The wisdom of routinely giving our horses annual vaccinations is being questioned. In small animals, serious problems have been linked to overvaccination, including vaccine-induced fibrosarcoma in cats and autoimmune diseases.
As a result, more animal owners are requesting veterinarians check antibody titers before vaccinating. However, relying solely on titers as an indication of your horse’s degree of protection from disease has its own problems.
First, while titers measure the level of circulating antibody to a specific organism or toxin, antibodies are far from the entire picture. Before they even come into play, the horse uses a variety of nonspecific immune-system reactions that recognize invading or foreign substances and attack.
There are also specific antibodies along the surfaces of the respiratory tract and digestive tract that can’t be measured by blood tests (titers are done through blood tests). These mucosal antibodies are a strong line of defense, but they’re only produced if there’s exposure to the organism on the mucosal surfaces, e.g. a natural respiratory infection, or an intranasal vaccine.
With the exception of Merial’s West Nile Virus vaccine, all vaccines for viral diseases in use are killed vaccines. They don’t produce much, if anything, along the lines of cellular immunity when given intramuscularly. Also, because challenge studies aren’t a requirement for vaccine licensure, researchers don’t know whether the development of a titer actually means the horse will be protected from disease. (Challenge studies refer to experiments where an animal is vaccinated and then deliberately exposed to the organism to see if it is protected from disease.) For most vaccines, testing for titers is a waste of time and money.
A 2003 study from the School of Veterinary Medicine at Tuskegee looked at titer responses to booster doses of EEE/WEE vaccine. Some horses failed to respond to the revaccination at all, in terms of their serum-neutralizing (SN) titers. Others responded but had SN titers at six months after boostering that were already low or nonexistent.
In contrast, Boehringer Ingelheim published the results of their studies regarding response to an experimental DNA vaccine against equine viral arteritis, where they found high levels of SN antibodies within two weeks of the initial vaccination in all horses. Titers were still detectable at one year.
A similar picture of poor-to-no response to vaccination has been found with some vaccines for respiratory viruses and Potomac horse fever. Clearly it’s time for more manufacturers to follow the lead of Boehringer Ingelheim and Merial and get rolling on development of more effective vaccines.
Where To Get Titers
Your veterinarian will have to draw your horse’s blood, ship the blood to the appropriate laboratory and request the titers. He or she can then advise you on whether or not a vaccination should be done. We advise you to err on the side of caution for most healthy horses. Not vaccinating for a deadly disease like rabies or tetanus is foolish.
The prices we list in this article reflect only pricing at the specific laboratory mentioned. They are what the lab bills your veterinarian. They do not take into account veterinary fees, your veterinarian’s handling costs, staff labor or shipping fees.
Titer: Several tests are available. The most frequently used is FAVN ?'' Fluorescent Antibody Virus Neutralization. Many rabies-free countries require this titer be done, within specific time intervals after vaccination, before an animal can be imported. Titer of 0.5 IU/ml is acceptable. Kansas State University offers a good price, $40 cost to the veterinarian.
Titer: Not currently offered for horses by commercial labs. Dr. David Miller of the USDA’s National Veterinary Diagnostic Laboratory, www.aphis.usda.gov/vs/nvsl/ said they plan to have an ELISA assay for equine tetanus titers soon. ELISA titers considered to be protective in other species are greater than 0.1 IU/ml. Higher titers may be recommended for horses because of their extreme sensitivity to tetanus.
West Nile Virus
Titer: VN (virus neutralization). Most state labs offer this test, as does the National Veterinary Diagnostic Laboratory of the USDA at Ames, Iowa. State costs range from free to $20 or more cost to the veterinarian with a $12 cost to the veterinarian at Ames. West Nile HI from http://tvmdlweb.tamu.edu/schedule.php costs $15.
Titering for WNV in a vaccinated horse won’t tell you if your horse has a protective titer or not because the titer reflecting protection from disease really is not known. The introduction of Merial’s vaccine further complicates interpretation of titers because of its greater potential to stimulate cellular immunity, not measured by titers. However, if the horse wasn’t vaccinated but has a positive titer from natural exposure that should be protective for life.
Titer: HI (hemagglutination inhibition). A $15 per individual titer cost to the veterinarian from the laboratory from http://tvmdlweb.tamu.edu/schedule.php. A complete equine encephalitis panel is $50. Many university and/or state veterinary diagnostic labs offer these titers. However, many state veterinarians working in areas with a high risk of exposure, e.g. Florida, caution that vaccination recommendations based on titer do a poor job of predicting protection from disease. According to Dr. Arturo Angulo from the Texas A&M diagnostic laboratory, post vaccination HI titers to VEE, EEE or WEE typically run 1:40 to 1:80 six to eight months after vaccination.
Potomac Horse Fever
Titer: IFA (indirect fluorescent antibody). An $18 cost to the veterinarian from the Animal Health Diagnostic Laboratory at Cornell (http://diaglab.vet.cornell.edu/). Titers of up to 1:640 have been found as a result of vaccination, but there’s tremendous variation from lab to lab. False positives are also a problem. Protection can’t be estimated from IFA titers. Studies looking specifically at SN (serum neutralizing) antibodies in response to PHF vaccines found poor-to-no response. SN antibodies are the first line of defense in the blood, binding and “neutralizing” the organisms before they can get to the tissues. Even more importantly, infectious strains other than the one used to make the vaccine have been identified. Poor vaccine response and strain variation are believed to explain why many horses aren’t protected by this vaccine.
Titer: HI (hemagglutination inhibition). Cost to veterinarian is $13 cost from the Animal Health Diagnostic Laboratory at Cornell. Protective titers are essentially unknown because challenge studies have never been done, but working with a laboratory such as Cornell’s, which is involved in monitoring vaccine responses, will help your veterinarian with interpretation of results.
Strangles (S. equi)< /B>
Titer: Not widely available. As with PHF, titers aren’t indicative of prevention from disease. The ability of serum from intramuscularly vaccinated animals to neutralize the bacteria begins to wane six to nine months after vaccination, but even animals with good titers may not be protected. Intranasal vaccine offers better protection despite low or no blood titer responses.
Titers have limited usefulness in determining vaccination needs, although a blood titer can tell you if your horse has protection due to a natural exposure to the West Nile virus.
Strangles and Potomac Horse fever titers won’t indicate protection, even if they appear to be good. While the intranasal flu and strangles vaccines are better than intramuscular vaccines, they don’t induce blood antibody titers. Epidemiologists believe killed vaccines provide reliable protection for four to six months.
Horses with chronic laminitis and a history of prior vaccine reactions are at highest risk for adverse effects from vaccine, which makes titer results more important to them. Talk with your vet.