While we’ve learned a lot about the susceptibility of many different animal species to SARS-CoV-2, horses have been a bit of an unknown. We’ve had concerns about potential susceptibility based on the nature of the receptor the virus uses to enter host cells, but study in horses has still been limited. A lot of that has been because people didn’t really want to know – I’ve run into a lot of roadblocks to research because people were more concerned about the implications of a positive test than the virus itself.
A couple of studies have provided a little bit of information. An Italian study didn’t find evidence of SARS-CoV-2 infection in 34 horses on two farms where there was exposure to infected people. Another study using PCR testing didn’t detect any SARS-CoV-2 in samples from 667 horses with respiratory disease, but there was no known exposure of the horses in this study to the virus – they just tested samples submitted for routine diagnostic testing of sick horses.
A recent study (Pusterla et al. 2022) has finally shown “proof of principle” with regard to infection of horses with SARS-CoV-2. The researchers used serological testing (for antibodies) to detect recent infection in a horse. The advantage of this method is you don’t have to get respiratory swabs during the likely very narrow window that an infected horse may be shedding the virus. The downside is the potential for a false positive test, which is of particular concern when trying to interpret results for individual animals, versus an entire group.
The paper identified antibodies against SARS-CoV-2 in one of two healthy horses owned by a person who had COVID-19, caused by the Delta variant. Both horses were healthy at the time of sampling and hadn’t had any noticeable illness around the time the owner was sick. Both horses were negative on PCR tests (that look for viral nucleic acids). However, one horse tested positive on two different antibody tests, including a plaque reduction neutralization test (PRNT), which is a pretty specific test. So, I think we can be fairly confident that the horse was infected, presumably from exposure via the owner, and either had very mild disease that wasn’t noticed or had no disease whatsoever.
What does this tell us?
It confirms what I think we suspected all along: that horses have at least some albeit limited susceptibility to SARS-CoV-2. The antibody response was pretty low and short-lasting, in comparison to other susceptible species. While we have to take care interpreting results from a single horse, it’s suggestive that horses are not as susceptible as some other species but can be infected.
Does this mean the virus can’t cause disease in horses?
Not necessarily. Subclinical infections are common even in species that can get severe disease. With only 1 positive horse, we can’t say much. However, given what we have to assume is very widespread exposure of horses to people with COVID-19, and the lack of any apparent concerns about increases in respiratory disease in horses, I think we can still be confident that this is not a serious health problem for this species.
- Maybe it never causes disease in horses.
- Maybe it causes disease sporadically but not commonly enough that we’d notice.
- Maybe it causes mild disease that never gets identified or isn’t serious enough that testing gets done.
Can horses infect people?
Who knows? All we know is that one horse was presumably infected. Some species can be infected and spread the virus to others. However, some species can be infected but not to the degree that they are able to infect others (known as a “dead end host”). Given what is likely a low level of susceptibility, I assume the risk posed by horses to people is very low.
What do we do now?
The same things we’ve been recommending since January 2020. If you have COVID-19, stay away from others as much as possible. “Others” includes humans AND other animals. Humans are driving this pandemic, and infections in animals are largely a by-product of that. We’d like to keep it that way by limiting exposure of infected people to animals. Some basic measures can reduce the unknown risks.