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What is the current state of H5N1 infections among wild and domestic birds in the United States and internationally?
In the United States, highly pathogenic avian influenza A(H5N1) virus has been detected in wild birds in 41 states. This includes migratory waterfowl, such as ducks and geese, as well as other bird species, including resident birds of prey like owls, eagles, hawks, and falcons. These infections generally have not been detected in suburban or urban birds that you might see coming to a bird feeder.
There are 36 states with detected H5N1 infections in commercial poultry or backyard bird flocks in 2022. Noncommercial backyard flocks can be ducks, chickens, and turkeys not raised for commercial purposes, as well as nonpoultry bird species, such as pheasants, geese, and guineafowl. Commercial poultry outbreaks or noncommercial backyard bird flock outbreaks have been confirmed in 36 states.
What is the current state of H5N1 infections among humans?
There has been one human infection reported in England in January of this year (2022) that occurred in December 2021. That infection was detected in an 80-year-old man who raised ducks. After a few of his ducks became sick with H5N1 virus infection, the man also was found to have the virus but was asymptomatic.
In April 2022, a Colorado man involved with poultry disposal operations reported fatigue and had an upper respiratory tract specimen collected, which tested positive for H5N1 virus. When he was resampled, however, he tested negative for influenza. He was isolated and treated with oseltamivir, which is an antiviral drug used for treatment of seasonal influenza. It is unclear whether he was actually infected or if the test was positive because of transient detection of H5N1 virus contamination.
There have been no other cases of H5N1 virus infection reported worldwide with the currently circulating virus. So, it appears that H5N1 virus infection, at least with this virus that’s circulating in birds, referred to as clade 126.96.36.199b, is very rare in people. The CDC considers the risk to the public to be low.
Nevertheless, we are working with local and state public health partners to monitor people that have been exposed to H5N1 virus–infected birds. So far, there have been more than 3000 people monitored by local state health partners in collaboration with CDC after exposure to birds with the virus infection. About 90 people who developed symptoms after their exposures were tested and none of them had detection of H5N1 except for this one individual in Colorado. It’s important to remember that there are many other causes of acute respiratory illness symptoms in humans, which include common respiratory virus infections and SARS-CoV-2.
Are H5N1 infections generally seasonal or perennial?
Previous H5N1 viruses have circulated in wild birds and infected poultry during cooler months, so in the fall, winter, and spring, but not exclusively.
But it’s important to note that the viruses causing these backyard and commercial flock outbreaks are different than H5N1 viruses from previous years. These current-clade 188.8.131.52b viruses circulating not only in the US but in other places in the world appear to the quite well adapted to spread in wild birds, and they appear to be less well adapted to infect people than previous H5N1 viruses.
How does the number of cases we have seen so far in 2022 compare with previous years?
From 2003-2021, there were 864 sporadic H5N1 cases with 456 deaths reported to the World Health Organization in 19 countries.
Since 2020, there have been very few (three) reported human cases of H5N1, and one person died in 2021. There was one case and one death reported globally in 2019, no cases reported in 2018, four cases and two deaths reported in 2017, and 10 cases and three deaths in 2016. In 2015, there were 145 reported cases of H5N1 virus infections in humans worldwide, and 42 people died.
But as I mentioned before, the H5N1 viruses that caused human infections before 2020 are different from the clade 184.108.40.206b viruses that are spreading in wild birds now, which appear to be less adapted to infect humans.
What does H5N1 infection look like in birds?
Highly pathogenic avian influenza A viruses, which includes H5N1 virus, can cause a range of illness when they infect wild birds: anywhere from no illness (asymptomatic infection) to death. (The categorization of ‘highly pathogenic’ in this context only refers to how a virus can cause severe disease and mortality in infected birds and is based upon specific virus characteristics; it does not apply to disease severity in infected people.) These avian viruses can infect domestic poultry and cause outbreaks with high mortality in certain poultry species, like chickens and turkeys. Signs of infection include loss or lack of energy, lack of appetite, difficulties in coordination, purple discoloration and/or swelling of different body parts, diarrhea, and rapid death. H5N1 viruses are present in respiratory secretions and feces of infected birds and can be present in bird tissues and contaminate their feathers.
How contagious is H5N1?
H5N1 viruses are very contagious among birds. However, it has been very uncommon for H5N1 virus to spread from a bird, whether it’s a wild bird or poultry, to infect a human. When that has occurred, it has been in the context of unprotected, close, or direct exposure, meaning the person was not wearing personal protective equipment. There is either aerosolization of the virus and then inhalation of the virus, or an individual could potentially self-infect themselves by having the virus on their hands and fingers and, by contact transmission, infect their mucous membranes.
Human-to-human H5N1 virus transmission is even rarer. These infections have been limited and nonsustained, meaning that the infection stopped after one or two infections and did not spread in the community. Most of these clusters of H5N1 virus infection in humans have occurred in blood-related family members through prolonged unprotected close exposures. These infections have occurred in households or when a family member is taking care of a hospitalized infected individual for a prolonged period without using personal protective equipment. There have been two instances of third-generation transmission, or where one individual infected another person who then infected a third person, all in blood relatives. Though these are very rare, H5N1 clusters are of great concern and it’s very, very important to investigate every single case and follow and monitor the close contacts very closely to assess whether additional infections have occurred.
We think the risk of both bird-to-human transmission is low and risk of human-to-human transmission is very low and lower with currently circulating H5N1 viruses compared with those in the past.
Créditos: Comité científico Covid