Japanese authorities have culled 500,000 birds and plan to cull about 1 million more to stop an outbreak of H5N2 avian influenza, a milder form than the deadly H5N1, which has infected a number of backyard poultry flocks in Thailand recently.

Half a million birds have already been culled in two Japanese prefectures: Ibaraki, where the virus surfaced in late June, and Saitama, according to an International Herald Tribune/Asahi Shimbun online report that appeared on ProMED-mail. Another 1.02 million birds will be culled in the affected areas, which contain about 30 farms with 4.4 million chickens, the report said.

Japan´s most recent report to the World Organization for Animal Health (OIE), dated Aug 23, said the outbreaks are in Ibaraki town and Mitsukaido city in Ibaraki prefecture and in Kounosu city in Saitama prefecture. Both areas are near Tokyo, Saitama to the northwest and Ibaraki to the northeast. The reports listed no poultry deaths attributed to the virus.

H5N2 viruses have not been known to infect people. The concern is to stop the spread of the low-pathogenic virus before it can evolve into a highly pathogenic form.

Officials in Japan´s Ministry of Agriculture, Forestry and Fisheries have raised the possibility that the infection was caused by use of an unauthorized, defective vaccine that contained an active virus, the report said.

An analysis by Japan’s National Institute of Animal Health found that the virus is 94% to 97% genetically similar to a virus found in Central and South American countries such as Mexico and Guatemala, according to a Sep 4 story from the Japanese newspaper The Yomiuri Shimbun.

Hiroshi Kida, a Hokkaido University professor and head of the agriculture ministry’s advisory panel, said a vaccine based on the Central and South American virus was brought into Japan and possibly used on poultry, the newspaper reported.

The ministry has a vaccine stockpile, but approved vaccines have not been distributed. The virus detected on the farms differs from those in vaccines sold outside Japan, so ministry officials speculated that it was developed outside the formal process, the paper reported. Unauthorized use of vaccines is illegal.

Although local authorities have checked to see if vaccines have been used, one official told the paper that no evidence had been found, and owners of several farms where the virus has been detected said they did not use any vaccines, the paper noted.

Meanwhile, the H5N1 virus continues to spread among poultry in Thailand, with a spate of new outbreaks in villages in Kampaengphet and Ayudhaya provinces between Aug 23 and Sep 1, according to a Thai report to the OIE.

More than 300 chickens died or were culled in four different villages, the report said. A ProMED-mail posting from China’s Xinhua news service cited Thai speculation that the transport of fighting cocks from one area to another might have triggered the outbreaks.

The infected poultry were described as backyard or free-range birds living under minimal biosecurity conditions.

In nationwide surveillance of outbreaks from Jul 1 to Sep 1, the Thai government found that 29 outbreaks occurred in four provinces (the two previously mentioned, plus Chainat and Suphanburi). All cases involved free-range poultry, poor sanitation, and "insufficient biosecurity," the report to OIE said. Most of the outbreaks involved chickens, but a few involved quail, fighting cocks, and laying ducks.

See also:

Sep 5 ProMED-mail report on H5N2 outbreaks in Japan

Aug 23 OIE report on Japan
http://www.oie.int/eng/info/hebdo/aIS_56.htm#Sec4

Sep 1 OIE report on Thailand
http://www.oie.int/eng/info/hebdo/aIS_55.htm#Sec3