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A new strain of bird flu virus called H7N8 has been found at an Indiana poultry farm — the first case of avian influenza at a poultry farm since last year’s devastating multi-state outbreak.

The U.S. Department of Agriculture said it confirmed the diagnosis Friday morning, and workers have already isolated the farm and begun slaughtering all the birds there and destroying their carcasses.

"This is a different strain of virus than the strain that we saw during the 2015 outbreak," said Dr. T.J. Myers, associate deputy administrator of the USDA's Animal and Plant Health Inspection Service (APHIS).

"This particular case is an H7N8 virus. It is a significant virus that does need an immediate response in order to contain it and prevent spread to other facilities," Myers said.

About 48 million birds died or were slaughtered in the outbreak of highly pathogenic H5N2 avian flu that spread to several states last year. The outbreak caused trade partners to stop buying U.S. poultry products and drove up the price of eggs.

"There’s no single thing that you can point to that is the one and only factor that explains this."

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The USDA is eager to prevent that from happening this time.

Myers says no one has figured out how H5N2 got into U.S. flocks in the first place or how it spread from farm to farm, but he says poultry producers have cracked down on biosecurity measures and hope that they will be enough to prevent spread this time.

"This is a fairly poultry-dense area," Myers told reporters.

"With the outbreak last year we did quite a bit of epidemiological work. There’s no single thing that you can point to that is the one and only factor that explains this," Myers said.

There are many different strains of influenza that infect birds. They’re designated not only by the names — H5N2 and H7N8, for instance — but also by whether they kill birds.

The H7N8 strain has been found in wild birds, mostly ducks, in the U.S. but it’s been a low-pathogenic strain that doesn’t make the birds sick.

Highly pathogenic bird flu is rare in the U.S. and Myers says it’s not clear how this particular strain mutated into a highly pathogenic form.

The H5N2 strain that caused last year’s outbreaks was originally from Asia and probably first came into the country inside the bodies of migrating wild birds. This H7N8 strain appears to be of North American origin, although officials are testing it now to find out its precise characteristics. These genetic fingerprints can sometimes give clues as to where a virus came from.

H7N8 is not known to have ever infected people. Avian influenza viruses do — H5N1 and H7N9 bird flu are both causing occasional human cases and they can be very deadly.

There are avian influenza vaccines for poultry but Myers says the U.S. doesn't have an H7N8 vaccine and says it's not clear that the ones that exist would match this strain. Flu viruses mutate wildly and vaccines must be made fresh for each strain.

Poultry producers usually only vaccinate as a last ditch effort because it's expensive and once birds are vaccinated they cannot be exported, because testing cannot differentiate a vaccinated bird from an infected one.