A deadly disease hitting greenfinches and chaffinches in the UK has spread to Europe.

Trichomonosis, which is transmitted by a parasite, first emerged in Britain in 2005, causing significant population declines in their populations.

In some counties, greenfinch populations dropped by one-third within a year of the disease emerging and across the country numbers have not bounced back from the 35% decline seen since 2006.

Research carried out by the Zoological Society of London (ZSL), the British Trust for Ornithology (BTO) and the University of East Anglia along with other organisations in Europe, has revealed the disease has spread to Norway, Finland and Sweden.

The research, published in the journal EcoHealth, suggests that chaffinches migrating from Britain to the other countries have carried the trichomonosis parasite.

Large numbers of chaffinches who winter in Britain fly back to breeding grounds in Europe, while only small numbers of greenfinches migrate – making chaffinches the most likely source of transmission.

Analysis of the parasite showed no differences between British and European samples, while recent research has shown that all UK birds have the same strain.

The emergence of the disease in southern Scandinavia means new populations and even other species could be exposed to the parasite, the experts warn.

Birds suffering from trichomonosis look lethargic and have fluffed-up feathers. They may show signs of struggling to feed and breathe. Outbreaks are most frequent and severe between August and October.

Mike Toms, the head of garden ecology at the BTO, said the disease causes lesions in the top part of the throat, which prevents them swallowing their food.

It is not yet known why greenfinches and chaffinches are particularly susceptible to the disease, and this is the subject of future research.

Gardeners can take steps to help reduce the spread of the disease. Mr Toms said: "If people have an outbreak, the advice is to stop feeding [the birds].

"But there is also a need for routine cleaning of feeders every couple of weeks, they should move them around the garden so you don't get a build-up of food and faeces below the feeder and generally keep things clean."