but a smaller number of people own or have contact with horses compared to household pets. Experimental studies aren’t commonly undertaken because trials in horses are generally very expensive due to their size and upkeep.
So, what do we actually know about SARS-CoV-2 and horses?
Not much. As I’ve mentioned in several other posts, there are studies that have looked at the composition of the ace2 receptor in different animal species. Ace2 is the part of the SARS-CoV-2 virus that it uses to attach to (and ultimately invade) 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. Not all studies have included horse, but one ace2 receptor study suggested that horses might be susceptible to SARS-CoV-2 – possibly even more susceptible than a few species we already know are quite susceptible, such as cats and ferrets. Another study based on ace2 receptor analysis 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 their being a host for another zoonotic coronavirus, Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV)).
We have to take such predictive studies with a (big) grain of salt, because the real world situation hasn’t always mirrored what was predicted. Those studies basically tell us we should pay more attention to horses and see if there’s a problem, not that a problem is necessarily likely.
Have any horses been tested for SARS-CoV-2?
Maybe. I haven’t tested any, and I haven’t heard of anyone who has, but it’s possible someone’s looked but not found the virus, or antibodies against it, in horses. If there was a positive, I assume it would have been reported somehow somewhere. I suspect few, if any, horses have been tested.
What should be done with horses?
Some surveillance would be good. Testing horses that have been exposed to infected people would be interesting, and tell us more about interspecies spread of the virus. There have been outbreaks of COVID-19 in grooms in racing stables, a human population that’s probably very high risk for infection, and for working while sick. They usually have close and frequent contact with horses, so testing horses from stables with outbreaks in the grooms or other staff would be a good start.
However, as for all animal species, the most important thing to do is stay away from them if you have COVID-19 or if you’ve had high-risk exposure to someone with COVID-19. It’s better to prevent a problem than have to figure out how to deal with it after it happens. If we reduce the number of infected people who have contact with horses, we reduce any potential problems.