H5N1 Bird Flu in U.S. Dairy Cattle and the First Two Farm-Worker Cases, Spring 2024

This summarizes a CDC report published on May 24, 2024; its figures are as of May 22, 2024.

In late March 2024 the U.S. Department of Agriculture reported an outbreak of highly pathogenic avian influenza A(H5N1) in dairy cows across several states. Within days, two dairy farm workers were found to be infected — the first known cases of presumed cow-to-human spread of an avian influenza A virus. CDC judged the risk to the general public low, but higher for people exposed to infected animals or contaminated materials, including raw milk.

The two human cases

• Texas, April 1, 2024. A commercial dairy farm worker exposed to cattle presumed infected tested positive by RT-PCR; CDC confirmed it by RT-PCR and sequencing. The only symptom was conjunctivitis. The worker isolated, took oseltamivir and recovered. Household members, all given oseltamivir as prevention, did not fall ill.
• Michigan, May 22, 2024. A worker on a farm with confirmed infected cattle, enrolled in a daily text-message monitoring program, reported only eye symptoms. The investigation was continuing.

Sequencing of the virus from the cattle and the Texas worker found clade 2.3.4.4b, which has circulated in U.S. wild birds, poultry, backyard flocks and other animals since January 2022. The virus was also found in barn cats, birds and other animals — one raccoon and two opossums — that died on affected farms.

In the herds

• Though first reported in March 2024, infection of U.S. dairy cows may have begun as early as December 2023.
• By May 22, infected cows had been found in 52 herds in nine states: Colorado, Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, South Dakota and Texas.
• Signs were vague: less milk, less cud-chewing, thick milk like colostrum, and in some cows a clear nasal discharge.
• Raw milk from infected cows carried high levels of the virus.

H5N1 in people worldwide

From 1997 to late April 2024, 909 sporadic human H5N1 cases were reported from 23 countries, and 52% were fatal; 26 of them, including seven deaths, came from eight countries after 2022 began. Nearly all had recent contact with poultry. The United States has had three human H5 cases, all mild, none hospitalized, all fully recovered: a Colorado poultry worker in April 2022 whose only symptom was fatigue, and the two dairy workers.

Watching for spread

• Exposed people. Since 2014, when these viruses first reached North American birds, CDC has monitored people exposed to infected poultry. People exposed to infected birds or animals are watched for 10 days after their last exposure, and anyone with symptoms is tested with H5-specific RT-PCR. From February 2022 to May 2024, about 9,400 people in 52 jurisdictions were monitored; by May 22 about 350 farm workers exposed to infected cows or raw milk had been, a number still rising. Monitoring is either daily contact from the health department or instructions for self-monitoring and where to get tested and treated.
• National surveillance. Human infection with a novel influenza A virus has been nationally notifiable since 2007, and every case is investigated. About 300 clinical laboratories report the share of flu tests that are positive; the National Syndromic Surveillance Program tracks emergency-department diagnoses, including where animals are infected; and wastewater testing tracks influenza A — though it cannot tell H5N1 from other influenza A viruses, or human from animal sources. As of May 18, 2024, none of these systems showed unusual flu activity in people.
• Laboratories. CDC's tests are used in more than 100 public health laboratories in every state, nine of them also sequencing. Each state tests enough flu-positive specimens every week to have 95% confidence of catching a novel virus.
• Through spring and summer, CDC is asking commercial laboratories to send more flu-positive specimens for subtyping; about 140,000 H5-specific tests are already with state and local health departments, and 750,000 more are available. Hospital surveillance (FluSurv-NET), normally run from October 1 to April 30, continues, and doctors are being urged to think of flu in patients with conjunctivitis or respiratory illness after risky exposures — including agricultural fairs.
• In the lab, CDC is infecting ferrets to study how severe and transmissible the virus is.

Vaccines and drugs

• Vaccine candidates. Two existing clade 2.3.4.4b candidate vaccine viruses cross-react well with the Texas worker's virus (A/Texas/37/2024). The Administration for Strategic Preparedness and Response (ASPR) has shared them with licensed pandemic vaccine makers and finished initial production of bulk antigen for the national pre-pandemic stockpile. Trials of a vaccine from one of them, combined with stockpiled adjuvants, are fully enrolled, and more were to start in late summer 2024. If needed, and if the FDA allows, these could be the first doses used while full-scale production ramps up.
• Antivirals. The Texas virus is susceptible to baloxavir marboxil and to neuraminidase inhibitors including oseltamivir. Oral oseltamivir is recommended to treat confirmed or suspected H5N1 infection and, at treatment dose, to protect close contacts such as household members. With older H5N1 clades, starting oseltamivir within 2 days of symptoms was linked to better survival. All four FDA-approved flu antivirals are in the Strategic National Stockpile and many state stockpiles. NIAID is also studying new antivirals and antibodies against H5N1.

What is hard about this outbreak

• Reaching workers. People exposed on dairy farms may be hard to reach — in rural or remote places, or migrant, transient or undocumented workers — and may not know the risks or symptoms. Dairy farmers had not been major partners in bird flu outreach before; worker-protection guidance has just been updated.
• Defining exposure. Infected poultry flocks are culled quickly, which leaves a clear 10-day window to monitor workers. Sick cows stay ill for 2–4 weeks, how long they shed live virus is unknown, and some infected cows show no signs — so workers may not know they were exposed. Retail testing found no live virus, but viral fragments in about one in five retail milk samples nationwide, which suggests infection of cattle may be widespread. Monitoring could be long and costly.
• Reading the data. H5N1 symptoms resemble other infections. Unusual signals are easier to see in spring and summer, when little flu circulates, than in the fall and winter flu season.

The bigger risk

Current H5N1 viruses do not bind easily to the receptors that dominate the human upper airway, so they do not pass easily to or between people. But with the virus so widespread in birds and other animals, more sporadic human cases are expected, and a virus that gained the ability to spread easily and steadily between people could start a pandemic. That is why every human case is investigated.

CDC and state partners were planning field studies with affected farms to answer five questions: is H5N1 infecting people, how sick they get, how many infections cause no symptoms, how the virus reaches people on farms and dairies, and which behaviours — protective equipment included — prevent infection. CDC ran a similar study after the 2022 poultry outbreaks.

What to do

• The risk to the general public is low.
• People who work with, or otherwise handle, infected birds, poultry, dairy cattle or other animals or contaminated materials such as raw milk should use the recommended protective equipment, watch for symptoms, and see a doctor promptly if they get sick, for flu testing and antivirals if needed.
• Pasteurization inactivates H5N1, and the FDA has confirmed the commercial milk supply is safe. Nobody should drink raw milk or eat products made from it.

The response is a One Health effort — human, animal and environmental health together — involving CDC, USDA, FDA, ASPR, the Health Resources and Services Administration, NIAID, and state and local public and animal health officials.

Sources

• Shikha Garg, Carrie Reed, C. Todd Davis, Timothy M. Uyeki, Casey Barton Behravesh and colleagues, "Outbreak of Highly Pathogenic Avian Influenza A(H5N1) Viruses in U.S. Dairy Cattle and Detection of Two Human Cases — United States, 2024," MMWR, 2024; MMWR Early Release, May 24, 2024. https://www.cdc.gov/mmwr/volumes/73/wr/mm7321e1.htm
• CDC, worker protection and personal protective equipment for H5: https://www.cdc.gov/flu/avianflu/h5/worker-protection-ppe.htm