A bacterial infection traditionally associated with contaminated food and water is increasingly spreading through sexual contact, while some strains are becoming much harder to treat.
The infection is shigellosis, caused by bacteria from the Shigella group. It can cause severe diarrhea, stomach cramps, fever and vomiting, and in serious cases can lead to dehydration, bloodstream infections and other life-threatening complications. In the United States, roughly 450,000 people acquire shigellosis each year.
What has particularly concerned researchers is the changing pattern of transmission and the rapid growth of antibiotic resistance.
Shigella is increasingly spreading through sexual contact
Shigella is transmitted when tiny amounts of infected fecal matter enter another person's mouth. This can happen through contaminated food or water, person-to-person contact and sexual activity.
The bacteria are particularly easy to transmit because only a small number of organisms may be enough to cause infection.
Although young children, international travelers and people living in crowded or difficult sanitary conditions have historically been important groups affected by shigellosis, health authorities have increasingly documented transmission among gay, bisexual and other men who have sex with men.
Researchers say sexual transmission has become an important route for several strains of the bacteria.
A study highlighted in July found that sexually transmitted Shigella infections increased about 15% faster each year between 2015 and 2020 than infections associated with other transmission routes, including travel, food and childcare outbreaks.
The true scale may be difficult to measure because patients may not realize that Shigella can be sexually transmitted, while clinicians may not routinely record sexual exposure when investigating diarrhea.
Antibiotic resistance is becoming the bigger concern
The growing spread of Shigella is being accompanied by a worrying increase in strains resistant to multiple antibiotics.
According to U.S. surveillance data, extensively drug-resistant (XDR) Shigella accounted for 0% of reported isolates in 2011 but had risen to 8.5% by 2023, according to research highlighted this week.
The CDC had already warned in 2023 that XDR Shigella was increasing in the United States. At that time, about 5% of reported Shigella infections in 2022 were caused by XDR strains, compared with none in 2015.
XDR strains are particularly concerning because they can resist all commonly recommended antibiotics used to treat Shigella. The CDC says there are currently no clinical-trial data establishing an optimal treatment strategy for XDR infections.
That means doctors may have significantly fewer options when a patient develops a severe infection.
Why sexual transmission may be helping resistance spread
Scientists are investigating several possible explanations for the relationship between sexual transmission and antibiotic resistance.
One possibility is what researchers describe as “bystander resistance.” This can occur when a person receives an antibiotic for another infection. The drug may expose bacteria in the gut to antibiotics without effectively eliminating them, creating conditions in which resistant organisms survive and multiply.
Repeated infections and antibiotic exposure can further increase opportunities for resistance to develop and spread.
Research has also found extensive international circulation of multidrug-resistant Shigella strains within sexual networks.
Geneticist Kate Baker of the University of Cambridge, who led recent research into sexually transmitted Shigella, has warned that the disease has moved from being relatively easy to treat to becoming increasingly difficult to manage.
Some patients require hospital treatment
Most Shigella infections eventually resolve with supportive care, particularly adequate fluids and rest. Antibiotics are not necessary for every case.
However, severe infections may require antimicrobial treatment, particularly when doctors are trying to shorten the illness or reduce the period during which an infected person can transmit the bacteria.
The CDC says some patients with drug-resistant Shigella have required hospitalization, while infections can become particularly serious in people with weakened immune systems.
Recent research has also found that sexually transmitted Shigella can lead to lengthy hospital stays, with some studies reporting average hospitalizations of four to five days.
How people can reduce the risk
The CDC recommends taking precautions because Shigella can remain in a person's stool for up to two weeks after diarrhea has stopped.
People with bloody or prolonged diarrhea should avoid sexual activity, and the CDC recommends waiting at least two weeks after diarrhea ends before having sex.
Handwashing and washing the genital and anal areas with soap and water before and after sexual activity can also reduce transmission. Shared sexual toys should be thoroughly cleaned between users.
People experiencing symptoms should also seek medical advice rather than automatically taking leftover antibiotics. Unnecessary or inappropriate antibiotic use can contribute to antimicrobial resistance.
A growing global public-health challenge
The emergence of sexually transmitted, drug-resistant Shigella is not simply a problem involving one community or one country.
Researchers have identified increasingly resistant strains in the United States, the United Kingdom and other parts of Europe, while genetic studies indicate that some resistant strains are circulating internationally.
The concern for public-health officials is therefore twofold: Shigella is finding an additional route of transmission, while the bacteria are simultaneously becoming more resistant to the medicines used against them.
For now, most infections remain treatable or resolve without antibiotics. But the continued emergence of extensively drug-resistant strains means prevention, accurate diagnosis and responsible antibiotic use are becoming increasingly important.
The CDC has urged clinicians to remain alert for XDR Shigella and, where possible, obtain antibiotic susceptibility testing so treatment can be selected according to the individual strain.
