Chlamydia felis PCR in cats

What it shows

C. felis cannot live long outside a cat, so it spreads through close contact, mostly with eye discharge. It is found most often in young cats, especially under one year of age, and in places with many cats, such as breeding catteries and shelters. Cats older than five years are very unlikely to be infected.

The typical picture is conjunctivitis: red, swollen lining of the eye, a third eyelid that partly covers it, discharge that starts watery and becomes thick. It often starts in one eye and then involves both. Sneezing and a runny nose are possible but usually mild. Other germs — feline herpesvirus and calicivirus — cause similar signs, which is why a test is useful.

PCR is the preferred test: it is very sensitive and does not need live bacteria, unlike culture. The swab has to collect enough cells, so how it is taken matters.

Higher than reference — what it may be a sign of

• “Detected” (positive) — C. felis DNA is in the sample. In a young cat with conjunctivitis this supports chlamydia as the cause. It is found in up to 30% of cats with conjunctivitis, especially long-lasting cases.
• Healthy carriers — because PCR is so sensitive, some cats without signs are positive: under 2–3% in most studies, though one study found 9.8%. Cornell gives about 3% for healthy-looking cats and about 20% for cats with upper respiratory signs. A positive result is read together with the eyes, not alone.
• Lasting infection — in most cats shedding from the eye stops about 60 days after infection, but some cats stay infected for months and can relapse.
• Co-infection — a positive result does not exclude herpesvirus or calicivirus; damage to the cornea is more typical of herpesvirus.

Lower than reference

“Not detected” (negative) means no C. felis DNA was found in this swab. It makes chlamydia less likely, but does not rule it out completely: a swab with too few cells, a long-lasting infection with few bacteria, or medicines given before the swab can lower the chance of finding it. If the signs fit, the vet may repeat the test or look for other causes of conjunctivitis.

Look at together with

• Feline herpesvirus PCR and calicivirus PCR — the other common infectious causes of eye and nose problems in cats; they are often tested from the same swab in a panel.
• Cytology of the conjunctiva — under the microscope the lab may see typical inclusions inside cells, but PCR is more reliable.
• Antibody tests — sometimes used in unvaccinated cats, but the sources disagree on their value: Merck considers commercial antibody tests not useful. Vaccination against chlamydia can make antibody results hard to read. This is a different test from PCR.

Units and conversion

This is a qualitative test, so the result has no unit. On the form it is written as “detected” / “not detected”, or “positive” / “negative”. Some labs add a comment about the amount of DNA found; there is no single scale for that between labs. The app stores the wording printed on the form.

Reference range

The reference is on your lab’s form — for this test it is “not detected”. Read the result together with the cat’s eyes and age: a positive result in a kitten with conjunctivitis means something different from a positive result in a healthy adult cat from a large group. The vet decides what to do next.

Cats and dogs

Sources

1. ABCD (European Advisory Board on Cat Diseases) — Guideline for Chlamydia felis
2. Merck Veterinary Manual — Chlamydial Conjunctivitis in Animals
3. Cornell Feline Health Center — Respiratory Infections
4. VCA Animal Hospitals — Chlamydial Conjunctivitis in Cats