Wastewater and Genomic Data Guide a Hepatitis A Outbreak Response in Los Angeles County

From Morbidity and Mortality Weekly Report (MMWR), Notes from the Field, by Jordan B. Braunfeld (CDC Epidemic Intelligence Service) and colleagues at the Los Angeles County Department of Public Health, the Council of State and Territorial Epidemiologists and the California Department of Public Health.

Summary

• Known before: routine hepatitis A virus (HAV) surveillance misses people who don't seek care, aren't tested, or aren't reported.
• New here: during March 12–April 18, 2024, three HAV cases among people experiencing homelessness (PEH) shared a matching subgenotype, pointing to one chain of transmission. At the same time, wastewater HAV rose about seven times above baseline, while a similar number of cases were reported — showing wastewater can reveal unreported spread.
• Implication: genomic analysis and wastewater testing can complement case-based surveillance to find and characterize HAV outbreaks.

Background

HAV is a highly infectious liver virus spread by the fecal-oral route. Most people recover fully, but it rarely causes liver failure and death. Poor access to hygiene and sanitation raises the risk. Case-based surveillance struggles because people are infectious for up to 2 weeks before symptoms, about 30% of infected people have none, early symptoms (fever, nausea, abdominal pain, diarrhea) are vague, and 30% of symptomatic cases never develop jaundice.

Investigation

• Surveillance: LA County monitors hepatitis A among about 10 million residents in 86 of 88 cities. Labs must report positive IgM anti-HAV tests; each is investigated for risk factors and contacts needing post-exposure prophylaxis, and specimens go to the state lab for sequencing.
• Wastewater: HAV testing began in September 2023 at two treatment plants — plant A, serving about 4 million residents including the City of Los Angeles, and plant B, about 3.5 million.
• The signal: the three spring cases among PEH (versus one in the prior 6 months) shared a previously unreported subgenotype IA strain. A concurrent rise at plant A, which serves where they lived, suggested more undetected cases — so the county defined an outbreak and stepped up case-finding.

Hepatitis A virus in wastewater and outbreak-associated cases, Los Angeles County, 2024. CDC.

Outcomes

• April 26–June 25: eight more outbreak cases, for 11 in all. Of 10 sequenced, all were genetically indistinguishable.
• Who: six reported homelessness and illicit drug use, four homelessness only, one neither. Nine had hepatitis symptoms; six were hospitalized. Of nine with known locations, seven lived in the City of Los Angeles.
• Wastewater: in April, when seven cases (including non-outbreak ones) were reported, peak HAV at plants A and B was three and seven times the September 2023–March 2024 baseline maximum — a period of 2–5 cases a month countywide. May and June peaks were lower.

Conclusions and actions

• Evidence of hidden spread: the April rise was disproportionate to reported cases, pointing to transmission in a population facing major barriers to care.
• Caution on declines: wastewater only catches people who shed virus within a monitored plant's area; someone exposed who becomes infectious elsewhere is missed. Wastewater data must be read alongside case data.
• Response: backed by genomic and wastewater data, the county launched mobile vaccination outreach for people at higher risk, countywide alerts to clinicians and homeless service providers, and more environmental health inspections of shelters in the outbreak area.
• Now: outbreak cases continue, but falling wastewater levels suggest transmission may be declining.

Sources

Based on "Notes from the Field: Genomic and Wastewater Surveillance Data to Guide a Hepatitis A Outbreak Response — Los Angeles County, March 2024–June 2024," MMWR, Centers for Disease Control and Prevention (PDF). MMWR material is in the public domain. The original's conclusions say "April 2023," evidently meaning April 2024, the month described in its results.