STI Awareness: Intestinal Parasites

This is a colorized scanning electron micrograph of Giardia lamblia, which is in the process of reproducing via binary fission. Image by Dr. Stan Erlandsen, provided by the CDC's Public Health Image Library.

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

Most sexually transmitted infections 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 by Dr. Stan Erlandsen from the CDC’s Public Health Image Library.

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.

3 responses to “STI Awareness: Intestinal Parasites

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  2. Serena Freewomyn

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