Experts in European law said the regulations do not permit mixing of bird consignments from different countries because of the risks of spreading avian flu.
But that appears to have happened at a licensed quarantine facility in Essex where a consignment of parrots from South America was kept in the same quarantine unit as a consignment of exotic birds from Taiwan.
The Department for Environment, Food and Rural Affairs (Defra) reported on Monday that a parrot from Surinam was probably infected with the deadly H5N1 strain of avian flu when it was kept in quarantine with a separate consignment of birds from Taiwan.
A senior government vet said mixing of consignments was allowed under EU quarantine regulations provided that all birds were kept under quarantine for a minimum of 30 days.
However, the EU regulations do not mention the possibility of mixing consignments of birds, said Cristiana Senni of the World Parrot Trust, a charity that has tried to ban the trade in wild birds.
"It's unbelievable that Defra can't understand the detail of the EU regulation on quarantines, which clearly states that each quarantine unit should hold only birds of the same consignment," Ms Senni said.
"This, of course, is an obvious requirement, otherwise the whole principle of quarantine would be useless, as this latest incident showed," she said.
Ms Senni cited the wording of the EU regulations governing bird quarantine, which states quarantine units much be kept operationally and physically separated from each other "and in which each unit contains only birds of the same consignment, with the same health status and being therefore one epidemiological unit".
Officials at Defra are investigating the circumstances that gave rise to the case of the H5N1 virus, which has killed 62 people in Asia, at a bird importer's quarantine facility in Essex.
Philip Tod, a spokesman for the European Commission, said: "The European Commission has so far received no information which suggests the UK authorities have breached the requirements. The UK authorities are still investigating the circumstances in this case and the Commission will wait for the outcome of that investigation to properly assess the situation."
The wording of the regulations distinguish between a quarantine centre, operating several units, and a quarantine facility, which is designed for small-scale importers bringing in one consignment at a time.
Ms Senni said the Essex importer was bringing in hundreds of birds in each consignment and was doing it professionally so his operation should have been considered a centre rather than a facility.
Meanwhile, Margaret Beckett, Environment Secretary, told MPs yesterday that Britain would introduce new measures to prevent the spread of bird flu.
Bird markets and fairs in Britain are to be banned to prevent an outbreak, a move that comes a day after the EU decided to outlaw the importation of exotic birds into Europe for the pet trade.
Mrs Beckett also said the question of why consignments of imported birds were allowed to be mixed was also to be considered.
It also emerged yesterday that preliminary tests conducted on three people who returned to the Indian Ocean French island of La Reunion after a trip to Thailand indicated they may be infected with the H5N1 bird flu virus.
The three people visited a bird park in Thailand during their trip there and had close contacts with birds, said Helene Monard, a French Health Ministry spokeswoman.
La Reunion authorities said that a 43-year-old man was taken to hospital with a fever and strong headaches on Saturday, three days after spending a week in Thailand. Nineteen others who also took the same trip were questioned about their health. Among them, two had flu symptoms and Mme Monard said their preliminary tests also were positive for H5N1.
"Although the symptoms are not very significant, there is thus a suspicion of flu of avian origin in the framework of a close contact with birds," the French Health Ministry said in a statement.
Bird flu is difficult to diagnose properly in preliminary tests, and false positives are not uncommon.
The EU said bird flu virus found in Croatia was the deadly H5N1 strain and advised citizens not to eat raw eggs or uncooked poultry.
African poultry farmers face economic disaster
Researchers fear the next stop for the bird flu which threatens a global pandemic will be Africa, where it could have a devastating impact.
The H5N1 virus is expected to be carried by migratory birds into the Middle East and east Africa within weeks, the United Nations' Food and Agriculture Organisation (FAO), said.
The journal Nature said the health and economic consequences could be worse than in south-east Asia where the virus is already widespread.
Ward Hagemeijer, from the conservation group Wetlands International in Wageningen, in the Netherlands, said: "Africa is on the main migratory route for birds. The first birds are already in east Africa." The pattern of the virus's spread points strongly to wildfowl travelling south-west from northern Russia to the African continent.
Outbreaks have been found along the route in Romania and Turkey, and controlled by culls and quarantine orders. Middle Eastern states have planned for possible outbreaks, but there has been little response in more vulnerable African countries, the report in Nature said.
Rural communities around the lakes of the Rift Valley depend heavily on poultry to survive, and live closely with domestic and migratory birds. An endemic H5N1 bird flu virus in Africa would greatly increase the chances of it mutating into a form that can spread between humans, triggering a pandemic, say experts.
The most immediate threat is economic disaster. Lea Borkenhagen, of Oxfam, told Nature: "Losing poultry would have a devastating effect on livelihoods. Women would be badly affected, because poultry are the only assets they can possess."
Few east African countries have systems to test for H5N1, and efforts to control outbreaks would be hampered by difficult terrain and low levels of education.
East African nations are to meet next month in Rwanda to develop a regional strategy to meet the threat.
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