Core Vaccine Titer Testing

Rabies Titer Test

What do the Micro Rabies Screen Test (MRS) results mean?

The MRS test is a serum neutralization assay based on the RFFIT designed to give a rapid screening for evaluating the need for rabies boosters. Work done at Kansas State Veterinary Diagnostic Laboratory (KSVDL), at Kansas State University, indicates dogs and cats that have been vaccinated for rabies in the past but are not current on their rabies vaccination status respond similarly to a booster as animals that are current on their rabies vaccination. We (KSVDL) suggest that both groups (non-current and current) can now be referred to as “pre-exposure” vaccinated animals.

Although revaccinating dogs and cats that have been exposed to rabies is effective for disease management, the concern is how to protect our pets from unknown exposures. In human medicine, titers are recommended on a periodic basis to confirm adequate response to rabies vaccination. The level defined as an adequate response is 0.5 IU/mL. This titer level is also recognized by OIE as protective in currently vaccinated dogs and cats based on challenge studies.

A review of rabies challenge-studies in currently vaccinated dogs and cats indicates that there is a positive correlation between rabies virus neutralizing antibody (RVNA) titers and the level of protection after virus challenge.

Pre-exposure vaccination coupled with a RVNA titer at or above 0.5 IU/mL is expected to provide greater assurance of protection than does current vaccination status alone.

The MRS test measures RVNA.

The recognized RVNA level of protection for rabies is 0.5 IU/mL. If the level is below 0.5 IU/mL a booster may be indicated depending on the pets rabies exposure risk level. Consult local rabies vaccination laws and regulations for final guidance.

Currently whether an animal requires a rabies booster vaccination is not determined by the level of RVNA in the serum, but by local regulations which can vary by interval and credentials of who may administer the vaccine. Some states allow exemptions to their rabies vaccine requirement based on the potential to compromise an animal’s health status. Veterinarians are obligated to be familiar with and follow legal requirements for vaccine administration accordingly. The Compendium of Rabies Prevention and Control recommendations, used as guidance for local regulations, are routinely reviewed with consideration of new information. Kansas State Veterinary Diagnostic Laboratory is part of this process to influence change that will provide the best guidelines for rabies booster requirements.

Viral Serologic Tests

Historically our profession has considered vaccination a relative innocuous medical procedure as evidenced by the fact that in most states, most vaccines can still be purchased over the counter. The standard operating procedures in veterinary practices has been “when in doubt, vaccinate the patient” believing the benefits of vaccinations always outweigh any possible risks. However the emerging awareness of acute and chronic adverse reactions to vaccines is changing the way vaccine decisions are being made in small animal practices. The current trend is to carefully assess each patient’s disease risk to determine if vaccine(s) are necessary and if so, which vaccine(s) would be appropriate. The use of serological titers can be a valuable tool in making those vaccine decisions.

Vaccine-induced immunity is a multifaceted process involving antigen processing, humoral immunity, cell mediated immunity, local immunity and cell memory. Predicting whether a patient is protected against a disease based solely on a single serum titer is a gross over simplification of this complex immune process. However, despite the drawbacks, the use of serum titers in making logical and informed vaccine–related decisions is gaining some popularity especially in animals with histories of a previous vaccine reaction, or in those individuals prone to allergic reactions, or in those specific cases where concerns of “over vaccinations” persist.

When interpreting any serum vaccination titer, you must take the following key points into consideration;

• Titer results will vary depending on the laboratory and testing methodology used.
• Even among the experts, there is no clear consensus on exactly what titer is considered protective for each specific disease.
• Titers measure the patient’s immune status at a single point in time and may not necessarily reflect the patient’s future immune status one month or one year later.
• A positive titer will protect against development of the clinical signs of that disease in the patient, however it may not prevent the patient from becoming infected or even transmitting the agent.
• Titers only measure circulating humoral antibodies and not cell mediated immunity, local immunity, cell memory or the anamnestic response to a viral challenge. Therefore patients with low titers may in fact be protected.

Serological titers can be useful in determining whether an animal is protected against a specific disease (positive or protective titer) at that point in time.

CANINE: There is an excellent correlation between a “positive” titer and protection against viral challenge with canine distemper virus (CDV); canine adenovirus (CAV-2); and canine parvovirus2 (CPV), and Rabies (RV).

CDV: SN > 1: 32
CAV-2: SN > 1: 32
CPV2: HI > 1: 80

FELINE: In cats there is an excellent correlation between a “positive” titer and protection against challenge with the feline panleukopenia virus (FPL) and rabies virus (RV) but only a good correlation with feline herpesvirus (FHV1) and feline calicivirus (FCV) protection.

FPL: HI > 1: 16
FCV: SN > 1:16
FHV1: SN > 1:16

Serological titers can be used to identify potentially susceptible animals (a negative titer). Because the titer only measures circulating humoral immunity, patients with negative titers may or may not be protected if challenged and therefore may be considered possible candidates for re-vaccination.

References:
AAFP Feline Vaccination Advisory Panel Report: 2013
http://www.catvets.com/guidelines/practice-guidelines/feline-vaccination-guidelines
AAHA Canine Vaccine Guidelines: 2011 https://www.aaha.org/professional/resources/canine_vaccine.aspx#gsc.tab=0
Intervet® website, www.continuum3.com, 2006
Lappin, M.; Use of serologic tests to predict resistance to feline herpesvirus, feline calicivirus, and feline parvovirus infections in cats. JAVMA 2002, vol 220, pgs 38-42.
Mouzin E.; Duration of serologic response to five viral antigens in dogs. JAVMA 2004, vol 224, pgs 55-60.
Mouzin,E.; Duration of serological response to three viral antigens in cats. JAVMA 2004, vol 224, pgs 61-66.
Schultz, R. ;Antibody titers – What do they mean? Proceedings of the Western Veterinary Conference 2005, pgs 542-543.
Scott, F.; Long term immunity in cats with an inactivated trivalent vaccine. AmJVR 1999, vol 60, pgs 652-658
Swan, K.; Leptospiraosis Techincal Bulletin: Proven Protection After Titers Have Waned. Fort Dodge Animal Health, 2005.
Twark, L. Clinical use of serum parvovirus and distemper virus antibody titers for determining revaccination strategies in healthy dogs. JAVMA 2000, vol 217. pgs 1021–1024.