They found that many such people believed they were on a mission to rescue animals, but are often prosecuted for animal cruelty because they cannot cope.

Some of the men and women studied had as many as 100 animals ranging from cats and dogs to birds and farm animals, the researchers said.

The team, based in the US, now plans to establish a national surveillance system so animal shelters can notify them of any future cases.

Public health issue

Dr Gary Patronek of the Tufts University School of Veterinary Medicine, in Massachusetts, led the study, which was published in the journal Public Health Reports.

He said: "Public health professionals should recognise that animal hoarding may be a sentinel for mental health problems or dementia, which merit serious assessment and prompt intervention.

"Animal hoarders often deny reality, and insist that their animals are healthy."

The team looked at 54 cases - all of which were referred by animal control agencies and humane societies - with an average of 39 pets each.

The team said the animals were often kept in filthy conditions.

Common factors

They also identified several features the subjects had in common:

• 76% are women
• 83% are middle-aged
• 50% live alone
• In 80% of cases animals were found dead, malnourished or injured
• Many are prosecuted, but usually move house and start again

'Impossible to judge'

An RSPCA spokeswoman said the society was campaigning for legislation to monitor people who took in large numbers of animals.

"There is at the moment no monitoring or licensing of that situation. We want legislation to guarantee the welfare of animals in that position," she said.

However, it was impossible to make sweeping judgements about the mental health of people who took in more than a few pets.