Avian Influenza A(H5N1) caused mild illness in the exposed U.S. population
1. In this case series, agricultural workers who contracted A(H5N1) experienced mild illnesses of short duration, with no reported fatalities.
2. No evidence of human-to-human transmission was identified.
Evidence Rating Level: 4 (Below Average)
Study Rundown:Â Avian influenza A(H5N1) viruses are known for causing disease in poultry and dairy cows in the United States. Since their first identification as a cause of human illness in 1997, these highly pathogenic viruses have infected over 900 individuals across 24 countries. Notably, A(H5N1) viruses from clade 2.3.4.4b have recently emerged as a more frequent cause of human infections. The most recent outbreak in the United States believed to have originated from cow-to-human transmission in March 2024, prompted this study to examine human cases of A(H5N1) reported between March 28, 2024, and October 31, 2024. During this period, 46 adult cases were documented. Among these, 25 patients reported exposure to infected cows, 20 had contact with infected poultry, and one had no known direct contact with either source. Importantly, no secondary human-to-human household transmissions were identified. Most affected individuals were otherwise healthy adults with no underlying medical conditions. While all patients experienced at least one symptom associated with A(H5N1), there were no critical cases or fatalities. Oseltamivir was the most commonly prescribed treatment, initiated within days of symptom onset. This study faced several limitations, including potential under-detection or underreporting of cases, incomplete case reports, data collection before symptom resolution, challenges in identifying behaviors linked to increased exposure risk, and limited capacity to confirm human-to-human transmission. Despite these constraints, the findings highlight the association between exposure to infected animals and the risk of illness, providing valuable insights for future disease prevention efforts.
Click here to read the study in NEJM
Relevant Reading: The Emerging Threat of H5N1 to Human Health
In-Depth [case series]: This case series examined human A(H5N1) cases in the United States reported between March 28 and October 31, 2024. Patients were included if they were symptomatic and tested positive for A(H5N1) using the Centers for Disease Control and Prevention’s Human Influenza Virus Real-Time RT-PCR Diagnostic Panel Influenza Subtyping assay. No explicit exclusion criteria were applied. During the study period, 46 adult cases were identified across California (47%), Colorado (22%), Washington (24%), Michigan (4%), and Texas (2%). Among the 25 patients exposed to cows, 21 had contact with both cows and raw milk, while only 4 had contact with cows alone. Personal protective equipment (PPE) usage among these patients was inconsistent: 60% reported using gloves, but only 48% used eye protection, 24% wore face masks, 28% used boots, and 16% wore gowns. In contrast, patients exposed to poultry reported higher PPE usage, with gloves used by 85% and gowns by 80%. The average number of household contacts per patient was three, but no secondary cases were identified among the 97 contacts. Conjunctivitis was the most common symptom among patients (93%), followed by fever or feeling feverish (49%) and headache (44%). Among 16 patients with available data, the median illness duration was 4 days. Seventeen patients sought medical attention, and 39 received oseltamivir treatment. Interestingly, patients exposed to poultry had a shorter median time to treatment initiation by 1.5 days compared to those exposed to cows, although this difference was not statistically significant. Poultry-exposed patients also reported a median illness duration of 2 days, three days shorter than those exposed to cows, but again, this difference was not statistically significant. Laboratory analysis successfully sequenced the hemagglutinin (HA) gene in 57% of cases, confirming all belonged to the H5 clade 2.3.4.4b. These findings highlight the need for improved public health measures to address gaps in PPE use among workers exposed to infected animals, emphasizing the importance of prevention strategies in minimizing risk.
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