Diagnosis of Giardia infection is important in the management of dogs presented for investigation of diarrhea and for controlling cyst shedding to limit environmental contamination and infection of kenneled dogs. Traditional approaches involve zinc sulfate (ZnSO4) fecal flotation to examine for cysts being shed by the patient. Tests that have been developed to detect Giardia antigen (coproantigen) in feces are specific for Giardia species, but may provide false-positive results with persistent antigen excretion for several weeks or even months after successful elimination of the parasite. This study compared 2 methods of Giardia cyst detection (ZnSO4 flotation versus immunofluorescent antibody [FAB] coproscopy) and 2 methods of fecal Giardia antigen detection (IDEXX SNAP [idexx.com] versus quantitative enzyme-linked immunosorbent assay [ELISA]) in dogs with naturally acquired chronic subclinical giardiasis. Data showed that the ZnSO4 flotation method, even when done by a trained technician, performed poorly when compared with FAB coproscopy. Both coproantigen tests performed as well or better than the ZnSO4 flotation test, but there was substantial discordance for both tests in various dogs. Agreement between the IDEXX Giardia SNAP test and FAB coproscopy was fair, whereas agreement between the TechLab (techlab.com) ELISA and FAB coproscopy was similar to that of ZnSO4. These results illustrate the challenge of correctly identifying dogs that are chronically infected with Giardia. Most commonly used in-house Giardia diagnostic tests have poor agreement with the gold standard method (FAB coproscopy). In most clinical settings, where probability is low, commonly available diagnostic tests, such as ZnSO4 flotation and IDEXX Giardia SNAP tests, will help rule out giardiasis as a cause of clinical signs but will not be able to rule in a diagnosis. When compared with FAB coproscopy, all the in-house diagnostic tests had excellent positive predictive values at the study prevalence (89%). At lower prevalence rates common in most clinical settings, ZnSO4, SNAP, and ELISA tests all had good negative predictive values but poor positive predictive values.
Commentary: This research investigated the diagnostic performance of 4 tests to detect Giardia species in a colony of 20 dogs with subclinical chronic infection. The findings highlight challenges associated with diagnosing low-prevalence diseases and with tests that are based on diagnostic material that may be shed intermittently (eg, cysts). When evaluating dogs with diarrhea in the clinical setting, a negative result from commonly available diagnostic tests is useful to exclude the possibility that giardiasis is contributing to clinical signs.
Comparison of 4 Giardia diagnostic tests in diagnosis of naturally acquired canine chronic subclinical giardiasis. Rishniw M, Liotta J, Bellosa M, et al. J VET INTERN MED 24:293-297, 2010.