Center for Disease Control
Diagnosis of parasitic infections is definitively made by identification of parasites in host tissue or excreta. Such identification is not generally possible in diseases such as toxoplasmosis or toxocariasis, in which parasites are located in deep tissue sites, and is not initially recommended in diseases such as cysticercosis or echinococcosis, in which invasive techniques with some risk to the patient are necessary to obtain material. Detection of antibodies can be very useful as an indicator that an individual has been infected with a specific parasite. A positive result in a person with no exposure to the parasite prior to recent travel in a disease-endemic area may be interpreted as indicating recent infection. However, detection of specific antibodies in a person native to an area where the parasite is endemic may reflect only a past infection unrelated to current clinical status. In general, detection of antibodies to parasitic diseases indicates only infection at some indeterminate time and not necessarily acute or current infection. Levels of antibodies to parasites slowly decline after the patient is cured of the infection but generally last for at least six months to many years, depending on the infecting parasite, and thus are not generally useful, real-time indicators of successful cure.
The detection of specific IgM and IgA antibodies may be of value in determining the approximate time of initial infection with Toxoplasma gondii, but is not recommended for any other parasitic disease. If infection with a parasite is suspected and blood film, stool or urine examinations are either not indicated or are negative, then the appropriate serology test for specific IgG antibodies should be requested. Tests for parasite-specific IgM, IgA, or IgE are generally not useful for diagnosis and should not be requested. If the parasite-specific IgG is negative, a positive IgM, IgA, or IgE result is generally a false-positive reaction and should not be considered when determining patient management.
The majority of antibody detection tests for parasitic diseases in the United States are now performed in several commercial laboratories: Focus Technologies, Mayo Medical Laboratories, Parasitic Disease Consultants, Quest Diagnostics, and Specialty Laboratories. Only a few commercial kits are available in the U.S. for antibody detection of parasitic diseases. Consequently, the majority of tests performed at the commercial labs and at CDC use reagents which are not universally standardized and are produced and evaluated in-house. This has often resulted in discrepant results obtained by different laboratories due to different reagents. There is no external Proficiency Testing program for any parasitic diseases except Toxoplasma. If an unexpected result occurs, have the sample retested in another laboratory to confirm the result before accepting it as valid.
Immunodiagnostic testing for parasitic diseases is complicated by the general assumption that serology tests for a disease will detect all infections caused by any of the species that infect humans. That assumption is not true; some serologic tests, such as those for schistosomiasis detect only species-specific antibodies. The same specimen can have a negative result in one test and a positive result in another. For example, samples of serum, stool, and urine from a patient suspected of having schistosomiasis were submitted for testing to a commercial lab. Although the serum was reported as antibody negative and the stool exam as no parasites found, the urine exam revealed eggs of Schistosoma haematobium. The serum specimen was submitted to CDC for blind retesting and was immunoblot positive for S. haematobium and immunoblot negative for S. mansoni. In this situation, tests for schistosomiasis in the two labs were not equal in detecting the 3 schistosome species that infect humans, with the potential result of an inaccurate immunodiagnosis.
For most parasitic diseases, the antigen is generally the assay component which has the most influence on test sensitivity and specificity. Parasites generally have more than one life cycle stage which may have both mutually shared antigens and stage-specific antigens. The matrix to which antigens are bound for use in a specific procedure also physically selects for which antigen subset will be available for antibody binding. The decision as to which parasite stage and antigen preparation will be used in a specific assay should not be made without extensive review of the published literature. Evaluation of a procedure should be made with specimens from patients in whom parasites have been observed. Unfortunately, this is not possible for diseases such as toxoplasmosis, toxocariasis, or trichinosis because the parasites are sequestered in muscle or organ tissues and are generally not detectable. Specimens from well-defined clinical cases are acceptable for assay evaluations of these diseases, but are usually difficult to obtain. The patient specimens should be characterized to suit the particular disease. Sensitivity of a procedure may be affected by the stage and type of the patient's disease. For example, patients who have undergone surgery for an echinococcal (hydatid) cyst in the liver almost always will have detectable antibodies, but may have been negative prior to surgery. A test evaluated with only post-surgery case specimens will have close to 100% sensitivity, but a test evaluated with pre-surgery case specimens will have a lower sensitivity more indicative of how efficient the assay is as an aid to establishing a diagnosis prior to surgery. Duplication of a published procedure does not necessarily mean that the results of several laboratories are identical without comparable evaluation and, ideally, exchange of reagents and sera. An organized, impartial proficiency program for parasitic serology assays other than Toxoplasma does not exist to aid in determining comparability between tests and laboratories.
Serology tests for the following diseases or organisms are not available at CDC, but may be available elsewhere within the United States or abroad:
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