What are antibodies?
Antibodies are specialized proteins, also called immunoglobulins, that are primarily found in the bloodstream. They are produced by specialized white blood cells called plasma cells, which are a form of lymphocyte.
Specific antibodies are generated in response to the different foreign substances that the body encounters (e.g., bacteria, viruses, toxins) and provide varying lengths of immunity against these substances.
What is an antibody titer?
An antibody titer is the most dilute serum sample that causes a positive test reaction. Serum is the liquid portion of a blood sample from which the red blood cells, the white blood cells, and the platelets have been removed. After separation, the serum will still contain antibodies.
"An antibody titer is the most dilute serum sample that causes a positive test reaction."
A positive test reaction differs for each test. For example, one of the most used antibody titer tests is for detection of antibodies against Leptospira bacteria – the cause of leptospirosis. In this titer test, different strains of Leptospira are exposed to serial dilutions of test serum. The end, or titer, is the highest dilution of serum that can produce a positive test reaction. For leptospirosis, a positive reaction is when there is clumping of at least 50% of bacteria.
What do antibody titers indicate?
Since antibodies are only produced after exposure to “foreign” material, including organisms such as bacteria and viruses, they indicate exposure to that organism.
Antibodies indicate exposure to foreign bacteria and viruses.
Because antibodies remain in circulation for quite some time, the presence of an antibody titer to a particular organism does not necessarily mean that that organism is the cause of the illness that your pet is currently experiencing.
The lack of an antibody titer to an organism can indicate one of two things.
- No exposure to that organism has occurred and your pet's current clinical condition is due to another cause; or
- Your pet has been exposed to the organism that there hasn’t been enough time for antibodies to be produced in sufficient quantities to be detectable in the serum.
Therefore, if a particular disease is suspected to be causing illness in your pet (e.g., leptospirosis), it is important to obtain both acute and convalescent serum titers. This means that blood samples for antibody titer testing are required at the onset of your pet's illness and again two to three weeks later.
If your pet has been infected with the suspected organism, then a rising antibody titer (of at least four-fold magnitude) over this period will confirm the infection. If there is no increase in antibody titer, or if there is no significant antibody titer at either time, then the suspected organism is not likely the cause of your pet's illness.
What diseases do we commonly test for using antibody titers?
Antibody titers are often used to test for leptospirosis and ehrlichiosis in dogs, and toxoplasmosis in cats and occasionally in dogs.
Why do we need antibody titers to diagnose these diseases?
The clinical signs associated with these diseases are variable and can be confused with other diseases. Therefore, it is important to confirm the presence of a specific disease through laboratory testing. By doing so, your veterinarian can optimize the therapeutic plan for your pet, offer a better estimation of the clinical outcome (prognosis), and alert you to any possible zoonotic potential.
What does zoonotic mean?
Zoonotic means that the disease is potentially transmissible or contagious to people.
Although most people with leptospirosis acquire the infection from contaminated water or through occupational exposure to wildlife or domestic animal hosts, leptospirosis can be transmitted to people by contact with contaminated urine or urine-soaked materials from an infected pet dog. Therefore, if your dog may have the disease, it is important that you wear rubber gloves when cleaning up areas that the dog has soiled.
Ehrlichiosis is not directly transmissible from dogs to humans, but infection of both may occur through the bite of an infected brown dog tick. Therefore, infected dogs serve as sentinels to indicate the presence of infected ticks in the area.
Although toxoplasmosis can be spread to people through infected oocysts (eggs) shed in the feces of cats, this is not a common route of infection in humans. A more common method of infection is consumption of contaminated, poorly cooked meat or unpasteurized dairy products, especially those from sheep or goats. Like cats, most people do not develop clinical disease even if they become infected. However, if a woman is infected for the first time during pregnancy, the fetus is also placed at risk. Therefore, your family physician may recommend that you and/or your cat be tested for toxoplasmosis if you are a woman of childbearing age.