The first cases of rabbit virus have been confirmed in hares in the UK, highlighting a major new threat to the UK’s rapidly dwindling brown hare population.

Two cases of the deadly rabbit haemorrhagic disease type 2 have been confirmed in Dorset and one in Essex, so it may already be taking hold in the wild, but more testing will be needed to determine its spread.

Hare numbers have plunged by about 80% in recent decades, largely as a result of changing farming practices and more intensive agriculture, which has affected their food supply and habitat. Any further threat from disease would strike another blow at what was once a common fixture of the countryside – hunted by some and harried by farmers, but much-loved and with a special place in British folklore, from the mad March hare to the nut-brown hare of the bestselling children’s book Guess How Much I Love You.

Mountain hares are native to the UK, and are now largely restricted to Scotland, but brown hares were, like rabbits, introduced and are estimated to number about 800,000.

Suspicions that the virus, which causes lung haemorrhaging and hepatitis, may have jumped to the hare population were raised in September, when sightings of sick and dying hares were first reported. The rabbit virus is known to have made the leap to European brown hares in countries such as Italy, France, Spain and Australia.

Brown hares can be distinguished from rabbits by their larger size, longer hind legs and black-tipped ears. Photograph: Richard Bowler/REX

Dr Diana Bell of the University of East Anglia, who led the research reported in the journal Vet Record, said it was too soon to say whether rabbit virus was the primary cause of the recently observed hare die-off, as other pathogens could be involved.

Last year, reports suggested myxomatosis may also have passed from rabbits to hares. In time, populations are likely to develop some natural immunity, but with numbers already under threat from other factors, hare populations face a race against time.

Bell praised members of the public who reported sick and dead hares, which are still being collected for postmortem examination. “We are enormously grateful for the continuing tremendous response – this is a good example of citizen science,” she said.

Members of the public who find dead hares are urged to report them for testing by emailing Bell. Hares can be distinguished from rabbits by their larger size, longer hind legs and black-tipped ears, which are at least the length of their heads. Instead of burrowing like rabbits, they live in “forms”, shallow scrapes in the earth surrounded by undergrowth, which makes them more vulnerable to mechanised agriculture.

• This article was amended on 28 January 2019. An earlier version said that mountain hares were restricted to Scotland. This has been corrected to say “largely restricted to Scotland” as there are small populations in northern England.