Diagnosis and Treatment Information for Medical Professionals
Healthcare providers can order laboratory tests to identify Giardia parasites in the stool of someone who is sick. Health professionals should consult a diagnostic laboratory with questions about appropriate testing. If appropriate testing is beyond the capacity of the diagnostic laboratory, the diagnostic laboratory should contact public health officials (state or county as appropriate) for information and guidance on specimen submission, including submission to CDC if appropriate.
Types of Tests
There are several tests to detect Giardia and some are more sensitive and specific than others. Microscopy with direct fluorescent antibody testing (DFA) is considered the test of choice for diagnosis of giardiasis since it provides increased sensitivity over non-fluorescent microscopy techniques.
Other alternate methods for detection include:
- Rapid immunochromatographic cartridge assays
- Enzyme immunoassay (EIA) kits
- Microscopy with trichrome staining
- Molecular assays
CDC recommends collecting three stool specimens from patients over several days for more accurate test results. Commercial products (DFA, EIAs, and rapid tests) are available in the United States for the diagnosis of giardiasis. Only molecular testing (DNA sequencing, for example) can be used to identify the strains of Giardia. Retesting for Giardia is only recommended if symptoms persist after treatment.
Following diagnosis of laboratory-confirmed giardiasis, healthcare providers and laboratories are required to report those cases to their local or state health departments, which in turn report the cases to CDC.
Giardia duodenalis (also referred to as Giardia intestinalis or Giardia lamblia) can be subdivided based on molecular analysis into what are known as different genetic assemblages (A-H). Some of these assemblages can be classified even further into subtypes, such as A-I, A-II, A-III. Each strain (assemblage or subtype) is capable of infecting certain animal groups, and some strains are more commonly seen than others. Giardia strain information can tell us about how the germ is spread, for example from animal-to-people or people-to-people, for better public health guidance.
Given the difficulty of diagnosing giardiasis, treatment is often provided to patients based on appropriate history and clinical signs. Different factors may shape the effectiveness of a drug regimen, including medical history, nutritional status, and condition of the immune system. Therefore, it is important for doctors to discuss treatment options with patients.
Several drugs can be used to treat Giardia infection. Effective treatments include metronidazole, tinidazole, and nitazoxanide. Other medications include paromomycin, quinacrine, and furazolidone. Some of these drugs may not be readily available in the United States.
Recurrent Giardiasis Infections and Treatment Failures
People treated for giardiasis may continue to experience illness symptoms or have positive tests for Giardia. In such cases, before switching therapies doctors should consider the following steps:
Dehydration due to diarrhea can be a particular risk among pregnant women and can be life-threatening for infants. For this reason, rehydration is especially important for these groups.
- Determine if the patient is still infected. Test 3 stool samples over several days by antigen testing or microscopy.
- If Giardia is not found after 3 stool exams, and if a parasite concentration method is used to process the stool specimen before the exam, it is highly probable that the patient is no longer infected.
- Please note that the patient may remain symptomatic for weeks to months following clearance of infection.
- Consider possible reinfection through the environment—home or daycare—or household members, rather than treatment failure.
- Consider inadequate dosing and duration of treatment. Confirm that the patient took the entire course of medication as prescribed.
- If Giardia is confirmed by a positive stool test, reinfection and inadequate dosing have been ruled out, and the patient remains symptomatic, consider combination therapy. Combination therapy can be safe, effective, and useful in the case of treatment failure. The treating physician can wait at least 2 weeks after the last dose of anti-Giardia medication is taken by the patient and then re-examine stool specimens as outlined in Step One for the presence of Giardia.
- If stool specimens remain positive for Giardia, consider an underlying immunodeficiency that may be impeding clearance of the infection. Workup for other diseases may be conducted during a combination therapy regimen.