As well as infecting humans, coronaviruses can cause a wide range of diseases in other species including dogs, cats, pigs, cows, mice, bats, birds and ferrets,
Despite the US buy-up of the drug remdesivir, studies are still underway to understand if it is beneficial in treating human COVID-19 patients, and which ones.
But it could still be a ‘magic bullet’ for cats suffering from a deadly disease caused by a different coronavirus.
Feline coronavirus is a particular challenge for veterinarians and cat owners. Most cats who are infected with coronavirus have no signs or only mild signs of illness.
However, in some cats, the same infection can cause a severe illness – feline infectious peritonitis (FIP).
In these cats, the virus enters and replicates in macrophages, which are cells involved in the immune response, and causes an inflammatory response centred on blood vessels, known as vasculitis.
This inflammatory response is sometimes accompanied by excess fluid production in the abdomen or chest. In most cats, the disease is widespread, affecting multiple organs such as the kidneys, liver, lungs, gastrointestinal tract and central nervous system.
Affected cats are typically lethargic, with a reduced appetite and fever which can wax and wane.
If there is abdominal fluid accumulation, they may develop a distended or bulging abdomen. If there is fluid accumulation in the chest or inflammation in the lungs, they may have difficulty breathing.
Research has shown that in households or other environments, like animal shelters with more than six cats, most are infected with coronavirus – but FIP is much less common.
One theory why some cats develop FIP is that in these cats the virus undergoes mutation to a more virulent strain after the initial infection.
This may occur weeks to years after infection, as some cats can be persistently infected and shed the virus continuously, even without signs of illness.
Cats who may be immunosuppressed due to stress, another infection or genetic factors may be more at risk of ongoing viral replication increasing the risk of mutation.
Another theory suggests there may be more and less virulent strains of feline coronavirus circulating in the cat population; so, a combination of infection with a virulent strain plus any unique features of an individual cat and its environment may result in FIP.
At present, there is no reliable way to distinguish strains of feline coronavirus that cause FIP rather than the more common mild illness.
Until relatively recently, there was no effective treatment for FIP, and it was considered to be uniformly fatal.
Corticosteroids like prednisolone (and dexamethasone, as used in people with severe COVID-19) can improve clinical signs and prolong life in some cases, but most affected cats die within weeks or months of showing clinical signs. Currently, a proven effective vaccine does not exist.
In 2018, researchers from the University of California Davis School of Veterinary Medicine and research-based biopharmaceutical company Gilead Sciences published research showing that a compound called GS-441524 was effective in inhibiting feline coronavirus replication in the lab – it cured cured 10/10 cats experimentally infected with coronavirus causing FIP.
GS-441524 is one of a group of antiviral drugs which block the virus’ ability to make copies of its genetic material, essential for its spread and survival.
GS-441524 is related to the compound GS-5734, now known as remdesivir, one of the drugs showing promise for treatment of COVID-19.
Researchers chose to focus on GS-441524 rather than remdesivir as a treatment for FIP as it is ‘chemically less complex’. Unlike remdesivir, there’s no commercially available formulation of GS-441524 and so veterinarians cannot prescribe it to treat cats affected with FIP, despite the promising results.
However, a black market for the production and sale of GS-441524 has emerged. Reportedly, cat owners across the world are purchasing and using the drug in affected cats, largely through social media groups.
Based on results for GS-441524, remdesivir should also be effective against FIP, but to date has not been tested in cats for safety or efficacy.
In future, though, it is possible that rather than use unregulated, untested, black-market GS-441524 – owners and veterinarians caring for cats with FIP could use remdesivir.
Off-label use of pharmaceuticals licensed for treatment of human diseases is common in veterinary medicine, especially in companion animals.
Dogs and cats share many diseases and biological processes with people, and where a licensed drug in animals does not exist, veterinarians can prescribe the human drug to an animal with care.
For example, for dogs and cats with cancer, there are very few licensed drugs available and most chemotherapy is off-label use of the same drugs used to treat people.
Remdesivir was provisionally approved in Australia in July for treatment of COVID-19 in severe human cases.
One positive of the COVID-19 pandemic that many people report is that we are spending more time with our pets.
In the future, we may look back to find that another unforeseen positive is access to an effective treatment for what was previously a death sentence for some of these same beloved pets.