In this week's spotlight, CDC confirms 3 additional cases of H5 bird flu in California, bringing the total in that state to 6; shares a summary of genetic sequencing data of H5N1 bird flu viruses from California available to date; shares information about a new pilot program to test symptomatic farm and dairy workers for H5 bird flu; and provides an update on a program to offer free seasonal flu vaccines this season to farm workers in states affected by H5N1 bird flu.
Situation Update
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11 Oct 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, 20 human cases of avian influenza A(H5) virus infection have been reported in the United States; 21 in total since 2022. Ten of these cases were associated with exposure to H5N1 bird flu-infected poultry and 9 were associated with exposure to sick or infected dairy cows (1,2). This includes 6 cases in California, 2 of which were confirmed by CDC on Thursday [3 Oct 2024], 2 on Wednesday [9 Oct 2024], and 2 on Thursday [10 Oct 2024]. All California cases occurred in dairy workers on affected farms. All of the California
cases are from different farms, except for 1 case reported this week, which was from a farm that had a case reported previously. The 2 cases from the same affected farm worked on different parts of the farm and are not close contacts of each other. The epidemiology of the situation continues to suggest sporadic instances of animal-to-human spread. All 6 California cases are reported to have experienced mild symptoms, including eye redness or discharge (conjunctivitis), and none were hospitalized. Additional testing is ongoing in California,
and presumptive positives are being routinely forwarded to CDC for confirmatory testing. Going forward, CDC will track and report confirmed cases, by state and source of exposure, in a table on its website. The source of the exposure in one case, which was reported by Missouri on 6 Sep 2024, could not be determined. Serological tests of the contacts of the Missouri case are pending. CDC believes the immediate risk to the general public from H5N1 bird flu remains low, but people with exposure to infected animals are at higher risk of infection.
On the animal health side, USDA is reporting that 299 dairy herds in 14 U.S. states have confirmed cases of H5 bird flu virus infections in dairy cows. The number of affected herds continues to grow nationally, fueled by increases in California. Last week, USDA reported 56 affected dairy herds in California as of 3 Oct 2024. That number increased to 100 by 11 Oct 2024. USDA reports that since April 2024, there have been H5 detections in 36 commercial flocks and 26 backyard flocks, for a total of 18.75 million birds affected. Among other activities previously reported in past spotlights and ongoing, recent highlights of CDC's response to this include:
Laboratory Update
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CDC has performed genetic sequencing on samples from the 2 human cases of H5 bird flu in California confirmed on Thursday [3 Oct 2024], and 1 of 2 human cases in California confirmed on Wednesday [9 Oct 2024]. Efforts to sequence additional cases are in progress. The results confirm that all 3 viruses sequenced to date are clade 2.3.4.4b H5N1
viruses, closely related to those detected in dairy cattle. The whole genome was sequenced from 1 of the 1st 2 cases (A/California/135/2024) and was confirmed to be a B3.13 genotype virus.
CDC has publicly posted in GISAID and submitted to GenBank the HA, NA, and NS gene segments for A/California/134/2024 (GISAID EPI_ISL_19463619; NCBI_PQ435213- PQ435215) and the whole genome sequence for A/California/135/2024 (GISAID EPI_ISL_19463618; NCBI_PQ435216-PQ435223). Additional sequencing data will be posted as it becomes available.
While the hemagglutinins (HAs) of the 3 sequenced viruses contained new amino acid changes compared to closely-related candidate vaccine viruses (CVVs) -- 2 in /California/135/2024, 3 in A/California/134/2024, and 3 in A/California/146/2024 -- none contained changes associated with increased infectivity or transmissibility among humans. CDC identified no changes associated with reduced susceptibility to neuraminidase inhibitors or polymerase acidic inhibitors, nor were there any changes associated with mammalian adaptation in other gene segments in samples from any of the cases based on currently available sequence data. CDC successfully isolated virus from samples from the 1st 2 confirmed cases in California. Attempts to isolate virus from subsequent specimens are pending. Antigenic characterization and antiviral susceptibility testing of the 2 viruses that have been isolated to date are underway. Antigenic characterization will inform whether existing H5 bird flu CVVs are still well-matched to these viruses.
CDC also has completed development and quality control testing of a reverse genetics-generated virus with the HA mutations (HA P136S and A156T) identified in the virus isolated from the H5N1 bird flu case in Missouri reported on 6 Sep 2024. Initial testing showed that the virus had reduced cross-reactivity to ferret antisera raised to viruses without these changes. To ensure optimal match with the Missouri H5N1 bird flu virus, this reverse genetics-generated virus will be used for serology testing of serum samples from the confirmed case in Missouri
and close contacts.