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Bird flu cases surge in the U.S. What we know so far.

The CDC said it's monitoring the worst avian influenza outbreak since 2015, which has devastated poultry farms along the East Coast and in the Midwest.
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Federal health officials are closely watching a highly lethal type of bird flu that’s devastated poultry farms along the East Coast and the Midwest in recent weeks. There are no signs the strain of avian influenza poses a danger to people yet, but experts are on the lookout for potential mutations of the virus that could make it more of a threat.

Although the risk to humans remains low, the Centers for Disease Control and Prevention said it will monitor people who've been exposed to domestic and wild species infected with H5N1 — a highly pathogenic avian influenza virus that spreads easily among birds.

Since February, the H5N1 virus has been detected in commercial and backyard flocks in at least 17 states, according to data from the U.S. Department of Agriculture, making it the worst bird flu outbreak since 2015, when nearly 50 million birds were slaughtered or died. The spread of the disease has largely been blamed on the migration of wild birds flying over domestic flocks and transmitting the virus through their droppings.

At this time, no human cases of H5N1 have been detected in the United States, CDC spokesperson Kate Grusich said in a statement to NBC News. “Based on past experience with earlier H5N1 bird flu viruses — and what is known about this group of viruses from existing epidemiologic and genetic sequence data — CDC believes the health risk to the general public is low,” she said.

The worry, experts say, is that a continued spread among birds could give the virus more chances to pick up new mutations and become more of a risk to people.

“New strains of influenza that are introduced to the human population and can cause global pandemics often originate from these animal sources, in particular birds,” said Dr. William Schaffner, an infectious diseases expert at the Vanderbilt University Medical Center. “There is a risk that some of these bird flu strains may pick up the genetic capacity to infect humans.”

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More than a dozen types of bird flu have been identified, according to the CDC. Since 2015, there have been sporadic cases in humans from bird flu viruses, though most often occur in people who had prolonged exposure to infected poultry.

Schaffner said the virus often has trouble infecting humans — once the virus is inhaled, it must latch on to the cells located in the back of the throat, the nose and the upper parts of the bronchial tubes, which are in the lungs. He noted that the disease can't be transmitted to humans through properly cooked food.

But when the bird flu does infect people, it can be quite severe, with a mortality rate estimated to be at approximately 60 percent, according to the CDC.

The CDC said data so far shows the current H5N1 virus lacks mutations seen in the past that have been associated with bird flu viruses spreading easily among poultry, infecting people more easily and causing severe illness. 

The agency said it is working closely with the Agriculture Department and states to track the spread and conduct additional laboratory work to be ready in case human infections occur.

‘Explosion’ of cases expected

Meanwhile, farms across the U.S. are taking steps to increase biosecurity measures.

Dr. Mendel Miller, the assistant state veterinarian for South Dakota, told NBC News that the state has euthanized just short of 200,000 infected birds.

As soon as state health authorities detect a potential case of bird flu, they immediately quarantine the facilities and prevent the movement of poultry products, he said.

"Everybody's trying to do what they can biosecuritywise, trying to prevent the introduction of the virus," Miller said. "Whether it's a backyard flock or more of a large, commercial type flock, everybody needs to do their part to prevent the spread of the disease."

The measures may do little to stop the spread of the virus, according to Henry Niman, a virologist and biochemist in Pittsburgh who has been tracking the bird flu’s spread. He is expecting an "explosion" of bird flu cases in birds in the coming weeks.

"This outbreak I think is likely going to be bigger than 2015," he said, noting that the disease is already widespread in other regions of the world, such as Europe and Canada.

The virus likely began circulating in Europe before it came down to the East Coast of North America, Niman said.

"There's a lot of birds that have been positive and there have been some outbreaks in the Delmarva Peninsula, which is in Delaware and Maryland. That peninsula has a lot of poultry," he said. "It is starting to take off in the Midwest."

Niman anticipates a "limited" number of human cases in the U.S., likely people who have close contact with birds who may then transmit the virus to close family members.

Mike Stepien, a spokesperson for the Animal and Plant Health Inspection Service, an agency within the Agriculture Department, said in an emailed statement that the virus does not present an immediate public health concern at this time.

Bird owners can take some simple steps to protect their birds from the virus, he said, including:

  • Practice good biosecurity, such as cleaning up feed spills to avoid attracting wild birds.
  • Prevent contact between domestic birds and wild birds.
  • Report sick birds or unusual bird deaths to state/federal officials, either through their state veterinarian or through the Agriculture Department’s toll-free number at 1-866-536-7593.

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