{"id":21412,"date":"2020-08-11T19:55:53","date_gmt":"2020-08-11T19:55:53","guid":{"rendered":"https:\/\/todaysveterinarypractice.com\/?p=21412"},"modified":"2022-04-05T17:58:42","modified_gmt":"2022-04-05T17:58:42","slug":"vaccine-titers-in-companion-animal-practice","status":"publish","type":"post","link":"https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/diagnostics\/vaccine-titers-in-companion-animal-practice\/","title":{"rendered":"Decoding Vaccine Titers: A Literature-Based Review of Vaccine Titers in Companion Animal Practice"},"content":{"rendered":"

Vaccination of companion animals has proven to be effective and generally safe for the prevention or control of common and potentially life-threatening diseases. The benefits reach past the individual animal and contribute to herd health, establishing an immune population that helps prevent spread of disease and\/or outbreaks. However, the potential for adverse events (ranging from mild to life-threatening) and vaccine failure exists,1<\/sup> and development of a vaccination protocol should be an informed medical decision established between the client and the veterinarian and tailored to patients on an individual basis. <\/span><\/p>\n

As part of this decision-making process, public attention has shifted toward the utility of vaccine titer testing. Vaccine titers are measures of the levels of serum antibodies against pathogens, which clinicians can use to determine whether a vaccination is required. The 2017 American Animal Hospital Association (AAHA) canine vaccination guidelines2<\/sup> and the 2015 World Small Animal Veterinary Association (WSAVA) vaccination guidelines3<\/sup> incorporate recommendations for this diagnostic testing in their protocols; in addition, vaccine titer testing is advocated by the American Holistic Veterinary Medical Association. This article provides a literature-based review of the limitations and benefits of this type of testing and guidance for interpreting results. <\/span><\/p>\n

What Viruses Do Vaccine Tests Apply to?<\/h2>\n

In the companion animal sector, core vaccine titer testing is well established; diagnostic tests measure antibodies against the common viruses covered in a canine core vaccine (canine distemper virus [CDV]; canine parvovirus [CPV]; and canine adenovirus type-2 [CAV-2], which also provides cross-protection against canine adenovirus type-1) and feline panleukopenia virus (FPV).1<\/sup> Diagnostic laboratories can also determine levels of antibody against feline herpesvirus (FHV) and feline calicivirus (FCV).<\/span><\/p>\n

What Titer Tests Are Available?<\/h2>\n

The gold standard for titer testing for antibodies against the core vaccine viruses CDV, CAV-2, FHV, and FCV is virus neutralization (VN).4<\/sup> For detecting antibodies against CPV and FPV, the preferred method is hemagglutination inhibition (HI).4<\/sup> These diagnostic tests are available through reference laboratories and have the benefit of detecting functional antibodies and providing an end-point titer, which helps quantify the antibody concentration. Enzyme-linked immunosorbent assay (ELISA) point-of-care tests provide rapid, qualitative (positive\/negative) results.<\/span><\/p>\n

VN Testing<\/h3>\n

VN detects neutralizing antibodies, which may also be described as antibodies that inhibit virus replication. In the laboratory, serial dilutions of the patient\u2019s serum (e.g., 1:8, 1:16, 1:32) are incubated with live virus and subsequently exposed to cells in culture.5<\/sup> If the virus is appropriately neutralized, the cells survive and the resulting dilution is considered that patient\u2019s antibody titer. However, if the virus is not neutralized, it will kill the cells. Cell death is detected by microscopic visual inspection, use of dye, or fluorescence, depending on the laboratory. <\/span><\/p>\n

HI Testing<\/h3>\n

HI is useful for assaying antibodies against viruses that cause hemagglutination, or the formation of a lattice between red blood cells. In this type of testing, live virus is incubated with serial dilutions of the patient\u2019s serum and exposed to red blood cells.5<\/sup> If levels of antibody are adequate, the virus is complexed (bound to antibodies), leaving a pellet of red blood cells, and the patient is considered protected at the level of that dilution. However, if levels of antibodies are inadequate, the virus will cause agglutination of the red blood cells, which is detected on the microwell plate as a mesh of cells. Therefore, the patient\u2019s titer is the greatest dilution at which there are sufficient antibodies to inhibit virus agglutination of red blood cells. <\/span><\/p>\n

Understanding the methods used for VN and HI testing is helpful for interpretation of results (<\/span>FIGURE\u00a01<\/b><\/span>). <\/span>Quantitative titers are reported as the dilution value (i.e., 1:160) or the inverse whole number (i.e., 160). Generally, the higher the number (in this example 160), the greater the amount of antibodies present at greater dilutions and the higher the likelihood that the patient is protected from that virus. The reference ranges established by a given laboratory are valuable because the method used can affect these values. For example, if a laboratory\u2019s reference range for response to vaccination is >1:80, a result of 1:160 is considered positive. However, a patient with even more antibodies and a result of 1:320 is not necessarily more protected than the patient with the result of 1:160; both are positive results. Numerical titers are beneficial for tracking patients\u2019 detectable immune responses over time. To reduce laboratory error, VN and HI typically are performed in triplicate.<\/p>\n

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Figure 1. Titer testing by virus neutralization (VN) and hemagglutination inhibition (HI). Both tests require serial dilutions of the patient\u2019s serum that are subsequently incubated with cells from culture (VN) or red blood cells (HI). In VN testing, cell survival indicates that the patient\u2019s antibodies neutralized the virus. In HI testing, lack of agglutination indicates that the patient\u2019s antibodies bound the agglutinating virus. The greatest dilution with cell survival (VN) or lack of agglutination (HI) is the patient\u2019s titer.<\/p><\/div>\n

ELISA<\/h3>\n

In-clinic ELISA tests yield positive or negative results, which cannot differentiate between neutralizing and non-neutralizing antibodies.6<\/sup> Despite this limitation, in a study that compared ELISA to gold standard VN\/HI testing for 431\u00a0shelter dogs, overall accuracy of ELISA testing was 95% for CDV and 98% for CPV antibodies.7<\/sup><\/span><\/p>\n

Other Tests<\/h2>\n

Aside from canine and feline core vaccine titer testing, veterinarians may be familiar with titer testing for rabies and Leptospira<\/i>. One must consider the legal and diagnostic nuances of these options when considering their use.<\/span><\/p>\n

Rabies titer testing.<\/b> Several options for rabies titer testing exist. Fluorescent antibody virus neutralization (FAVN) testing is most commonly used for animal export purposes and is performed at specialized laboratories and yields an end-point titer that is converted to IU\/mL. This option is associated with a moderate cost and turnaround time. The rapid fluorescent focus inhibition test (RFFIT) is used for rabies titer testing in humans and animals; it is available with end-point (quantitative) and screening (greater than or less than a reference value associated with protective immunity) options. Because of the legal implications of forgoing rabies vaccination, this type of titer testing has limited usefulness with regard to informing vaccination decisions.<\/span><\/p>\n

Leptospira<\/i> titer testing.<\/b> Readily available laboratory microscopic agglutination testing (MAT) for Leptospira <\/i>measures primarily IgM and some IgG levels and is more accurate for assessing exposure or infection rather than protection.6<\/sup> MAT titers against Leptospira<\/i> do not<\/i> correlate well with protective immunity. In addition, MAT does not differentiate between natural infection <\/span>and vaccination. Typically, MAT titers against Leptospira<\/i> decline within 4 to 6 months after vaccination, despite manufacturers\u2019 recommendations for annual vaccination, which are based on challenge studies.8<\/sup><\/span><\/p>\n

Are Titers a Measure of Immunity?<\/h2>\n

The goal of vaccination is to create protection through immunologic memory by stimulating both humoral and cell-mediated immunity. Titer testing measures humoral immunity through levels of blood antibodies against a particular pathogen. Antibodies are formed by the body in response to protection (i.e., vaccination) or infection\/exposure. Titer testing alone does not differentiate between these mechanisms. Detection of functional antibodies (e.g., with VN and HI titer testing) at appropriate levels indicates that a dog or cat should be protected when exposed to a given virus. The amount of circulating antibody is upregulated or <\/span>down-regulated, according to exposure. The immune system also responds to attack with cell-mediated immunity through memory B and T cells, which cannot be measured reliably. Full assessment of vaccine efficacy would require a challenge study, which is not feasible for client-owned animals.6<\/sup> Therefore, a patient with lower titer results may be able to mount an appropriate immune response,9<\/sup> but succumbing to the given disease is unlikely for a patient with positive VN\/HI results.<\/p>\n

The type of disease produced by an infection can help determine whether circulating antibodies (primarily IgG, some IgM) should be indicative of immune status. Infections most susceptible to protection by circulating antibodies are systemic infections.6<\/sup> According to comparison with challenge studies, antibody titers against viruses such as CDV, CPV, CAV,10,11<\/sup> and FPV12<\/sup> have a well-established correlation with protective immunity (<\/span>BOX 1<\/b><\/span>). In a study of 10 specific pathogen-free (SPF) puppies vaccinated against CDV, CPV, and CAV, serologic testing indicated that all vaccinated puppies maintained immunity and all resisted challenge at 4.5\u00a0years, except for 1 dog that did not respond to CDV vaccination.11<\/sup> In another study, researchers vaccinated SPF cats against the feline core vaccine antigens and monitored serologic response and response to challenge at 30 to 36 months.12<\/sup> The positive predictive value of HI serology for resistance to challenge with FPV in this study was 100%.<\/span><\/p>\n

BOX 1 Viruses with Excellent Correlation Between VN\/HI Titers and Protective Immunity<\/div>
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