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Understanding the use of antibody titers in veterinary practice
Antibody titers are a tool to assess an individual patient’s immune response to vaccination for some common diseases. As titers grow in popularity, veterinarians must learn when to perform them and how to interpret the results to maintain patient health.
As concerns about over-vaccination and vaccine-associated injuries have grown in human medicine, vaccine compliance has decreased. In some areas, these concerns have spilled over into veterinary medicine and led pet owners to question the vaccine protocols recommended by their veterinarian. Due to these concerns, vaccine titers may be requested in lieu of vaccination. In other cases, the veterinarian may recommend a titer due to a patient’s history.
No matter who suggests performing a titer, it is imperative that we as veterinarians understand which titers are reliable and how to interpret them to best protect our patients. The AAHA Canine Vaccination Guidelines, published in 2017 and updated in 2018, include a section on antibody testing for vaccine-preventable diseases.
In a lecture at the 2020 Midwest Veterinary Conference, held in February in Columbus, Ohio, lead guidelines editor Richard Ford, DVM, MS, DACVIM, DACVPM (Hon), discussed utilization of antibody titers in companion animal practice. He encouraged practitioners to shift their thinking away from the dichotomy of vaccination versus titer testing and instead consider “using antibody testing to assess response to vaccines.”
Which titers are reliable?
The only titers for which a positive test result has a high correlation with protective immunity are for the vaccine-preventable diseases canine distemper virus, canine parvovirus, canine adenovirus and feline panleukopenia virus. In practice, both quantitative and qualitative titers are available for these diseases.
Quantitative titers are laboratory-based tests that provide results in a matter of days. Dr. Ford cautions that the amplitude of the antibody titer should not be equated to a level of immunity. A patient is either immune or not.
Qualitative titers are also available as point-of-care tests that can be performed in the clinic and provide results much more quickly. These tests have been validated, and a positive result correlates well with the results of quantitative tests.
How should titer results be interpreted?
Dr. Ford cautions practitioners that a positive result can have different meanings depending on the titer performed. Only positive antibody titers for canine distemper virus, parvovirus, adenovirus and feline panleukopenia can be interpreted as protection. Based on the current laws, a positive rabies antibody titer can only be interpreted as exposure to the vaccine. These results cannot be interpreted as an indication of protective immunity.
When a titer produces a negative result for canine distemper, parvovirus, adenovirus or feline panleukopenia, clinicians should not assume an adult patient with previous vaccination history is susceptible to the disease. Over time, antibody levels circulating in the blood will decrease in the absence of exposure, but the immune system possesses memory that can produce an immune response should it encounter the disease. A negative titer in a puppy undergoing its initial vaccine series indicates that the immune system has not yet responded to vaccinations or may be unable to respond.
Why should titers be performed?
In Dr. Ford’s experience, the most common reason for practitioners to perform titers is at the request of an owner. The AAHA guidelines discuss 12 indications for titer testing and guide practitioners on how a positive or negative result should be used to guide patient management in each case. Dr. Ford highlighted several of these indications during his lecture.
Assess immunization status in puppies
Dr. Ford repeatedly emphasized that just because we are vaccinating pets does not mean they are immunized against a disease. Immunization occurs after a pet’s immune system has mounted a response to the organism(s) in the vaccine.
Research has shown that at 12 weeks of age, only 50% of puppies have been immunized against canine distemper virus and parvovirus. This lack of immunization is thought to be due to interference from maternal antibodies. The goal of a puppy booster series for modified-live vaccines is to administer at least one vaccine after maternal antibodies are no longer present, leading to the recommendation of vaccination through at least 16 weeks of age. However, at 16 weeks of age, Dr. Ford stated that 15% of puppies are still not immunized.
When an owner needs to know if immunization has been achieved, a titer could be performed two to four weeks after completion of the initial vaccine series. These cases could include especially valuable animals, those that will be traveling extensively, or those living in endemic disease areas. If titers are negative, an additional dose of the vaccine should be administered. An additional titer can be performed two to four weeks after this booster to assess immunity.
Identify genetic nonresponders
A small subset of dogs are genetic nonresponders to parvovirus vaccine (but will respond to vaccination for other preventable diseases, including distemper and adenovirus). These pets are at high risk for infection if they are exposed to the disease, and lifestyle modifications to minimize exposure are needed to protect these dogs. At this time, these dogs can only be identified through serial vaccine titers. If a negative titer is obtained to parvovirus following the final puppy booster, one additional booster should be administered. If a negative titer is again obtained, the patient can be identified as a nonresponder.
Assess immunization status in adults
Titers can be used to determine whether a pet has protective immunity to canine distemper virus, parvovirus, adenovirus or feline panleukopenia. This can be helpful if the vaccination history of an adult dog is unknown or if the client requests a titer in lieu of vaccination. If the titers are positive, the pet does not need to be revaccinated. If a titer is negative, a booster is recommended, especially if the pet's vaccine history is unknown.
Determine whether vaccination is necessary in adult patients with comorbidities
At times, there may be contraindications to vaccinations, including a history of severe adverse vaccine reaction or immune-mediated disease, a chronic illness or a patient undergoing immunosuppressant therapies. In these cases, a titer can help guide patient management.
In cases in which a positive titer is obtained, the pet is protected and revaccination is not needed. When a negative titer is obtained, the pet may not be protected and the clinician and client should work together to make the best decision for the pet in light of the comorbidities present.
Dr. Ford encouraged veterinarians to consider titers as a tool that can be used to assess a pet’s response to previous vaccination and determine if revaccination is necessary. Positive titers for canine distemper, parvovirus, adenovirus and feline panleukopenia are indicative of protective immunity and can give a clinician confidence that revaccination is not currently needed. Questions as to how frequently titers should be performed remain unanswered. The AAHA Vaccine Guidelines offer extensive information on antibody titers for vaccine-preventable diseases and can be used as a guide for practitioners on when to perform titers and how to interpret them.
Dr. Boatright, a 2013 graduate of the University of Pennsylvania, is an associate veterinarian and freelance speaker and author in western Pennsylvania. She is actively involved in the AVMA House of Delegates as well as her local and state veterinary medical associations. She is a former national officer of the Veterinary Business Management Association.