Guidebook: Canine Parvovirus
Chapter 4: Recognition and Diagnosis
Diagnosis of CPV is fortunately reasonably straightforward in most cases. While not perfect, in-house fecal ELISA tests are reportedly quite specific and sensitive even for recently emerged strains. A recent study found that the Idexx SNAP test detected 80%, 78% and 77% of parvovirus 2a, 2b, and 2c respectively.5 As with any test, false results are possible. Negative results will occur later in the course of disease when virus is bound by antibody or no longer being shed. This should not be interpreted to contradict earlier positive results. Weak false positives may also reportedly occur due to recent vaccination. However, this is likely uncommon, particularly with the Idexx brand SNAP test.6,7 In general, positive results should be taken seriously even in recently vaccinated dogs.
Of course, in all cases, history, signalment and clinical signs should be considered along with test results. With much at stake, confirmatory diagnostic testing should be performed, especially if the result does not fit the rest of the clinical picture. Other accessible in-shelter diagnostic tools include blood smear/CBC looking for leukopenia and, if a dog dies or is euthanized, in-house necropsy for characteristic segmental enteritis.
Fecal samples can be submitted to a laboratory for PCR with rapid turn-around time; this method is sensitive to detect CPV infection but also more likely to detect vaccine virus in recently vaccinated dogs. PCR analysis is the only method to distinguish between the various strains of parvovirus; however, this has minimal clinical relevance as the approach to prevention and treatment is identical regardless of strain. Histopathology and immunohistochemistry on a necropsy specimen is the gold standard for diagnosis, and should be performed in atypical outbreaks if any dog dies or is euthanized (e.g. apparent infections in well-vaccinated animals; persistent outbreak in the face of good control measures).
In general, testing should be reserved for dogs with clinical signs or recently exposed/high risk dogs. Because viral shedding can occur a few days before clinical signs appear, it can be helpful to test very high risk puppies even if they are showing no overt signs of infection, for instance unthrifty looking puppies from known high risk locations in a community. However, routine CPV testing in healthy appearing dogs or puppies is costly, ineffective for controlling parvovirus in a shelter (as animals may test negative, then begin shedding hours or days later), and likely to result in an increased rate of false positive results.
Although diagnostic tests are quite effective to confirm or rule out CPV infection, this must go hand in hand with daily or more frequent monitoring of the population. If an unthrifty animal goes undetected for hours or days, the opportunity for spread is hugely magnified. Teach all staff and volunteers to be alert for dogs with signs of illness and provide clear instructions for what to do should CPV be suspected. Train staff in correct use and interpretation of diagnostic tests and perform formal daily medical rounds during which the health of all dogs in the shelter is evaluated.