Horses can be infected by the virus that causes Covid-19, but appear to show no clinical signs, American researchers report.
The findings, reported in the journal Animals, are based on an analysis of samples from more than 1200 horses.
“Similar to dogs and cats, horses do not seem to develop clinical SARS-CoV-2 infection,” Kaila Lawton and her fellow researchers at the University of California, Davis, reported.
“However, horses can act as incidental hosts and experience silent infection following spillover from humans with Covid-19.
“SARS-CoV-2-infected humans should avoid close contact with equids during the time of their illness,” they said.
The study team noted that more and more studies are reporting on the natural transmission of SARS-CoV-2 between humans with Covid-19 and their companion animals, notably dogs and cats.
Understanding the host range for the virus is important in order to control the ongoing pandemic and to protect animal populations, both domestic and wild.
Horses, they noted, are apparently susceptible to SARS-CoV-2 infection based on similarities between the human and equine ACE-2 receptor – the cell protein that provides the entry point for the coronavirus to hook into and infect cells.
However, no clinical or subclinical infection has yet been reported in equine species.
To investigate the possible clinical role of the SARS-CoV-2 virus in equids, nasal secretions from 667 horses with acute onset of fever and respiratory signs were tested for the presence of the virus.
The samples were collected from January to December of 2020 and submitted to a commercial molecular diagnostic laboratory for the detection of equine influenza virus, equine herpesvirus, equine rhinitis A and B virus, and Streptococcus equi subspecies equi.
Not a single horse tested positive for SARS-CoV-2, while 36% of the sick animals were positive for at least one of the common respiratory pathogens.
In the second part of the study, serum samples were analyzed from 587 healthy racing Thoroughbreds in California whose track personnel had tested positive for the virus.
In all, 35 of the Thoroughbreds – that’s 5.9% of the racehorses – had detectable antibodies to SARS-CoV-2.
Studies assessing seroprevalence in companion animals living in households with infected owners reported seropositivity rates for SARS-CoV-2 of 3.4 to 23.5% for dogs, and 4 to 43.8% for cats.
“This relatively high percentage of suspect seropositivity in horses could be related to the large number of infected jockeys and track workers having contact with the racing horses,” the authors said.
Horses appear to be susceptible to the virus when in close contact with humans with the infection, the study team concluded.
“Experimental challenge studies using pure inocula are needed in order to study the clinical, hematological, molecular, and serological features of adult horses infected with SARS-CoV-2,” they said.
Discussing their findings, the researchers noted that no outbreak of respiratory disease was reported in the racehorses in the second phase of the study, suggesting that horses with antibodies to SARS-CoV-2 likely experienced a subclinical infection.
The susceptibility to developing Covid-19 in companion animals is a complex interplay between various viral and host factors, they said.
“While data is limited on the susceptibility to SARS-CoV-2 infection in domestic animals, it appears that equids are incidental hosts because of occasional SARS-CoV-2 spillover from humans.
“However, continuous surveillance is necessary in order to monitor the possible transmission of SARS-CoV-2 infection in equids.
“From a biosecurity perspective, it is highly recommended that humans with clinical and asymptomatic SARS-CoV-2 infection avoid close contact with any companion animals.
“From an epidemiological standpoint,” they continued, “it is important to continue to monitor the possible transmission of SARS-CoV-2 infection in equids and other domestic animals and to emphasize the risk of SARS-CoV-2 transmission from humans with clinical or asymptomatic SARS-CoV-2 infection to susceptible animals.”
The study team comprised Lawton, Rick Arthur, Benjamin Moeller, Samantha Barnum and Nicola Pusterla.
Lawton, K.O.Y.; Arthur, R.M.; Moeller, B.C.; Barnum, S.; Pusterla, N. Investigation of the Role of Healthy and Sick Equids in the COVID-19 Pandemic through Serological and Molecular Testing. Animals 2022, 12, 614. https://doi.org/10.3390/ani12050614