Avian Influenza: Is a Pandemic Among Birds Under Way?
By Eric Toner, M.D., June 20, 2005
The epidemic of H5N1 avian influenza in Vietnam continues with 7 new human cases in the north and a new poultry outbreak in the south reported in the last few days. There is great concern that efficient human to human transmission may evolve in Vietnam and spread the disease worldwide; however, new information suggests that a human pandemic could start far from Southeast Asia.
China recently reported a new outbreak of H5N1 highly pathogenic avian influenza in the Tacheng district of Xingjian Province (which is on the border with Kazakhstan in the northwestern most corner of China). This outbreak involves 1000 domestic geese. In response, 13,000 geese have been culled. It is not clear how or if this new outbreak is related to the recent outbreak among migratory water birds in Qinghai. Although Xingjian and Qinghai provinces border each other, the affected areas are approximately 1000 miles apart. It is now reported that there is a cluster of human pneumonia cases in Tacheng, although the cause is unclear.
Migratory birds follow predictable paths or flyways. The affected areas of western China are where the East Asian-Australian and Central Asian-Indian flyways overlap .The original birds found to be infected in Qinghai were bar headed geese which migrate to Qinghai from India. At least 4 other species of bird were found to be infected in Qinghai as well. Chinese scientists have said that the genotype of the Qinghai virus is similar to the strain seen in southeast China last year. Since there are no farms in the Qinghai Lake area, it is presumed that the disease was brought in by migratory birds, however the affected species all migrate from India and Bangladesh, not the southeast.
Although it is not clear at this point how the virus got to western China, the presence of the disease among migratory birds in this flyway “hub” clearly suggests that the H5N1 virus is either more widespread than thought or it is likely to become so. Further analysis and release of the genetic sequences of specimens from these birds is essential to determine the relationship to the ongoing outbreak in Southeast Asia. So far no 2005 avian H5N1sequences have been submitted to Genbank.
H5N1 now extends over an area that is approximately 3000 miles, north to south, by 2000 miles, east to west, (from the southern most islands of Indonesia to the border of northwestern China and Kazakhstan to Korea) [map] and may be much larger.
In the past, highly pathogenic avian influenza was solely a disease of domestic poultry (chickens and turkeys) and migratory water birds were the natural reservoir. Now it appears that multiple species of migratory birds, including both water birds and raptors, are also susceptible. How far these birds can travel once infected and how contagious they are is unknown.
The East Asian-Australian flyway connects to the Central North American flyway across the Bering Strait and from there heads into the heart of the central U.S. The Central Asian-Indian flyway connects the southern tip of India to Siberia as far west as the border of Europe.
The ever enlarging range of H5N1 among birds suggests that the risk of human H5N1 may not be confined to Viet Nam and Thailand. Might the first case of human H5N1 in the U.S. pop up in Kansas, spread not from a human traveling from Asia but from an infected migratory bird? Much more information about the extent of H5N1 in migratory birds is needed.