Canine distemper tests (PCR and virus neutralization titer)

What it shows

• Distemper PCR — looks for the genetic material (RNA) of the virus. The sample is chosen by the signs: swabs from the eye, nose or throat for respiratory signs; EDTA blood or faeces for vomiting and diarrhoea; urine, EDTA blood or an eye swab for nervous-system signs. Some labs run a quantitative PCR that estimates how much virus is in the sample.
• VN titer (virus neutralization) — measures antibodies that stop the virus from infecting cells in the lab. The dog’s serum is diluted step by step, and the titer is the highest dilution that still fully blocks the virus. Most samples sent for it come from healthy dogs, to check the response to vaccination or the antibodies a puppy got from its mother.

Higher than reference — what it may be a sign of

• PCR “detected” in a dog with fitting signs — supports distemper, most of all in an unvaccinated dog or one with unknown vaccination status.
• Recent vaccination — a modified live vaccine contains a weakened virus, and for several weeks after vaccination PCR can be positive; one lab reports this in about 20% of dogs. Standard PCR may not tell vaccine virus from natural infection. A quantitative PCR can help: natural infection usually gives much more virus. Some forms print a Ct value; lower values (teens and low 20s) point more to a severe active infection, values over 30 to very early or long-standing infection or to vaccination.
• VN titer positive — the dog has neutralizing antibodies. Cornell University’s lab suggests that any positive titer indicates protection from disease; Michigan State University’s diagnostic laboratory uses a CDV titer of 32 or higher as its cut-off for protection. Antibody tests can be positive within 3 weeks of a vaccine, so they do not prove infection.

Lower than reference

PCR “not detected” is the expected result in a healthy dog. The chance of finding the virus depends on the right sample for the signs, and the vet decides whether to test again or test another sample. A low or negative VN titer may mean that a dog has not responded to vaccination; in puppies, antibodies from the mother can block a vaccine and are usually gone by 12–14 weeks. What to do next is for the vet to decide.

Look at together with

• Lymphocytes and white blood cells — a fall in white cells, mainly lymphocytes, can occur with the early fever 3–6 days after infection.
• Vaccination record — type and date of the last vaccine, for both PCR and titer.
• Parvovirus — one of the diseases the vet may consider when a dog has gut signs.
• Spinal fluid — in nervous-system disease, relatively more antibody in spinal fluid than in blood is typical of natural infection rather than vaccination.

Units and conversion

Both results are qualitative and have no unit. PCR is reported as “detected” or “not detected”; a quantitative PCR may add the amount of viral RNA, and some forms a Ct value. The VN titer is the reciprocal of the dilution: a titer of 1:64 can appear as “Pos 64”, and 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”, and for the VN titer the lab’s comment explains how it reads the number. A result is read together with the signs, the vaccine date and the dog’s age.

Cats and dogs

Sources

1. Merck Veterinary Manual — Canine distemper
2. Kansas State Veterinary Diagnostic Laboratory — Distemper diagnostic dilemma
3. IDEXX — Canine distemper virus (CDV) Quant RealPCR test
4. Cornell University Animal Health Diagnostic Center — Canine distemper virus serum neutralization
5. Michigan State University Veterinary Diagnostic Laboratory — Serological responses to canine vaccination
6. UC Davis School of Veterinary Medicine — Canine distemper
7. AAHA — 2022 Canine vaccination guidelines: canine distemper virus