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Scanning electron micrograph image of Giardia duodenalis (formerly G lamblia and G intestinalis) from the 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 | February 1, 2022 | 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. Presenting symptoms are commonly diarrhea, weight loss, rash, eosinophilia, skin ulcers, or fever.

This image shows trophozoites of G duodenalis resembling a monkey face identified in the stool. Image courtesy of Russell W Steele, MD.

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

Russell W Steele, MD | February 1, 2022 | Contributor Information

Giardiasis

G duodenalis is regarded as the most significant parasitic pathogen in the United States, especially prevalent in daycare centers, institutional care facilities, and areas with overcrowding. Infants and toddlers may be an important reservoir for infection, as up to 50% are asymptomatic. Clinical symptoms vary with age and include diarrhea, weight loss/failure to thrive, anorexia, vomiting, abdominal cramps, belching, and malodorous stools.

Diagnosis is most readily achieved by antigen detection in stool. Direct examination of stool samples by a laboratory with expertise in parasitology will establish the diagnosis in 85% of cases. Two medications are preferred for treatment: tinidazole and nitazoxanide.[1-3]

Entamoeba histolytica cysts with ingested red cells are shown. Image from the 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 | February 1, 2022 | Contributor Information

Amebiasis

E histolytica is a protozoan parasite that primarily produces disease in the colon but may also cause liver and cerebral abscesses. The presence of ingested red blood cells in the endoplasm of a large trophozoite is the most reliable morphologic feature.

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

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

Russell W Steele, MD | February 1, 2022 | 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.

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 by 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.[4-6]

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

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

Russell W Steele, MD | February 1, 2022 | Contributor Information

Balantidiasis

Gastrointestinal colonization with B 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. 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.[7-9]

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

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

Russell W Steele, MD | February 1, 2022 | Contributor Information

Cryptosporidiosis

Cryptosporidium species (spp) are protozoa that are increasingly reported as causes of watery diarrhea, which can be severe in immunocompromised patients. Two species, C hominis and C parvum, account for 90% of infections. Other symptoms are abdominal cramping, fever, vomiting, and weight loss. Organisms are transmitted via contaminated food or water, including recreational pools and lakes, and from animals. Oocyst organisms are identified in stool or antigen detected by enzyme immunoassay (EIA).

Treatment is with nitazoxanide, but mild cases do not require antimicrobial therapy as they will generally resolve in 2 to 3 weeks.[10-12]

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

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

Russell W Steele, MD | February 1, 2022 | Contributor Information

Cyclosporiasis

C 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.

Humans are the only hosts of these organisms, as 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.[13-15]

A Dientamoeba fragilis trophozoite in stool is shown. Image from the CDC DPDx dientamebiasis image library.

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

Russell W Steele, MD | February 1, 2022 | Contributor Information

Dientamebiasis

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

Diagnosis is made by identifying D fragilis trophozoites in the stool.

The antimicrobial agent of choice for patients requiring treatment is metronidazole.[16-18]

A motile Trichomonas vaginalis protozoan in a 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 | February 1, 2022 | Contributor Information

Trichomoniasis

T 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. The organism is predominantly isolated from vaginal secretions in women and urethral secretions in men. It has also been isolated from the rectum, although 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. The sexual partner(s) should also be treated even if they are asymptomatic.[19-21]

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