STD Awareness: Intestinal Parasites

Editor’s Note: If you’re wondering if there is a sexually transmitted disease (STD) that causes maggots, please see our new article, “Is There an STD That Causes Maggots?”

This colorized scanning electron micrograph shows Giardia lamblia reproducing asexually. Image: Stan Erlandsen, CDC’s Public Health Image Library

Most sexually transmitted diseases are caused by bacteria or viruses, but some are caused by organisms that are classified as completely different lifeforms. Trichomoniasis, for example, is caused by a protozoan organism; protozoa occupy their own kingdom, separate from plants, animals, and bacteria. Intestinal parasites are often protozoan organisms, but can also include parasitic worms (which are members of the animal kingdom). They are spread through contact with fecal matter — and as such, they can be transmitted sexually as well as nonsexually. Intestinal parasites are usually transmitted by fecal contamination of food or water, and are most common in areas with insufficient sewage treatment and untreated water in the wilderness. Some pathogens, however, have low infectious doses, making their sexual transmission more likely.

What has eight flagella and can live in your intestines?

Oral contact with the anus, also called anilingus or rimming, is the primary means of the sexual transmission of these pathogens. Putting fingers or hands in your mouth after they have had contact with the anus is also risky. Other modes of transmission include oral sex, as genitals can be contaminated with feces, as well as sharing sex toys and other equipment. For these reasons, it is very important to use dental dams or latex gloves during contact with the anus; to clean the anus before engaging in rimming; to clean or use condoms on shared sex toys; and to use condoms or dental dams during oral sex.

Many people whose intestines are habitat to dangerous parasites don’t exhibit symptoms, which means they could unknowingly infect their partners. Symptomatic individuals could experience diarrhea, abdominal pain or bloating, nausea, and vomiting. If you suspect you might have contracted an intestinal parasite, a health-care provider can take a stool sample and examine it for the presence of parasites. More sensitive tests, such as antibody tests or DNA tests, might be performed. You can seek diagnosis and treatment at a Planned Parenthood health center, as well as other clinics, private health-care providers, and health departments.

Below we will discuss three intestinal parasites that are relatively easy to transmit sexually. The conditions they cause are called giardiasis, amebiasis (sometimes referred to as amoebic dysentery), and cryptosporidiosis.

A scanning electron micrograph shows the mucosal surface of a gerbil’s small intestine, which is almost entirely obscured by Giardia that have infested the intestinal epithelial surface. Photograph: Stan Erlandsen, CDC’s Public Health Image Library.


A common intestinal parasite is Giardia lamblia (also known as Giardia intestinalis), a protozoan that uses a “sucker” to adhere to the intestines of mammalian hosts, including humans. This organism has two nuclei and eight flagella, and can cause a persistent diarrheal disease called giardiasis (colloquially known as “beaver fever”). About half of carriers are asymptomatic, while those who experience symptoms usually do so one to three weeks after infection. G. lamblia reproduces by dividing itself in two (a process called binary fission), and when their numbers are high enough they can interfere with the host’s ability to absorb nutrients during digestion. It can exit the host via the feces, in which it is excreted in a protective capsule called a cyst. If the cyst is ingested, G. lamblia can infect its new host’s intestines.

Giardiasis is diagnosed by examining stool samples (at least three, as this test is the least sensitive and prone to false negatives). More reliable diagnostic methods include antigen-detection tests or a “string test.” The string test involves the patient swallowing a capsule filled with a tiny string, one end of which is attached to the skin outside of the patient’s mouth. The capsule dissolves in the stomach and releases a rubber bag that is able to collect samples from the upper bowel. A few hours later, the string is used to remove the bag from the patient’s body; the bag’s contents are then examined for G. lamblia. Giardiasis is treated with metronidazole, the same drug that treats trichomoniasis.

Entamoeba histolytica eats its host’s red blood cells, which can be seen inside the organism in this photograph. Image courtesy of the CDC’s Parasite Image Library.

Entamoeba histolytica eats its host’s red blood cells, which can be seen inside the organism in this photograph. Image: CDC’s Parasite Image Library.


Entamoeba histolytica colonizes the human intestine and is a leading cause of parasitic death. Its cysts, when ingested by a host, can withstand the acidic environment of the stomach; when they arrive in the intestines, the walls of the cyst are digested, releasing the organism into the gut. Here E. histolytica is able to colonize and feed on the cells lining the colon, causing lesions that develop into ulcers. The cellular damage they do to the tissue in the gastrointestinal tract opens the door for bacterial infections, which can lead to complications if not treated. Cysts can be shed with the feces to continue E. histolytica‘s lifecycle through the infection of a new host. About 5 percent of U.S. residents are estimated to be asymptomatic carriers of E. histolytica; worldwide this estimate climbs to about 10 percent.

When symptomatic, hosts can suffer from severe dysentery, called amebiasis, which usually includes bloody diarrhea. When these symptoms are chronic, they may be misdiagnosed as ulcerative colitis or irritable bowel syndrome. Diagnoses relying on the examination of stool samples are not authoritative and must be confirmed by testing for the microorganism’s DNA. However, genetic testing is more expensive and requires a more highly trained diagnostician.

Infection by this parasite is treated with medications such as metronidazole and diloxanide furoate. If you are infected with this parasite, you should notify any sexual partners from the past several months so they can be tested and treated as well.

Tiny, round Cryptosporidium organisms line the outside of host cells. Image by Edwin P. Ewing, Jr., courtesy of the CDC’s Public Health Image Library.

Tiny, round Cryptosporidium organisms line the outside of host cells. Image: Edwin P. Ewing, Jr., CDC’s Public Health Image Library.


Protozoan species of the genus Cryptosporidium, such as Cryptosporidium hominis and C. parvum, can cause diarrhea in human hosts who have ingested as few as 10 microscopic oocysts (egglike structures that are made of chitin, the same protein that forms the exoskeletons of ants and other insects) contained in feces or contaminated water. After ingesting an infectious dose of Cryptosporidium oocysts, symptoms could appear within three weeks and can include a brief bout with diarrhea. People with weakened immune systems, however, could experience severe, life-threatening diarrhea.

Once the oocysts reach the small intestine, they release the organisms into the gut, where they embed themselves into the cells lining the intestinal wall. Eventually, these organisms release oocysts that can leave the body via the feces, where they can infect new hosts.

Cryptosporidium is so called because the oocysts used to be impossible to see under a microscope — the name is Latin for “hidden seed.” It wasn’t until the 1980s that a team of scientists from the University of Arizona, led by Joan Rose, developed a method to detect their presence in water samples. This technique involves attaching fluorescent antibodies to the oocysts’ surfaces; the antibodies glow under UV light.

Unfortunately, there is currently no treatment for cryptosporidiosis that has been proven to be broadly effective, although a new drug called nitazoxanide, which can also be used to treat giardiasis, is showing promise in treatment of cryptosporidiosis.

Click here to check out other installments of our monthly STD Awareness series!

28 thoughts on “STD Awareness: Intestinal Parasites

  1. Pingback: Back with something too cool to let by | Mom’s Tinfoil Hat

  2. This is the best-written article on Intestinal Parasites that I have come across on the web. It answered questions that other bigger sites left unclear (like how long does it take to become symptomatic after infection?). Thanks also for the casual, yet professional, tone.

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  4. Pingback: STDs 101: An Introduction to Sexually Transmitted Diseases | Planned Parenthood Advocates of Arizona | Blog

  5. Very good article, ? though can a man get trich from
    intestinal parasites from having anal sex? reason I
    have never cheated on my girl and she swears she has never cheated I believe her could there be and
    explanation for her being positive for trichomoniasis thanks in advance Anna

    • Trichomoniasis is not caused by an intestinal parasite — it’s caused by a different organism, called Trichomonas vaginalis, that usually lives in the vagina. Men can get trich, though they usually don’t have symptoms and can usually fight the infection off within a couple of weeks. If your girlfriend’s trich infection is in her vagina, you could get it from vaginal intercourse. If her trich infection is in her rectum, you could get it from anal intercourse.

      She can’t really know when she became infected with trichomoniasis. She might have acquired it recently, but she could have had it since before your relationship began. It is possible that she has had trich for a long time but never knew about it until being tested or until symptoms appeared. It seems that women can have the infection for a long time without knowing it. There haven’t been a lot of studies, because it would be unethical to infect women with trich and monitor the infection to see when it goes away, but it’s thought that women can harbor a trich infection indefinitely.

  6. Three months ago i had fever with chills nd after 4 days i had profusely bleeding diarrhea, nd i was hospitalized since then no blood or fever but mild ibs now i remember that 7 days before this incident i had rimming ass lick of my wife.. She washes her anus before anilingus nd I hv been doing it for past one it because of rimming or i hv to investigate other way, i m 38 nd my wife 32 year old nd pregnant.. Pls answer thanks in advance..

    • Hi Shiv, there is no way to know the cause of your diarrhea unless a health care provider took a stool sample and tested it. A laboratory test might have been able to find a microbe that could have caused your symptoms. You couldn’t know for sure how you got it — if your wife tested positive for the same microbe, you might have gotten it through sexual contact, but you could have gotten it through other contact (food preparation, direct contact, etc.). Or you could have gotten it first and given it to her, but she didn’t have symptoms.

      • At that time my stool cultre was tested but it had no bacteria organisam , in hospital i was given antibiotics nd i responded to medicines pls suggest wht tests i should do, so i can be sure that this does nt happen in future…

        • Since this was three months ago, it’s probably too late to do further tests. If you have questions, you should talk to a health-care provider in person. Some people use dental dams (a thin piece of latex that can cover the anal area) during anilingus to have safer sex. It acts as a barrier between the mouth and anus to minimize contact with bacteria and other microbes.

          • Doctorwho gave me ttreatment in hospital advised to have colonoscopy then i consulted another hospital adviced that if ur fecal occult blood is negative nd hemoglobin nt going down no need for furtheriinvestigation so i tested fecal occult blood it was negative nd hemoglobin also in normalrange..can u pls advise wht do i do?

  7. I just wana ask, does annilingus or rimming causes dysentery even if other partner is healthy, or in short , does all human feacal mouth contact causes infection or the feacel needs to be bacterial??

    • The partner receiving anilingus must be infected in order for there to be a possibility of transmitting it to the partner performing anilingus. The fecal mater must contain infectious particles, which can be bacteria, parasites, or other microorganisms.

      • You mean to that , to lick human fecal which has no bacteria, parasite or microorganisms causes no harm to human being.

  8. If my boyfriend has parasite say Giardia, is it safe to have sex with him? Can it be spread to me via vaginal only intercourse? Sorry if this seem naive/off the wall, the parasite can’t climb from his anus to mine right?

  9. I have had about three sexual partners this year that gave me oral sex. I’m female 24 years old. My digestive problems started one day I had sexual contact with a guy he put his penis near my vagina without a condom but didn’t penetrate. And I also ate some food from a restaurant. I experienced loud constant stomach gurgling and severe constipation. I took laxatives. My constipation became diarrhea. 7 months later I still have the same problem. My white blood cell count is low. But no blood disease . I got an endo/colonoscopy. Nothing. Bone marrow biopsy. Nothing . But I noticed that all my partners that gave me oral sex get gassy bloated and constipated right after oral sex or unprotected vaginal sex. They get my same symptoms. So my question is which doctor should I go to? What test should I get? What are some of the things it can possibly be? And are there any natural alternatives to regular medicine ?

  10. I’ve recently been treated for Trichomonas but the past two nights I’ve noticed when I urinate I’ve seen a black small worm in the toilet… Not once but twice what is going on with me? And I’ve drunk alot of faucet water lately and I do have a sewer problem in my home. Maybe you can she’d a little light on my situation…

  11. Great article, is it possible that these get misdiagnosed as IBS? I tried this 3months ago, 2 days later bad diarrhea for a week, was given ciprofloxacin then doc immediately gave PPI meds after completing 5 day antibiotic course. Diarrhea n gastro problems increased, stopped PPIs after 2 weeks, still had recurring gastro issues for 2months finally received colonoscopy to which they found nothing (stool test only for C.difficile neg) and said it was IBS. Is it possible i am misdiagnosed? I have since been scibed rifaximin but still experience some symptoms. PLEAS HELP

    • Always get a 2nd and 3rd opinion (If POSSIBLE!) Getting a 2nd opinion for me is out of the question because I am on a fixed income.

  12. I have just been diagnosed with Blastocystis Hominis most likely picked up from my trip to Bali 12 months ago. I would like to know if it is contagious sexually be it rimming or anal. I am currently on tablets for it and hopefully it will be gone after the treatment. Thankyou

  13. So can you get paracitic worms from oral contact with your own feces!? (Even if you are healthy and in clean surroundings?)

    • You would have to be infected with these parasitic worms first. Otherwise they would not show up in your feces.

  14. Hey I really need to know. Can you get parasitic worms from coming into contact with your own feces?

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