Avian Influenza 2024-2025 Outbreak in the United States of America

Summary

This section is a summary of the current situation with key developments since the last update.

Highly pathogenic avian influenza (HPAI) outbreaks have primarily affected wild and domesticated bird populations, with occasional human spillover. In 2024, the United States (U.S.) reported the first known infection of H5N1 in dairy cattle, rapidly evolving into a multispecies outbreak among cattle and poultry with spillover into humans.

Currently, there are 70 confirmed and 8 probable human cases in 15 states.

Table 1. Outbreak Summary. Number of new and total cases of key epidemiological measures for avian influenza. Green icons indicate no new data during this reporting period. Data as of February 22, 2025.

Human Confirmed Cases New: 1 Total: 70 # States Affected: 13	Human Probable Cases New: 0 Total: 8 # States Affected: 4	Human Deaths New: 0 Total: 1
Farm Worker Infections New: 1 Total: 65	Positive Commercial Poultry Farms Week 7: 304 Week 8: 312 New: 18 Total: 312 # States Affected: 36 # Territory: 1	Positive Commercial Cattle Farms Week 7: 973 Week 8: 973 New: 0 Total: 973 # States Affected: 17

Epidemiological Update

This section includes an epidemiological update with an epi curve, weekly case incidence, age and gender distribution, data availability chart, symptoms figure, and genomic data.

Table 2: Number of confirmed human cases by state and exposure source and change in cases since the last report. Data as of February 22, 2025.

Table 3: Number of probable human cases by state and exposure source and change in cases since the last report. Data as of February 22, 2025.

Case Definitions: According to Council of State and Territorial Epidemiologists (CSTE) guidance. When a human case tests positive for H5 at a public health laboratory and the result is confirmed by CDC testing, a case is reported as confirmed. When a human case tests positive for H5 at a public health laboratory but testing at CDC is not able to confirm H5 infection, a case is reported as probable.

Context & Background

Highly pathogenic avian influenza (HPAI) H5N1 is a zoonotic virus primarily infecting wild and domestic bird populations, with occasional spillover events in mammals, including humans.

To learn more, follow these links to CDC pages on causes and spread, symptomatology, and updates on the current situation.

In March 2024, the H5N1 avian influenza virus was detected in dairy cattle in the U.S. for the first time, quickly evolving into a multi-state outbreak among humans, animals, and poultry linked to spillover events at commercial agriculture and related operations. This event marked the first instance of cow-to-human spread of bird flu globally. And prior to this outbreak, only one human case of avian influenza had ever been detected in the U.S.

Within the U.S., multiple local, state, and federal agencies are responsible for protecting and monitoring human, environmental, and animal health. The resulting data collected by responding agencies is siloed, which in turn can hamper timely analysis and response, highlighting the need for an integrated One Health data collection approach. The outbreak emphasizes the need for integrated human-animal-environmental surveillance.

As avian influenza infects more people, experts have raised concern over risk for mutation or viral reassortment through co-infection with the seasonal flu to create a viral strain that is transmissible between humans. Likewise, while most cases have been mild, the death of an individual from Louisiana underscores that there is a risk of severe disease as the virus continues to spread. Therefore, individuals, farms, and authorities must take precautions to protect humans from the virus and reduce pandemic risk. Enhanced cross-species surveillance and early intervention remain key to containing spread.

Data & Analytical Notes

G.h relies on publicly available data, and some variables may not be available for all cases. Therefore, figure sample sizes are based on data availability. For example, only 6% of confirmed cases have data on both age and gender, so the sample size for Figure 5 is relatively small.

Curators have faced many challenges with building an emerging infectious disease dataset in real-time, including: delays in reporting for human cases, animal outbreaks, and wastewater detections, delays in confirmatory testing, limited data standardization within and across government organizations, lack of precision of language in the media (e.g., H5, H5N1 being used interchangeably with avian influenza), presentation of aggregated case data, limited case metadata, and retroactive removal and addition of impacted herds, flocks, and wastewater sites.

G.h has conducted regular data reconciliation efforts to mitigate retroactive, unannounced, and unexplained changes and data discrepancies between G.h and our primary sources (WastewaterSCAN and USDA). USDA publishes the total number of cattle outbreaks on their website, and we are able to align counts on a daily basis. However, cumulative numbers are not available from USDA (Poultry) or WastewaterSCAN. Therefore, public data that are currently available from these sources may differ from the above counts.

Curator Notes

Probable Cases Discrepancy

Update 2025-01-17: We are trying to update case information. Curators have attempted to contact the Los Angeles and Stanislaus County departments of health, but no reply. For now, the Los Angeles case is captured as 'Probable' Status in the G.h dataset (ID66). The G.h count for probable CA cases is 2, which does not align with CDPH number, and we are aware of this discrepancy.

Entry 2024-12-23: On December 23rd, CDPH and CDC's websites for confirmed cases (related to cattle) increased by 1 case.

However, news media reported two "confirmed" avian influenza cases on December 23rd - one case in Los Angeles, and another in Stanislaus County (Ref ID66 and ID67, Source I).

Stanislaus explicitly states that CDC has confirmed the case.

Los Angeles is less clear if CDC has confirmed the case or if the investigation is ongoing.

So, to make sure the G.h confirmed count did not exceed CDPH and CDC totals, curators had to pick one case to add to our linelist.

One possibility is that these 2 cases are the 2 confirmed cases from 12/20 and there was a delay in announcement.