By contrast, the relatively elusive equine coronavirus (ECoV) — which does not infect humans — can spread between horses and cause serious illness. With a fatality rate of 9%, “the virus should not be neglected,” said Nicola Posterla, DVM, PhD, Dibble. ACVIM, associate professor in the department of medicine and epidemiology at the University of California, Davis.
“The ‘real stuff’ (for horses) is the equine coronavirus,” Busterla said during the 2021 American Association of Equine Practitioners conference, which was held in Nashville, Tennessee, and around last December.
Horses test positive for COVID-19
Scientific models have suggested that horses may be susceptible to SARS-CoV-2, the COVID-19 virus. Researchers have shown that dogs, cats, mink, and other animals acquire SARS-CoV-2 from humans and may contribute to the spread of the virus. In some cases – with minks in particular – the virus can mutate within a species before returning to humans in a mutated form.
Until now, the horse’s role in the current epidemic was unknown. So Pusterla and his colleagues decided to test more than 600 horses exposed to people with confirmed active COVID-19 infection. The researchers determined that horses can, in fact, become seropositive for SARS-CoV-2 — meaning they display specific antibodies to the virus in their blood, which indicates infection — from being around humans who have been infected with the serocontaining virus.
However, seropositivity rates in horses are rare — less than 7% despite being surrounded by humans with COVID-19 — and horses have no detectable viral antigens in nasal secretions. Moreover, the virus did not appear to make the animals sick, as none of the infected horses showed clinical signs, he said.
For these reasons, Busterla said, horses having any significant contribution to the current pandemic is highly unlikely.
“If we look at cats and dogs that share households with a COVID-19 patient, up to 40% of these companion animals will become infected and show seroconversion, and the biggest risk factor is for the companion animal to sleep with you,” said Posterla. The practice is commonly in the horse business, so I think we’re fairly safe there.”
On the other hand, he said, the equine coronavirus can cause serious illness in horses. Affected animals can present with a fever, lethargy, loss of appetite, and a variety of other signs, including watery diarrhea and neurological problems. While most horses survive ECoV infections with supportive care, including anti-inflammatories, antibiotics and fluids, Pusterla emphasized that about 9% have to be euthanized humanely.
“If you think about it, 9% isn’t much, but that’s higher than what the world’s population is experiencing from the SARS coronavirus, so this isn’t a virus that should be neglected,” he said.
Identifying the disease and limiting its spread
Because of its vague and variable clinical signs, Pusterla said, ECoV can be difficult to recognize at first. “This virus is fooling you,” he said. “It’s a gastrointestinal (intestinal) organism that doesn’t always show intestinal signs unfortunately, which makes it a little more difficult.”
With good protocols in place, veterinarians can quickly identify signs of a viral infection and ultimately determine its cause.
This means that any horse with fever and lethargy – which are suggestive of possible infectious disease – is immediately isolated and samples taken for a complete blood count (CBC) panel. A low lymphocyte count in the CBC should strongly indicate the presence of a virus.
“Every time I see a horse with a fever and lymphopenia (an abnormally low white blood cell count), I have to think of a virus, no matter what caused it,” he said.
In addition to checking for other viruses, veterinarians can run quantitative polymerase chain reaction (qPCR) tests on horse feces samples to look for ECoV. If the test is positive, the entire farm must go into quarantine for two weeks, after which time the epidemic usually ends naturally, Posterla said. Seropositive horses should be isolated for a week and then re-tested before being released from isolation to ensure that they have turned sero-negative. About 15 to 20% of horses in an outbreak shed virus without clinical symptoms.
He said that the equine coronavirus has a fecal-oral transmission route. Biosecurity should include thorough hand washing between handling horses and disinfection of equipment and stalls – keeping in mind that ECoV-positive horses tend to shed the greatest amount of virus two or three days after they are infected with the serotype. “These horses will paint the stalls with the coronavirus,” said Busterla. “And be aware, especially if you have a horse (with) watery diarrhea, there’s a lot of volume, (and) there’s a lot of virus, that speaks to the infection of that virus.”
Fortunately, ECoV does not live long on surfaces and succumbs to heat and disinfectants easily, he said.
Take the letter home
Although horses do not seem to play a major role in the current COVID-19 pandemic, Pusterla said it makes sense for people to exercise biosecurity with horses if they test positive for SARS-CoV-2. “I still say that if we had someone who got infected and he was a horse owner, I would really encourage him to take the same precautions that we do among ourselves,” he said.