By Ethan Covey
The use of genomic analysis and wastewater testing data aided in identifying and addressing an outbreak of hepatitis A virus (HAV) in Los Angeles County (MMWR Morb Mortal Wkly Rep 2025;74[5]:66-68).
During April 2024, officials with the LA County Department of Public Health (DPH) identified three cases of HAV infection among people experiencing homelessness (PEH), compared with one case that had been confirmed among PEH during the preceding six months. The cases were identified via routine surveillance.
At the same time, wastewater surveillance at two water treatment plants in LA County revealed maximum HAV wastewater concentrations of three (wastewater plant A) and seven (plant B) times the maximum concentrations recorded during the baseline period of September 2023 through March 2024.
This increase in wastewater concentration resulted in LA County’s DPH developing an outbreak case definition and putting in place enhanced case-detection measures.
The resulting public health response resulted in the identification of eight additional outbreak-associated cases from April 26 through June 26.
“Hepatitis A is a highly contagious liver infection that can be challenging to detect by traditional case-based surveillance because many hepatitis A cases remain asymptomatic,” said lead author Jordan B. Braunfeld, MD, an epidemiologist with the CDC’s Epidemic Intelligence Service. “Wastewater surveillance data indicated that the amount of community transmission of hepatitis A was potentially greater than the number of cases detected by traditional surveillance. The disproportionate increase in wastewater concentrations relative to reported cases supported enacting a prompt broad-reaching outbreak mitigation response.”
In addition, Dr. Braunfeld said, “sequencing hepatitis A isolates helped link cases that did not otherwise have a clear link with each other in place and time.”
More Needed to Understand HAV
Although the genomic and wastewater surveillance helped inform an organized and timely response, Dr. Braunfeld noted that additional research is needed to better define hepatitis A shedding dynamics, such as concentration in stool, variation in concentration by symptom status and duration of shedding.
“This information will help improve interpretation of hepatitis A wastewater testing data,” he said, “such as the limit of detection and the number of cases a positive detection represents within a defined geographic area.”
Dr. Braunfeld reported no relevant financial disclosures.
