Vietnam requested the help of the FAO after reporting an outbreak of deadly "blue ear" pig disease in four provinces in recent months, amid fears the animal virus might have travelled south from neighbouring China.
An expert team from the FAO and World Organisation for Animal Health (OIE) has visited Vietnam and sent samples to a US laboratory for sequencing of the viral strain and to identify a vaccine, said the FAO in a statement.
The team, which reported its findings to Vietnam's communist government Tuesday, said "it appears that there are a number of swine diseases that may be interacting to cause the unusual signs being observed."
"Improving overall disease management can bring many of the diseases under control, including the blue-ear disease' or Porcine Reproductive and Respiratory Syndrome (PRRS)," said the statement.
They stressed that "PRRS does not infect humans" and said the pig mortality in recent outbreaks of about 10-15 percent of affected animals was "not considered unusually high for livestock in Southeast Asia."
Vietnam last month reported at least two human deaths and more than 20 human cases of another pig-borne disease, Streptococcus Suis.
The FAO/OIE team said blue-ear disease "reduces immunity in pigs and leads to secondary infections" of bacterial diseases like Streptococcus Suis and salmonella and was also associated with other viruses.
The team said testing of the samples was ongoing in the United States, where preliminary tests had so far tested negative for African swine fever, classical swine fever and foot and mouth disease.
It said "one of the possible causes of the disease being reported in Vietnam may be PRRS," a disease that was first recognised in the United States in the mid-1980s and is now present in most pig-producing countries.
To control the problem, the FAO recommended "good pig production practices on small farms."
"Sick pigs can be cured from secondary infections by appropriate use of approved antibiotic injections, and this would greatly reduce the losses," said team leader Dr Carolyn Benigno.
The FAO and Vietnam called on the international community to support future disease investigation and laboratory work and to undertake a communication campaign for farmers, para-veterinarians and the general public.