Canine parvovirus tests (PCR and HI antibody titer)

What it shows

• Parvovirus PCR — detects the DNA of the virus, most often in faeces. It is more sensitive than the quick faecal antigen tests done in the clinic, which can miss the virus. Some labs also report the variant: CPV-2a, CPV-2b or CPV-2c. Current vaccines are considered to protect against all of them.
• HI titer (haemagglutination inhibition) — measures antibodies to parvovirus in serum. Antibodies can come from vaccination, from infection or, in young puppies, from the mother. Diagnosing infection by antibodies needs a fourfold rise in titer over 14 days, so the titer is used more often to check whether a dog responded to vaccination. Laboratories note that a positive titer correlates well with protection against parvovirus.

Higher than reference — what it may be a sign of

• PCR “detected” in a dog with diarrhoea, vomiting and low white cells — supports parvovirus infection.
• Recent vaccination — modified live vaccines use a weakened virus, and PCR may find it for a while; in general most of it is cleared within about two weeks, but this varies with the vaccine. Quick faecal antigen tests can also be falsely positive 4–10 days after a modified live parvovirus vaccine. This is why the vet asks for the date of the last vaccine.
• HI titer positive — the dog has antibodies to parvovirus. In a healthy vaccinated dog this is expected. Michigan State University’s diagnostic laboratory considers a CPV-2 titer of 80 or higher a sign of protection.

Lower than reference

PCR “not detected” is the expected result in a healthy dog. Shedding in faeces falls within 10–12 days after infection, so a test taken late can miss the virus; early on, before shedding peaks, large amounts of diarrhoea can dilute it for quick antigen tests. A low or negative HI titer may mean that a dog did not respond to vaccination: some dogs are non-responders, and in young puppies antibodies from the mother can block a vaccine. What to do next is for the vet to decide.

Look at together with

• White blood cells — most dogs with parvovirus develop a moderate to severe fall, with low neutrophils and lymphocytes.
• In-clinic faecal antigen test — often done first; a negative result in a dog with typical signs may be followed by PCR.
• Vaccination record — type and date of the last vaccine, for both PCR and titer.
• Distemper — can also cause diarrhoea and is one of the diseases the vet may consider.

Units and conversion

Both results are qualitative and have no unit. PCR is reported as “detected” or “not detected” (sometimes “positive” / “negative”), at times with the variant. The HI titer is written as 1:N or just N (for example 80): it is the highest dilution of serum that still shows antibody, so a larger number means more antibody. In the app these results are stored as text, as printed on the form.

Reference range

The reference range is on your lab’s form; for PCR it is “not detected”. For the HI titer the lab’s comment explains how it reads the number. A PCR result is read together with the signs, white cell count and vaccine date; a titer — with the age and vaccination history.

Cats and dogs

Sources

1. Merck Veterinary Manual — Canine parvovirus infection (parvoviral enteritis in dogs)
2. Kansas State Veterinary Diagnostic Laboratory — Canine parvoviral infections
3. UC Davis Real-time PCR Research and Diagnostics Core Facility — Diagnostic FAQs
4. Michigan State University Veterinary Diagnostic Laboratory — Serological responses to canine vaccination
5. Kansas State Veterinary Diagnostic Laboratory — Indications for core vaccine titer testing in dogs and cats
6. ABCD (European Advisory Board on Cat Diseases) — Guideline for feline panleukopenia