Potential Covid-19 spillover from a woman to her horse described in study

The Quarter horse in California mounted an immune response to the virus, but showed no signs of illness.
Photo by Tatiana

A potential spillover of the Covid-19 virus from a woman to one of her two horses in California has been described by researchers, who suggest infected people should avoid horses and other pets as a precaution.

The Quarter horse’s immune system mounted an antibody response to the virus, laboratory testing showed. The animal showed no signs of illness.

The results suggest that horses can become silently infected with SARS-CoV-2 following close contact with humans infected with the virus, Nicola Pusterla and her fellow researchers concluded.

“As a precautionary measure, humans infected with SARS-CoV-2 should avoid close contact with equids and other companion animals during the time of their illness to prevent viral transmission.”

The researchers, writing in the journal Viruses, described the circumstances surrounding the case.

In October 2021, a middle-aged woman developed fever, fatigue, muscle aches and a loss of smell. Shortly after, she tested positive for Covid-19 via a rapid antigen test (RAT).

The infection was confirmed through laboratory testing, with the results of genotyping being consistent with the temporarily circulating Delta B.1.617.2 variant.

Before developing clinical signs, the woman had been caring for her two Quarter horse mares, aged 8 and 21. Her two-hour daily routine included twice-daily feeding, stall cleaning, grooming, and exercising.

Due to the woman’s concern for possible exposure of her two horses to the virus, Pusterla was contacted in order to monitor the horses.

The animals were monitored daily via a routine physical assessment. Additionally, blood, nasal secretions, and feces were collected weekly for three weeks following the woman’s Covid-19 diagnosis. The various samples were tested in a laboratory for the coronavirus.

Both horses remained healthy throughout the three-week monitoring period, the researchers reported.

However, serum testing in the younger of the horses showed an antibody response to the virus seven days after the owner had tested positive. Peak antibody levels were reached 21 days after diagnosis.

Additional blood samples were collected from the infected horse for 60 days following diagnosis, with antibody levels elevated up to the last sample collection.

“Despite the low susceptibility of equids to SARS-CoV-2 and little evidence for natural infection, this present case report describes a potential spillover from a Covid-19 patient to one of her two horses,” the researchers said.

Various companion animal species such as dogs and cats have shown little disease expression, despite being susceptible to SARS-CoV-2. “Apparently, horses follow the same pattern after infection with SARS-CoV-2,” they said.

This observation is also supported by the inability to detect the SARS-CoV-2 virus by quantitative reverse transcription polymerase chain (RT-qPCR) testing in 667 horses with fever and respiratory signs in a previously reported study.

Indeed, neither of the two horses in the present study returned a positive RT-qPCR test for the virus.

“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,” they said.

“A similar pattern comprising seroconversion with no to very short viremia and nasal shedding has recently been reported in dogs and pigs, which relate to the low susceptibility of these animal species to SARS-CoV-2.”

They continued: “While various host, viral, and environmental factors can predispose infection, it was interesting to notice that only the younger of the two horses seroconverted to SARS-CoV-2.

“While the number of infected horses is too low to draw any age-related association, it is interesting to notice that younger cats are apparently more vulnerable than older ones.”

The authors stressed that without the detection and characterization of SARS-CoV-2 from the horse, the source of infection remains speculative. However, the circumstances suggest direct transmission from human to animal.

They noted that while a previous attempt to experimentally infect a single horse with SARS-CoV-2 failed, it is possible that certain human-adapted variants are more likely to replicate in equids.

It will be interesting, they said, to determine if newer variants, such as Omicron, which is known to have an increased rate of virus transmission, will be more likely to induce silent infection in equids.

The antibody response observed in the young Quarter horse mare was, compared to more susceptible animal species, low.

“This observation likely reflects a short infection phase, supported by the lack of clinical disease and absence of detectable virus. However, the antibody response persisted during the entire study period.”

The study team noted that antibodies to the virus have been shown to persist up to 10 months in some dogs and cats from Covid-19 positive households.

“At the present time and based on the limited scientific data available, it appears that the overall risk of SARS-CoV-2 transmission between humans with Covid-19 and equids is low,” they wrote.

However, it is important to continue to study the impact of potential spillover via longer studies aimed at sampling horses at regular intervals once caretakers or horse owners have been diagnosed with Covid-19.

“It is imperative to determine the ability of SARS-CoV-2 to infect and become established in different animal species such as horses, especially in light of possible animal-to-human transmission.

“While there is no evidence for horse-to-horse transmission of SARS-CoV-2, the current guidelines recommend that owners infected with SARS-CoV-2 avoid any close contact with their animals, including horses.”

The study team comprised Pusterla, Antoine Chaillon, Caroline Ignacio, Davey Smith, Samantha Barnum, Kaila Lawton, Greg Smith and Bradley Pickering. Pusterla, Barnum and Lawton are with the Department of Medicine and Epidemiology at the University of California, Davis, School of Veterinary Medicine; Chaillon, Ignacio and Smith are with the Division of Infectious Diseases and Global Public Health at the University of California San Diego; while Smith and Pickering are with the National Center for Foreign Animal Disease, part of the Canadian Food Inspection Agency.

Pusterla, N.; Chaillon, A.; Ignacio, C.; Smith, D.M.; Barnum, S.; Lawton, K.O.Y.; Smith, G.; Pickering, B. SARS-CoV-2 Seroconversion in an Adult Horse with Direct Contact to a COVID-19 Individual. Viruses 2022, 14, 1047. https://doi.org/10.3390/v14051047

The study, published under a Creative Commons License, can be read here



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