Feline herpesvirus-1 (FHV-1) PCR in cats

What it shows

PCR detects the DNA of FHV-1 in a swab. ABCD names the oropharynx as the preferred site if only one sample can be taken; adding conjunctival and nasal swabs significantly increases the chance of finding the virus. PCR is more sensitive than growing the virus in the laboratory.

The key to reading the result is latency. After the first infection the virus settles in nerve cells, mainly in nerve nodes of the head, and all cats with a primary infection are likely to become lifelong latent carriers. There are no direct tests for latency. Cornell writes that up to 97% of cats are exposed to the virus during their lives, that it causes lifelong infection in up to 80% of exposed cats, and that up to 45% of these shed it from time to time, usually when stressed.

Higher than reference — what it may be a sign of

For PCR, “higher than reference” means the DNA was detected. Possible explanations:

• Current FHV-1 disease — in a cat with respiratory or eye signs, a positive result fits an active infection.
• Reactivation in a carrier — a latent virus can start shedding again. In studies, shedding was induced in about 70% of latently infected cats by corticosteroids, and occurred after lactation (40%) and moving to a new home (18%).
• Healthy shedder — a Spanish study found FHV-1 in 6% of healthy cats, 28% of cats with upper respiratory disease and 24% of cats with conjunctivitis; a Swiss study found it in 9% of cats without signs. A positive result alone does not prove that the virus causes the current problem.

Vaccination is an unlikely explanation: PCR can detect FHV-1 DNA in modified live vaccines, but two studies found none after vaccination, so recent vaccination is not thought to interfere with PCR in sick cats.

Lower than reference

“Not detected” does not mean the cat is free of FHV-1. A latent virus is not shed, and shedding is intermittent, so a carrier can test negative on the day of the swab. A general rule of veterinary PCR laboratories is that a pathogen may be present below the test’s detection limit. Sampling more than one site raises the chance of finding the virus. Fluorescein and local anaesthetic eye drops may lower PCR sensitivity, so the laboratory is consulted before they are used. The vet decides whether to repeat the test.

Look at together with

• Calicivirus PCR (FCV) — another respiratory virus; the two infections are often hard to tell apart by signs.
• Antibody tests — of limited value: they cannot distinguish infected from vaccinated cats.
• Recent events — stress, a move, a new cat or surgery before the swab help to explain a positive result.

Units and conversion

PCR has no units. The form shows “detected” or “not detected” (sometimes “positive” / “negative”). “Not detected” is a negative result, and “detected” means the test found its target. Some forms add a Ct value: the lower the Ct, the more DNA in the sample. Close to the test’s limit, the form may note that the result may not be reproducible. In the app the result is stored as text, as printed on the form.

Reference range

The reference range is on your lab’s form; for this PCR it is usually “not detected”. Because many healthy cats carry the virus, a positive result is read together with the signs, and a negative one does not rule out carriage. The result helps the vet choose the next steps.

Cats and dogs

Sources

1. ABCD (European Advisory Board on Cat Diseases) — Guideline for feline herpesvirus infection
2. Tonozzi CC. Feline Respiratory Disease Complex. Merck Veterinary Manual, professional version
3. Cornell Feline Health Center — Respiratory Infections
4. Cornell University Animal Health Diagnostic Center — Interpretation of PCR results
5. UC Davis Real-time PCR Research and Diagnostics Core Facility — Diagnostic FAQs