Investigation of the Role of Healthy and Sick Equids in the COVID-19 Pandemic through Serological and Molecular Testing

Abstract

More and more studies are reporting on the natural transmission of SARS-CoV-2 between humans with COVID-19 and their companion animals (dogs and cats). While horses are apparently susceptible to SARS-CoV-2 infection based on the homology between the human and the equine ACE-2 receptor, no clinical or subclinical infection has yet been reported in the equine species. To investigate the possible clinical role of SARS-CoV-2 in equids, nasal secretions from 667 horses with acute onset of fever and respiratory signs were tested for the presence of SARS-CoV-2 by qPCR. The samples were collected from January to December of 2020 and submitted to a commercial molecular diagnostic laboratory for the detection of common respiratory pathogens (equine influenza virus, equine herpesvirus-1/-4, equine rhinitis A and B virus, Streptococcus equi subspecies equi). An additional 633 serum samples were tested for antibodies to SARS-CoV-2 using an ELISA targeting the receptor-binding domain of the spike protein. The serum samples were collected from a cohort of 587 healthy racing Thoroughbreds in California after track personnel tested qPCR-positive for SARS-CoV-2. While 241/667 (36%) equids with fever and respiratory signs tested qPCR-positive for at least one of the common respiratory pathogens, not a single horse tested qPCR-positive for SARS-CoV-2. Amongst the racing Thoroughbreds, 35/587 (5.9%) horses had detectable antibodies to SARS-CoV-2. Similar to dogs and cats, horses do not seem to develop clinical SARS-CoV-2 infection. 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.

SARS-CoV-2 Seroconversion in an Adult Horse with Direct Contact to a COVID-19 Individual

“…A recent serological survey from China, did not find antibodies specific to SARS-CoV-2 from 18 healthy horses [ 12 ]. This was in sharp contrast to a study evaluating silent transmission of SARS-CoV-2 between racetrack workers with asymptomatic COVID-19 and racing Thoroughbred horses [ 10 ]. The latter study showed that 5.9% of healthy racehorses had detectable antibodies to SARS-CoV-2, suggesting possible spillover from COVID-19 humans to horses.…”

“…Apparently, horses follow the same pattern after infection with SARS-CoV-2. This observation is also supported by the inability to detect SARS-CoV-2 by RT-qPCR in 667 horses with fever and respiratory signs [ 10 ]. The lack of SARS-CoV-2 detection via RT-qPCR in the horse that seroconverted likely related to the short shedding period and the weekly sample collection interval.…”

“…Additional blood samples were collected up to 60 days post-COVID-19 diagnosis. The various biological samples were tested for SARS-CoV-2 via RT-qPCR as previously described [ 10 ]. Weekly serum samples were tested for antibodies to SARS-CoV-2 using an ELISA targeting the receptor-binding domain (RBD) of the S protein.…”

“…Weekly serum samples were tested for antibodies to SARS-CoV-2 using an ELISA targeting the receptor-binding domain (RBD) of the S protein. The cut-off value for the ELISA was determined as six times the standard deviation above the mean value of reactivity of 88 seronegative samples from a pre-COVID-19 cohort of healthy adult horses as previously reported [ 10 ]. The serum samples were also tested via the plaque reduction neutralization test (PRNT).…”

“…While experimental studies are needed to establish susceptibility and investigate risk of horse-to-horse and/or horse-to-human transmission, such protocols never reproduce natural conditions. Recent studies looking at the molecular detection of SARS-CoV-2 in nasal secretions and/or feces from healthy horses in contact with infected keepers and from horses with acute onset of fever and respiratory signs did not find evidence of SARS-CoV-2 shedding [ 9 , 10 ]. Seroepidemiological studies have been shown to be more accurate at documenting past exposure to SARS-CoV-2 in companion animals and livestock [ 11 ].…”