Avian Influenza A(H5N1) Outbreak Among US Farm Exposures: Clinical Findings and Early Treatment Outcomes

Background: Highly pathogenic avian influenza A(H5N1) has reemerged in the United States with documented infections in poultry and dairy cows since 2021. From March through October 2024, 46 human cases were identified, most of whom were workers engaged in poultry depopulation or dairy-farm activities where infected or presumably infected animals were present.

Objective: To characterize the clinical presentations, exposure settings, and outcomes of individuals with laboratory-confirmed H5N1 infection and to investigate potential routes of transmission, disease severity, and risk to public health.

Methods: Using a standardized case-report form, data were collected on exposure history, symptom onset, and use of personal protective equipment (PPE). Respiratory and conjunctival swabs from symptomatic persons underwent real-time RT-PCR for H5 subtyping at both state laboratories and the Centers for Disease Control and Prevention (CDC). Genetic sequencing was performed on available samples. Investigators also monitored close household contacts to evaluate the risk of secondary transmission. An additional hospitalized patient with no identifiable exposure source was detected through routine influenza surveillance.

Results: Of the 46 adult case patients, 20 were exposed to infected poultry, 25 to infected or presumably infected dairy cows, and 1 had unknown exposure. Among the 45 occupationally exposed patients, illness was mild, with no hospitalizations or deaths. Conjunctivitis was present in 93% of cases; 49% reported fever, and 36% had respiratory symptoms. Fifteen patients had only conjunctivitis, highlighting the utility of conjunctival specimens for detection. Early antiviral therapy with oseltamivir was common, initiated at a median of two days after symptom onset. No additional cases were found among 97 closely monitored household contacts, indicating no evidence of sustained human-to-human transmission. Genetic analyses revealed clade 2.3.4.4b viruses, with some genotypic differences between poultry-related (D1.1 genotype) and cow-related (B3.13 genotype) infections.

Conclusions: In this observational study, H5N1 infections in US adults were generally mild, self-limited, and predominantly associated with conjunctivitis. The absence of critical illness or fatalities contrasts with historical reports of more severe H5N1 disease. Although no ongoing person-to-person transmission was documented, continued vigilance is warranted, given the virus’s potential for rapid adaptation.

Implications for Practice: Occupational health measures, such as consistent PPE use (especially eye protection), timely surveillance, and prompt antiviral treatment, may reduce the impact of H5N1 infections among exposed workers. Clinicians should consider conjunctival sampling for symptomatic patients with relevant animal contact. Policy efforts should focus on improving biosecurity practices in both poultry and dairy settings.

Study Strengths and Limitations: Strengths include systematic surveillance, robust laboratory testing of both respiratory and conjunctival specimens, and early antiviral administration. Limitations involve possible underreporting of mild or asymptomatic cases, incomplete details on exposure duration, and limited data on specific routes of cow-to-human transmission.

Future Research: Further studies should explore viral evolution in cows, the significance of raw milk as a transmission vehicle, and the potential for more severe infections, as highlighted by sporadic reports of severe H5N1 illness worldwide.

Reference: Garg S, Reinhart K, Couture A, Kniss K, Davis CT, Kirby MK, Murray EL, et al. Highly Pathogenic Avian Influenza A(H5N1) Virus Infections in Humans. New England Journal of Medicine. Published December 31, 2024. Link: https://www.nejm.org/doi/full/10.1056/NEJMoa2414610

• Editorial: Ison MG, Marrazzo J. The Emerging Threat of H5N1 to Human Health. New England Journal of Medicine. Published December 31, 2024. Link: https://www.nejm.org/doi/full/10.1056/NEJMe2416323