As the flu season winds down, the emergence of a concerning gastroenteritis outbreak, primarily caused by norovirus, is gaining attention.
Recent reports from the Pacific Crest Trail Association indicate that hikers in Southern California have experienced an uptick in norovirus infections.
However, those who traverse this famed trail aren’t the only victims of this outbreak.
New wastewater monitoring data show that elevated levels of norovirus are on the rise across various regions nationwide, particularly in the Northeast.
From August 1 to May 7, the Centers for Disease Control and Prevention (CDC) recorded 1,194 norovirus outbreaks, a decrease from 2,534 cases in the previous year and consistent with historical averages.
According to Amanda Bidwell, scientific program manager for WastewaterSCAN—a collaborative initiative between Stanford University and Emory University—the influx of norovirus cases on a national level is significant, with concentrations remaining high over the past three weeks.
Despite being dubbed the “winter vomiting disease,” Bidwell notes that norovirus can persist well into late spring, highlighting a shift in seasonal dynamics of the virus.
Dr. Linda Yancey, an infectious disease expert at Memorial Hermann Hospital in Houston, emphasized that the prevalent wastewater figures reflect a notable incidence of norovirus in the San Francisco Bay Area, even as national trends appear average for this time of year.
Yancey underscored that, while California’s situation may seem unique, it simply reflects unfortunate local circumstances.
The CDC maintains that current national levels of norovirus are not disproportionately high compared to previous seasons, though actual figures may be underreported as many cases go unconfirmed.
Given the challenges in monitoring this highly contagious virus, Bidwell highlights the necessity of tracking norovirus in wastewater; most affected individuals recover at home without medical intervention.
Circulating Mutant Strains
Dr. Yancey points out that multiple norovirus strains are currently circulating, including the long-standing GII.4 and the more mutated GII.17 strain, which has the ability to partially circumvent existing immunity due to lower public exposure.
During the 2024-25 season, GII.17 overtook GII.4 in the United States, emerging as the predominant strain responsible for approximately 75% of outbreaks.
Although GII.4 and GII.17 exhibit similar symptoms, norovirus is known for its rapid mutation rates.
Yancey noted that while new variants aren’t necessarily more transmissible, their ability to spread more broadly is attributable to the reduced immunity within the population.
Factors Contributing to the Outbreak
Traditionally a winter-related illness, norovirus may see surges into late spring, according to Aaron Glatt, chief of infectious diseases at Mount Sinai South Nassau in Long Island, New York.
Dr. Glatt pointed out that the virus’s ease of transmission between individuals compounds the problem.
Transmission rates parallel those of highly contagious diseases like measles, with one infected person allegedly spreading the virus to up to seven others.
Seasonal travel and heightened temperatures may also be contributing factors to the current outbreak.
Dr. Scott Roberts, associate medical director for infection prevention at Yale University, explained that increased holiday travel and gatherings can facilitate close contact, thereby escalating the virus’s transmission, while rising heat may confine people indoors, creating ideal conditions for outbreaks.
While Dr. Roberts noted no recent evidence of new viral strains, he cautioned that evolution is a natural process, and emphasized that immunity to norovirus is often short-lived.
Transmission Pathways of Norovirus
Bidwell stated that while norovirus can circulate throughout the year, it typically peaks between November and May in the Northern Hemisphere.
Common transmission settings include crowded environments such as cruise ships. The virus can spread through direct contact with an infected individual or by touching contaminated surfaces and subsequently transferring it to the mouth.
Foodborne transmission is also prevalent, particularly when food is prepared by an infected person or if drinking water is inadequately treated.
Individuals are at heightened risk if they consume contaminated ready-to-eat foods, such as salads and sandwiches.
Symptoms and Recommended Care for Norovirus
Norovirus stands as the leading cause of foodborne illness in the United States.
Symptoms, which typically manifest 12 to 48 hours after exposure, include vomiting, diarrhea, and abdominal pain. While some may experience isolated symptoms, the hallmark of norovirus is severe vomiting.
Diagnosis usually occurs without laboratory testing; however, stool tests can confirm the presence of the virus.
Most healthy adults recover within a few days, but they can continue to transmit the infection for up to two weeks post-recovery.
Given that the infection arises from a virus rather than bacteria, there is no specific treatment, and antibiotics are ineffective.
Health experts recommend oral rehydration solutions to replenish lost nutrients and minerals due to vomiting and diarrhea, although beverages like Gatorade may offer relief for mild dehydration.
In severe cases of dehydration, intravenous fluids may be necessary. Vulnerable populations, including young children and the elderly, should monitor for signs of dehydration:
- Reduced urination
- Dry mouth
- Dizziness when standing
- Crying without tears
To prevent norovirus infection, individuals are advised to steer clear of raw shellfish and maintain strict hand hygiene with soap and water. Hand sanitizers may not be effective against norovirus, making the use of EPA-registered disinfectants crucial for sanitizing surfaces.
