Brucella canis antibody test (canine brucellosis) in dogs

What it shows

The test looks for antibodies — proteins the immune system makes against B. canis. Several methods exist, and the line on the form may name one of them:

• Screening tests — the rapid slide agglutination test (RSAT), the tube agglutination test (TAT), the indirect fluorescent antibody test (IFA) and one type of agar gel immunodiffusion (AGID). They are sensitive but give false positives fairly often. For the IFA, sensitivity and specificity have not been established.
• Confirmatory tests — a modified slide test with 2-mercaptoethanol (2ME-RSAT) and another type of AGID. They rarely give false positives but miss more infected dogs.
• Finding the bacterium itself — blood culture or PCR. This is considered the gold standard, but the bacteria appear in the blood only from time to time, so these tests can also be negative in an infected dog.

The infection spreads mainly through mating and contact with vaginal discharge, aborted tissue, semen and, less often, urine or milk; puppies can be infected before birth.

Higher than reference — what it may be a sign of

• Positive screening test — the dog may have met B. canis, but this is not a diagnosis. Antibodies to other bacteria, including Bordetella bronchiseptica and Pseudomonas, can cross-react and make the result falsely positive. A positive screening result is followed by a confirmatory test, culture or PCR.
• Positive confirmatory test or bacterium found — supports infection. Signs that fit include infertility, late abortion (typically at 45–55 days of pregnancy), swelling of the testicles or epididymis, eye inflammation, back pain from spinal infection (discospondylitis) and enlarged lymph nodes. Fever is not typical.
• Agglutination titer — on the TAT, a higher titer has been linked with a greater chance of finding the bacterium in blood, while low titers can reflect past or long-standing infection.

The vet weighs all results together and decides on the next steps. In some places, including many US states, B. canis infection must be reported to the authorities.

Lower than reference

A negative result does not rule out infection. Antibodies usually appear 2–4 weeks after infection but can take 2–3 months, so false negatives are common in the first weeks. Dogs with long-standing infection may also have low antibody levels. When exposure is suspected, testing is repeated after an interval — the vet decides when.

Look at together with

• Routine blood tests — a complete blood count, biochemistry and urinalysis are usually normal in canine brucellosis, so normal results do not exclude it.
• History — breeding, a recent litter, abortion or contact with dogs of unknown status matter for reading the result.
• Culture or PCR — finding the bacterium in blood or other material, used to confirm a positive antibody result.
• Signs that point to a site — with back pain or lameness the vet may image the spine; with eye inflammation, examine the eyes.

Units and conversion

The result has no unit. Screening slide tests and AGID are reported as “positive” or “negative”. Agglutination tests such as the TAT are reported as a titer, written 1:N (for example 1:200) — the highest dilution of serum at which antibodies still clump the bacteria; the larger N, the more antibody. PCR is reported as “detected” or “not detected”. 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: the lab states which result or titer it calls negative. One result is read together with the test type (screening or confirmatory), the time since possible exposure and the dog’s signs.

Cats and dogs

Sources

1. Merck Veterinary Manual — Brucellosis in dogs
2. Brucella canis: an update on research and clinical management (review, 2018)
3. Canine brucellosis: an update (review, open access)
4. CDC — Veterinary guidance for brucellosis