Mr Johnson’s rescued Jack Russell Terrier was wheezy, exhausted and too tired to go for walks.
The dog is believed to have caught Covid in April when Mr Johnson was fighting for his life at St Thomas’ Hospital in London.
Dilyn was pictured at the time with Mr Johnson’s then-pregnant fiancée Carrie Symonds while the 32-year-old was self-isolating for the virus.
A source told The Sun: ‘It’s well known that the virus did the rounds through Downing Street, but it appears to have affected little Dilyn too.
Dilyn was pictured with Mr Johnson’s fiancée Carrie Symonds while the 32-year-old was self-isolating for the virus, when she was heavily pregnant in April. ‘Self-isolating isn’t so bad with this one,’ she captioned the Instagram post
Mr Johnson arrives with his dog Dilyn at Westminster Central Hall, London, December 12, 2019
‘He wasn’t tested so we can’t be sure, but around the same time as Carrie came down with it he was very lethargic, tired and didn’t hop about as much.
‘Looking back, it’s pretty likely he had it.
‘Fortunately, like the PM, he’s made a full recovery and is fit as a fiddle.’
It is widely known that cats and dogs can catch coronavirus, however they only usually have mild symptoms and cannot pass it on.
The government website says: ‘It is rare for an animal to contract coronavirus, and they tend to show only mild symptoms and recover within a few days.
There is no evidence that coronavirus passes between pet animals or that pets transmit the virus to humans.’
Dilyn was rescued from a breeder by animal charity Friends of Animals Wales before being adopted, and was moved into Downing Street in September last year.
Carrie Symonds regularly posts images of the dog on an Instagram page
The dog has its own Instagram page run by Ms Symonds which has attracted more than 2,000 followers.
His name is a Welsh word, a nod to his Welsh origin, which means ‘to follow’ or ‘pursue’. It is also said to mean ‘loyal’ when used as a name in Ireland.
The PM has previously revealed that Dilyn has helped him reduce his stress levels by taking regular walks with his dog.