Waterborne hepatitis A virus (HAV) outbreaks have declined significantly in the United States. However, those with private water supplies should still be on the lookout for potential hazards, according to the Centers for Disease Control and Prevention’s (CDC) latest Morbidity and Mortality Weekly Report (MMWR).

Hepatitis A is a liver virus primarily transmitted through the fecal-oral route, meaning it is most often spread via food, shared drinks, close contact, drugs, sex and unsanitary conditions. Although rare, drinking water–related outbreaks have been known to occur, and HAV can remain infectious in water for months. In order to prevent potential waterborne outbreaks, researchers keep a close eye on the data. This risk study was part of that effort.

For the study, CDC researchers analyzed data on illnesses reported to the Waterborne Disease and Outbreak Surveillance System between 1971 and 2017. They identified 32 water-related hepatitis A outbreaks, resulting in 857 illnesses and no deaths. The most recent outbreak they identified took place in 2010. Outbreaks occurred in 18 states, the majority of them in the South, Southeast and Appalachia.

Untreated groundwater was linked to 72% of water-associated HAV outbreaks and 68.3% of reported cases. All individual water systems with outbreaks were supplied by private wells or springs. Individual water systems accounted for 41% of outbreaks, while community outbreaks made up 31%. Study authors also noted that one recreational waterborne outbreak and one environmental outbreak were also reported during this period, but they were excluded from the analysis.

Factors that contributed to fecal contamination of groundwater, and therefore hep A risk, include nearby septic tanks or sewage, heavy rainfall, improper well construction and maintenance, surface water seepage and hydrogeologic formations such as limestone pockets and caves.

Overall, the incidence of drinking water–associated hepatitis A outbreaks has dropped significantly over the past three decades, owing largely to, study authors say, the introduction of national water treatment rules in 1989 and the introduction and widespread adoption of the hepatitis A vaccine in 1995.

That said, CDC researchers noted that public health officials should continue to raise awareness of the risks associated with drinking untreated groundwater, especially among people who use private wells. The agency recommends that those with a private groundwater supply test their wells annually for fecal contamination and that those who have not yet been vaccinated against the virus consider getting a protective HAV shot.

What’s more, person-to-person transmission of hepatitis A has increased in recent years. To read more about ongoing HAV outbreaks in the United States, click here.