Animal rescuers are being inundated with calls to severely sick seal pups being washed up along the north Norfolk coast - but remain baffled as to what is causing them to become so ill.

Volunteers and members of the Seal and Bird Rescue Trust have been attending beaches from Winterton to Sheringham to pick up the emaciated and malnourished mammals, many of which have not survived.

The Ridlington-based charity said it has been rescuing between two and four seals a day and has pointed to lungworm, a parasite known to be present in most seals, as a possible cause for the influx in the poorly pups.

Beverley Cosse, Trust chairman, said: 'We are losing common seals at a great rate and until the reason for this problem is discovered, work cannot begin on addressing it although research continues with great vigour.

'The lungworm parasite leaves the seals weak, starving and fighting for breath. Why they are being affected so badly we just don't know yet.'

Seals rescued by the Trust have been taken to the RSPCA wildlife rescue centre in East Winch, west Norfolk, which has taken in 41 sick pups since June.

A vet there was due to carry out post mortems on some of the dead seals to try and discover if there was a common theme for their sickness, but staff said this would be unusual.

Last year the RSPCA centre took in an unprecedented 121 seals, which were suffering from a number of ailments including pneumonia, and it is hoped 2011's sudden influx of sick babies will not result in a repeat of 2010's high numbers.

Alison Charles, centre manager, said: 'Because there's been so many in quickly it's been really hard work for the staff here and in Ridlington, so everyone's run off their feet.

'We can't explain it. Last year there was all sorts of reasons and (there) wasn't one specific disease. The only common theme is they're absolutely emaciated.

'We had one last week come in at nine and a half kilos, it should be born at 10 kilos.'

The Trust has now launched an emergency appeal for donations to help members to continue their work to rescue the seals. Cheques for The Seal and Bird Rescue Trust can be sent to The Barns, Mill Common Road, Ridlington, North Walsham, Norfolk, NR28 9TY or visit www.sealbirdrescuetrust.co.uk

Ill or sick seals, if spotted by the public, will be lethargic, unresponsive and found fairly high up the beach. People are advised to keep themselves and dogs away from the animals and not to touch them. If it looks as though the seal needs help call the RSPCA helpline on 0300 1234999.