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Scanning electron micrograph of Giardia duodenalis from Centers for Disease Control and Prevention (CDC) | Dr Stan Erlandsen. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Parasitic Diseases

Most parasitic diseases have virtually disappeared in developed countries as a result of concentrated efforts to improve sanitary conditions. Only pinworm infection (enterobiasis) and giardiasis are seen with any frequency in general practice, with occasional cases of amebiasis, strongyloidiasis, toxocariasis, ascariasis, tapeworm infection, and hookworm infection requiring management.

Unusual parasitic infections are more likely to be seen in immigrants and individuals who have traveled to endemic areas. Common presenting symptoms are diarrhea, weight loss, rash, eosinophilia, skin ulcers, and fever.

Life cycle of Giardia. Image from CDC Public Health Image Library via Wikimedia Commons. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Giardiasis

Giardia duodenalis (formerly Giardia lamblia and intestinalis) is regarded as the most significant parasitic pathogen in the United States; it is especially prevalent in daycare centers, institutional care facilities, and areas with overcrowding.

Infants and toddlers may be an important reservoir for infection, in that as many as 50% are asymptomatic. Clinical symptoms vary with age and include diarrhea, weight loss/failure to thrive, anorexia, vomiting, abdominal cramps, belching, and malodorous stools. Reactive arthritis is also described.[1]

Trophozoites of G duodenalis resembling monkey face identified in stool. Image courtesy of Russell W Steele, MD.

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Diagnosis is most readily achieved through antigen detection in stool.[2] Direct examination of stool samples by a laboratory with expertise in parasitology will establish the diagnosis in 85% of cases.

Three medications are preferred for treatment: metronidazole, tinidazole, and albendazole, with nitazoxanide and paromomycin used for treatment failures or cases of known local resistance.[3-5]

Entamoeba histolytica cysts with ingested red cells. Image from CDC Division of Parasitic Diseases and Malaria (DPDx) amebiasis image library. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Amebiasis

Entamoeba histolytica is a protozoan parasite that primarily produces disease in the colon but may also cause liver and cerebral abscesses.[6] Identification of ingested red blood cells in the endoplasm of a large trophozoite (shown) is the most reliable morphologic feature.

E histolytica cyst invading colonic mucosa. Image from CDC DPDx amebiasis image library. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Transmission of amebiasis is via the fecal-oral route following ingestion of contaminated water, food, or surfaces. The pathogenicity of the organism is related to its ability to attach to the colonic mucus layer (shown).

Infected individuals may be asymptomatic carriers, may experience mild symptoms, or may be affected by the dysenteric form. Symptoms of severe disease include the following: abdominal pain, tenesmus, diarrhea with blood and mucus, fever, and abdominal distention.

Treatment varies according to the extent of the infection: Patients with asymptomatic intestinal colonization are treated with paromomycin or iodoquinol, whereas those with colitis are treated with metronidazole or tinidazole followed by paromomycin or iodoquinol.[7-9]

Photomicrograph of Balantidium coli cyst from CDC. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Balantidiasis

Gastrointestinal colonization with Balantidium coli, a ciliated protozoan, is usually asymptomatic but may result in watery or bloody diarrhea with abdominal pain or chronic diarrhea. Occasional cases of intestinal perforation and secondary bacterial abscesses have been reported. Urinary tract infection (UTI) has also been described.[10] Infection occurs after contact with pigs, which are the primary hosts, or via the fecal-oral route.

Diagnosis is made by identification of B coli trophozoites in the stool.

If needed, the treatment drug of choice is tetracycline.[11-13]

Cryptosporidium oocysts in stool. Image from CDC DPDx cryptosporidiosis image library. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Cryptosporidiosis

Cryptosporidia are protozoa that are increasingly reported as causes of watery diarrhea, which can be severe in immunocompromised patients. Two species, Cryptosporidium hominis and Cryptosporidium parvum, account for 90% of infections. Other symptoms are abdominal cramping, fever, vomiting, and weight loss.

Life cycle of Cryptosporidium. Image from CDC DPDx cryptosporidiosis image library. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Organisms are transmitted via contaminated food[14] or water, including recreational pools and lakes, and from animals. Oocysts are identified in stool or through antigen detection by enzyme immunoassay (EIA).

Treatment is with nitazoxanide, but mild cases do not require antimicrobial therapy, because they will generally resolve in 2-3 weeks.[15-17]

Cyclospora cayetanensis oocyst in stool. Image from CDC DPDx cyclosporiasis image library. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Cyclosporiasis

Cyclospora cayetanensis—a coccidian protozoan—is a common cause of travelers' diarrhea. It is characterized by watery diarrhea that can become chronic. About half of infected patients experience fever, but nausea, vomiting, and abdominal pain are more common. Foodborne outbreaks have been reported in developed countries from imported fresh produce such as fruit and lettuce.

Diagnosis is made by identifying oocysts in stool. Molecular assays (PCR) are also available but have limitations.[18]

Humans are the only hosts of these organisms; animals have not been identified as sources of transmission.

Most Cyclospora infections do not require treatment, but for moderate-to-severe infection, trimethoprim-sulfamethoxazole is the drug of choice.[19-21]

Dientamoeba fragilis trophozoite in stool. Image from CDC DPDx dientamebiasis image library. [Public domain.]

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Dientamebiasis

Dientamoeba fragilis is a trichomonad parasite whose mode of transmission has not been fully defined. Humans and nonhuman primates are the only well-documented natural hosts. Transmission is via the fecal-oral route and, interestingly, through infection of Enterobius vermicularis (pinworm) eggs. Because asymptomatic colonization is so common, many experts question the true pathogenicity of this organism. Major symptoms are diarrhea and abdominal pain.

Diagnosis is made by identifying D fragilis trophozoites in stool.

The antimicrobial agent of choice for patients requiring treatment is metronidazole.[22-24]

Motile Trichomonas vaginalis protozoan in wet mount microscopy evaluation of vaginal discharge. Image courtesy of Russell W Steele, MD.

Your Unwanted Internal Pets: Intestinal Parasites (Other Than Nematodes)

Russell W Steele, MD | October 13, 2023 | Contributor Information

Trichomoniasis

Trichomonas vaginalis is a motile parasitic protozoan identified as one of the most common causes of sexually transmitted infections (STIs) in the United States and worldwide.[25] The organism is predominantly isolated from vaginal secretions in women and urethral secretions in men. It has also been isolated from the rectum, though this is not a common area of infection. The rectal prevalence of T vaginalis among men who have sex with men (MSM) is quite low.

Diagnosis is easily made by identifying the organism in genital secretions.

Treatment is with tinidazole or metronidazole. Any sexual partner should also be treated, even if asymptomatic.[26-28]

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