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Intestinal parasites
OVERVIEW: The class of infectious agents called parasites is divided into two parts:
• Protozoa are single cell animals which characteristically divide and multiply within the host, are usually direct fecal-oral in transmission, and do not cause an eosinophilia
• Helminths (worms) are multi-cellular animals and with rare exceptions (i.e., Strongyloides stercoralis, Hymenolepis nana) do not multiply within the host and are often associated with some degree of eosinophilia. The level of eosinophilia is associated with the degree of mucosal invasiveness. The worms have a limited life span within the host and without reinfection would eventually die on their own.
• Not all of the parasites that start out by ingestion in the bowel will remain in the bowel. Some are invasive and some do not release their infective forms into the bowel. This later group, including Toxoplasma gondii, Echinococcus, Trichinella spiralis will not be covered in this topic.
• Most worms require either a prolonged incubation period outside the host before being infectious or need a specific vector for transmission. A notable exception to this rule is Enterobius vermicularis (pinworm), the eggs of which are infectious shortly after being passed, so auto-infection occurs readily.
• Direct person-to-person transmission of worms is uncommon
• The likelihood of acquiring an intestinal parasite depends on several factors - the presence of the specific infectious agent, an appropriate vector or mode of transmission, and a host who is susceptible to the infectious agent. The world-wide distribution of parasites is determined by geographic factors, socioeconomic factors, age, and crowding with poor food preparation and a break in the standard of water and personal sanitation being the major factors.
System(s) affected: Gastrointestinal
Genetics: Genetic factors play a minor role in the acquisition, pathogenesis and clearance of these infections
Incidence/Prevalence in USA:
• From laboratory statistics: 5-30% general population
• From day care surveys: Asymptomatic 20-30%; symptomatic 50-80%
• Intestinal protozoa account for the majority of parasitological findings in North America (most considered to be non-pathogenic)
• In a random sampling, at least one parasite would be found in the stools of 5-10% of all people. If Blastocystis hominis were included in this accounting, 20-30% of specimens examined in parasitology will be positive.
• Helminths are considerably rarer and are highly dependent on population demographics and prior geographic exposure risk factors. In general, less than 10% of all parasitology reports include a helminth.
Predominant age: Pediatric
Predominant sex: Male = Female
SIGNS AND SYMPTOMS: • Diarrhea
• Abdominal pain/tenderness
• Excessive gas - bloating, eructation, flatulence, borborygmi
• Nausea or vomiting
• Weight loss and anorexia
• Dysentery, i.e., bleeding (rare, but associated with Entamoeba histolytica, Balantidium coli)
• Pruritus ani (E. vermicularis, Trichuris trichiura, S. stercoralis, tapeworms)
• Passing a worm or a worm segment
• Increased bowel sounds
• Perirectal or vulvar rash
CAUSES: • Protozoan pathogens:
• Giardia lamblia
• Entamoeba histolytica
• Cryptosporidium
• Isospora belli
• Balantidium coli
• Cyclospora cayetanensis
• Microsporida
• Possible protozoan pathogens:
• Dientamoeba fragilis
• Probable non-pathogenic protozoa
• All other Entamoeba species
• Endolimax nana
• All other intestinal flagellates
• Helminthic pathogens - nematodes (roundworms):
• Enterobius vermicularis
• Trichuris trichiura
• Ascaris lumbricoides
• Hookworm (Necator americanus, Ancylostoma duodenale)
• Strongyloides stercoralis
• Capillaria philippinensis
• Trichostrongylus spp
• Helminthic pathogens - trematodes (flukes)
• Fasciolopsis buski
• Clonorchis sinensis
• Opisthorchis viverrini
• Heterophyes heterophyes
• Fasciola hepatica
• Paragonimus westermani
• Schistosoma mansoni
• S. japonicum
• S. hematobium
• S. mekongi
• Helminthic pathogens - cestodes (tapeworms)
• Taenia saginata
• Taenia solium
• Diphyllobothrium latum
• Hymenolepis nana
• Hymenolepis diminuta
• Dipylidium caninum
RISK FACTORS: • Age (children)
• Low socioeconomic status
• Poor sanitation - personal, food, water
• International travel
• Crowding - day care centers, institutional care
• Intercurrent medical conditions, pregnancy, gastric hypoacidity, immunosuppression (AIDS)
DIAGNOSIS DIFFERENTIAL DIAGNOSIS: • Other intestinal infections
• Food poisoning
• Malabsorption
• Inflammatory bowel disease
• Hemorrhoids
• Rectal fissures
LABORATORY: • Examination of a single stool specimen collected into a preservative (i.e., sodium acetate formalin [SAF]), well mixed to fix and preserve all elements, will provide an accurate diagnosis in 90% of patients. Additional specimens will need to be examined for greater diagnostic accuracy.
• Newer techniques of lab exam of stool specimens (such as monoclonal antibodies, other antigen detection techniques, DNA detection) are exciting developments but currently provide little advantage over routine techniques and represent additional costs. These techniques are needed to differentiate E. histolytica from E. dispar
• Serology - for specific infections, especially if they do not produce a patent infection in the bowel (i.e., no eggs or parasites released into the stool), or if low numbers of parasites. May be indicated rarely, but are usually available only through referral centers.
Drugs that may alter lab results: Use of antibiotics, oil based laxatives, and the presence of barium in the stool may make a parasitological diagnosis difficult or impossible
Disorders that may alter lab results: N/A
PATHOLOGICAL FINDINGS: • Majority of intestinal parasites are not invasive and produce no or non-specific changes in the histology of the bowel
• Invasive amebiasis of the bowel produces a classical endoscopic and histological picture of ulceration and inflammation in the colon
• Protozoa and helminths may be seen in bowel biopsies
SPECIAL TESTS: • Special techniques for the detection of Cryptosporidium, Isospora belli, Cyclospora, and microsporidia often require that the laboratory be informed of the risk profile of the patient before these tests will be done
• Pinworm paddles provide a greater diagnostic yield when Enterobius vermicularis is being considered. Multiple tests (5) may be needed to exclude the diagnosis of pinworms.
• Parasite culture is possible for a few organisms - Giardia lamblia, Entamoeba histolytica, Strongyloides stercoralis, but are rarely indicated and are usually available only in referral laboratories
• String tests and upper bowel intubations are rarely needed to diagnose the upper intestinal parasites
• Rarely, a biopsy will demonstrate the presence of an invasive helminth on tissue section. Worms can be extremely difficult to diagnose in this manner, usually needing the expertise of a tissue parasite pathologist. The other parasites may be visualized on the mucosa or in the mucous layer.
IMAGING: Diagnostic radiology rarely needed. Exception is for invasive infections such as amebiasis where colitis, amebomas and liver abscesses may be demonstrated by the appropriate techniques.
DIAGNOSTIC PROCEDURES: • Invasive diagnostic procedures are rarely needed or indicated
• With hemorrhagic colitis and a possible diagnosis of invasive amebiasis, sigmoidoscopy will reveal a mucopurulent colitis with ulceration. A scraping from an ulcer, promptly examined by microscopy, will reveal the motile hematophagous trophozoites of E. histolytica.
• Upper intestinal endoscopy can yield fluid to be examined for Giardia lamblia and Strongyloides stercoralis. Impression smears and biopsies obtained with the endoscope can also be examined.
TREATMENT APPROPRIATE HEALTH CARE: Outpatient except for rare surgery or inpatient medical treatment
GENERAL MEASURES: • Therapy must be assessed in the best interest of the patient. Not all patients need to be treated with drugs.
• Symptomatic treatment is indicated for patient comfort once specific therapy has been initiated
• Bowel paralyzing drugs, for diarrhea caused by invasive organisms, are relatively contraindicated
SURGICAL MEASURES: • Surgical procedures play little role in treatment except when amebic liver abscesses need to be drained, e.g., multiple or large abscesses not responding to medical management, or threatened rupture, especially left lobe abscesses. Drainage of such abscesses is often accomplished by directed catheter placement in radiology, with surgical back-up as required.
• Surgery may be required if bowel or other organ obstruction occurs, as can be seen with Ascaris lumbricoides migration
ACTIVITY: As tolerated
DIET: • Nutritional support may be required
• Many patients during and following bowel infections, especially when infected with Giardia lamblia, will experience irritable bowel syndrome and/or lactose intolerance. The majority of these patients will respond to a lactose free diet, reduction of caffeine intake, and an increase in dietary fiber.
PATIENT EDUCATION: • Educating the patient is important to reduce the risk of reinfection or transmission
• Education will depend on the parasite, host characteristics and the environment that the two interact in
MEDICATIONS DRUG(S) OF CHOICE: • Protozoa
• Entamoeba histolytica asymptomatic needs individual assessment
• Entamoeba histolytica symptomatic intestinal - iodoquinol or diloxanide furoate
• Entamoeba histolytica invasive disease - iodoquinol or diloxanide furoate. Plus metronidazole, alone or dehydroemetine or emetine plus chloroquine phosphate.
• Giardia lamblia - metronidazole or tinidazole or furazolidone or quinacrine. Note: albendazole, available in US only from manufacturer, may have activity against G. lamblia.
• Cryptosporidium: none proven effective
• Isospora belli protozoa: trimethoprim-sulfamethoxazole
• Balantidium coli: tetracycline or iodoquinol or metronidazole
• Cyclospora: sulfamethoxazole-trimethoprim
• Microsporidia: albendazole (some species)
• Helminths
• Nematodes (except Strongyloides and Trichostrongylus): mebendazole or pyrantel pamoate or piperazine citrate or albendazole (available in US only from manufacturer)
• Strongyloides and Trichostrongylus: thiabendazole or albendazole (available in US only from manufacturer)
• Cestodes: praziquantel or niclosamide
• Trematodes: niclosamide or praziquantel
Contraindications: Refer to manufacturer's profile of each drug
Precautions: Refer to manufacturer's profile of each drug
Significant possible interactions: Refer to manufacturer's profile of each drug
ALTERNATIVE DRUGS: N/A
FOLLOW UP PATIENT MONITORING: Repeat examination, to ensure clearance, should be timed taking into account: The life cycle of the parasite (how long would it take to regenerate and become patent) and the risk of reinfection, as well as the specific test likely to find the parasite (stool, pinworm paddle, culture, serology)
PREVENTION/AVOIDANCE: The specific nature of the infection often dictates the specific methods needed to avoid reinfection. This usually involves matters of personal, food and/or water sanitation.
POSSIBLE COMPLICATIONS: Chronic persistent diarrhea
EXPECTED COURSE AND PROGNOSIS: See specific text on individual parasite
MISCELLANEOUS ASSOCIATED CONDITIONS: N/A
AGE-RELATED FACTORS:
Pediatric: Most common age group affected
Geriatric: Illness may cause more severe debilitation
Others: AIDS - susceptibility to infection, severity of disease
PREGNANCY: Some of these infections can be particularly serious in pregnancy. Many of the drugs are contraindicated in pregnancy.
SYNONYMS: N/A
ICD-9-CM: 129 Intestinal parasitism, unspecified
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