The worst bird flu outbreak in American history is "wiping out everything in numbers we've never seen before," according to a University of Maryland professor, Jennifer Mullinax.
The H5N1 strain has killed more than 58 million birds, according to reports.
That comes less than a decade after the H5N8 strain killed 50 million birds in 2015.
"Unlike H5N8, this disease is heavily impacting wild birds," University of Maryland researcher Johanna Harvey told Maryland Today.
"It's difficult to estimate how many birds are truly affected across wild populations, but we're seeing dramatic disease impacts in raptors, sea birds, and colonial nesting birds. And we now have the highest amount of poultry loss to avian influenza, so this is a worst-case scenario."
This is not thought to be a burgeoning problem for humans, but a Chinese woman has become the first person to die from bird flu, according to the World Health Organization.
The 56-year-old woman from the southern province of Guangdong was the third person known to have been infected with the H3N8 subtype of avian influenza, WHO said in a statement Tuesday.
All of the cases have been in China, with the first two cases reported last year.
The Guangdong Provincial Centre for Disease Control and Prevention reported the third infection late last month but did not provide details of the woman's death.
The patient had multiple underlying conditions, said WHO, and a history of exposure to live poultry.
Sporadic infections in people with bird flu are common in China where avian flu viruses constantly circulate in huge poultry and wild bird populations.
Samples collected from a wet market visited by the woman before she became ill were positive for influenza A(H3), said WHO, suggesting this might have been the source of infection.
Though rare in people, H3N8 is common in birds in which it causes little to no sign of disease. It has also infected other mammals.
There were no other cases found among close contacts of the infected woman, WHO said.
"Based on available information, it appears that this virus does not have the ability to spread easily from person to person, and therefore the risk of it spreading among humans at the national, regional, and international levels is considered to be low," WHO said in the statement.
The U.S. government is testing four potential bird flu vaccines for poultry, officials said earlier this month.
The trials, conducted by the U.S. Department of Agriculture's (USDA) Agricultural Research Service, are the first step in a lengthy process toward the possible first use of vaccines to protect U.S. poultry from the lethal virus.
Bird flu, also known as highly pathogenic avian influenza (HPAI), has killed hundreds of millions of birds around the world, raising interest in vaccines. The virus is largely spread by wild birds that transmit it to poultry.
The USDA is testing one vaccine from Zoetis Inc, one from Merck Animal Health, and two developed by the department's Agricultural Research Service.
Zoetis said it previously supplied its vaccine to a USDA stockpile in 2016, following a massive 2015 outbreak, but it was never used.
Initial data from a study using a single dose of a vaccine are expected in May, while results from studies on two-dose vaccine regimens are expected in June, the USDA said.
If the trials are successful and USDA decides to continue development, it would take at least 18-to-24 months for a vaccine that matches the current virus to be commercially available, the agency said.
The government needs to ensure vaccinations would not disrupt trading with major buyers, said Greg Tyler, president of the industry group USA Poultry & Egg Export Council.
Governments have previously focused on culling infected flocks to control the virus due to concerns importers would block shipments of vaccinated poultry to avoid the risk of infections.
The USDA said its "current strategy of stamping out and eradicating HPAI... continues to be the most effective strategy because it works."
France said in early April it was launching an order for 80 million doses of vaccines to use in ducks in the autumn if final trial results are positive, the first EU member to start such a plan.
Information from Reuters was used in this report.
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