Margaret Stalker, Josepha DeLay, Hailey Bertrand

Animal Health Laboratory, University of Guelph, Guelph, ON (Stalker, DeLay);

Parkside Animal Hospital, North Bay, ON (Bertrand)

AHL Newsletter 2021;25(3):24.

Fecal testing for Canine parvovirus 2 (CPV-2) using an antigen detection enzyme-linked immunosorbent assay (ELISA) is a useful rapid test modality for investigating the possibility of parvovirus infection in the clinical setting.  However, confusion may arise when CPV-2 antigen is not detected by the ELISA in feces from animals exhibiting classic clinical signs, or typical lesions at postmortem.

The most likely cause for a false-negative fecal antigen ELISA is the presence of endogenous plasma antibodies which may develop as rapidly as day 3 following infection.  Significant viral-mediated damage to proliferating intestinal crypt epithelium disrupts the gut mucosal barrier by 6-9 days post-infection, allowing leakage of plasma antibodies.  If present in sufficient concentration to bind and neutralize remaining virus in the intestinal lumen, these antibodies may result in false-negative test results in the antigen ELISA.  Parvovirus can however persist inside the surviving intact mucosal epithelial cells, and may remain detectable by immunohistochemistry staining (Fig. 1) or by PCR testing, and may continue to be shed in feces.

This reinforces the importance of considering history, signalment and clinical signs in conjunction with results of laboratory testing when diagnosing infectious diseases.  A positive fecal antigen ELISA result is helpful to confirm CPV-2 infection; however, a negative result does not exclude CPV-2, particularly if clinical signs are consistent with parvoviral enteritis.

Figure 1: Histology (A; H&E) and parvovirus immunohistochemistry (B; Nova Red) of the small intestine of a 13-week-old puppy that tested negative on two sequential CPV-2 antigen ELISA tests, demonstrating late-stage infection with widespread crypt epithelial necrosis and complete collapse of intestinal villi (A).  Note positive immunostaining for canine parvovirus within surviving crypt epithelium (B).