September 8, 2024
Infectious Diseases & Vaccines / By Dodds
Over the past few years, we have seen serological titer testing partially embraced by major veterinary associations to discover if a companion dog or cat is immunized against a particular infectious disease.
Before we get into the debate of titer testing, let’s discuss the basics.
What is a titer test?
A titer test is a simple blood test that measures an animal’s antibodies to viruses or other infectious agents.
What is the difference between sterilizing immunity and non-sterilizing immunity?
Basics
Sterilizing immunity = prevents clinical disease and protects from infection. The vaccines: canine distemper, parvovirus, adenovirus-2 (covers hepatitis); and, feline panleukopenia (a parvovirus) produce sterilizing immunity.
Non-Sterilizing immunity = does not protect against infection; but should keep infection from progressing to severe clinical illness. Examples of vaccines that produce non-sterile immunity would be leptospirosis, Bordetella, rabies virus, influenza, herpesvirus and calicivirus – the latter two being upper respiratory viruses of cats.
Discussion
As previously stated, sterilizing immunity not only prevents clinical disease but also prevents infection, and only the presence of antibody can prevent infection. As stated by the late eminent expert Dr. Ronald Schultz in discussing the value of vaccine titer testing, these tests “show that an animal with a positive test has sterilizing immunity and should be protected from infection.”
The vaccines that do provide sterilizing immunity are distemper virus, adenovirus, and parvovirus in the dog, and panleukopenia (feline parvovirus) virus in the cat.
While non-sterile immunity may not protect the animal from infection, it should keep the infection from progressing to severe clinical disease.
Why titer test?
- History of Adverse Events to Vaccinations
- Vaccinosis – Dogs and cats with immune-mediated diseases or predisposed to them are especially vulnerable to vaccinosis, since over-vaccination places additional stress on their already compromised immune systems and has been linked to autoimmune disease.
- Sterilizing Immunity – An animal with a positive test to the sterilizing vaccines should be protected from infection. If that animal were vaccinated it would not respond with a significant increase in antibody titer, but may develop a hypersensitivity to vaccine components (e.g. fetal bovine serum and the excipients). Furthermore, the animal doesn’t need to be revaccinated and should not be revaccinated since the vaccine could cause an adverse reaction.
Testing
Serological antibody titer testing has become more readily available over the past several years with veterinary reference laboratory and the availability of in-house testing kits. While this in-house testing is convenient, the gold standard is still laboratory testing because several of the available kits have been documented to vary in accuracy.
Interpreting Titer Results
Research has shown that once an animal’s titer stabilizes to sterilizing vaccines or infections, it is likely to remain constant for many years. It is often said that the antibody level detected is “only a snapshot in time”. That’s simply not true; it is more a “motion picture that plays for years”.
Protection – as indicated by a positive titer result – is not likely to suddenly drop-off unless an animal develops a medical problem such as cancer or receives high or prolonged doses of immunosuppressive drugs. Viral vaccines prompt an immune response that lasts much longer than that elicited by classic antigen. Lack of distinction between the two kinds of responses may be why practitioners think titers can suddenly disappear.
In all actuality, interpreting titers correctly depends upon the disease in question and if the vaccine produces sterilizing or non-sterilizing immunity. Some titers must reach a certain level to indicate immunity, but with other agents like those that produce sterile immunity, the presence of any measurable antibody shows protection.
The positive titer test result is fairly straightforward, but a negative titer test result is more difficult to interpret, because a negative titer is not the same thing as a zero titer and it doesn’t necessarily mean that animal is unprotected. Indeed, one of the drawbacks of titer testing is that it does not measure cellular immunity. Anyway, a negative titer result usually means the titer has failed to reach the threshold of providing sterile immunity.
This is an important distinction, because for the clinically important distemper and parvovirus diseases of dogs and panleukopenia of cats, a negative or zero antibody titer indicates that the animal is not protected against canine parvovirus and may not be protected against canine distemper virus or feline panleukopenia virus. While non-sterile immunity may not protect the animal from infection, it should keep the infection from progressing to severe clinical disease.
Puppy Recommendations
Since 2016, the World Small Animal Veterinary Association (WSAVA) Vaccination Guidelines have partially embraced serological titer testing for canine distemper (CDV), parvovirus (CPV), and adenovirus (CAV) for puppies. For kittens, the guidelines mention titer testing for antibodies against feline panleukopenia (a parvovirus), but it is not discussed to the degree that puppy titer testing is.
In a snapshot, the WSAVA states that a companion pet caregiver can choose to have their puppy titer tested for those three diseases four or more weeks after the final set of initial vaccinations. If a puppy has a positive test result, the puppy does not need to be revaccinated for another three years. If they choose not to titer test, WSAVA recommends vaccinating once more at 26 weeks and then three years after that.
For instance, the association suggests the last set of puppy vaccinations against CDV, CPV, and CAV-2 (covers ICH = CAV-1) be given at 16 weeks. At 20-26 weeks of age, titer test or opt for a final vaccination at 26 weeks.
One of the key points WSAVA emphasizes is that laboratory testing is the gold standard for serological titer testing and is much preferred over the in-house tests – particularly for CDV and CAV.
Dog and Cat Recommendations
For years, the WSAVA has recommended triennial vaccinations of core vaccines: canine distemper, parvovirus, and adenovirus-2 (covers infectious canine hepatitis/ICH); and feline panleukopenia (a parvovirus) after the initial vaccination series given to puppies and kittens.
We applaud this recommendation because it essentially extended the interval from one year to every three. However, we think it fails to include a valid step: titer testing.
Indeed, we prefer that individual dogs and cats be titer tested every three years to determine if they do in fact need this vaccination after the initial vaccination series given to puppies and kittens.
The AAHA Stance
In contrast to WSAVA, AAHA does not embrace titer testing…except in certain instances.
The AAHA Canine Vaccination Guidelines of 2022 discuss the drawbacks of titer testing such as: interpretation; differences between the various laboratories and the results they output; issues with in-house testing; lack of new research; and, questions of the test’s validity. The Association has stated, “It can be misleading to forecast an outcome on the basis of one cofactor: a titer.”
Yet, AAHA’s triennial vaccination protocol for feline panleukopenia, canine distemper, canine parvovirus, and canine adenovirus-2 is based on titer testing.
Further, while the Association does not advise to routinely rely on or use titer testing, it does provide exceptions, “except in cases in which dogs have a history of adverse responses to vaccination, there is a suspicion of vaccine-related autoimmune disease, or when owners express resistance or hesitancy to having their dogs vaccinated or boostered—in which case client communication and education may help overcome this hesitancy.”
They appear to be equivocating.
Reminder
Dr. Dodds still wants puppies properly vaccinated against canine distemper and parvovirus, and cats against feline parvovirus (panleukopenia). Remember: more often than not, these diseases are worse than the vaccinations against them! We want to immunize wisely and not overly vaccinate. That’s why we advocate for laboratory titer testing, which is a service Hemopet provides.
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