Outbreak of Highly Pathogenic Avian Influenza A(H5N1) Viruses in U.S. Dairy Cattle and Detection of Two Human Cases — United States, 2024

Investigation and findings

Identification of two human cases of influenza A(H5) virus infection

On April 1, 2024, the Texas Department of State Health Services reported, after confirmation from the CDC, that an employee at a commercial dairy farm tested positive by real-time reverse transcription–polymerase chain reaction (RT-PCR) for highly pathogenic avian influenza (HPAI). ) Infection with the A(H5N1) virus after exposure to dairy cattle suspected of being infected with the A(H5N1) virus*,; The CDC confirmed the laboratory results by RT-PCR and sequencing (1). The patient experienced only conjunctivitis with no other signs or symptoms, was asked to self-isolate, was treated with oseltamivir, and recovered. No illnesses were identified among members of the patient’s household, all of whom received oseltamivir post-exposure prophylaxis. A week earlier, the U.S. Department of Agriculture reported an outbreak of the A(H5N1) virus in dairy cows in several states.§ A(H5N1) viruses have also been detected in barn cats, birds, and other animals (e.g., a raccoon and two opossums) that lived in and around human habitations and died on affected farms . Genetic sequencing of the A(H5N1) virus from infected cattle and the farm worker** identified clade; this clade has been detected in wild birds, commercial poultry, backyard flocks, and other animals in the United States since January 2022 (2). On May 22, 2024, the Michigan Department of Health and Human Services reported an A(H5) case in a dairy farm employee on a farm confirmed to have A(H5N1) virus in cattle; this individual was enrolled in a text-based active monitoring program and reported only ocular symptoms.†† The investigation into this second case is ongoing. These two cases are the first known cases of suspected spread of the avian influenza A virus from cows to humans.

Influenza A(H5N1) virus in dairy cattle in the United States

Although first reported in March 2024, A(H5N1) virus infection in dairy cows in the United States may have been occurring since December 2023, according to preliminary data (3). As of May 22, 2024, infected dairy cows have been identified in 52 dairy cattle herds in nine states.§§ (Colorado, Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, South Dakota and Texas). Signs in cattle were nonspecific and included decreased milk production, reduced rumination, and thickened milk consistency (colostrum-like); some cows also had clear nasal discharge. High levels of A(H5N1) virus have also been detected in unpasteurized (raw) milk from infected cows (4).

Human cases of influenza A(H5N1) worldwide

From 1997 to the end of April 2024, a total of 909 sporadic human cases of influenza A(H5N1) were reported worldwide in 23 countries; 52% of human cases were fatal (2); of the 909 cases, 26 human cases of A(H5N1) have been reported in eight countries, including seven deaths, since 2022. Since these figures were last updated, two additional human cases of A(H5) have been detected, including the case in Michigan and one case in Australia. Almost all reported human cases of A(H5N1) virus were from recent exposure to poultry. In the United States, three human cases of A(H5) have been identified to date; all patients had mild illness, were not hospitalized, and made a full recovery. The first occurred in April 2022 in a person in Colorado with direct exposure to infected poultry, who reported only fatigue,¶¶ and the second and third occurred in dairy farm workers with conjunctivitis referenced in this report.

Outbreak response activities in the United States

Activities implemented using a One Health*** approach to respond to this epidemic††† include monitoring infections in exposed individuals, conducting syndromic and laboratory surveillance, planning epidemiological investigations, and evaluating the performance of existing medical countermeasures, including diagnostic tests, vaccines, and therapeutic products. To evaluate the pathogenesis, severity, and transmissibility of the A(H5N1) virus in an animal model of infection, the CDC is also conducting laboratory experiments on ferrets.

This activity was reviewed by the CDC, deemed not to be research, and was conducted in accordance with applicable federal law and CDC policy.§§§ The ferret studies were approved by the CDC Institutional Animal Care and Use Committee.

Surveillance of people exposed to influenza A(H5) viruses

In 2014, CDC began monitoring people exposed to infected poultry when HPAI A(H5) viruses were first detected in poultry and wild birds in North America (5). Recommendations are to monitor people exposed to infected birds, poultry or other animals for 10 days after their last exposure and to test symptomatic people for influenza A viruses by RT-PCR testing using primers and probes specific to H5, in coordination with national or local authorities. ministries of health (6).

From February 2022 to May 2024, approximately 9,400 people in 52 jurisdictions were monitored. As of May 22, 2024, approximately 350 agricultural workers have been or are currently being monitored for illness following exposure to infected cows or infected raw cow’s milk; the number of people under surveillance continues to increase; data is updated weekly.¶¶¶ Monitoring is conducted either through direct daily contact with state or local health departments or by providing individuals with information on how to self-monitor and where to seek testing and treatment possible in the event of symptoms. The most recent A(H5) human case was identified through active, daily surveillance of exposed agricultural workers using a text-based disease surveillance program in Michigan (7).

National surveillance activities

CDC’s influenza surveillance systems**** collect information to track trends in influenza activity and detect changes in circulating influenza viruses, including the detection of new influenza A viruses throughout the year. ‘year. Human cases of infection with the new influenza A virus have been reportable nationally since 2007; Each identified case is investigated and reported to the CDC.

Through approximately 300 clinical laboratories, the CDC monitors the percentage of flu tests with positive results in clinical settings. The National Syndromic Surveillance Program collects data from emergency departments and other health care settings, making it easier to detect unusual trends in influenza diagnoses, including in jurisdictions where A(H5N1) viruses have been identified in animals.

CDC National Wastewater Monitoring System†††† complements other existing human influenza surveillance systems by monitoring influenza trends. These surveillance methods detect influenza A viruses but do not distinguish between influenza A subtypes, meaning that current wastewater testing can detect influenza A(H5N1) viruses but cannot detect them. distinguish from other influenza A viruses or determine the source of influenza A viruses (e.g., humans). in relation to animals or products of animal origin). Together, these systems provide visibility into influenza activity in the United States. As of May 18, 2024, no indicators of unusual human influenza activity, including A(H5N1), have been detected in humans by these systems.

CDC molecular diagnostic tests are used in more than 100 public health laboratories in all 50 states and other U.S. jurisdictions to detect seasonal and novel influenza A viruses; nine centers also carry out genetic sequencing for virus characterization. Statistical methods are used to determine the number of samples needed to have 95% confidence that at least one new influenza A virus among all influenza-positive samples per week would be detected given the prevalence flu variable; the number varies depending on the time of the season. Each state’s contribution is proportional to its population and has been set as a national weekly target for public health laboratory testing.§§§§

Spring and summer activities

Multiple efforts are underway to enhance influenza surveillance activities in the spring and summer as part of this response. The CDC is working with commercial laboratories to increase the submission of positive influenza test samples to public health laboratories to increase the number of samples available for virus subtyping. Approximately 140,000 of these H5-specific tests are already pre-positioned nationally and locally, with another 750,000 tests available for distribution if needed. CDC also continues to collaborate with commercial diagnostic test manufacturers with the goal of having an A(H5N1) test widely available if needed. Surveillance of laboratory-confirmed influenza-associated hospitalizations will also continue in the spring and summer through the Influenza-Related Hospitalization Surveillance Network (FluSurv-NET), which typically conducts surveillance from October 1 to April 30 of each flu season. Additionally, CDC is working with state and local public health partners, through outreach to providers and clinics, to increase awareness about influenza A(H5N1) so that influenza is considered in patients suffering from conjunctivitis or respiratory illness after exposures, including attendance at agricultural fairs, which could increase the risk of infection with the novel influenza A virus.

Medical countermeasures

As a World Health Organization Collaborating Center and in partnership with the Administration for Strategic Preparedness and Response (ASPR), CDC regularly develops new influenza A vaccine candidate viruses (CVV) for preparedness to a pandemic. Antigenic characterization of the A(H5N1) virus isolated from a Texas farm worker (A/Texas/37/2024) with ferret antisera raised against existing CVVs confirmed that two CVVs from the A(H5) clade have good cross-reactivity with this virus. virus. As part of the National Pre-Pandemic Influenza Vaccine Stockpile (NPIVS) program, ASPR shared these CVVs with manufacturers of pandemic influenza vaccines approved by the Food and Drug Administration (FDA) and completed initial production of bulk antigens. ASPR also supports the clinical evaluation of the safety and immunogenicity of vaccines using an antigen made from one of these CVVs, the A/Astrakhan/3212/2020 influenza virus vaccine, in…

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