In this week´s Spotlight, we confirm three additional cases of H5 bird flu in poultry farm workers at a second poultry farm in Colorado, for a total of 13 human cases since April 2024. All three people were working to depopulate infected poultry as part of a culling at a commercial egg layer operation that had reported an outbreak of H5 bird flu among poultry. All three people have mild illness and have been offered antiviral medication.

Today´s edition includes details about CDC´s genetic sequence analysis of the virus obtained from the sixth human case of H5N1 bird flu that was reported in association with exposure to infected poultry at a poultry farm in Colorado on July 19th. The findings are reassuring, showing it is closely related to the first human case from Colorado and that it does not have changes in the neuraminidase associated with antiviral resistance. The sequence was uploaded to public databases so that researchers in the United States and around the world can analyze it.

Finally, we include information about Emergency Use Instructions (EUI) issued for the flu antiviral drug, oseltamivir, and describe a new MMWR Notes from the Field report.

July 26, 2024 – CDC continues to respond to the public health challenge posed by a multistate outbreak of avian influenza A(H5N1) virus, or "H5N1 bird flu," in dairy cows, poultry and other animals in the United States. CDC is working in collaboration with the U.S. Department of Agriculture (USDA), the Food and Drug Administration (FDA), Administration for Strategic Preparedness and Response (ASPR), state public health and animal health officials, and other partners using a One Health approach. Since April 2024, 13 human cases of avian influenza A(H5) infection have been reported in the United States. Four of these cases were associated with exposure to sick dairy cows and nine were associated with exposure to avian influenza A(H5N1)-infected poultry. A BThis includes three additional cases in Colorado that were confirmed by CDC this week. The three new cases were in poultry workers who were working directly with infected poultry at a commercial egg layer operation that had reported an outbreak of H5 bird flu among poultry. Similar to previous cases, all of the people have mild illness. Based on the information available at this time, CDC´s current assessment is that the risk to the general public from H5 bird flu remains low. On the animal health side, USDA is reporting that 171 dairy cow herds in 13 U.S. states have confirmed cases of avian influenza A(H5N1) virus infections in dairy cows as the number of infected herds continues to grow. USDA reports that since April 2024, there have been A(H5) detections in 35 commercial flocks and 19 backyard flocks, for a total of 18.37 million birds affected.

Among other activities previously reported in past spotlights and ongoing, recent highlights of CDC´s response to this include:

Colorado Investigation Update
More than 600 workers performing depopulation activities on three poultry farms have been screened for symptoms. Of these, 103 had symptoms and were tested, and nine were positive for influenza A(H5). Other respiratory viruses such as SARS-CoV-2 (the virus that causes COVID-19) were also detected among symptomatic workers. Symptom onset began within two-three days and peaked within five days after the start of depopulation. Among nine workers who tested positive for avian influenza A(H5), all reported mild symptoms (predominantly conjunctivitis) and were tested within three days of symptom onset. All cases have been provided oseltamivir and are recovering. More in-depth summaries of the investigation are in process with Colorado Department of Public Health and Environment.

Laboratory Update
CDC sequencing of the virus from the sixth human case reported from Colorado confirms the neuraminidase (the N in the subtype) is an N1 and the virus is a HPAI A(H5N1) virus from clade 2.3.4.4b. Six of the eight influenza virus gene segments (the majority of the flu genome) were successfully sequenced. The sequences are closely related to the virus sequence described last week, A/Colorado/109/2024, as well as viruses detected in recent poultry outbreaks and infected dairy cattle herds made available from USDA. The sequence maintains primarily avian genetic characteristics and lacks changes that would make the virus better adapted to infect or spread among humans.

The influenza virus sequence contains a change at PB2 M631L – which is the same marker of mammalian adaptation identified in more than 99% of dairy cow sequences and that was also identified in the first Michigan human case. The influenza virus sequence does NOT have the PB2 E627K change that was seen in the virus from the case in Texas but not subsequent human viruses. That change is known to be associated with viral adaptation to mammalian hosts and has been found before in people and other mammals, but with no evidence of onward spread among people.

Furthermore, there are no markers known to be associated with influenza antiviral resistance to neuraminidase-inhibitors found in the virus sequence from the patient´s specimen. The genetic sequences for the virus from the patient (A/Colorado/134/2024) were posted to two sequence databases: GISAID (EPI_ISL_19280426) and GenBank (PQ060139). Influenza virus isolates and sequences were only successfully generated from two of the six human cases from the first poultry farm in Colorado. Overall, the genetic analysis of this HPAI A(H5N1) virus in Colorado supports CDC´s conclusion that the human health risk currently remains low.

CDC also continues to meet with commercial laboratories to discuss H5 assay licensing agreements and interest in development of commercial H5 tests. CDC´s Technology Transfer Office and the Influenza Division are actively pursuing establishing licensing agreements with multiple companies, and several have been completed.

Clinical Update
On July 19, 2024, CDC issued Emergency Use Instructions (EUI) for the flu antiviral drug, oseltamivir (generic for brand name drug Tamiflu?), for treatment or post-exposure prophylaxis (PEP) of pandemic influenza A viruses and novel influenza A viruses with pandemic potential. EUI provide information about emergency use of FDA-approved medical products that differ from or go beyond the information provided in the FDA-approved labeling (package insert). CDC-issued oseltamivir EUI fact sheets for health care providers and recipients and caregivers are available on CDC´s website: Emergency Use Instructions (EUI) for Oseltamivir.