Bulgaria shut off wetland areas on Monday and investigated the deaths of scores of birds after announcing its first case of H5 avian flu in a dead swan last week, officials said.

Veterinarians also prepared to send samples from the infected swan -- found partially paralysed in the Danube river -- to Britain to test whether it had the deadly H5N1 strain of the virus.

"The samples from the swan will be sent to the United Kingdom on Monday. We expect the results in a week´s time," said Margarita Kozhuharova, spokeswoman of the agriculture ministry.

With its Black Sea neighbours Turkey and Romania battling outbreaks of the deadly H5N1 since October, Bulgaria has long been considered a potential destination for the disease.

Since the first H5 case was announced on Friday, vets have tested dozens of waterfowl found dead in wetland areas, including 30 ducks which died at a farm on Sunday.

"The birds, domesticated ducks, died at a farm near the Black Sea port of Varna," Dinko, Neshovski, head of the veterinary office in Varna, told Reuters. "We´ve sent samples for tests to Sofia." He would not elaborate.

On orders from the state veterinary office, police began guarding the lakes of Shabla and Durankulak near the northeast border with Romania to prevent people from coming into contact with more than 100,000 wild birds that spend the winter there.

South of Europe´s largest wetland area in the Danube delta, the lakes like along the Pontic migratory route, by which scientists believe birds travelling south from northern Russia may have brought the disease to southeast Europe.

Hundreds of birds have died at the lakes since the start of the year. Veterinarians have blamed that on a severe cold snap in which temperatures dropped to around minus 20 degrees Celsius (minus 4F), but said they were stepping up measures anyway.

"We are getting ready for a possible outbreak," said Alexander Alexandrov, head of the regional veterinarian office in Dobrich.

"People should forget about taking farm animals to water at the two lakes until spring, when the last duck is gone."