Most of us have heard about the usual culprits when it comes to sexually transmitted infections (STIs) that cause diarrhea in men who have sex with men (MSM)—things like Giardia, Shigella, or Campylobacter. But there’s another microscopic troublemaker that often flies under the radar: Blastocystis hominis.
What Is Blastocystis hominis?
Blastocystis hominis is a single-celled parasite that lives in the intestines. It’s found worldwide, more common in developing countries, and can spread through contaminated food or water. In MSM, it can also be transmitted sexually—especially through oral-anal contact (“rimming”), sharing sex toys without proper cleaning, or during group sex where fecal-oral exposure is possible.
While some people can carry Blastocystis without symptoms, others can develop persistent diarrhea, abdominal cramps, bloating, excess gas, and fatigue—often for weeks or months.
Why Many Doctors Overlook It
Here’s where it gets tricky:
Old research confusion: For decades, Blastocystis was thought to be harmless. Many medical textbooks still describe it as a “commensal” (a harmless hitchhiker) rather than a pathogen.
Inconsistent lab reporting: Some labs don’t even mention Blastocystis unless it’s seen in large numbers, and some providers dismiss a positive test result as irrelevant.
Self-limited in some cases: Because some people improve without treatment, many providers assume it doesn’t need intervention at all.
The truth? In symptomatic patients—especially in MSM with sexual exposure risk—Blastocystis can absolutely cause problems, and treatment can help.
Symptoms to Watch For
Loose or watery stools
Abdominal cramping or discomfort
Increased gas and bloating
Nausea
Fatigue (from chronic diarrhea and fluid loss)
If you’ve been tested for “all the usual suspects” and come up empty—but your gut still feels like a war zone—Blastocystis could be the missing piece.
How It’s Spread in MSM
Blastocystis spreads through the fecal-oral route. In the MSM community, risk factors can include:
Oral-anal contact (rimming)
Sharing anal sex toys without thorough cleaning
Group sex where fecal material may be exchanged
Inadequate handwashing after anal contact before touching food or one’s mouth
Yes, it’s awkward to talk about—but it’s reality, and knowing the risks means you can take steps to protect yourself.
Testing and Treatment
Diagnosis is usually made by stool ova and parasite (O&P) testing or newer PCR-based stool panels.
Important: Because testing for Blastocystis is not very sensitive or specific, proper testing requires three separate stool specimens collected on different days. This increases the odds of catching the parasite if it’s there—especially in infections where parasite numbers fluctuate from day to day.
If Blastocystis is found and you have symptoms, treatment may include:
Metronidazole (Flagyl) – most commonly used, but not always effective
Trimethoprim-sulfamethoxazole (Bactrim) – another option
Combination regimens for resistant cases
Clearing the parasite often leads to resolution of symptoms—sometimes dramatically so.
Prevention Tips
Use barriers (dental dams) for oral-anal contact
Wash hands thoroughly after sexual activity
Clean sex toys between partners and between uses
Avoid sharing anal toys unless sanitized
Get tested if you have persistent gastrointestinal symptoms
Bottom Line
Blastocystis hominis is more than just a lab curiosity—it can be a sexually transmissible cause of persistent diarrhea in gay and bi men. Many providers still overlook its role in gut symptoms, but for those who are symptomatic, testing with a full three-sample stool series and treating if positive can be life-changing.
At Tom of P-Town Health, we believe in looking beyond the usual suspects when it comes to your health. If your stomach has been giving you grief and no one can figure out why, it’s worth asking about Blastocystis- contact us or your local provider for help.
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