Rocky Mountain spotted fever antibodies (IFA) in dogs and cats
Rocky Mountain spotted fever (RMSF) is a tick-borne infection of dogs and people caused by the bacterium <i>Rickettsia rickettsii</i>. It occurs in North, Central and South America. The IFA test looks for antibodies to the bacterium in the blood; the result is given as a titer, and it is usually read as a pair of samples taken a few weeks apart.
What it shows
The bacterium lives inside the cells lining small blood vessels and causes inflammation of the vessel walls (vasculitis). It is passed on by ticks: in the USA mainly the American dog tick and the Rocky Mountain wood tick, and in some areas, such as Arizona, the brown dog tick. Despite the name, most cases come from the south-eastern and south-central USA, not from the Rocky Mountains.
In the indirect immunofluorescence test (IFA), the dog’s serum is diluted step by step and placed on slides with the bacteria. If antibodies are present, they stick to the bacteria and glow under a special microscope. The titer is the highest dilution that still glows. Antibodies show that the immune system has met the bacterium or a close relative; on their own they do not prove that the current illness is RMSF.
Higher than reference — what it may be a sign of
- Rising titer between two samples — a fourfold or greater rise between the first sample and one taken 1–3 weeks later supports a recent infection. This often confirms the diagnosis only in hindsight.
- Single positive titer — says little on its own. Antibodies to other spotted fever rickettsiae, including less harmful and harmless ones, cross-react in the test and are common, and antibodies can stay for months after an infection.
- Very high single titer — CAPC regards a single IgG titer above 1:1024, like a fourfold rise, as confirming infection; the vet reads it together with the signs.
Signs are often vague — fever, joint and muscle pain, lethargy, poor appetite — so the vet weighs the titer together with the examination, tick exposure, season and other tests and decides on the next steps.
Lower than reference
A negative result early in the illness does not rule RMSF out: dogs are often still seronegative in the first days, because antibodies take time to appear. This is why the vet does not wait for serology in an acutely ill dog and may repeat the test later. A negative result in a second sample taken weeks after the illness makes recent RMSF less likely.
Look at together with
- Platelets — a low platelet count is the most common blood finding in RMSF.
- Albumin and urine protein — low albumin and protein in the urine also occur.
- PCR on blood — may be positive very early, before antibodies appear; a negative PCR does not rule infection out, because the bacteria are not always in the bloodstream.
- Other tick-borne infections with similar signs — Ehrlichia, Anaplasma, Lyme disease; the vet chooses which to test.
- The tick itself, if one was found — tick identification shows which infections it can carry.
Units and conversion
The result has no unit. It is reported as “positive” / “negative” or as a titer written 1:N (for example 1:128) — the highest dilution at which antibodies are still detected; the larger N, the more antibodies. Two titers are compared step by step: a rise from 1:64 to 1:256 is a fourfold rise. In the app this result is stored as text, as printed on the form.
Reference range
The reference range is on your lab’s form: the lab states from which titer it calls the result positive. Paired titers are best compared within the same lab, because different labs and methods give different numbers.
Cats and dogs
Cats may have antibodies to R. rickettsii, but they are not known to develop visible disease, and the infection is rarely diagnosed in cats. A positive titer in a cat mainly shows contact with infected ticks.
Dogs are highly susceptible. Signs usually start within days to two weeks after the tick bite; severely affected dogs may have bruise-like spots on the gums and eyes, swelling of the face or legs and neurological signs. People cannot catch RMSF directly from a dog — a tick is needed — but an infected dog shows that infected ticks are around the household.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.