"Infectious disease is common in horses; for this reason we use vaccination as an important component of our preventive strategy. Nevertheless, we're often left with a lack of studies and information that could be important (in decision-making regarding vaccination)," said Paul Lunn, BVSc, MS, PhD, head of the Department of Clinical Sciences at Colorado State University's College of Veterinary Medicine. He and other veterinarians saw that need for research, and at the 2003 American Association of Equine Practitioners' convention, Lunn presented studies that he and six other scientists completed comparing immune responses between commonly used, commercially available vaccines.

The scientists performed two studies. First, they did a seroconversion study of equine influenza virus (EIV), equine herpesvirus-type 1 (EHV-1), and Eastern equine encephalitis (EEE), vaccines. Second, they did a study looking at the immune responses to two EHV-1 anti-abortion vaccines.

The horses used in both studies consisted of aged, non-pregnant mares whose full vaccination histories were not available, although it was thought that the majority would have received annual spring vaccinations in the past. The mares were all maintained in a single barn in one large group and were run through a chute system for sample collection and vaccination. Each had an electronic chip implanted for identification.

For the first study, a total of 40 mares were used, divided into the following groups of 10:

Group A: Intervet Vaccines: Encevac T (EEE, Western equine encephalitis, or WEE, and tetanus), Prestige II (EHV-1, EHV-4, and EIV)
Group B: Fort Dodge Animal Health (FDAH) vaccines: Equiloid (EEE, WEE, and tetanus) and Fluvac EHV-4/1 Plus (EIV, EHV-1, and EHV-4)
Group C: Boehringer Ingelheim (B-I) vaccines: Cephalovac EWT (EEE, WEE, tetanus) and Calvenza EIV/EHV (EHV-1, EIV)
Group D: The control group received a placebo.

Two shots were given, one month apart, and blood samples were taken at the time of each vaccination and at monthly intervals for six months after the second vaccine. "None of the vaccines in either of the two studies in this relatively small number of horses demonstrated any adverse effects of vaccination," said Lunn.

Influenza--"In this experiment all of the horses had a background level of antibody to influenza, indicating prior exposure through exposure or vaccination. Against this background antibody level, the responses to vaccination were generally low, although Boehringer Ingelheim's Calvenza vaccine (group C) did produce a statistically significant response compared to the control horses. Nevertheless, the responses were all relatively low and the statistically the three vaccines could not be distinguished from each other in any other comparison."

EHV-1--Responses to EHV-1 test were even less impressive. Over the entire course of the experiment, EHV-1 antibody responses were not elevated in any vaccine group when compared to the control levels.

EEE--Antibody responses to vaccination in the Intervet (group A) and Fort Dodge (B) groups were both significantly greater than the control group over the time course of the experiment.

Tetanus--Antibody responses to tetanus demonstrated the greatest difference between the vaccines. All the vaccines generated antibody responses greater than the controls, but the Intervet vaccine (group A) had higher tetanus antibody levels than either of the other two vaccine groups.

"There were certainly significant differences between different products, and no single manufacturer had a product that excelled in terms of antibody response to all of these antigens. Each had some strengths, and weaknesses," he concluded about the first study.

"The responses to respiratory products were generally lower," Lunn said. "In defense of the B-I vaccine--it is labeled for a three-dose regimen, but the authors used it extra-label (only two doses). We would recommend that all of these vaccines be administered three times for the primary course, as is stated in current AAEP recommendations."

EHV-1 Abortion Vaccines
In the EHV-1 anti-abortion vaccine study, the scientists compared Intervet's Prodigy (Group E) to the Fort Dodge Pneumabort K vaccine (Group F). Three groups of 5 horses each received either the Intervet or the Fort Dodge vaccine or a placebo (controls) three times at two-month intervals; blood was drawn before administration of the first vaccine, one month after the second vaccine, and one month after the third vaccine. Antibody responses were measured, as was interferon-gamma production (INF-g is produced by T-lymphocytes and fights viral infection by preventing virus multiplication, which may be critical for preventing EHV-1 abortions).

Both vaccination groups demonstrated a strong antibody response to the first vaccine, with no further increase after the second dose. In the Intervet group, there was a response after a third dose, and antibody levels in this group were significantly different from the controls over the course of the study. The Fort Dodge vaccine responses did not quite achieve statistical significance compared to control horses, which may have been a result of the relatively small number of horses in this study.

"Generally, a INF-g response is something you want to see," said Lunn. "The only responses (we saw) were after the third vaccination" in both vaccination groups, although this increase was statistically different from the controls only in the Intervet group (E). This is the first demonstration of a INF-g response to EHV-1 in horses. The Intervet and FDAH vaccines were not found to be statistically different from one another, and it might need a bigger study to establish whether one of these vaccines is clearly superior to the other or not.

The virus neutralization titers of these vaccines were much higher than the responses seen in the respiratory products in the conventional seroconversion study. "Given the greater immune responses to vaccination, it may be wise to consider using the anti-abortion EHV-1 formulations for control of other forms of EHV-1 infection as well as abortion," the researchers suggested.

This study was generously supported by Intervet.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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