infectious diseases
Approach to Parasitic Infections
Diagnosis of Parasitic Infections
Microscopic examination Antigen and DNA tests Methods used to diagnose specific parasitic diseases are summarized in the table. Parasitic infections should be considered in the differential diagnosis of clinical syndromes in residents of or travelers to areas where sanitation and hygiene are poor or where vector-borne diseases are endemic. For example, fever in a traveler returning from an endemic area suggests the possibility of malaria. People who have immigrated from endemic areas to nonendemic countries and who return home to visit friends and relatives are at particularly increased risk. They frequently do not seek pretravel vaccines, medications, and advice on disease prevention and are more likely to spend time in high-risk areas compared with tourists who stay at resort facilities ( Reference ). Although less frequent, the possibility of locally acquired parasitic infection must also be considered in residents of countries with modern sanitation systems who present with suggestive clinical syndromes, even if they have not traveled; some parasites are endemic in these countries and others (mainly those transmissible by the fecal-oral route) may be acquired from infected travelers. Historical information, physical findings, and laboratory data may also suggest specific parasitic infections. For example, eosinophilia is common when helminths migrate through tissue and suggests a parasitic infection in an immigrant or returning traveler. The diagnosis of parasitic infections was once based on the identification of ova, larvae, or adult parasites in stool, blood, tissue or other samples or the presence of antibodies in serum, but diagnosis is being increasingly based on identification of parasite antigens or molecular tests for parasite DNA. Clinicians with expertise in parasitic infections and tropical medicine are available for consultation at many major medical centers, travel clinics, and public health facilities. For detailed descriptions of diagnostic methods, see the Centers for Disease Control and Prevention (CDC)Laboratory Identification of Parasites of Public Health Concern. Various stages of protozoa and helminths that infect the gastrointestinal tract are typically shed in the stool. Routine detection requires examination of stool specimens, preferably 3 collected on different days, because shedding can vary. Sensitivity of stool examination for ova and parasites is low enough that when clinical suspicion is strong, empirical treatment should be considered. Sensitive and specific assays are now available to detect antigens ofGiardia,Cryptosporidium, andEntamoeba histolyticain stool. Although expensive, molecular tests also are available forGiardia,Cryptosporidium,E. histolytica, andCyclospora. Tests for one or more of these organisms are typically included in multiplex polymerase chain reaction (PCR)-based screens for enteric bacterial, viral, and parasitic pathogens in stool samples (see table). Freshly passed stools uncontaminated with urine, water, dirt, or disinfectants should be sent to the laboratory within 1 hour; unformed or watery stools are most likely to contain motile trophozoites. If not examined immediately, stools should be refrigerated, but not frozen. Portions of fresh stools should also be emulsified in fixative to preserve gastrointestinal protozoa. Concentration techniques can be used to improve sensitivity. Anal cellophane tape or swabs may collect pinworm or tapeworm eggs. If Strongyloidiasis is suspected, one or more specialized stool tests should be done if larvae are not seen on direct examination of fresh stool. Antibiotics, x-ray contrast material, purgatives, and antacids can hinder detection of ova and parasites for several weeks. should be considered when routine stool examinations are negative and Amebiasis is suspected in patients with persistent gastrointestinal symptoms. Sigmoidoscopic specimens should be collected with a curet or spoon (cotton swabs are not suitable) and processed immediately for microscopy. Duodenal aspirates or small-bowel biopsy specimens may be necessary for diagnosis of such infections as Some parasites can be detected by serologic tests (see table). 1.Bacaner N, Stauffer B, Boulware DR, et al: Travel medicine considerations for North American immigrants visiting friends and relatives.JAMA291(23):2856-2864, 2004. doi:10.1001/jama.291.23.2856 