Diarrhea is a common symptom with many possible causes, making its link to sexually transmitted infections (STIs) often overlooked. Some STIs can directly infect the digestive tract, while others impact the body’s defenses, leading to severe gastrointestinal issues. Understanding the different ways an STI can cause diarrhea is important for accurate diagnosis and treatment.
STIs That Directly Affect the Digestive System
Certain infections traditionally associated with food or water contamination can be transmitted sexually, directly inflaming the intestinal lining. These are often referred to as sexually transmitted enteric infections because they affect the gut. Transmission typically occurs through sexual activities involving oral-fecal contact, such as anilingus.
The bacterium Shigella causes shigellosis, characterized by diarrhea that may contain blood and mucus. This bacterium invades the cells lining the colon, causing inflammation and resulting in frequent, painful bowel movements. The parasites Giardia lamblia and Entamoeba histolytica can also be passed through sexual contact, colonizing the intestines and leading to persistent watery diarrhea, abdominal cramping, and gas. Hepatitis A virus, which primarily targets the liver, is easily transmitted through sexual contact involving the anus and can cause diarrhea alongside flu-like symptoms.
Infections like Gonorrhea and Chlamydia, while commonly known for affecting the urethra or cervix, can also infect the rectum, causing proctitis. This inflammation of the rectal lining can lead to symptoms like pain, discharge, and an urgent need to defecate, which may present as watery stools or diarrhea. In rare cases, a severe form of Chlamydia called lymphogranuloma venereum (LGV) can cause significant inflammation and destruction of rectal tissue, leading to pronounced gastrointestinal distress. The direct invasion of these pathogens disrupts normal absorption and secretion, leading to loose, watery stool.
The Role of Immune Suppression and Opportunistic Infections
Infections that compromise the body’s overall immune defenses can lead to diarrhea by making the gut vulnerable to common environmental pathogens. The most prominent example is the Human Immunodeficiency Virus (HIV), which systematically targets and destroys CD4+ T-cells, the primary coordinators of the immune response. When the CD4+ T-cell count falls, the body can no longer effectively fight off microbes that are usually harmless.
These common microbes, referred to as opportunistic infections, can thrive unchecked in the gastrointestinal tract, causing severe and chronic diarrhea. Parasites like Cryptosporidium parvum and Isospora belli are frequent culprits, causing profuse, watery diarrhea that can be life-threatening if untreated. Viruses, such as Cytomegalovirus (CMV), can also cause severe inflammation and ulceration of the colon, known as CMV colitis.
Diarrhea in the context of advanced HIV infection may also be due to HIV enteropathy, a non-infectious condition caused by the virus damaging the lining of the small intestine. This damage impairs the gut’s ability to absorb nutrients and water, leading to malabsorption and chronic diarrhea even when no specific opportunistic pathogen is identified. This indirect mechanism highlights how a systemic STI can have profound, long-term effects on digestive health.
Diarrhea as a Side Effect of Treatment
Sometimes, the medication used to treat an STI, rather than the infection itself, can cause digestive upset. Antibiotics, used to cure bacterial STIs like Chlamydia and Gonorrhea, can disrupt the natural balance of bacteria in the gut microbiome. This imbalance can lead to antibiotic-associated diarrhea (AAD), as beneficial gut flora are inadvertently destroyed along with the target bacteria.
A more serious form of AAD is caused by the overgrowth of the bacterium Clostridioides difficile (C. diff), which produces toxins that inflame the colon. Antiretroviral therapy (ART) used to manage HIV infection is also associated with gastrointestinal side effects. Certain classes of antiretrovirals, particularly protease inhibitors, are known to interfere with the transport of water and electrolytes across the intestinal lining.
This interference results in increased fluid secretion into the bowel, causing a secretory diarrhea that is a common reason for patients to stop taking their medication. While this diarrhea is not a sign of treatment failure or a new infection, it remains a significant obstacle to adherence for many people managing a viral STI. The diarrhea caused by these treatments is typically temporary and manageable but requires close communication with a healthcare provider.
Recognizing Warning Signs and When to Get Tested
Unexplained or persistent diarrhea, especially when coupled with other symptoms, should prompt a conversation with a healthcare professional. Any case of diarrhea lasting more than a few days, or any episode accompanied by severe abdominal pain, high fever, or blood in the stool, requires immediate medical evaluation. These symptoms can be markers of severe intestinal inflammation or infection, whether related to an STI or another cause.
If you are sexually active and experience these digestive symptoms, particularly after engaging in sexual contact involving oral-anal exposure, STI screening is highly recommended. Testing for enteric pathogens, along with standard STI panels, can help identify the cause of the diarrhea. Early diagnosis of an STI is important for preventing complications and limiting transmission. Addressing unexplained gastrointestinal symptoms with a comprehensive sexual health assessment is the most responsible step toward maintaining health.