Results
Identification of the Index Patient
The index patient’s illness began on December 13, 2024. Signs and symptoms included, fever, abdominal pain, myalgias, and conjunctivitis, which lasted approximately 1 week and resulted in two health care visits (December 16 and 17). The first visit was to a local emergency department on December 16, where testing for influenza, respiratory syncytial virus, and COVID-19 was performed on a nasopharyngeal specimen. That day, an influenza A–positive rRT-PCR result was received, and the specimen was subsequently sent to SFDPH PHL for further testing as a part of enhanced surveillance. On January 9, 2025, the original specimen tested positive for A(H5) (cycle threshold [Ct] value approximately 29, with Ct values <38 considered positive) using the CDC assay at SFDPH PHL and was confirmed by CDC using the same A(H5) primer and probe set on January 14. A second specimen (oropharyngeal) collected by SFDPH on January 10 (25 days after the first specimen, when the child had been asymptomatic for 21 days), was positive for A(H5) (Ct value approximately 37); specimens collected 4 days later were negative.
Sequencing revealed clade 2.3.4.4b, genotype B3.13 viruses, closely related to B3.13 viruses detected in humans and animals in California (Figure 1). Phylogenetic analyses revealed that the sequences clustered together on an independent branch relative to other California human and dairy cattle sequences. Nucleotide and amino acid changes in the hemagglutinin (HA) and nucleoprotein (NP) genes were observed between the two sequences, consistent with viral replication, and no critical markers of mammalian adaptation (increased virulence or transmission risk) were identified.
Index Case Investigation
The index patient lived in an urban environment, did not travel, and had no reported exposure to dairy cows, cats, poultry, birds or other wild animals in the 10 days prior to the illness onset; the family had a pet dog. There were no animals at school, and the patient’s family did not work in occupations that increase risk for A(H5N1) virus infection (handling, slaughtering, defeathering, butchering, culling, caring for, or milking infected animals). A member of the patient’s family purchased raw poultry at a live bird market 2 weeks before the child’s illness onset; the poultry was cooked and consumed the same day it was purchased.
Investigation of Close Contacts
Among 84 persons identified as possible contacts of the index patient (seven household, 53 school, and 24 health care), 67 (80%) met the close contact definition (Figure 2). No household contacts reported illness. School absences were reported for 34 (64.2%) school contacts, 26 (76.5%) of whom were interviewed (one teacher and parents of 25 children). All interviewed parents reported respiratory illnesses in their children, including seven who were symptomatic at the time of interview. The teacher had had influenza-compatible symptoms but was asymptomatic at the time of interview. Four persons were tested for one or more respiratory viruses (COVID-19, RSV, or influenza) previously while ill; all test results for influenza were negative. Among the 24 health care worker contacts from three facility visits (two urgent care, one emergency department), 11 (45.8%) completed a survey, including seven who had close contact with the patient; none reported influenza-compatible symptoms. All 11 available respiratory (oropharyngeal and nasal) specimens from close contacts (seven household and four school) were A(H5)-negative by rRT-PCR.
Serum specimens were collected from the index patient (32 days from onset to convalescent serum collection), three adult household contacts, two school contacts, and four health care contacts. Among these nine contacts, the median interval between their first exposure to the index patient and serum collection was 45 days (range = 9–47 days), and the median interval between their last exposure and serum collection was 26 days (range = 0–46 days). The patient had antibodies to all three wild-type A(H5N1) viruses, with elevated antibody titers in all assays, consistent with recent H5N1 infection: A/Texas/37/2024 (B3.13) (MN titer = 160, HI titer = 320); A/Michigan/90/2024 (B3.13) (MN titer = 320, HI titer = 226); and A/Washington/240/2024 (D1.1) (MN titer = 113, HI titer = 320). All nine close contacts’ serology results were negative for all three wild-type A(H5N1) viruses.