HANOI (AFP) - Vietnam's first bird flu patient in 18 months has made a full recovery, but health authorities warned Tuesday that the deadly virus is spreading in the country's fowl population.

Avian influenza has infected birds in 16 of the country's 64 provinces and municipalities since last month, said veterinary officials, warning of widening gaps in national vaccination campaigns, especially of ducks.

Authorities fear the virus may have hit the capital after more than 300 white-winged ducks died since Sunday in two Hanoi backyard farms, but animal health officials stressed that they were still testing samples.

Vietnam's first human known to have been infected with the H5N1 strain since November 2005, a 30-year-old man, was released Monday from Hanoi's Bach Mai hospital, where he had been treated since mid-May, said a physician.

A second patient, who fell ill in late May, is still undergoing treatment.

The World Health Organisation (WHO) is carrying out laboratory tests to confirm the two new human H5N1 infections in Vietnam, where 93 people were infected and 42 killed by the virus between 2003 and 2005.

The current major wave of poultry outbreaks in Vietnam, the fifth since late 2003, has meanwhile spread to 88 communes nationwide, with infections concentrated in the populous northern Red River Delta around Hanoi.

The UN Food and Agriculture Organization (FAO) blamed the surge on a rise in often unvaccinated ducks grazing in newly harvested rice paddies after Vietnam in March lifted a ban on waterfowl hatching.

"Free range duck production is an excellent system for farming, but there are risks and challenges involved," said FAO Vietnam chief Andrew Speedy.

Experts warn that ducks can be "silent carriers" of bird flu, spreading the virus through their faeces as they roam across rice fields and ponds, often without showing symptoms of illness themselves.

The FAO warned Vietnam must ensure good surveillance and response mechanisms, that vaccination campaigns must match the fowls' breeding cycles and that hatcheries, slaughterhouses and markets must be clean and bio-secure.

Communist Vietnam, once the country worst hit by the disease, has won praise for containing earlier bird flu outbreaks through culls of more than 50 million birds and mass vaccination and public awareness campaigns.

But the number of cases has spiked again this year, and Vietnamese health officials have recently warned that many regions have grown complacent and failed to follow through on nationwide vaccination campaigns.

Worldwide, the virus has killed 190 people out of 311 known infections, according to WHO figures, but experts fear the death toll would rise sharply if the virus were to mutate and become easily transmitted between humans.

Up to now, H5N1 does not appear to have significantly mutated in Vietnam.

"So far genetic sequencing of recent virus isolates have shown no significant change in the antigenicity of the virus", said Dr Jeffrey Gilbert, the FAO's chief technical on avian influenza in Vietnam.